~o~lo@calJournal
ofthe Linnean Suciep (1999), 126: 225-250. With 7 figures
Article ID: zjls. 1998.0159, available online at http://u?h?Y.idealit,rary.com on
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What makes an ophiuroid? A morphological
study of the problematic Ordovician stelleroid
Stenaster and the palaeobiology of the earliest
asteroids and ophiuroids
JULIETTE DEAN
Department o f Earth Sciences, UniversiQ o f Cambridge, Downing Street, Cambridge, CB2 3EQ
and Department of Palaeontology, T h e Natural History Museum, Cromwell Road, London,
S W7 5BD
Received October 1997; accepted f o r publication June 1998
Extant asteroids and ophiuroids [Echinodermata] are distinguished by differences in arm
support, water vascular system structures and in details of arm and jaw structure. However,
some lower Palaeozoic taxa show combinations of both asteroid-like and ophiuroid-like
characters and their morphology and functional biology is poorly understood. This paper
redescribes one such taxon, the middle-upper Ordovician stellate echinoderm Stenaster and
clarifies its phylogenetic status. Characters in common with extant and Ordovician ophiuroids,
include arm support due primarily to ambulacral ossicles, presence of extensive longitudinal
arm musculature, a mobile jaw and an internalised radial water vessel with internalised
podial pores. In addition, Stenaster lacks several characters which are conventionally considered
to be asteroid-like, for example an axillary, madreporite, marginal ossicles and a true
ambulacral groove. However, in overall shape Stenaster is remarkably asteroid-like, showing
short, broad-based arms shared podial basins and a small disc. A cladistic analysis of early
asteroids, ophiuroids and somasteroid taxa consistently places Stenaster within the ophiuroids
and suggests secondary convergence to asteroids. In functional terms, Stenaster is interpreted
as an ophiuroid which has secondarily adopted a semi-infaunal, deposit-feeding mode of life,
analogous to that of some extant paxillosid asteroids.
0 1999 The Linneari Society of London
ADDITIONAL KEY WORDS:-Ophiuroidea
- morphology - phylogeny.
~
Asteroidea
~
Somasteroidea palaeontology
~
CONTENTS
Introduction . . . . . .
Abbreviations . . . . .
Material studied and method
Age and occurrence . . .
Systematic description
. .
Ambulacral ossicles . .
Adambulacral ossicles .
. . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . .
226
227
227
227
227
228
230
Correspondence to Dept. Earth Sciences. E-mail: jd [email protected]
0024-4082/99/060225
+ 26 $30.00/0
225
0 1999 The Linnean Society of London
226
J. DEAN
Aboral and oral disc plating . . . . .
Mouth frame plating . . . . . . .
Functional morpholocgy . . . . . . . .
Water vascular system . . . . . . .
Body wall support and general morpholocgy
Arm structure . . . . . . . . . .
Jaw structure . . . . . . . . . .
Cladistic analysis . . . . . . . . . .
Results . . . . . . . . . . . . .
Discussion . . . . . . . . . . . .
Palaeobiological implications . . . . .
Acknowledgements
. . . . . . . . .
References . . . . . . . . . . . .
Appendix 1: Synonymy list for Stenaster obtusus. .
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233
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246
247
249
INI‘RODUCTION
Asteroids (starfish or sea-stars) and ophiuroids (brittle- and basket-stars) are two
extant classes of echinoderm, collectively known as stelleroids, in which the ambulacra
are confined to the ventral surface and are usually developed into distinct arms.
Although modern asteroids and ophiuroids are readily distinguishable on the basis
of a large number of morphological characters, a few early stelleroids show a mixture
of features and do not appear to fit easily into either class. In many cases, a lack of
understanding of the morphology of such fossil has led to their elevation to artificially
high rank. For example, the enigmatic European ‘somasteroids’ of Osek (Czech
Republic) and the St. Chinian region (southern France) have been given equal rank
to asteroids and ophiuroids within the stelleroids and interpreted as their common
ancestors (Spencer, 1951; Spencer & Wright, 1966). In fact, debate over whether
asteroids and ophiuroids form a monophyletic group continues (David & Mooi,
1997; Littlewood et al., 1997); ophiuroids may be more closely related to echinoids
(sea-urchins) and holothurians (sea-cucumbers) than to asteroids. Placement of
problematic fossil taxa in both a functional and morphological sense is essential if
we are understand the early evolution of each sub-class and decide between
competing hypotheses of echinoderm class-level relationships.
