Palaeogeography, Palaeoclimatology, Palaeoecology 192 (2003) 187^219
www.elsevier.com/locate/palaeo
Arthrophycus in the Silurian of Alabama (USA) and
the problem of compound trace fossils
Andrew K. Rindsberg a; , Anthony J. Martin b
a
b
Geological Survey of Alabama, P.O.Box 869999, Tuscaloosa, AL 35486-6999, USA
Department of Environmental Studies, Emory University, Atlanta, GA 30322, USA
Received 12 February 2002; accepted 6 December 2002
Abstract
Arthrophycus brongniartii (Harlan, 1832) is common in marginal-marine deposits in the Silurian Red Mountain
Formation of Alabama. The ichnospecies, the second to be named in North America, is revived and emended after
long disuse. Transitional forms to Rusophycus isp. and other morphologic evidence indicate that the maker of
Arthrophycus was an arthropod, perhaps a trinucleine (raphiophorid?) trilobite. Interconnection of Arthrophycus and
Nereites biserialis, as well as intergradation of Arthrophycus with Cruziana aff. quadrata, Phycodes flabellum, and
Asterosoma ludwigae, indicate that these Red Mountain trace fossils were made by the same species of arthropod.
Possible relationships with Arthrophycus alleghaniensis (Harlan, 1831) in the Silurian belt from Ontario to Tennessee
are also explored. Ichnofamily Arthrophycidae Schimper, 1879 is emended. The ichnofamily is interpreted as chiefly
the work of arthropods. Arthrophycus and other trace fossils from the Silurian of Alabama constitute a test case to
build criteria for recognizing the members of complexes of trace fossils. In general, criteria such as interconnection of
different forms, intergradation among unconnected forms, similarity of size, similarity of morphologic elements, and
co-occurrence should be examined in order to determine the biologic and ethologic interrelationships of trace fossils.
7 2003 Elsevier Science B.V. All rights reserved.
Keywords: ichnology; taxonomy; Arthrophycus; Alabama; Lower Silurian; Trilobita
1. Introduction
The classic parable of the six blind men and the
elephant tells how each man gave his interpretation of the nature of an elephant by feeling only
part of it, a method that resulted in an incomplete
* Corresponding author. Tel.: +1-205-349-2852;
Fax: +1-205-349-2861.
E-mail addresses: [email protected]
(A.K. Rindsberg), [email protected] (A.J. Martin).
model of a complex animal. The tale illustrates
the problem of how to name forms that intergrade or interconnect, a di⁄culty that has long
vexed biologists, paleontologists, and ichnologists
alike (e.g. Seilacher, 1970; Pickerill, 1994). For
paleontologists, the need to deal with fragmentary
and misleading material makes the problem even
more di⁄cult, and for ichnologists, the fact that
the material was never living makes the problem
harder yet. In principle, we could examine a
broad array of case studies in order to determine
the best procedural methods. However, in prac-
0031-0182 / 03 / $ ^ see front matter 7 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0031-0182(02)00685-5
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tice, even a single test case can suggest potential
procedures and rule out others as valueless.
As de¢ned by Pickerill (1994, p. 23), compound
trace fossils are those that comprise several parts
that together form a whole, as opposed to composite trace fossils, each of whose parts can be considered as whole. Forms that are connected and
intergrade continuously, such as some Cruziana
and Rusophycus, are compound, and this is strong
evidence of a single maker. In addition, ichnotaxa
that intergrade only through a series of morphologically similar forms, but are not physically connected, are in some cases also considered as evidence of a common tracemaker, but this evidence
is neither as clear nor as strong. In this paper, we
use the term interconnection to refer to specimens
that are physically connected, and the term intergradation to refer to suites of specimens that are
related through a morphologic series.
Arthrophycus Hall, 1852 and related ichnotaxa
in the Lower Silurian Red Mountain Formation
of Alabama (USA) constitute a productive test
case for the taxonomic treatment of compound
trace fossils. The group includes apparently disparate traces as series of Rusophycus, Cruziana a¡.
quadrata, Asterosoma ludwigae, and Nereites biserialis, some of which are interconnected, and
others of which are intergradational in form or
share morphologic elements. As will be shown,
this complex of trace fossils was probably made
by one species of arthropod, most likely a trinucleine trilobite.
2. Arthrophycus and related traces in the Silurian
of Alabama
2.1. Setting
The Red Mountain Formation consists largely
of interbedded shale and sandstone with minor
oolitic hematite ironstone and shelly limestone
beds (Butts, 1926, 1927; Chowns and McKinney,
1980) (Fig. 1). Fossils indicate that the formation
is Llandoverian to Pridolian and contains four
stratigraphic sequences separated by widespread
unconformities (Table 1 ; Ulrich in Butts, 1926;
Burchard and Andrews, 1947; Berry and Boucot,
1970; Chowns and McKinney, 1980; Rindsberg
and Chowns, 1986; Berdan et al., 1986; Chowns,
1999, written communication, 2002). The faunal
assemblages are poorly understood. Arthrophycus
and related trace fossils studied here are from the
lower two sequences of the Red Mountain Formation (Rhuddanian and/or Aeronian).
The ichnology of the Red Mountain Formation
was previously studied in large, fresh roadcuts of
northwestern Georgia and southeastern Tennessee
by Frey and Chowns (1972), Rindsberg (1983),
and Rindsberg and Chowns (1986). Frey and
Chowns (1972) reported uncommon ?Arthrophycus (A. brongniartii) from Ringgold Gap, Georgia.
Most of our knowledge of Silurian ichnology in
Alabama comes from three sites on Red Mountain in Je¡erson County. Only the freshest sites
contain abundant Arthrophycus brongniartii,
which occurs mainly in relatively friable sandstone. From southwest to northeast, the Alabama
sites include the tailings pile of former Sloss Mine
No. 2 in Bessemer, Red Mountain Expressway cut
on U.S. Highway 31 in Birmingham (Thomas et
al., 1971), and Chalkville Road cut east of Chalkville at Red Gap (Fig. 1). One of us (A.K.R.)
mapped the geology of Red Mountain in the Birmingham metropolitan area from Bessemer to
Chalkville (Rindsberg and Osborne, 2001; Rindsberg, unpublished data). Other exposures are too
weathered to yield much information about Arthrophycus.
More than 30 ichnotaxa have been recognized
in the lower Red Mountain Formation (Table 2).
Represented behaviors include resting, dwelling,
feeding, and locomotion. The high diversity and
abundance of trace fossils, their inferred behaviors, and associated lithofacies indicate that
most of these animals lived in a normal, shallow-marine environment (Frey and Chowns,
1972; Chowns and McKinney, 1980).
Trace fossil assemblages of the Red Mountain
Formation correspond to a continuous gradient
from shallow continental shelf to shoreface
(Rindsberg and Chowns, 1986 ; Table 2). The
storm-dominated shelf facies of alternating shale
and sandstone yields the most diverse assemblage,
commonly including Planolites, Chondrites, and
Scotolithus. Hematitic sandstone (lower shoreface)
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189
Fig. 1. Location map showing major outcrops of the Red Mountain Formation in Birmingham area, Je¡erson County, Alabama.
strata contains a higher percentage of sandstone
beds in which Asterosoma, Monocraterion, Arthrophycus, and Petalichnus are common. Upper shoreface beds, consisting largely of ripple-marked
sandstone, are dominated by relatively large Arthrophycus and accompanied by Lockeia and Lingulichnus. In contrast, oolitic ironstone (tidal
channel) facies contain almost no trace fossils.
Table 1
Stratigraphy of Silurian and adjacent units in the Birmingham area, Alabama (Chowns, 1999; Rindsberg and Osborne, 2001)
Period: Stage
Formation, Member
Mississippian
Fort Payne Chert
Maury Formation
Upper Silurian: Pridolian
Red Mountain Formation: sequence 4
Lower Silurian (Llandoverian):
late Aeronian^Telychian
sequence 3, including Hickory Nut and Ida ironstone seams at base
Rhuddanian or Aeronian
sequence 2, including Irondale and Big ironstone seams at base
Rhuddanian
sequence 1
Upper Ordovician
Middle Ordovician
Sequatchie Formation
Chickamauga Limestone
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Table 2
Gradient of trace fossil assemblages in the lower Red Mountain Formation in the Birmingham area, Alabama
Hematitic sandstone (upper shoreface): large Arthrophycus brongniartii (Harlan), Lingulichnus isp., Lockeia isp., Rusophycus ispp.
Hematitic sandstone (lower shoreface): Arthrophycus brongniartii, Asterosoma ludwigae Schlirf, Dictyodora major Mc Coy, Diplocraterion isp., Lockeia isp., Monocraterion isp., Nereites isp., Petalichnus isp., Rusophycus isp.
Oolitic hematite ironstone (tidal channel): almost no trace fossils; Rusophycus isp., obscure hypichnia
Alternating gray sandstone and shale (shallow shelf): Arenicolites ispp., Arthraria isp., small Arthrophycus, Asterosoma ludwigae,
Chondrites £exuosus (Emmons), C. gracilis (Hall), C. a¡. intricatus (Brongniart) Sternberg, Cochlichnus isp., Cruziana a¡. quadrata Seilacher, Cruziana isp., Curvolithus? isp., Cymataulus isp., Dictyodora major Mc Coy, Diplocraterion ispp., Lockeia isp., Monocraterion isp., Nereites missouriensis Weller, N. biserialis Seilacher, Olenichnus isp., Palaeophycus (‘Halopoa’) imbricatus Torell, P.
tubularis Hall, Petalichnus isp., Phycodes £abellum (Billings), Phycodes? isp., Planolites beverleyensis Billings, P. montanus Richter,
Protovirgularia isp., Ptychoplasma isp., Rosselia? isp., Rusophycus a¡. carleyi (James), R. pudicus Hall, Scotolithus mirabilis Linnarsson
2.2. Behavior represented by the Alabama
Arthrophycus
Arthrophycus and other trace fossils in the Red
Mountain Formation form a group related by
interconnection, intergradation, similar dimensions, and morphologic similarities that, taken together, strongly suggest a common tracemaker.
Some of them qualify as compound trace fossils
as de¢ned by Pickerill (1994) ; others are closely
related morphologically. Our inferences regarding
such interrelationships to Arthrophycus depend
only on material from the Red Mountain Formation of Alabama and Georgia. Although we compare this material with classic Arthrophycus from
the Silurian of New York and Pennsylvania, our
conclusions regarding compound ichnogenera do
not in any way depend on this comparison.