Stenaster obtusus (Forbes) (Figs 1-5) is arguably the most outstanding example of a
successful early stelleroid with unresolved taxonomic affinity. Originally designated
an asteroid, several workers have debated its taxonomic position. Schuchert (1914)
suggested Stenaster was a cryptozonate asteroid. Spencer (1914), however, identified
morphological features which were ophiuroid-like, including the possession of tall
blocky ambulacral ossicles and an arm cross-section comprising four main ossicles
(two ambulacrals and two adambulacrals). Spencer (1 95 1) subsequently included
Stenaster in the order Stenurida, a taxon in which he placed all Arenig ophiuroids.
This classification was repeated by Spencer & Wright (1966)) and followed by
Lockley (1980). Hotchkiss (1976) created the suborder Scalarina to include stenurids
with opposite ambulacrals and concluded by comparison with the Devonian genera
Klasmura and Antiquaster that Stenaster was an ophiuroid. More recently, however,
Branstrator (1 979) reverted to interpreting Stenaster as an asteroid.
Is Stenaster a primitive ophiuroid showing shared primitive characters with early
asteroids, a stem group member prior to the asteroid/ophiuroid split, or is it a
derived ophiuroid, showing ecological, functional and morphological convergence
WHAT MAKES AN OPHIUROID?
227
on an asteroid body form? To what extent does it show convergence in form to
contemporaneous asteroids and ophiuroids?
T o solve this taxonomic problem a thorough revised systematic description of
Stenaster obtusus, the type species, is presented here. This work aims to reveal the true
taxonomic position of Stenaster and suggest its relationship to known Palaeozoic
asteroids and ophiuroids and to the somasteroids.
ABBREVIATIONS
a
ab
ad
adf
adg
b
ch
d
df
dg
h
If
m
mc
ambulacral ossicle
aboral boss
adambulacral ossicle
adambulacral facet
adambulacral groove
ambulacral boss
perradial channel
dental plate
distal podial flange
distal groove
adambulacral head
latero-oral flange
marginal ossicle
medial concavity
medial process
median ridge
first podial pore
second podial pore
proximal podial flange
proximal groove
perradial region
proximal shelf
proximal tongue
pustules
ambulacral socket
transverse bar
transverse groove
transverse process
MATERIAL STUDIED AND METHOD
Restudied material was drawn from the large collection of Stenaster obtusus held in
the Natural History Museum, London (BMNH). These specimens are mouldic with
part and counterpart usually present and include the material described by Spencer.
Latex casts of the specimens were whitened using ammonium chloride sublimate
before camera lucida drawings were made.
AGE AND OCCURRENCE
Trenton (Middle Ordovician) of North America and Canada, and Ashgillian and
Caradocian (Middle to Upper Ordovician) of Britain (Table 1, see also Hotchkiss,
1976).
SYSTEMATIC DESCRIPTION
Specimens are generally pentaradial, with broad petalloid arms showing a high
degree of separation from the disc (Fig. IA, B). Mean radial length measured from
the disc centre (‘R’) ranged from 7 mm to 13 mm, and interradial radii ((r’)
J. DEAN
228
TABLE
1. Ordovician locality and stratigraphic data for Stenaster
Country
Stratigraphy
Location
America
(California)
Johnson Spring Formation, Middle
Ordovician.
Mazourka Canyon, Independence Quadrangle,
Inyo County Inyo Mountains, measured section
JS-11 of D.C. Ross (1966), unit 2, basalfoot of
limestone.
America
(Kentucky)
(1) & (2) Curdsville Limestone Member
of Lexingtone Limestone (Trenton).
(1) Curdsville, Wilmore Quadrangle, Mercer
County
(2) USGS locality 5101-C0, Nicholasville
Quadrangle, Jessamine County.
America
(Vermont)
(1) Trenton. May be from Glen Falls
limestone
Panton, Addison County
Lourdes Limestone, Long Point Group
Canada
(Newfoundland) (Portefieldian).
Long Point, Newfoundland
Canada
(Ontario)
(1) ‘Dalmanella Beds’ and perhaps
,
‘Crinoid Beds’, Bobcaygeon Formation,
Upper Member;
(2) Verulam Formation;
(3) ‘Cystid Beds’, upper Couberg heds,
Ottawa Formation.
(1) Near Kirkfield, Carden township, Victoria
County, Lake Simcoe district;
(2) Belleville, Thurlow township, Hastings County;
(3) Ottawa region and Philemon Island, Hull,
Quebec.
Ireland
Tramore Limestones (Caradoc).
Pickardstown, Drumcannon, County Watersford.
Kazakhstan
(1) & (2) Lower Anderkenyn Horizon,
containing Isolelus mnanouski Weber,
likely late Llandeilo.
(1) Anderkenyn-Akchoku tract, Chu-Iliyskiye
Mountains;
(2) Chagyrly Mountains, Chetsky region.
Scotland
Ashgdl, Late Rawtheyan Stage,
Dicellograptus anceps zone, Ardmillan
Series, Upper Drummuck Group,
Ladyburn Starfish Bed.
Thraive Glen, Girvan District, Strathclyde,
Ayrshire.
Wales
(1) Lower Bala Group (Caradoc);
(1) In lane to Gelli Grin Farmhouse, 2.5 miles
southeast of Bala Moel-y-Garnedd, Gwynedd,
North Wales;
(2) In the stream at Ty Nant (SH 906262);
(3) Glyn Ceiriog.
(2) Beds immediately above the
Fronddenv Ash;
(3) Dolhir Beds, Ashgill.
from 3 mm to 5 mm. Specimens are dorso-ventrally flattened although several are
preserved with the arms bent towards the aboral surface. In two such individuals
(BMNH E524 10 and E53976) the arms are straight-sided with ambulacral ossicles
united across the entire perradial length. The majority of specimens have petalliod
arms with the proximal ambulacrals not closely associated perradially on the oral
surface. BMNH E52410 has six arms.
Ambulacral ossicles
Ambulacral ossicles (a)are blocky and form a biserial row arranged oppositely
about a straight perradial suture. In oral aspect each ossicle is superficially ‘T’shaped, composed of a prominent perradial region (pr) and transverse bar (tb)
(Figs lC, D; 2A). The transverse bar separates two abradially positioned deep
flanges. Each flange forms half of a podia1 basin.
The perradial region is sub-triangular, with a blocky adradial end which tapers
abradially and is continuous with the transverse bar. Successive arnbulacrals articulate
WHAT MAKES AN OPHIUROID?
229
Figure 1. Stenark7 obtusw. (Forbes), Ashdl of Girvan, Scotland. A, B BMNH E52400. A, oral, x 2.5.
B, aboral, x 2.5. C, D BMNH E52409. C, oral, x 6. D, aboral, x 7. Latex casts whitened with
ammonium chloride sublimate. For abbreviations see text and p. 227.
via a boss-socket (b, skt) arrangement on the proximal and distal faces of the ossicle
in an adradial position. Opposite ambulacrals have concave adradial faces forming
a cylindrical perradial channel (ch).A pore passes from this channel to each podia1
basin at the suture between successive ambulacrals.
J. DEAN
230
R
distal
r adial
proximal
pt
‘skt
Figure 2. Stenaster obtusus, camera lucida drawings of ambulacral ossicles. A, oral view tilted distally
upwards. B, aboral view. Scale bar= 1 mm.
The transverse bar is bordered adradially by a distinct process (tp), separated
from the perradial region by a groove (tg) which runs obliquely across the ossicle
towards the distal podial flange. Abradially a triangular boss (b)is developed onto
which the adambulacral ossicle articulates.
The transverse bar separates the proximal and distal podial flanges (pf, df). Each
basin is completely floored, its abradial side completed by an adambulacral. The
larger proximal flange is dominated by a central proximally-facing tongue (pt),
bordered adradially by the socket. The narrower distal flange possesses a corresponding depression to receive the proximal tongue and terminates adradially in
the boss. A small groove undercuts the perradial region from the pore at the
ambulacral suture to the distal flange following the boss proximal margin.
In aboral view ambulacrals have flat adradial and abradial regions, separated by
medially-positioned processes (mp) concavities (mc) (Figs 1D, 2B). The abradial
face is curved and closely abuts attaching adambulacrals (ad).
In transverse section the ambulacrals are sub-spherical. Aboral and oral ossicle
relief is similar and the podial basin flanges occur mid-way down the ossicle width.
With the exception of the ambulacral ossicles involved in jaw formation, serial
variations in ambulacral morphology are absent, although aboral medial processes
and concavities show greater development in the proximal arm.
Adambulacral ossicles
The adambulacral ossicles (ad)are differentiated into an aboral boss (ab)and
latero-oral flange (If) separated by a medial groove (mg) (Figs 1, 3, 4A-C). The
ossicles are tilted in longitudinal section, the latero-oral flange flares distally with
respect to the vertically set aboral boss so that the aboral ossicle surface is more
proximal to the disc centre than the oral side. As a result the adambulacrals are set
alternately to the ambulacrals aborally but are opposite orally.