Arthrophycus brongniartii Harlan, 1832 is a
nearly straight to curved burrow having a complex internal structure. As elucidated by Seilacher
(2000), the internal structure forms a shallow
spreite, which in the Alabama material is retrusive. The burrow can be wall-like and the cross
section accordingly subquadrangular. The regularly annulate lower surface, which represents
the ‘search image’ most researchers have for Arthrophycus, is only rarely seen in the Alabama
material, mostly owing to the common preservation of the Alabama specimens as epichnia. The
annulate lower surface is only the outer surface of
a sheath around the lower part of an inner core
that has a much coarser, imbricate spreite structure (Fig. 2A), an observation made by Hall
(1852, p. 5, pl. 1, ¢g. 1) but long overlooked.
Horizontal sections show a series of imbricated
sediment pads about two to three times as long
as they are wide, surrounded by a sheath of ¢ne,
oblique laminae (Fig. 2B,C). As seen in horizontal
sections from above, Arthrophycus presents a very
di¡erent appearance from that of its lower surface, one that shows a considerable range of
form (Fig. 3).
The tracemakers’ direction of movement is
shown by the lunate form of the sediment wedges
or pads, whose concave surfaces faced in the same
direction as the animals’ locomotion. The animals’ course commonly followed the upper surfaces of wave-rippled sandstone, in some cases
plowing through crests and moving sand upward
to form wall-like sandstone spreites in and above
troughs. As seen now with overlying shale weathered away, the burrows commonly begin and end
as if the animal had moved through the super¢cial
mud layer before and after digging along the
sand^mud interface for some distance. This must
have expended considerable e¡ort, which suggests
that the sand^mud interface was their goal.
In contrast, specimens from Ontario to Tennessee are almost exclusively reported as hypichnia
on the bases of sandstone beds, although R.R.
McDowell (written communication, 2002) documented an example of an endichnion in a Silurian
shale bed in western Virginia. In epichnia and
hypichnia alike, the animals plowed along the
sand^mud interface for relatively long distances,
displacing and packing sand extensively, expending far too much e¡ort for simple locomotion.
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Fig. 2. Arthrophycus brongniartii from the Red Mountain Formation, Chalkville, Alabama, in diverse epichnial forms. Scale
bars = 1 cm. (A) A. brongniartii showing overall stellate pattern from overlapping burrows; individual burrows exhibit annulate
lower surfaces, imbricated upper surfaces, and sedimentary pads (evident as shallow spreite) surrounded by laminated sheaths.
(B) Crosscutting relationships of burrows, with close-up view of imbricated upper surfaces and sedimentary pads. White lines parallel individual segments of one burrow. (C) Burrow parallelism in A. brongniartii. Note angular segments along the course of
burrow near the top of the picture, and abrupt turn of other burrow toward top burrow preceding parallelism.
Evidently, the tracemakers fed preferentially on a
mix of sand and mud.
Close observation clari¢es that the Alabama
traces are made up of many short segments that
correspond to the large, imbricate sediment pads
(Figs. 2B, 4A and 5A ; compare Fig. 6). The makers were evidently not capable of smoothly and
gradually changing direction, but had to do so
in discrete increments. These increments are of
regular length compared to burrow width, and
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Fig. 3. Diagram of Alabama Arthrophycus brongniartii showing range of appearance in di¡erent sections. (A) Reconstruction in full relief. There is a great deal of variation within individual burrows and between di¡erent burrows, but this is
representative. (B) Transverse section showing subquadrangular outline. (C) Longitudinal section through middle part of
burrow. The large crescentic pads of the inner zone face the
opposite direction from the small crescentic pads of the outer
zone. The outer zone was evidently made by the tips of digging appendages while the main part of the body was involved in the inner zone. (D) Longitudinal section through
lower part of burrow. (E) Reconstructed hypichnion showing
zipper-like construction of sediment pads. Pads can also be
arranged in a nearly transverse pattern.
thus may approximate the length of the tracemaker. For example, one 5-mm-wide burrow
showed successive increments of 17, 11, and
15 mm, suggesting a sti¡-bodied tracemaker that
was two to three times longer than it was wide
(Fig. 4A,B). Looping behavior is also evident in a
New York specimen ¢gured by Hall (1852), but
with a smoother course (Fig. 4C,D).
A recurrent behavioral pattern, here named
multiple-crossing behavior, is the tendency for animals to cross previous paths at a single point,
forming an apparently radiating pattern (Figs.
2A and 5). Harlan (1832) mentioned this super¢cially stellate pattern in his original description
of Arthrophycus brongniartii, and it has been ¢gured without comment several times (e.g. Faill
and Wells, 1974, ¢g. 6a; Cotter, 1983, ¢g. 7A).
In the Alabama material, these nodes consist of
as many as seven overlapping burrows, each having about the same width. Remarkably, the burrows commonly show a sharp bend toward the
point of overlap from at least 12 cm away, demonstrating that the animals were aware at a distance of several body lengths not merely of previous burrows, but of their point of intersection.
Spreites show roughly equal numbers of burrows
oriented inward and outward, which would not be
expected in burrows radiating from a central shaft
(Fig. 5D). Multiple-crossing behavior is common
in crisscrossing earthworm trails today (A.K.R.,
personal observations).
The senses used for this purpose might have
been chemoreceptive (animal buried within the
sediment) or visual (animal scouting atop the sediment), with the former more likely. The organs
were probably not tactile or auditory, because
other animals would already have passed by
such points. Multiple-crossing behavior might
have been the simplest way to minimize ingestion
of previously digested sediment while crossing a
network of many older paths (A. Uchman, oral
communication, 2001). This is supported by another common tendency, paralleling behavior, that
can also be interpreted as avoidance of previous
burrows (Figs. 2C and 4C). Looping behavior
does not support this model, but the other behaviors are much more common. Alternatively, multiple-crossing behavior may be social, a way for
animals of one cohort to make a census of their
neighbors and maintain their distance, or to seek
potential mates. In either case, the behavior indicates tracemakers with fairly complex reactions to
stimuli that most likely represent chemotaxis.
2.3. Arthrophycus and series of Rusophycus
In Alabama, Arthrophycus intergrades with
progressively shallower spreite burrows, some of
which amount to a connected series of the resting
trace Rusophycus with underlying imbricate structure. That is, the spoon-like pads of sediment that
constitute the inner spreite (Fig. 2A,C) become
elongated and somewhat £attened, and a few of
these show the details that identify them as Rusophycus. Similarly, Cruziana a¡. quadrata (see Section 2.5) intergrades with hypichnial series of Rusophycus (Fig. 6).
These resting traces indicate a trilobite- or limuline-shaped maker with a large, broad head having genal spines, and a thorax having ¢ve to seven
pairs of digging appendages. These features show,
as nothing else short of a preserved body would,
the morphology of the tracemaker’s ventral surface (cf. Rusophycus morgati Baldwin, 1977a,b).
The presence of genal spines demonstrates that
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193
D
Fig. 4. Arthrophycus as a looping burrow. Scale bars = 1 cm. (A) Arthophycus brongniartii from the Red Mountain Formation of
Chalkville, Alabama, forming a counter-clockwise loop, crossover point, and angular segments following crossover. (B) Close-up
of point of crossover, where tracemaker moved from left to right. (C) Specimen of A. alleghaniensis collected by Hall (1852)
showing a similarly proportioned loop and crossover point. Medina Sandstone, Medina, Orleans County, New York. NYSM
31 663. (D) Figure of same specimen from Hall (1852, pl. 2, ¢g. 1c), showing selectivity of the illustrator with respect to the original sample.
the whole venter is registered. Striae made by digging appendages indicate that the maker was an
arthropod, and their low number rules out nearly
all Early Silurian arthropod groups but trilobites
(superfamily Trinucleina) and xiphosurans.
2.4. Arthrophycus and Nereites biserialis
(Seilacher, 1960)
Actually connected with Arthrophycus are burrows whose inner core consists of a shallow, im-
bricate spreite and whose outer sheath is lobate
(Fig. 7A). These epichnial burrows resemble Nereites and therefore may shed light on its mode of
formation. Short Nereites-like segments are intergradational to specimens that could be identi¢ed
as epichnial N. biserialis (Fig. 7B), which show
regularly alternating lobes of sediment that have
a similar spacing to those of Arthrophycus (Fig.
7C). The lateral lobes may thus be interpreted as
recording jerky or episodic movement. Dawson
(1890, pp. 596^597) very brie£y noted a similar
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Fig. 5. Evidence that the ‘stellate’ form of Arthrophycus brongniartii is a compound trace fossil in the Red Mountain Formation.
All are epichnia. Sloss Mine No. 2, Alabama. Scale bars = 1 cm. (A) Parallelism followed by convergence on a single point by individual burrows. (B) Numerous intersecting and overlapping burrows, contributing to radiating patterns and much confusion.
(C) Close-up of one of the points of intersection. (D) Interpretation of tracemaker directions based on spreite and crosscutting
relations, with circles denoting major points of overlap between burrows. Eight burrows were directed inward and only six outward, suggesting that the tracemakers may have sometimes shifted vertically at the node; their subsequent courses would not be
visible at this horizon.
relationship between ‘Arthrichnites’ (Arthrophycus) and Nereites. One of us (A.J.M.) observed
juvenile limulids (Limulus polyphemus) making
similar Nereites-like burrows in newly emergent
intertidal sands on the Georgia coast.
2.5. Arthrophycus and Cruziana a¡. quadrata
Seilacher, 1970
Other Red Mountain trace fossils that show
similarities to Arthrophycus include hypichnial
and epichnial trail-like burrows, Cruziana a¡.
quadrata (Fig. 8). Frey and Chowns (1972) previously described these burrows from the lower Red
Mountain Formation at Ringgold, Georgia, dubiously identifying them as Gyrochorte, and noted
their similarity to Upper Ordovician forms described by Osgood (1970) as repichnia of Cryptolithus.
Like Arthrophycus brongniartii, Cruziana a¡.
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195
relatively narrow width of most Cruziana a¡.
quadrata and their occurrence mainly in relatively
¢ne-grained sandstone. Although they may represent the behavior of two species, another explanation is that these di¡erences represent the behavior of juveniles and adults of a single species.