Longitudinal adambulacral width is relatively constant over the arm length but
transverse width varies. Adambulacrals reach their greatest width 6.6 ossicles from
\VHAT W4KES AN OPHIUROID?
23 I
Figure 3. StenartRr o b t u m (Forbes), Ashgdl of Girvan, Scotland. A-C, BMNH E53 12 1. A, oral view of
disc centre, x 8. B, ahoral view of disc centre, x 8. C, lateral view of arm, oral surface uppermost,
x 6. D, E BMNH E53088. D, aboral view of disc centre showing first ambulacral ossicles and a
dental plate, x 8. E, oral view of same area, x 8. Latex casts whitened with ammonium chloride
sublimate.
J. DEAN
232
A
B
distal If
aboral ,ab
D
aboral
proximal
mr
Figure 4. Stenaster obtusus, camera lucida drawings of adambulacral ossicles. A oral view, B distal surface,
C aboral view. D adradial surface. Scale bar = 1 mm.
the disc centre and maintain this size until close to the arm tip where a sharp width
reduction occurs.
The aboral ossicle surface comprises the aboral boss, medial groove and, abradially,
the aboral termination of the latero-oral flange (Figs lB, D, 3B-D, 4C). The medial
groove is bound abradially by a median ridge (mr)and indented proximally and
distally creating a vertical groove between adambulacrals. This groove continues
laterally and onto the oral surface, and separates successive latero-oral flanges.
Smaller grooves are developed between longitudinally adjacent aboral bosses, creating
upraised flat facets on the proximal and distal ossicle surfaces which abut closely in
successive adambulacrals.
The aboral boss has a higher relief than that of the ambulacral ossicles and is
covered in a number of small pustules (pu),interpreted as spine bases, although the
inferred vertical spines are not observed. The medial groove and aboral latero-oral
flange are set at a similar relief to the aboral ambulacrals.
In oral view the latero-oral flange is asymmetric, pointed adradially and blunt
abradially (Figs lA, C, 3A, 4A). It shows a coarsely pitted ornament without pustules.
The adradial point faces proximally and continues for two thirds of the adradial
surface as a vertical ridge. Aborally to this ridge a square proximally-projecting
head (h) is developed (Fig. 4). A steep curving surface forms the distal face of the
head (Fig. 4D). In its upper (more aboral) half this surface is rimmed and concave
inwards, completing the proximal adradial wall to the podial basin. The lower (more
oral) half is undercut to form a curving facet (adf) which overlaps the distally
adjacent adambulacral. The adambulacral facet is received by a groove (adg) on
the oral head surface; successive adambulacrals therefore are stacked on each other.
The proximal head face completes the distal adradial wall to the podial basin. At
the junction between the proximal and distal faces of the head the adambulacral
abuts with the transverse boss of the ambulacral.
WHAT MAKES AN OPHIUROID?
233
Clear oral ‘groove spines’ are present on BMNH E53766b, generally dissociated
from the adambulacrals and covering the ambulacral groove. Each spine is relatively
broad and short and similar to the oral spines observed. BMNH E52409 has
small sub-rectangular platelets between adambulacrals, covering the aboral grooves,
however these are irregular in distribution and may represent included sediment.
Aboral and oral disc plating
From the disc centre fine rod-like elements form a densely packed single layer
extending to the arm tips (Fig. 1D). Each element is paxillate with an expanded
basal platform supporting a stalk-like middle region and expanded blunt end.
Mouth frame plating
Debate exists as to whether the major jaw plates are adambulacral (Blake, 1994)
or ambulacral (Smith &Jell, 1990) in origin. In Stenaster, the ossicles involved in the
jaw are clearly continuous with the arm ambulacrals in aboral and oral view and
correspond directly in position on both surfaces; I therefore support the views of
Smith &Jell (1990) and have followed their terminology in naming mouth frame
ossicles. Interradially adjacent first ambulacral ossicles are associated with a single
dental plate, observed in oral and aboral view on BMNH E53088a, b (Fig. 3D, E).
The first three ambulacrals comprise the jaw in Stenaster. In aboral view the most
proximal ambulacral forms an elongate prominent ‘mouth angle plate’ (Fig 3D, 5).
The second ambulacral is modified and appears slightly oblique to the remainder
of the ambulacrals, although this is brought about mainly by a broadening and
opening of the ambulacral channel. The third ambulacral directly abuts its opposite
along the perradius, but is slightly larger than the remaining ambulacrals (Fig. 3B,
D). Ambulacral four is identical to the remaining arm ambulacrals. A single dental
plate (torus) per mouth angle plate pair is identified, with attached tooth spines.