2.6. Arthrophycus and Phycodes £abellum
(Miller and Dyer, 1878)
Fig. 6. Series of Rusophycus, Red Mountain Formation,
Chalkville, Alabama. Scale bar = 1 cm. Each resting trace is
of similar length and includes registration of cephalic and
thoracic areas, indicating the approximate body length of the
tracemaker. Note abrupt changes of direction, which are
present in all Alabama material but are particularly evident
in this specimen. A photogenic hypichnion is ¢gured, but epichnial examples also exist.
quadrata shows a separation into a broad inner
zone £anked by narrow outer zones that correspond to the walking or digging appendages.
The burrows are bilobate to quadrilobate depending on cleavage relief. The inner pair of lobes is
relatively deep and comprises the bilobate undertrail (Rindsberg, 1983). The inner lobes are nodose and/or striate and the outer lobes are obliquely and ¢nely striate.
As in Arthrophycus, the course is a series of
short segments with a 2:1 to 3:1 length:width
ratio (Fig. 8C,D). The burrows are trail-like but
can occur at the bases of sandstone beds as thick
as 8.7 cm, disappearing upward into the sandstone. Di¡erences from Arthrophycus include the
At the Red Mountain Expressway, trail-like
Cruziana a¡. quadrata may branch in a fan-like
pattern that where densely repeated is recognizable as Phycodes £abellum, as redescribed by Osgood (1970) from the Upper Ordovician of Ohio
(Fig. 9). Forms with few and many branches are
seen in the same bed (unit 38 of Thomas et al.,
1971), and their behavioral identity is shown by
each having quadrilobate to bilobate construction. The spreite and subquadrangular outline of
the Phycodes galleries suggest an a⁄nity with Arthrophycus. In Alabama and Ohio forms alike, the
feeder gallery tends to have a vertical spreite and
the fan-like part a subhorizontal spreite (Fig. 9B).
The animal penetrated sand beds 4.3^8.7 cm thick
before reaching mud; like the maker of A. brongniartii, it excavated along the mud^sand interface,
although in this case the result was hypichnial
rather than epichnial.
2.7. Arthrophycus and Asterosoma ludwigae
Schlirf, 2000
The branching pattern of Phycodes £abellum
also seems to be intergradational with that of Asterosoma ludwigae in the same beds, so that
branched Cruziana a¡. quadrata can represent
an incipient form of Asterosoma ludwigae as well
as of Phycodes £abellum. Although Asterosoma
and Arthrophycus have not been found connected
in Alabama material, the axial burrow leading to
the fan of one specimen of Asterosoma (Fig. 10B)
has similar dimensions to both A. brongniartii and
C. a¡. quadrata, hinting at a possible relationship
among these ichnotaxa.
Asterosoma ludwigae is a back¢lled burrow
whose fan-like branches are arranged in a series
of one-sided fans rather than radiating from a
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Fig. 7. Intergradation between Arthrophycus and Nereites, Red Mountain Formation, Chalkville, Alabama. Interconnection also
occurs, but in less photogenic material. All are epichnial. Scale bars = 1 cm. (A) Basal part of partially preserved Arthrophycus
showing Nereites-like morphology, with lateral sediment pads resembling pelletal structure but in zipper-like pattern. (B) Nereites
biserialis, but with morphology and dimensions similar to Arthrophycus shown in (A). (C) Full expression of Nereites biserialis.
Compare both (B) and (C) to Fig. 5.
central area as in A. radiciforme von Otto, 1854
(Fig. 10A). Asterosoma ludwigae is common in
shoreface deposits at all three sites. The burrows
are relatively deep in these beds, occurring on the
bases of sandstone beds as much as 2.7 cm thick,
and penetrating as deep as 3.5 cm in thicker sandstone beds.
2.8. Arthrophycus and unrelated arthropod traces:
Rusophycus pudicus Hall, 1852, Rusophycus a¡.
carleyi (James, 1885), Petalichnus isp.
In addition to the foregoing ichnotaxa, arthropod resting traces (Rusophycus pudicus and R. a¡.
carleyi) and trackways (Petalichnus isp.) occur in
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Fig. 8. Cruziana a¡. quadrata, Red Mountain Formation, Chalkville, Alabama. Scale bars = 1 cm. Some of the di¡erences among
these trace fossils probably derive from di¡erences in the substrate, especially water content and subsequent compaction. (A) Epichnial trail-like burrow showing hints of quadrilobate morphology. (B) Crosscutting trail-like burrows of similar size, with better
de¢nition of quadrilobate form. (C) Trail-like burrows exhibiting lateral genal spine impressions and interiors showing similarities
to Nereites biserialis. Note the considerable range in width. Turns show abrupt changes in direction, evidenced especially in inner
lobes. (D) Bilobate trail-like burrows perhaps referable to C. a¡. quadrata, showing segments with 3:1 length:width ratio and
variable expressions related to depth of traces.
upper shoreface deposits of the lower Red Mountain Formation. Although these show some similarities to Arthrophycus brongniartii, their di¡erent size and morphologic elements suggest that
they were probably made by other species.
Well-preserved Rusophycus from the Red
Mountain Formation consists mostly of very
small specimens that are di⁄cult to assign to ichnospecies (cf. Seilacher, 1970). Larger, indistinct
specimens are also present. Most of the small
specimens are similar to Rusophycus carleyi
(James, 1885) or R. pudicus Hall, 1852. R. pudicus
has a tapering, calymenid-shaped outline that is
very di¡erent from the forms seen in the series of
Rusophycus described in Section 2.3.
Rusophycus carleyi has an unusual ellipsoid
outline, and a bilobate to quadrilobate transverse
section in which the outer lobes are relatively
prominent and the inner lobes are shallow and
nodose (Osgood, 1970, p. 306; Osgood and Drennen, 1985). James (1885) and Osgood (1970) attributed R. carleyi to isotelid trilobites. Although
the quadrilobate pattern of R. carleyi is super¢cially similar to that of Arthrophycus brongniartii,
the deep outer lobes (endite workings) and shallow inner lobes (coxal impressions) do not match
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Fig. 9. Phycodes £abellum showing a⁄nity to Cruziana a¡. quadrata. Hypichnia, Red Mountain Formation, Red Mountain Expressway, Alabama. Scale bars = 1 cm. (A) Form with evident quadrilobate morphology. (B) Form with less de¢nition of quadrilobate morphology but similar dimensions and geometry. Note shadow cast by subvertical spreite of main gallery on left, contrasting with subhorizontal spreite of branched galleries on right.
Fig. 10. Asterosoma ludwigae, Red Mountain Formation, Chalkville, Alabama. Scale bars = 1 cm. (A) Positive-relief hypichnion
(compare to Fig. 9). (B) Negative-relief epichnia (A); one feeder burrow (F) is visible on left edge. Associated trace fossils include
narrow trails (T) and a delicate form of Chondrites (C).
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Arthrophycus, which tends to have deep inner
lobes and shallow outer lobes.
Examples of Petalichnus are comparable in size
with the largest specimens of Rusophycus and Arthrophycus. Although the trackways are generally
poorly preserved in these relatively coarse sandstones, some might have been made by the same
animals, though no intergradational or interconnecting forms are known. The presence of Petalichnus is evidence that trilobites or trilobitiform
arthropods were part of the assemblage.
2.9. Summary of Arthrophycus and related forms
in the Red Mountain Formation
In sum, the Alabama material yields feeding
burrows (Arthrophycus, Nereites, Cruziana, Phycodes, Asterosoma), and connected resting traces
(series of Rusophycus) in interconnected, intergradational, or morphologically similar forms. These
relationships make it likely that at least some of
these trace fossils were made by the same species
of tracemaker. Nevertheless, if any or all of these
forms were assigned to one ichnotaxon on the
basis of interconnection, intergradation, or presumed maker, communication among scientists
would be hindered rather than enhanced.
3. The maker of Alabama Arthrophycus
With the preceding information in mind, a review of evidence for the animals that made Arthrophycus and related trace fossils in the Red
Mountain Formation is appropriate. The tracemakers must have had the following characteristics :
(1) The animals were bilaterally symmetrical
and had a £at or nearly £at venter, as shown by
the subquadrangular cross section of Arthrophycus (especially where series of resting traces are
included in the trace), and by the relatively £at
£oor of each imbricated pad of sediment.
(2) The tracemakers had at least ¢ve to seven
pairs of digging appendages, as shown by the
number of leg impressions in arthrophycid chains
of Rusophycus.
(3) These appendages could be moved indepen-
dently on each side, as shown by the zipper-like
structure of some Arthrophycus and the biserial
structure of Nereites biserialis.
(4) The animals had a broad head with genal
spines, as shown by impressions in chains of Rusophycus. Appendages extended about as far laterally as the genal spines.
(5) The animals were deposit-feeders, apparently preferring a mixture of mud and sand (though
this could be the result of taphonomic bias).
(6) The animals were about two to three times
as long as they were broad, as shown not only
by the dimensions of Rusophycus, but also by distances between minor changes of direction in Arthrophycus.
(7) The animals could sense previously formed
Arthrophycus, perhaps chemoreceptively, across a
lateral distance of at least 12 cm.
Taken together, these characteristics indicate an
arthropod tracemaker for the Alabama Arthrophycus, one that had at least ¢ve pairs of digging
legs and a broad head with genal spines, and
could perhaps sense chemoreceptively: a xiphosuran or trinucleine trilobite similar to the raphiophorids.
The appendages of only a few species of trilobites are known, but fortunately they include a
Late Ordovician trinucleid relative of the raphiophorids, Cryptolithus tesselatus (Fig. 11). Apart
from the di¡erence in age, this is not a possible
maker of Arthrophycus, because it has the wrong
number of appendages; and the genal spines of
this species are too outspread and far too long,
though some members of the genus have short
genal spines. However, as the closest trilobite
whose appendages are known, this species will
be used as an imperfect analog of the tracemaker.
The tracemaker probably had more strongly built
thorax and appendages, and much shorter genal
spines.
Richter (1919) and Schevill (1936) long ago already understood on the basis of functional morphology that the locomotory abilities of Cryptolithus must be similar to those of modern Limulus,
which is rather powerfully built for its size (Eldredge, 1970; Fisher, 1975). This assessment implies the ability to walk on the sediment, to dig
temporary burrows in the substrate, and even to
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Fig. 11. Cryptolithus, an imperfect model for the most likely
tracemaker of Arthrophycus, series of Rusophycus, Cruziana,
Phycodes, and Asterosoma in the Red Mountain Formation
of Alabama. From Treatise on Invertebrate Paleontology,
courtesy of and z1959, The Geological Society of America
and The University of Kansas.
swim. Bergstro«m (1975) analyzed the functional
morphology of xiphosurans and concluded that
only the limulines, ranging from the Carboniferous to present, could burrow.
The steeply sloping forward brim of the cephalon is no great hindrance to Limulus, which plows
into the sediment head¢rst with the aid of four
pairs of powerful walking legs (Eldredge, 1970).
The blind trilobite Cryptolithus was probably also
a burrower (Raymond, 1920). As Richter ex-
201
plained for Cryptolithus, such activity is not conducive to the construction of long-term open burrows, but to short-term burrows having a £attish
cross section. Shifting such a burrow vertically
would result in a spreite burrow having a subquadrangular cross section, as observed in Arthrophycus (Sarle, 1906; Seilacher, 2000). Campbell
(1975) pointed out that the elaborately pitted
cephalic fringe of Cryptolithus is a very strong
structure for its weight. Although he considered
that the pits would be a liability in burrowing and
it was therefore more likely to be a sensory array,
nature often ¢nds multiple uses for organs and
the fringe’s strength may be related to a burrowing function.
The appendages of Cryptolithus were discovered early and have been discussed often (Beecher,
1895; Raymond, 1920; StSrmer, 1939; Whittington, 1959; Osgood, 1970; Bergstro«m, 1972;
Campbell, 1975). As described by Bergstro«m, the
walking appendages are seated between coxal segments near the trilobite’s axis; there are about 20^
22, but the pygidial ones are quite small and the
total number of appendages involved in digging
might be ten. In Bergstro«m’s reconstruction, they
form a chevron pointed backward; Campbell
(1975) gave evidence suggesting that the pygidial
set is even smaller than previously supposed, and
was involved in fanning and/or ¢ltering a current
rather than digging. Each appendage bifurcates
into a clawed telopodite (walking leg) and a
brush-like exite. The tips of the walking leg have
bristles and very small claws that are not independently movable, and which therefore would have
made multiple parallel striae in the sediment. The
chevron of Cryptolithus appendages could have
made a trackway consisting of series of chevrons
of imprints (Petalichnus). Both Bergstro«m and
Campbell favored the idea that Cryptolithus excavated sediment with the thoracic and cephalic telopodites and fanned it backward with the comblike exites to ¢lter it for food.
Concerning diet, Raymond (1920) reported direct evidence : a specimen of Cryptolithus whose
gut was crammed with mud. The maker of Arthrophycus is also presumed to have been a deposit-feeder. Modern xiphosurans are carnivores,
feeding on infaunal invertebrates such as poly-
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chaetes and bivalves. Carnivory on infauna seems
unlikely for the maker of large Arthrophycus,
which commonly occurs without other trace fossils.
Osgood (1970) reported rare resting traces of
Cryptolithus in the Upper Ordovician of Ohio,
naming them Rusophycus cryptolithi, and his interpretation was enthusiastically con¢rmed by trilobite researchers (Bergstro«m, 1972; Campbell,
1975). The resting traces may have been compacted; they are now as deep as 4^5 mm. Curiously, they represent little more than the cephalon, not the thorax or pygidium. In Campbell’s
analyis (1975, ¢g. 2), the telopodites excavated a
pair of deep, inner longitudinal lobes, the exites, a
pair of shallow outer lobes. Arthrophycus-burrowing trilobites similar to Cryptolithus would have
made longer lobes.
Osgood’s (1970) reports of possibly Cryptolithus-made trails (Cruziana) and trackways (Trachomatichnus) are more controversial. Not enough
is known about the appendages of most trilobites
to rule out other groups as makers of Trachomatichnus. Bergstro«m (1972) accepted Cruziana a¡.
quadrata as quite possibly the work of Cryptolithus, but Campbell (1975) reasonably pointed out
that the long genal spines should have left more
of a lateral impression when the animal turned.
This argument is equally e¡ective in the case of
Arthrophycus, whose maker must have had relatively short genal spines. Shifting impressions of
long genal spines are indeed seen in a Carboniferous trail, Cruziana (‘Rusichnites’) acadica (Dawson, 1864, pp. 367, 458), whose author noted the
similarity to modern trails made by Limulus.
Campbell (1975) brie£y reviewed the evidence
for senses in Cryptolithus, concluding that the
adult may have had a light-sensitive area, but
was nearly blind, relying mainly on tactile and
chemoreceptive senses. As most shallow-marine,
epibenthic trilobites and xiphosurans had eyes,
this suggests a burrowing animal. Cryptolithus
also had antennae (Raymond, 1920), perhaps for
tactile and chemoreceptive senses.
Although the trinucleids themselves ranged
only from Early to Late Ordovician, their trinucleine relatives, the less familiar raphiophorids,
are similar in form and ranged from Early Ordo-
vician to Middle Silurian (Harrington et al.,
1959), approximately matching the known range
of Arthrophycus in the strict sense. If Arthrophycus is the work of xiphosurans instead of trilobites, then they must have been unusually specialized ones, because the range of Arthrophycus is
rather limited compared to the range of the morphologically conservative xiphosurans.
In contrast to the arthropod interpretation,
Sarle (1906), Seilacher (2000), and others inferred
a worm maker of Arthrophycus, and this is not
ruled out for New York Arthrophycus. However,
for all of the reasons given here, vermiform animals are unlikely makers of Alabama Arthrophycus.
4. Comparison with Arthrophycus of New York
and Pennsylvania
4.1. Early history of the ichnogenus
Harlan’s (1831, 1832) two species of Arthrophycus (originally placed in Fucoides) were the ¢rst
trace fossils to be named in North America.
Although the type specimens are lost, both ichnospecies were illustrated within a few years of their
discovery, and these ¢gures are reprinted here
(Fig. 12), perhaps for the ¢rst time in more than
175 years.
Harlan (1831) discovered Arthrophycus alleghaniensis in an ornamental stone in front of a tavern. The slab had rolled down a steep slope to
within a few meters of the house, and the landlord
placed it before his door to attract notice to his
tavern. This is evidently the slab shown in Harlan’s plate (Fig. 12A), which he described as
about half a foot (15 cm) thick.
The nomenclatorial history of Arthrophycus
and its synonym Harlania is complex, and only
a few highlights will be noted here. Diagnoses
and supporting evidence are given in the Appendix.
Harlan (1831) named Fucoides (Cladorytes) alleghaniensis for fan-like hypichnia from the Lower
Silurian Tuscarora Sandstone of Pennsylvania.
Soon afterwards, Harlan (1832) based another
species, Fucoides (Cladorytes) brongniartii, on rel-
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A
203
B
Fig. 12. Arthrophycus alleghaniensis and A. brongniartii as ¢gured by Harlan (1831) and Taylor (1834) from their type localities.
Both specimens are now lost; presumably they were hypichnia. (A) Arthrophycus alleghaniensis after Harlan (1831). Shade Mountain, Mi¥in County, Pennsylvania. Scale bars = 1 cm except for the large slab (10 cm). (B) Arthrophycus brongniartii after Taylor
(1834). Probably Lockport, Niagara County, New York. Scale bar = 1 cm.
atively simple forms from rocks of the same age in
Pennsylvania and New York. The new ‘section’
(ichnosubgenus) Cladorytes ^ the earliest ichnogeneric name to be based on American material ^
was apparently never used after 1835 (Harlan,
1835a,b) and is a nomen oblitum (see Section
A.2.5).
Conrad (1838) substituted the name Fucoides
harlani for F. brongniartii without stating a reason
for doing so. F. harlani is thus an objective junior
synonym.
In 1852, two researchers independently gave the
trace fossils new ichnogeneric names, Harlania
Go«ppert and Arthrophycus Hall. Go«ppert substituted Harlania hallii for previous names; Hall
preferred Arthrophycus harlani. Although both researchers described similar material from the
northern Appalachians, it is unknown whose
work was published ¢rst. In such cases, the decision of the ¢rst reviser must stand (ICZN, 1999,
Art. 24.2). The ¢rst reviser was apparently Miller
(1877), who considered Arthrophycus as the senior
synonym; Schimper (1879) echoed this decision.
Roemer (1880) mistakenly thought that Hall’s
work was published in 1853, and chose Harlania,
so most European researchers preferred that name
until Ha«ntzschel (1962) opted for Arthrophycus.
Arthrophycus has been the standard name since
1962.
4.2. What is Arthrophycus: an annulate burrow or
a fan-like burrow?
What trace fossils are called Arthrophycus depends on the relationship, if any, of simple and
branched, transversely sculpted forms. Harlan
(1831, 1832) distinguished them as di¡erent species, but Hall (1852) included all transversely annulate forms whether simple or fan-like in A. harlani. This historically led to divergent diagnoses of
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Arthrophycus, one based chie£y on annulation,
the other on fan-like branching.
Seilacher (1955) pointed out the similarity of
fan-like Arthrophycus with Phycodes and placed
the two in synonymy. Both can have transverse
sculpture and a median furrow on their lower
surfaces (Richter, 1850; Ma«gdefrau, 1934). Other
researchers followed Osgood (1970), who distinguished Arthrophycus from Phycodes on the basis
of the larger general size and more distinct annulation and bilobation of Arthrophycus, and both
names continue in use. Seilacher (2000) now includes them in the same ichnofamily, Arthrophycidae (see Section A.1).
In contrast, Frey (1970) and KsiaVzWkiewicz
(1970, 1977) retained usage of the name Arthrophycus only for simple annulate burrows. These
researchers extended the genus to include Cretaceous forms that were later recognized as Thalassinoides or decorticated Ophiomorpha with meniscate ¢ll. Taking a practical approach, Uchman
(1998, 1999) rede¢ned Arthrophycus as simple,
oblique to horizontal burrows having an annulate
exterior, without regard to internal structure. Reinvestigation of Arthrophycus by Seilacher (2000)
showed that the burrow ¢ll even of simple Arthrophycus is imbricate, not meniscate, and the outline
tends to be distinctively subquadrangular. Accordingly, post-Paleozoic reports of Arthrophycus
probably should be assigned to other ichnogenera.