1st ambulacral (mouth angle plate)
The first ambulacral ossicle is interradial in position, elongate radially and closely
abuts its interradial neighbour. In aboral view it is crossed by a two grooves. The
proximal groove (pg), mid-way along the ossicle, is deep and trends obliquely.
Adjoining proximal grooves of adjacent first ambulacrals form a circular canal,
inclined slightly towards the disc centre. The groove is bounded by a proximal shelf
(ps) and medial process (mp) (Fig. 5).
The distal groove (dg)is shallower but more vertically orientated. It vees towards
the interradius and is wntinuous with a broad, uncovered lateral channel (ch)on
the second ambulacral, which joins the covered perradial channel of subsequent
ambulacrals. The first and second ambulacrals contact distal to the groove where
an enlarged medial concavity and medial process are developed on the respective
ossicles.
In oral view each ossicle is sub-triangular, tapering distally where it abuts the
most proximal adambulacral. Each first ambulacral has a distinctive radial ridge
rimming the oral and lateral surface of the ossicle. Interracially adjacent plates meet
234
J. DEAN
aboral
distal
Figure 5. Stenaster obtusus, camera lucida drawing aboral view of ambulacral jaw plates. Scale bar=
1 mm.
enclosing a V-shaped gap, into which the dental plate articulates. In radial lateral
view a podial proximal flange is developed which is associated with a distal flange
from ambulacral two, forming the first podial basin. A pore, (pl),emerges onto the
open part of the perradial channel at the margin of the two ambulacrals.
In all examples first ambulacrals were closely associated interradially; the interradial
face was not observed. On the radially-facing surface a semi-circular tongue is
developed distally and aborally, which is continuous with the open part of the
perradial channel (ch)on ambulacral two. First ambulacrals are not spine-bearing.
Overlays of part and counterpart drawings show that the first ambulacrals are
not paired to adambulacrals.
2nd ambulacral
The second ambulacral ossicle is characterised by an open, wide perradial channel
and tapered radial surface, causing the ossicle to appear oblique with respect to the
mouth angle plate and remaining ambulacrals (Fig. 5). In fact only the abradial
regions of opposite second ambulacrals are dissociated. In perradial view each ossicle
is strongly indented and crossed by a channel (ch),continuous with that in the
perradial region of subsequent ambulacrals and also with the distal tongue and
groove of the first ambulacral. The second ambulacral contributes to two pores (pl
and p2), which emerge onto the open region of the perradial channel and are
associated with the first and second podial basins respectively in oral view. The
distal adradial margins of the second ambulacrals overlap the proximal region of
the third ambulacrals.
Aborally, a well-developed medial concavity occurs at the junction of the first
and second ambulacrals and the medial process of ambulacral two is correspondingly
larger. The second and third ambulacrals are very closely associated. At its interradial
distal margin the second ambulacral contacts with a lateral adambulacral; overlay
drawings reveal this to be the most proximal adambulacral in oral view (i.e. the
adambulacral which contacts the mouth angle plate).
In oral view the second ambulacral is relatively similar in morphology to the
remaining arm ambulacrals however the perradial region runs obliquely to the
adjacent mouth angle plate (i.e. the cross bar of the ‘T’ is inclined). Podia1 basins
WHAT MAKES AN OPHIUROID?
235
are shared with the mouth angle plate and the third ambulacral. Both these podial
basins are smaller than those of remaining arm and the first podial basin is swung
inwards and faces towards the mouth. The second podial basin occupies the normal
lateral position.
3rd ambulacral
The third ambulacral ossicle is the least modified of the elements involved in jaw
formation in Stenaster but is larger and broader than ambulacrals in the remaining
arm and is overlapped by the second ambulacral in aboral view. Oppositely adjacent
third ambulacrals meet directly along their entire length in oral and aboral view.
In perradial view, proximally the third ambulacral have an open perradial channel,
but this is united distally, enclosing the channel, as in the rest of the arm.
Dental plate and teeth
A single, hemispherical torus (or ‘dental plate’, d) attaches to each interradial
pair of mouth angle plates, giving five dental plates per jaw (Fig. 3D). Several
specimens show simple, peg-like teeth scattered in the oral area (BMNH E52409,
E52839) and in E52612 these may be in situ.