4.3. Distinction among ichnospecies of
Arthrophycus
Most workers after Conrad (1839) regarded Fucoides alleghaniensis and F. brongniartii as synonyms, though Prouty and Swartz (1923) distinguished the two as forms. They noted that the
two commonly occur together in the northern
and central Appalachians, but Arthrophycus
brongniartii is far more abundant at some horizons. Seilacher and Alidou (1988) considered the
simple and branched forms as stratigraphically
useful, and Seilacher (2000) later distinguished
them taxonomically, although he was apparently
unaware of the early name F. brongniartii for the
simple form and renamed it Arthrophycus linearis.
He also distinguished vertical and horizontal
spreites among the fan-shaped forms as A. alleghaniensis and A. lateralis, a useful distinction,
but one that cannot always be inferred in photographs, making reinvestigation based on ¢eld and
museum specimens necessary in critical cases.
4.4. Structure of Arthrophycus alleghaniensis
The earliest collection of Arthrophycus located in museums is that of Hall (1843, 1852), three
of whose ¢gured specimens have survived in
the American Museum of Natural History (Figs.
4C,D and 13A,B). As is immediately evident,
the New York illustrator drew only parts of the
specimens, and not wholly accurately. Although
Hall’s ¢gures have been reprinted many times in
well-circulated publications since the 1840’s (e.g.
Le Conte, 1878; Dana, 1895; Shimer and Shrock,
1944, to cite only a few), the ¢gured specimens are
here shown photographically for the ¢rst time.
Hall’s specimens occur in reddish, very ¢ne^
medium-grained sandstone (in one case including
granules). They show lower surfaces that are regularly annulate with coarse, apparently transverse
corrugations; these corrugations are wrinkled or
striate in cases of exceptional preservation. The
crosscutting pattern of wrinkles or striae shows
that the corrugations were formed biserially in a
zipper-like pattern rather than in a truly transverse manner (Fig. 14). These are the specimens
that have formed the ‘search image’ of Arthrophycus alleghaniensis for generations of geologists.
The outer corrugations mask a complex internal structure that has never been completely
understood. Sarle (1906) more or less correctly
interpreted the closely spaced, regular corrugations of the burrow’s lower surface as representing
successive positions of the burrow: a shallow
spreite, as pointed out again by Seilacher (2000).
Sarle (1906, p. 206) noted that the coarse corrugations on the underside of Arthrophycus are
accompanied in well preserved material by ‘¢ne
parallel or interfering wrinkles extending in the
same direction’. Seilacher (1997, 2000) illustrated
these wrinkles, interpreting them in 1997 as ¢ne
striae ‘probably related to ciliary action’ and in
2000 as molds of wrinkled body cuticle. Sarle’s
point that the wrinkles interfere (crosscut one an-
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other) is valid (Fig. 14), and the wrinkles are here
interpreted as scratchmarks (striae). An un¢gured
part of one of Hall’s ¢gured specimens from New
York (Hall, 1852, pl. 2, ¢g. 1b; AMNH 31 662)
also shows two small, ¢nely striate areas.
Hall’s fan-like specimen (Fig. 13) includes one
relatively thick, fusiform branch that may be
structured concentrically like Asterosoma, although it would be necessary to cut the specimen
to ascertain this. Considering the high visibility
of Hall’s image, and especially the juxtaposition of ¢gures of Arthrophycus and possible Asterosoma on the same page in the Treatise (Ha«ntzschel, 1962, 1975), it is surprising that no one
previously recognized the possibility of the two
ichnogenera interconnecting. Because super¢cially
similar Rusophycus also occurs in close association with Arthrophycus (Seilacher, 1969, 1997),
careful observation is needed to distinguish the
two.
4.5. Toponomy
Arthrophycus alleghaniensis is usually reported
as hypichnia (e.g. Taylor, 1834; Grabau, 1901,
1909, p. 239; Sarle, 1906; Bassler, 1909; Prouty
and Swartz, 1923; Swartz, 1923, p. 26; Pemberton
and Risk, 1982; Metz, 1998), only rarely as epichnia (Cant, 1980) or as endichnia in shale (R.
McDowell, written communication, 2002). It
even has uses as a geopetal feature in structurally
deformed terrain (e.g. Cooper, 1971, pp. 4, 10).
Judging from McDowell’s experience, Arthrophycus may be underreported in shale beds because of
low contrast and fragility.
4.6. Paleoenvironment
Arthrophycus is widespread in Silurian clastic
rocks and is nearly the only fossil known in
some formations, such as the Tuscarora Sandstone of the central Appalachians. As the most
abundant fossil at many sites, its paleoenvironmental interpretation has long been of interest.
Arthrophycus-bearing strata occupy a position intermediate between normal, shallow-marine facies
west of the Appalachians, and freshwater, eurypterid-bearing facies toward the east (Amsden,
205
1955; Pelletier, 1958; Yeakel, 1962; Martini,
1971). The consensus is that Arthrophycus indicates marginal-marine (shoreface) conditions,
though Cotter (1983) inferred a braided alluvial
setting. In the southern Appalachians, the occurrence of Arthrophycus with bidirectional ripplemarks and trace fossils that are normally considered to be marine (Table 2) suggests marginalmarine conditions.
4.7. Stratigraphic range
The stratigraphic range of Arthrophycus alleghaniensis provides another clue to its maker.
The trace fossil has long been touted as a Silurian
index fossil (Conrad, 1839, pp. 60^61; Bassler,
1915; Shimer and Shrock, 1944), and applied
not only in eastern North America, but also in
Africa and South America (e.g. Hubert, 1917;
Fritel, 1925; Kilian, 1931; de Oliveira e Silva,
1952; Borrello, 1967; Seilacher and Alidou,
1988; Seilacher, 1969, 2000), sometimes erroneously, as pointed out by Janvier and Melo
(1988) and Fernandes et al. (2000). Undoubted
records are distributed widely in Ordovician strata, but only in Gondwana; the ichnogenus
became cosmopolitan in the Early Silurian (Fernandes et al., 2000). No certain records of
A. alleghaniensis are known in post-Silurian rocks,
although the ichnogenus exists in Devonian and
possibly in Carboniferous strata (Turner and Benton, 1983; Orr, 1994 ; F. Ettensohn, written communication, 2000). The ichnospecies’ migration
out of Ordovician Gondwana mirrors that of
many trilobite and other invertebrate species,
and argues strongly that the makers of A. alleghaniensis consisted of only a small group of organisms having a unique set of behaviors.
4.8. Interpretations : earthworm, fucoid, crinoid,
polychaete burrow, trilobite burrow
Based on its form, Harlan (1831), Hall (1852),
and other early researchers regarded Arthrophycus
as a fucoid, that is, a brown alga or seaweed.
Eaton’s (1820) early suggestion relating the fossils
(‘naked vermes’) to earthworms was ignored until
James unearthed it in 1893. Eaton (1832) and de
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B
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207
Castelnau (1843) later considered the fossils to be
large but poorly preserved crinoid stems, hence
generic assignments to Encrinus, Encrinites, and
Crinosoma. The fucoid idea persisted in some
quarters until the 1950s (e.g. de Saporta and Martion, 1883; Nery Delgado, 1886^1887; Becker and
Donn, 1952). Later researchers (Dawson, 1864,
1890; Nathorst, 1881; James, 1893; Sarle, 1906;
Seilacher, 2000) deduced that Arthrophycus was a
trace fossil, perhaps the work of a polychaete or
other worm.
Dawson’s (1864, p. 366) early attribution of
Arthrophycus and Rusophycus to trilobites was
long overlooked. Nathorst (1881) noted the similarity of Arthrophycus to the traces of modern
dipteran larvae. Pemberton and Risk (1982)
pointed out that the lack of post-Paleozoic Arthrophycus suggests that the maker may have
been one of several ‘Paleozoic soft-bodied experiments’, and the possibility of a soft-bodied trilobite maker is not ruled out here. The idea that
Arthrophycus could be the work of an arthropod
did not reappear until long after Dawson (e.g.
Ha«ntzschel, 1962; Turner and Benton, 1983).
4.9. Northern hypichnia, southern epichnia
Fig. 14. Arthrophycus brongniartii: analysis of striate pattern
in a hypichnion ¢gured by Seilacher (2000, ¢g. 1b). Lower
Silurian of Rochester, New York. Universita«t Tu«bingen,
GPIT 1858/9. Scale bar = 1 cm.
Compared with specimens from Alabama, the
specimens from the northern Appalachians are
relatively regular, showing few signs of the angular segmentation that is so prominent in the Red
Mountain examples. Still, this may be the result
of comparing northern hypichnia with southern
epichnia. Fan-like Arthrophycus is prominent in
the northern Appalachians and unknown in the
southern Appalachians. Perhaps fan-like Asterosoma and Phycodes should be considered as analogs that ¢t such a role.
5. Implications for taxonomy of compound traces
Arthrophycus from the Red Mountain Forma-
Fig. 13. Comparison between Hall’s specimen and illustration of Arthrophycus alleghaniensis. Hypichnion. Medina Sandstone,
Medina, Orleans County, New York. NYSM 31 661. Scale bars = 1 cm. (A) Specimen collected by Hall showing fan-like, overlapping burrows. Compare to Asterosoma in Fig. 10A. (B) Lithograph of same specimen (Hall, 1852, pl. 2, ¢g. 1a).
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tion of Alabama has a complex internal structure
and is also only part of a multifaceted range of
forms that may represent di¡erent behaviors of a
single species of arthropod. These forms include
not only compound trace fossils in the strict sense,
but also morphologically related trace fossils. The
taxonomy of such trace fossil complexes is never
easy, but we may be guided by the following principles:
(1) The taxonomy should be as simple as possible; a minimum of names should be proposed,
not a series of names representing only slight morphologic di¡erences.
(2) The overall taxonomy should be reproducible, so that di¡erent researchers can use the system reliably.
(3) All other things being equal, the most conservative classi¢cation is preferred; previous
names and diagnoses are conserved where feasible.
(4) Thus, divisions between ichnotaxa should,
insofar as possible, follow natural, morphologic
boundaries between populations of objects, not
theoretical or arbitrary constructs.