FUNCTIONAL MORPHOLOGY
Several authors have considered differences in the functional morphology of the
extant stelleroid subclasses (Hyman, 1955; Lawrence, 1987) and their application
to Palaeozoic forms (Spencer, 1914-40; Schuchert, 1915; Fell, 1963; McKnight,
1975; Gale, 1987; Blake, 1982, 1987, 1994; Black & Guensberg, 1988, 1994). The
major morphological differences between extant asteroids and ophiuroids lie in: (1)
the water vascular system; (2) body wall support and general morphology; (3) arm
structure; (4) jaw structure. Taking each of these major structural differences in
turn, comparing these to those typical of Ordovician stelleroids and comparing with
the morphology of Stenaster (Table 2) is one approach to beginning to answer such
questions.
Water vascular system
In extant ophiuroids the water vascular system consists of a central circumoesophageal ring with five radial extensions. Each radial water vessel is housed in
a perradial conduit running within the vertebrae of each arm (each vertebra consists
of a fused pair of opposing ambulacrals). A single branch extension to each tube
foot also runs internally and emerges into the podial basin via a small lateral pore.
Basins are formed entirely by a single ambulacral ossicle. Tube-feet may be expanded
basally to form a muscular bulbous structure which is accommodated by the podial
basin. At the jaw, the circum-oesophageal water canal rests on an aboral circular
groove formed by interconnecting grooves on each jaw plate. The stone canal is
usually not ossified and leads from the ring canal to the aboral surface, terminating
in a simple, non-ossified hydropore. In fossil ophiuroids this outlet may be calcified
and, if so, is termed the ‘madreporite’.
J. DEAN
236
TABLE
2. Comparison of morphology of extant and Palaeozoic stelleroids to Stenaster. Asterisks indicate
states for Platanaster or Furcaster, where two or more morphological types exist
Extant
Asteroids
Ophiuroids
Ordovician
Asteroids
Ophiuroids
Open
Closed
Open
Closed
Closed
Present
Absent
Present
Absent
Podia1 basins
Shared
Within single Shared
ossicle
Ampullar pores
Present
Absent
Absent
Present or
absent (*)
Shared (*) or
within single
ossicle
Absent
Adaxial
(marginals,
actinals,
abactinals)
Present
(2 series)
Axial
(ambulacra)
Adaxial
Axial
Axial
Absent
Present (1 (*)
or 2 series)
Absent
Absent
Low
High
Low
Low to high (*) High
High
Low
Low
Low
Low
Opposite
Opposite
Opposite
Opposite (*)
or alternate
Opposite
Present
Oral
Absent
Lateral
Present
Oral or lateral
Absent
Lateral
Absent
Lateral
Abut
Overlap
Abut
About or
overlap (*)
Overlap
Dental plate
Absent
Present
Axillary/Odontophore
Mouth angle plate origin
Present
Absent
?Adamhulacral/ Amhulacral
ambulacral
Absent (*) or
Present
present
Present
Absent
?Adambulacral/ Ambulacral
ambulacral
WATERVASCULAR SYSTEM
Radial water vessel/lateral
branch
Madreporite
BODY WALL
Stenaster
Shared
Absent
SUPPORT AND
GENERAL MORE’HOLOGY
Arm support
Marginals
h
M STRUCTURE
Longitudinal ambulacral
muscle field development
Transverse ambulacral muscle
field development
Perradially adjacent
ambulacral arrangement
Ambulacral groove
Adambulacral position with
respect to ambulacral
Adambulacral-adambulacral
contact
i*)
JAW STRUCTURE
Present
Absent
Ambulacral
In Amphiura sp., tube foot extension is achieved primarily through an elastic
collapse of the radial water vessel (Woodley, 1967). Contraction of the proximal
bulb of the tube foot, separated from the radial vessel by a valve, enables further
localized extension. Opposite tube-feet work as independent functional units, isolated
from adjacent vertebrae by circular musculature. These act as ‘sphincters’,preventing
dissipation of the hydrostatic pressure between units.
Extant asteroids share the same basic water vascular structure of central and
peripheral systems (Nichols, 1972; Lawrence, 1987). However, in contrast to ophiuroids, each radial water vessel rests on an uncovered axial ambulacral channel;
lateral branches are simply external notches at the oral adradial podia1 junctions of
adjacent ambulacrals. Tube-feet lack proximal head bulbs but internal ampullae
are present and are involved in the complete extension of tube-feet, although radial
water vessels continue to play the key role (Nichols, 1972). Woodley (1967) noted
WHAT MAKES AN OPHIUROID?
237
that the head bulbs of ophiuroid tube-feet may be analogous rather than homologous
to asteroid internal ampullae. The stone canal and madreporite are usually present.
Palaeozoic stelleroids show significant differences in the water vascular system, in
comparison to Recent stelleroids. Firstly, although true internal ampullae are present
in all post-Palaeozoic asteroids (Gale, 1987) and some Carboniferous taxa, including
Calliasterella americana (Kesling & Strimple, 1966), (Blake & Guensberg, 1988), they
are absent in Ordovician asteroids (Blake, 1994). Smith & Jell (1990) described
large gaps between the ambulacral transverse bars of the Ordovician somasteroid
Archegonaster pentagonus Spencer, 1951, which could be interpreted as pores housing
internal ampullae. Similar pores are also present in edrioasteroids (Smith, 1985),
although were not interpreted as ampullar pores by Bell (1977). The possibility that
the podial pores of edrioasteroids and Archegonaster are homologous to ampullae
pores of neoasteroids has yet to be tested, but seems unlikely. Secondly, the majority
of Ordovician asteroids have madreporites.
In contrast to extant ophiuroids, all early asteroids, somasteroids and several
Ordovician ophiuroids (e.g. Eophiura bohemica Schuchert, 1915; Pradesurajacobi (Thoral,
1935);Phragmactis gryae, Spencer, 1940)have podial basins which are shared between
longitudinally-adjacent ambulacrals; the proximal half of each podial basin is formed
by a distal flange of one ambulacral ossicle and the distal half by the proximal
podial flange of the succeeding ossicle. In Ordovician stelleroids with the ‘shared’
condition, adjacent podial flanges meet closely or overlap, completely flooring the
podial basin or meet adradially but leave an abradial gap which is closed by the
walls of the adjacent adambulacrals (Blake & Guensberg, 1988). In fact the podial
basin morphology of apparently ‘primitive’ ophiuroid, asteroid and somasteroid taxa
are virtually identical. In asteroids with internal ampullae, podial flanges do not
meet, producing the ampullar pore. The lateral branch, be it internal (ophiuroid)
or external (asteroid),is therefore formed at the junction of ambulacral ossicles-each
ossicle margin is indented forming a half-pore (ophiuroids) or notch (asteroids)which
corresponds to a similar structure on the adjacent ossicle. In Ordovician ophiuroids
where podial basins are confined to a single ossicle (e.g. Hallaster cylindricus (Billings,
1857))the lateral branch extends through that ossicle alone to the perradius, as in
extant ophiuroids, and is visible as a discrete pore within that ossicle. In some more
problematic taxa, for example Palaeura neglecta Schuchert var bohemica Smith (see
Gutitrrez-Marco et al., 1984), the position of the lateral branch is within an ossicle
proximally, but is shared moving distally down the arm. In contrast to most extant
ophiuroids, the calcified stone canal and madreporite are present in some Ordovician
taxa, although lacking in others.
The water vascular system of Stenaster obtusus is identical to that of a typical
Ordovician ophiuroid: the radial water vessel is covered and housed in a perradial
channel; lateral branches are covered, although at the margin of adjacent ambulacrals,
since the podial basins are shared. Each podial basin is large in comparison to
ambulacral width and completely floored by ambulacrals adradially and adambulacrals abradially. Ampullar pores are absent. The large, cup-like basins suggest
substantial tube-feet, probably expanded proximally to form some kind of headbulb. At the jaw, a clear circum-oesophageal groove can be identified. A calcified
stone canal and madreporite are lacking, in common with derived ophiuroids.
Implications for mechanisms of tube foot protraction in Stenaster are unclear.
Opposite ambulacral ossicles would allow a similar system of tube-feet isolation as
is used by extant ophiuroids and lack of internal ampullae would argue for external
238
J. DEAN
control of tube-feet. Each podial basin is large and rounded, suggesting a head-bulb
mechanism, although this may not have been identical to the proximal bulbs of
extant ophiuroid podia, since the basins in Stenaster are not high-walled and they
are not confined to a single ambulacral as in extant ophiuroids and ‘derived
Ordovician ophiuroids’, e.g. Hallaster. Stenaster has large podial basins which are
partially bound by adambulacrals; this morphology may also be observed in the
somasteroids, several Ordovician ophiuroids including Eophiura, Falaeura and certain
asteroids (e.g. Cnemidactis girvanensis (Schuchert, 1915), Platanaster ordovicus, Spencer,
1919),suggesting a similar mechanism of tube foot operation in these taxa. Opposing
ambulacrals and the lack of a madreporite in Stenaster do not contradict this view,
since these are in common with Cnemidactis as well as with the ‘derived ophiuroids’;
however the somasteroids and early ophiuroids have madreporites suggesting there
may be some differences in water vascular system operation as a whole between
these taxa. In extant stelleroids, the role of the madreporite is not fully understood,
but may aid pressure equalisation (Nichols, 1972). The adaptive significance of a
calcified hydropore and/or stone canal is unknown.