Thus, interconnecting trace fossils that have
markedly di¡erent forms, but which have similar
dimensions and constructional elements, should
be given di¡erent names. Here the similarities
point to a common tracemaker, but this is not
enough to make it desirable to place di¡erent
forms under the rubric of one ichnotaxon. Such
trace fossils may have the same maker in one geologic occurrence and not in another, so lumping
them together would be complex and irreproducible. Moreover, it would not follow natural
divisions that were apparently caused by the
tracemakers themselves. In the case of interconnecting Arthrophycus and Nereites, the transitional zone is only a few centimeters long. Evidently,
the animals had only a limited repertoire of distinct behavioral programs, and ultimately it is this
that allows us to make relatively easy distinctions
among ichnotaxa.
Similarly, intergradational trace fossils can be
split into separate ichnotaxa where the two forms
represent distinct behaviors. High-order behavioral di¡erences, such as that between locomotion
and feeding, may be used to distinguish high-
order ichnotaxonomic di¡erences (Fu«rsich,
1974), e.g. between ichnofamilies. A di¡erence in
morphology that does not represent even a slight
di¡erence in behavior is of questionable value in
ichnotaxonomy. In the Alabama material, we ¢nd
it di⁄cult to draw the line between Arthrophycus
(which represents brief periods of feeding followed by short periods of locomotion) and series
of Rusophycus, which indeed may represent taphonomic variants. In fact, there is a greater behavioral di¡erence between a single Rusophycus
and a series of Rusophycus, even though we are
accustomed to call them by similar names.
Even without interconnection and intergradation, the a⁄nity of trace fossils can be indicated
by the presence of common structural elements and
dimensions. Resting traces, trails, and trackways
that have similar width, lobation, and/or morphologic elements (e.g. number and morphology of
leg imprints) may have had the same maker, as
Seilacher (1970) demonstrated in the case of
Cruziana and Rusophycus. The greater the number
of morphologic similarities, the more certain such
determinations can be. The inferred relationship
of Arthrophycus brongniartii and Cruziana a¡.
quadrata rests largely on similar morphologic features such as lobation; intergradational and interconnected forms have not been found.
None of the aforementioned features of compound trace fossils are very convincing except in
cases where they occur together. The closer, the
better : the same formation, the same bed, preferably the same slab. Thus, co-occurrence is an
important subsidiary consideration, because it encourages the observer to look for paleobiologic
links among ichnotaxa where none may have
been previously considered. As new information
becomes available, especially of intergrading and
interconnecting forms, a long ichnospecies list
may be shortened considerably. Ichnologic diversity does not accurately re£ect biologic diversity.
In a metaphorical sense then, an elephant becomes an elephant, rather than a disjointed accounting of its parts.
In conclusion, the main features that indicate
biologic a⁄nity in compound trace fossils are interconnection, intergradation, common structural
elements and dimensions, with co-occurrence
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being an important additional consideration. Distinctions among ichnotaxa depend not on biologic
a⁄nity but on morphologic di¡erences. However,
biologic and ethologic considerations can guide
which morphologic di¡erences are judged as signi¢cant at ichnospeci¢c, ichnogeneric, and ichnofamilial ranks, and which are taxonomically unimportant.
209
Daedalae Hundt, 1941, pp. 7^63 [partim].
*Arthrophycidae Seilacher, 2000, p. 240 [junior
homonym and synonym].
A.1.2. Type ichnogenus
Arthrophycus Hall, 1852 by original designation.
A.1.3. Original diagnoses
Acknowledgements
We are indebted to R.R. McDowell, C.T. Baldwin, and F. Ettensohn for supplying us with
specimens and photographs. Our work was enlivened by discussions with these scientists, and with
P. Strother, L. Borghi, R.G. Netto, and C.K.
Chamberlain. C.T. Baldwin and M.K. Gingras
graciously reviewed the manuscript; D.C. Kopaska-Merkel and A. Uchman reviewed an earlier
version. Special thanks to L.A. Herr for helping
with illustrations. R. Kaesler and J. Hardesty expedited reproduction of the ¢gure of Cryptolithus.
T.M. Chowns, R.G. Bromley, and A. Uchman
lent us their expertise in the ¢eld in Alabama
and Georgia. For their help in locating and lending type specimens, we thank E. Benamy (Academy of Natural Sciences of Philadelphia), N. Eldredge and B.M. Hussaini (American Museum of
Natural History), E. Landing (New York State
Museum), W.L. Taylor (Field Museum of Natural History), and B. SzczudIo, A. Setlik, and A.
Pacholska (Muzeum Geologiczne, Uniwersytetu
WrocIawskiego, Poland).
Arthrophyceae Schimper, 1879 (p. 53): ‘Phyllom einfach oder unter spitzem Winkel sparsam
zertheilt, mehr oder weniger lang, cylindrisch,
kurz quergegliedert, an der Spitze oft keulenfo«rmig oder kolbig verdickt, mit oder La«ngsrinne’.
(‘Phyllome simple or split under central angle,
more or less elongate, cylindrical, ¢nely crossjointed, often thickened at the center with knobs
or knots, or with longitudinal furrow.’)
Arthrophycidae Seilacher, 2000 (p. 240) : ‘Paleozoic worm burrows characterized:
b by regular transverse ridges, which are often
discontinuous, giving the casts a squarish cross
section,
b by teichichnoid back¢ll structures (spreiten)
resulting from transverse or oblique dislocation
of a J-shaped tunnel through the sediment. Depending on the behavioral programs, the back¢ll
structures may have linear, palmate, fan-shaped,
spiral, or multi-winged geometries. Also, their internal structures may be either protrusive or retrusive.’
A.1.4. Emended diagnosis
Appendix. Systematic ichnology
A.1. Ichnofamily Arthrophycidae Schimper, 1879,
nomen correctum
A.1.1. Synonymy
*Arthrophyceae Schimper, 1879, pp. 53^54;
Nathorst, 1881, pp. 32, 86^87.
*Daedaleae Zimmermann, 1892, pp. 58, 61
[partim].
Burrows with vertical to horizontal spreite resulting from regular, oblique back¢ll generally
having a £attish £oor and transverse sculpture;
striae common ; burrows of limited to inde¢nite
length, simple or branched, straight or curved,
in some cases composed of segments arranged angularly.
A.1.5. Discussion
Schimper (1879) named the Arthrophyceae
(Gliederalgen, ‘jointed algae’) in the belief that
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they were marine plants characterized by transverse ridges, and included Muensteria Sternberg
and Taenidium Heer along with Arthrophycus in
the group. Although Arthrophyceae Schimper,
1879 long ago fell into disuse, its junior homonym
and synonym, Arthrophycidae Seilacher, 2000,
cannot replace it, because the younger name has
not been used by at least ten authors during the
past ¢fty years (ICZN, 1999, Art. 23). Instead, the
older name must be emended to ¢t new information; the meniscate burrows do not belong with
the imbricate burrow Arthrophycus.
Zimmermann’s (1892) Daedaleae included walllike, scribbling or coiling traces such as Daedalus
Rouault, Dictyodora Weiss, and Vexillum
Rouault. Hundt (1941) interpreted the Daedalae
as trace fossils constructed as a J- or U-shaped
tube was shifted through the sediment. Accordingly, he included forms without a spreite, but
otherwise this matches the modern concept of
the Arthrophycidae rather well.
Seilacher (2000) grouped together trace fossils
characterized by their teichichnoid back¢ll structure and regular transverse ridges. These include
Arthrophycus, Daedalus, and Phycodes Richter. A.
brongniartii is a relatively simple spreite burrow
that nonetheless shares the basic morphologic elements and ethology of more complex members
of the group.
Arthrophycus Harlani, Hall, 1851, pp. 123^125
[nomen nudum].
Harlania Go«ppert, 1851, p. 189 [nomen nudum;
non viso; ¢de Andrews, 1970, p. 100].
*Arthrophycus Hall, 1852, p. 4, non p. 6, pl. 2,
¢g. 2; Miller, 1877, p. 23; Schimper, 1879, pp. 53^
54 [partim; ?non A. siluricus Schimper, 1879];
Nery Delgado, 1886, pp. 73^75; James, 1893,
pp. 82^86; Sarle, 1906, pp. 203^210, ¢g. 4; Bassler, 1915, pp. 70^71; Pe¤neau, 1946, p. 91, pl. 6,
¢g. 1; Ha«ntzschel, 1962, p. W184; Borrello, 1967,
pp. 10^11, pl. 1, pl. 2, ¢gs. 1^2, pl. 3^5, pl. 6, ¢gs.
1^2, pl. 7^8, pl. 9, ¢gs. 1^2, pl. 10, ¢gs. 1^2, pl.
11; Seilacher, 1969, p. 120, pl. 1; Ha«ntzschel,
1975, pp. W38^39, ¢g. 25,4; [?] Legg, 1985, p.
159, pl. 4E; Seilacher and Alidou, 1988, pp.
434^435, 438, ¢gs. 1d^f, 2d^f; Uchman, 1998,
pp. 109^110 [partim]; Seilacher, 2000, pp. 240^
241.
*Harlania Go«ppert, 1852, p. 98, pl. 41, ¢g. 4;
Schimper, 1869, p. 196, pl. 2, ¢g. 6; Roemer,
1880, p. 135 [non viso]; Lessertisseur, 1956, pp.
54^56, ¢gs. 32A^E, pl. 7, ¢gs. 9^11.
?non Rau¡ella, Ulrich, 1889, p. 235 [only Rauffella palmipes Ulrich, 1889 = trace fossil].
*Arthrichnites Dawson, 1890, pp. 596^597 [nomen vanum].
Phycodes, Seilacher, 1955, pp. 385^386 [partim].
A.2.2. Type ichnospecies
A.2. Ichnogenus Arthrophycus Hall, 1852, nomen
protectum
A.2.1. Synonymy
Fucoides (Brongniart, 1822) Sternberg, Harlan,
1831, p. 289.
*Cladorytes Harlan, 1831, p. 289 [nomen oblitum; type ichnospecies, Fucoides (Cladorytes) Alleghaniensis Harlan, 1831, by monotypy].
Encrinus, Eaton, 1832, p. 37 [partim; E. giganteus Eaton, 1832 only].
*Crinosoma, de Castelnau, 1843, p. 50, pl. 25,
¢g. 1 [C. antiqua de Castelnau, 1843 only; nomen
oblitum].
Encrinites, de Castelnau, 1843, p. 51, pl. 26,
¢g. 4.
Arthrophycus: In naming Arthrophycus and A.
harlani, Hall (1852, p. 4) designated ‘the species of
the Medina sandstone’ as the ‘typical form’, and
gathered the same three species of Fucoides together as Arthrophycus harlani (Conrad, 1838).
Fucoides brongniartii and F. harlani were collected
from the Medina. Thus, Fucoides harlani is the
type species of Arthrophycus. Fucoides harlani
Conrad is an objective junior synonym of F.
brongniartii Harlan.
Harlania : As noted by Ha«ntzschel (1975),
Go«ppert (1852) did not designate a type species
for Harlania, but this was unnecessary, because
he placed all previously named species in synonymy with his new species Harlania hallii. By original monotypy, H. hallii is the type species of
Harlania ; H. hallii is a junior synonym, whether
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of H. alleghaniensis or of H. brongniartii is unclear.
A.2.3. Original diagnoses
Cladorytes Harlan, 1831 (p. 289) : ‘Stipes ramosus; ramis subcylindraceis, transverse rugatis’.
(‘Stipe branched; branches subcylindrical, transversely wrinkled’.)
Crinosoma de Castelnau, 1843 (p. 50, pl. 25, ¢g.
1): ‘Ce corps est tellement di¡erent de tous les
crino|«des connus, que, malgre¤ le mauvais e¤tat de
conservation de l’e¤chantillon, j’ai cru qu’il e¤tait
ne¤cessaire d’en former un genre distinct’. (‘This
body is so di¡erent from all known crinoids,
that, despite the poor state of the specimen’s preservation, I believed it necessary that it form a
distinct genus’.)
Arthrophycus Hall, 1852 (p. 4): ‘Stems simple
or branching, rounded or subangular, £exuous,
ascending, transversely marked by ridges or articulations’.
Harlania Go«ppert, 1852 (p. 98): ‘Frons coriacea
simplex aggregata vel dichotoma, rami in statu
iuniori longitudinaler sulcati; rami adultiores subcylindrici interrupte transversim elevato-striati’
(‘Frond coriaceous [leathery], simple, bunched,
or dichotomous; branches in younger state longitudinally sulcate; more adult branches subcylindrical, interrupted transversely with elevated
striae’.
A.2.4. Emended diagnosis
Subhorizontal to oblique, straightish to curved,
bilaterally symmetrical burrows with ¢ll arranged
in an imbricate series of spoonlike pads; cross
section subquadrangular;
burrows simple,
sparsely branched, or densely branched in an
overlapping, fan-like pattern; lower half part of
burrow sheathed by outer zone of closely spaced
pads of sediment ; lower surface of burrow with
closely spaced transverse ridges, commonly with
median groove.
A.2.5. Discussion
Arthrophycus is a junior synonym of two for-
211
gotten names that are here invalidated for purposes of priority : Cladorytes Harlan, 1831 and
Crinosoma de Castelnau, 1843. Harlan diagnosed
Cladorytes as a ‘section’ (subgenus) of Fucoides,
following the practice of Brongniart (1828), who
named several sections in this form genus. Neither
Cladorytes nor Crinosoma has been cited as a valid name since 1843, but their junior synonym,
Arthrophycus, has been cited tens of times since
1852, meeting the conditions of ICZN (1999), Art.
23.9.1. Accordingly, Arthrophycus is hereby declared to be a nomen protectum and Cladorytes
and Crinosoma are invalidated as nomina oblita
(ICZN, 1999, Art. 23.9).
Although a full review of the named species of
Arthrophycus is beyond our scope, those that were
based on Silurian Appalachian material are considered here.
Encrinus giganteus Eaton, 1832 (p. 37) was described as ‘branching, red or gray: often compressed, whirls uniform and generally obscure:
branches of great length ; mostly lying in the direction of the layers, or nearly so’. Eaton found
these fossils in ‘saliferous rocks at Oak Orchard,
Mineral Hill in Blenheim, and a mile south of Mt.
Hesse’ in New York; i.e. within the Medina. In
the absence of specimens it is unclear whether he
referred to Arthrophycus alleghaniensis, A. brongniartii, or (probably) both.
Crinosoma antiqua de Castelnau, 1843 (p. 50,
pl. 25, ¢g. 1) was never described as such, merely
¢gured; the specimen came from the ‘Genesee’
(Genessee) River in New York and was therefore
probably Lower Silurian. Castelnau’s ¢gure shows
a small piece of fan-like Arthrophycus, very likely
A. alleghaniensis.
Other named species and subspecies attributed
to Arthrophycus include A. montalto Simpson in
Lesley, 1889 (Upper Cambrian, Pennsylvania),
A. siluricus Schimper, 1890 (Cambrian, Sardinia),
A. elegans Herzer, 1901 (Pennsylvanian, Ohio),
A. £abelliformis Hundt, 1940 (Ordovician, Germany), A. krebsi Hundt, 1941 (Lower Ordovician,
Germany), A. annulatus KsiaVzWkiewicz, 1977 (Cretaceous, Poland), A. strictus KsiaVzWkiewicz, 1977
(Cretaceous, Poland), A.? dzulynskii KsiaV zWkiewicz, 1977 (Cretaceous^Eocene, Poland), Sabularia tenuis KsiaVzWkiewicz, 1977 (Cretaceous^Oligo-
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cene, Poland), ‘A. corrugatus’ (Fritsch), Mikula¤s›,
1992, A. minoricensis Bourouilh in Orr, 1994, Arthrophycus qiongzhusiensis Luo et al., 1994 (Lower
Cambrian, Yunnan, China), A. hunanensis Zhang
and Wang, 1996 (Silurian^Devonian, Hunan,
China), A. unilateralis, Seilacher, 1997 (nomen nudum), A. linearis Seilacher, 2000, A. linearis protrusiva Seilacher, 2000 (Upper Ordovician, Benin
and Jordan), A. linearis retrusiva Seilacher, 2000
(Lower Ordovician, Libya and Algeria; Lower
Silurian, USA and Argentina), and A. lateralis
Seilacher, 2000 (Lower Silurian, Libya). Uchman
(1998) discussed some of these species more
fully; not all belong to Arthrophycus as emended
here.
A.2.6. Stratigraphic range
Possible examples in the Middle Cambrian of
Spain (Legg, 1985). Lower Ordovician (Arenigian) to Upper Ordovician of South America,
Africa, and Europe. Lower Silurian of eastern
North America, South America, Africa, Asia,
and Europe (including Iberia). Devonian of Libya
(Turner and Benton, 1983).
¢g. 4 [non viso]; Lesquereux, 1883, p. 29, pl. 2,
¢g. 3.
Arthrophycus alleghaniensis, James, 1893, p. 86
[partim]; Sarle, 1906, pp. 203^210, ¢g. 4; Bassler,
1915, pp. 70^71 [partim]; Prouty and Swartz,
1923, pp. 404^405, pl. 12, ¢g. 2 [partim]; Becker
and Donn, 1952, pp. 214^215, ¢gs. 1, 2; [non]
Ha«ntzschel, 1962, p. W184, ¢g. 111.3 [A. brongniartii]; Borrello, 1967, pp. 10^11, pl. 1, pl. 2, ¢gs.
1^2, pl. 3^5, pl. 6, ¢gs. 1^2, pl. 7^8, pl. 9, ¢gs. 1^2,
pl. 10, ¢gs. 1^2, pl. 11; Ha«ntzschel, 1975, p. W39,
¢g. 25,4; Baldwin, 1977a,b, p. 26, pl. 2a [?partim;
simple and branched forms]; Seilacher, 1997, p.
41, unnumbered ¢gure; Metz, 1998, pp. 104^
106, ¢g. 3E [partim; also A. brongniartii]; [?non]
Moreira and Borghi, 1999, p. 425, ¢g. 14 [A. lateralis?]; [?non] Nogueira et al., 1999, p. 137, ¢gs.
6A^C [¢g. 6B = A. lateralis?; ¢gs. A,C = A. brongniartii]; Arthrophycus alleghaniensis, Seilacher,
2000, pp. 243^244, ¢gs. 3, 4.
Arthrophycus, deep palmate form, Seilacher and
Alidou, 1988, p. 434, ¢gs. 1f, 2f.
[non] Arthrophycus unilateralis Seilacher, 1997,
p. 41, unnumbered ¢gure on p. 41 [nomen nudum].
[non] Arthrophycus lateralis, Seilacher, 2000, pp.
244^245, ¢gs. 5, 6.
A.3. Arthrophycus alleghaniensis (Harlan, 1831)
A.3.2. Original diagnoses
A.3.1. Synonymy
Fucoides (Cladorytes) alleghaniensis Harlan,
1831 (p. 289): ‘Fronde compressa, rugata; apice
recurva, obtusa; ramis inequalibus, digitatis et
fastigiatus, enervibus, nudatis’. (‘With frond £attened, wrinkled ; with apex curved back, blunt;
with branches unequal, digitate and bundled,
without nervure, bare’.)
Arthrophycus harlani Hall, 1852 (p. 5): ‘Plant
composed of strong rounded and articulated
stems, which divide near the base into numerous
elongated branches ; branches simple, £exible, articulated and diminishing very gradually, usually
appearing of the same dimensions throughout,
and more or less curved; diameter of the branches
1/4^1/2 inch [0.65^1.3 cm]’.
Harlania Hallii Go«ppert, 1852 (p. 98): ‘H.
fronde £abellato-digitata dichotome ramose,
ramis cespitose aggregates sulco longitudinali distinctis transversim rugoso-striatis, striis regulari-
*Fucoides Alleghaniensis Harlan, 1831, pp. 289^
292, pl. 15, ¢gs. 1^3; Taylor, 1834, p. 5; Conrad,
1837, p. 171.
Fucoides (Cladorites) alleghaniensis, Harlan,
1835a, pp. 393^395, unnumbered plate, ¢g. 1.
Fucoides harlani, Vanuxem, 1842, p. 71, ¢g. 10
[on p. 74 misspelled Fucoides harlanii]; Hall, 1843,
pp. 46^47, woodcut 5, ¢gs. 1^2.
*Crinosoma antiqua de Castelnau, 1843, p. 50,
pl. 25, ¢g. 1.
Arthrophycus harlani, Hall, 1852, p. 4, pl. 2,
¢gs. 1a,b, non pl. 1, ¢g. 1, pl. 2, ¢g. 1c [partim];
Schimper, 1879 ; p. 53, ¢g. 41 [partim]; Grabau,
1901, p. 132, pl. 16.
*Harlania Hallii Go«ppert, 1852, pp. 98^100, pl.
41, ¢g. 4; Schimper, 1869, p. 196, non pl. 2, ¢g. 6
[partim; A. brongniartii]; Roemer, 1880, p. 135,
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213
bus aeque distantibus elevates distinctissimis’
(‘H[arlania] with frond £abellately digitate, dichotomously branched; with branches bunched
in tufts, with distinct longitudinal sulcus, transversely wrinkled to striate ; with striae regular,
evenly spaced, very distinctly raised’.
Sandstone of Shade Mountain, next to the Lewistown Narrows near Macedonia, about 6.5 miles
from the center of Lewistown (40‡36.70’N,
77‡26.86’W).
A.3.3. Emended diagnosis
Lower Ordovician, Iberia, Jordan. Ordovician(?), Argentina. Lower Silurian, eastern North
America from Ontario to West Virginia, South
America, Africa.
Arthrophycus with sparse to extensive, palmately branched burrows; galleries straight to
gently curved, generally with subquadrangular
cross section, branching at acute angle, self-penetrating; internal structure expressed on lower surface as nearly transverse sculpture; lower surface
with median groove in some specimens.
A.3.4. Type specimens
Harlan’s (1831) types of Fucoides alleghaniensis
are lost; they are not in the collections of the
Academy of Natural Sciences of Philadelphia
(ANSP), American Museum of Natural History
(AMNH), Field Museum of Natural History
(FMNH), or New York Museum of Natural History (NYSM).
Of Go«ppert’s material, only one un¢gured
specimen of Harlania hallii from the Medina
Sandstone survived the Second World War in
the Muzeum Geologiczne of the Instytut Nauk
Geologicznych, Uniwersytetu WrocIawskiego
(WrocIaw, Poland). The specimen is available
for study only at the Museum (B. SzczudIo, A.
Setlik, and A. Pacholska, written communication,
2002).
A.3.5. Type locality
Harlan (1831, pp. 289^290) described his type
locality as ‘One of the eastern ridges of the Alleghany mountains, about 40‡ north latitude, and
about 77‡ west longitude, from Greenwich ; one
hundred and ¢fty miles from Philadelphia; ten
miles east of Lewistown, north side of the Juniata
river, Mi¥in county, state of Pennsylvania’. Taylor (1834) reinvestigated the type locality and provided an early geologic map. The fossils most
likely derived from the Lower Silurian Tuscarora
A.3.6. Stratigraphic range
A.4. Arthrophycus brongniartii (Harlan, 1832)
A.4.1. Synonymy
[?] Naked vermes, Eaton, 1820, pp. 211^212
[non viso].
*Fucoides (Cladorytes) Brongniartii Harlan,
1832 (January), p. 307.
[?]*Encrinus giganteus Eaton, 1832, p. 37, pl. 1,
¢g. 8.
[non]*Fucoides Brongniarti Mantell, 1833, pp.
95^96, unnumbered ¢gure [junior homonym;
plant].
[non] Fucoides Brongniartii Harlan, Hitchcock,
1833, pp. 233^234, pl. 13, ¢gs. 38^39.
Fucoides Brongniartii, Taylor, 1834, p. 14, pl. 3,
¢g. 6; Harlan, 1835b, pp. 398^399, unnumbered
plate, ¢g. 2; Conrad, 1837, p. 169 [on p. 168 misspelled F. Brongniartia]
*Fucoides Harlani Conrad, 1838, p. 113 [nom.
subst. pro F. Brongniartii Harlan]; Conrad, 1839,
pp. 60^61, 68 [partim; includes A. alleghaniensis].
Encrinites, de Castelnau, 1843, p. 51, pl. 26, ¢g.
4.
Arthrophycus harlani, Hall, 1852, p. 5, pl. 1, ¢g.
1, pl. 2, ¢g. 1c, non ¢gs. 1a,b [partim].
Harlania Hallii, Schimper, 1869, p. 196, non pl.
2, ¢g. 6 [partim]; Lesquereux, 1883, p. 29, pl. 2,
¢g. 3.
Arthrophycus cfr. Harlani, Nery Delgado, 1886,
pp. 75^77, pl. 23, pl. 35, ¢gs. 1^3, pl. 36.
Arthrophycus alleghaniensis, Bassler, 1915, pp.
70^71 [partim]; Prouty and Swartz, 1923, pp.
404^405, pl. 12, ¢g. 1 [partim]; Swartz, 1923, p.
26 [partim]; Ha«ntzschel, 1962, p. W184, ¢g. 111.3
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[partim]; Ha«ntzschel, 1975, p. W39, ¢g. 25,4 [partim]; [?] Baldwin, 1977a,b, p. 26, pl. 2a [retrusive
spreite] ; Nogueira et al., 1999, p. 137, ¢gs. 6A,C,
non B [partim].
Harlania alleghaniensis, Lessertisseur, 1956, p.
54, ¢g. 32A, pl. 7, ¢g. 11.
?Arthrophycus, Frey and Chowns, 1972, p. 37,
pl. 5C.
Arthrophycus, Pemberton and Risk, 1982, p. 99,
unnumbered ¢gure.
Arthrophycus, linear form, Seilacher and Alidou, 1988, pp. 434, 438, ¢gs. 1d, 2d.
*Arthrophycus linearis Seilacher, 1997, unnumbered ¢gure on p. 41 [nomen nudum]; [?] *Seilacher, 2000, pp. 241^243, ¢gs. 1, 2 [nomen nudum].
*Arthrophycus linearis retrusiva Seilacher, 2000,
pp. 241^242, ¢g. 2 (right) [nomen nudum].
A.4.4. Type specimens
A.4.2. Original diagnoses
A.4.5. Type locality
Arthrophycus brongniartii (Harlan, 1832):
‘Fronde elongata, sub-quadrangularis, canaliculata, transverse rugosa; ramulis inequalis, sparsis,
remotis, compressis, rugatis, recurvis, nudis’.
(‘With frond elongate, subquadrangular, canaliculate, transversely wrinkled; with branchlets unequal, sparse, scattered, compressed, wrinkled,
curved back, bare’.)
Arthrophycus harlani (Conrad, 1838): ‘The organic remains consist chie£y of fucoides Harlani,
nobis (F. Brongniartii, Harlan.) [sic].’
Arthrophycus linearis (Seilacher, 2000): ‘Shallow Arthrophycus burrows with no or few side
branches, running straight, or smoothly curving,
along bedding planes. Depending on whether the
animal burrowed with its head up or down, the
cross section of the low teichichnoid spreite is
either protrusive (seleniform back¢ll laminae convex-up) or retrusive (lamellae convex-down, in the
same direction as in the longitudinal section)’.
Western New York, including the vicinity of
Lockport, Niagara County (Harlan, 1832), in
the Lower Silurian Medina Group (Hall, 1852).
A.4.3. Emended diagnosis
Arthrophycus with simple to sparsely branched
burrows; spreite retrusive. Burrows with tendency
to parallel one another or to cross one another at
nodes of three or more burrows, giving a super¢cially stellate appearance.
Harlan’s (1832) types are lost. He listed specimens in the private collections of W.R. Johnson
and P.A. Brown, in Peale’s Museum (New York),
and in the ANSP. In 2001, they were not found in
the collections of the ANSP, AMNH, FMNH,
or NYSM. Peale’s Museum, renamed the (¢rst)
American Museum of Natural History, was
burned by a terrorist in 1865 (Sellars, 1980). It
would be desirable to collect and designate a neotype of Fucoides brongniartii from Lockport, New
York.
The NYSM formerly held the type of Fucoides
harlani Conrad, 1838 (Clarke and Ruedemann,
1903), but it was not found in 2001 (Ed Landing,
written communication).
A.4.6. Alabama material
Hundreds of specimens observed at Sloss Mine
No. 2, Bessemer; Red Mountain Expressway, Birmingham; and Chalkville Road, Chalkville. Epichnial full-reliefs in light gray to moderate grayish-red sandstone.
A.4.7. Description based on Alabama material
Subhorizontal, simple burrows with a shallow
vertical spreite and subquadrate to rounded cross
section. Path straightish to curved to looping,
commonly angling abruptly to intersect other burrows of the same ichnospecies at or near a single
point, yielding a super¢cially stellate pattern;
course composed of short, regular segments arranged angularly, about 2^3 times longer than
wide. Burrow including inner and outer zones,
with the outer zone present only on the lower
part of the spreite. Inner zone made up of successive wedges of sediment having a spoonlike lower
surface and a lunate longitudinal section. Sides of
outer zone laminated obliquely, with laminae ori-
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ented downward and outward from the points of
the inner lunes. Where laminae are composed of
alternating sand and clay, clay may weather out
to leave a series of sand ‘beads’. Lower surface of
outer zone transversely ridged.
A.4.8. Discussion
Seilacher (2000) designated no type specimens
for Arthrophycus linearis or its two ‘subichnospecies’ (ichnosubspecies), A. linearis protrusiva and
A. linearis retrusiva, which is required for new
species-group taxa named after 1999 (ICZN,
1999, Art. 72.3) ^ except for papers that were
submitted as manuscripts before 2000 (Art.
86.1.2). As this was probably the case, the names
are not thereby invalidated.
However, adjustments are still necessary. A. linearis retrusiva is here designated as the type ichnosubspecies of A. linearis and is renamed Arthrophycus linearis linearis Seilacher, 2000 to satisfy
the requirement for a nominotypical subspecies
(ICZN, 1999, Art. 47). Thus de¢ned, A. linearis
is a junior synonym of A. brongniartii, and
A. linearis protrusiva is a separate ichnosubspecies, Arthrophycus brongniartii protrusiva (Seilacher, 2000).
A.4.9. Stratigraphic range
Lower Ordovician ? (Arenigian?) of Spain
(Baldwin, 1977a,b). Lower Silurian of eastern
North America from Ontario to Alabama and
Indiana. Middle Silurian of Ontario (Canada)
and New York (USA).
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