Body wall support and general morphology
Arguably the most profound skeletal distinction between Recent asteroids and
ophiuroids lies in the method of support of the arms (Lawrence, 1987). Asteroid
arms are supported by adaxial elements (actinals, marginals, abactinals) causing the
arms to merge without distinction from the disc and accommodate a large internal
coelom. This restricts arm flexibility and consequently the reliance is on tube-feet
for locomotion and feeding. Suckered tube-feet in some taxa enable molluscivory
and locomotion on inclined hard substrates. Gale (1987) noted that this was a key
innovation by the Neoasteroidea. Arms are generally short and broad relative to
disc diameter and taper to a blunt point.
In contrast, ophiuroid arms are entirely supported by the axial skeleton (ambulacrals, adambulacrals); consequently the arms have little perivisceral coelom,
marginal ossicles are absent and the disc and arms appear highly separate. Arms
are therefore highly flexible and take the major role in feeding and locomotion.
Each is long in comparison to disc diameter and may end in a whip-like tail.
The majority of Ordovician stelleroids conform to these general morphologies.
The arms of Stenaster, significantly, are supported entirely by axial skeleton, restricting
the major organ systems to the disc centre, a very ophiuroid-like morphology. The
disc region is small and highly separated from the radii. Marginal ossicles are absent
and abactinals small relative to those of asteroids. However, each arm is broad and
relatively short, less suggestive of ophiuroid affinity.
Ann structure
Arms of extant ophiuroids are highly flexible and modified for food capture or
for clinging to a host. This is achieved primarily by the development of large aborallateral muscle fields, specialized inter-vertebral articulation structures, arm spines,
and the possession of lateral, tall adambulacrals, which are articulated flexibly to
ambulacrals. Ambulacrals are opposing and fused; there is no ambulacral groove.
\$’HAT MAKES AN OPHIUROID?
239
Adambulacrals bear short ‘groove’ spines, i.e. spines within the ambulacral groove,
protecting the podia, and elongate ‘vertical’ spines, which occur on the abradial
surface of the adambulacra and extend away from the arm. Tube-feet are inconspicuous and non-suctorial. In contrast, the arms of extant asteroids rely primarily
on tube-feet for locomotion and passing food to the mouth, hence tube-feet are
large, pointed or suckered, and ampullae-controlled (although the main role of
ampullae may be respiratory [Nichols, 19721). Ambulacrals are opposing and unfused
but articulated by dentition and oral and aboral transverse muscles (Gale, 1987).
Each ambulacral is lifted from the substrate, creating an ambulacral groove. This
is achieved primarily by the adambulacrals being set orally to the abradial regions
of the ambulacrals (Blake, 1989). Adambulacrals are blocky and spine-bearing.
Early Palaeozoic asteroids present several notable differences in arm architecture.
Firstly, there is no evidence for suckered tube-feet in early Palaeozoic asteroids (this
also extends to ophiuroids and somasteroids, the later definitely having pointed
tube-feet). Secondly, there is no evidence of cross-furrow musculature in these
asteroids, although rudimentary dentition does occur. Extensive longitudinal and
transverse asteroid ambulacral-ambulacral musculature together with true internal
ampullae appear to originate in the Carboniferous (Gale, 1987) and are thought to
relate to the development of extra-oral feeding and exploitation of hard substrates
by the neoasteroids. Thirdly, although adambulacrals are set orally to ambulacrals
in some taxa, others have lateral adambulacrals (examples are Platanaster, Cnemidactis
and Uranaster).
Early Palaeozoic ophiuroids show varying degrees of longitudinal ambulacralambulacral musculature; some, for example Furcaster trepidans Spencer, 1922, having
well-developed aboral-lateral ambulacral muscle fields. However, ambulacrals are
not fused to form vertebrae in Ordovician ophiuroids and zygospodylous/streptospondylous articulations have not been observed; articulation seems to occur through
simple but relatively flexible ball and socket joints. Transverse musculature is lacking
in Eophiura and related taxa, and has not been conclusively observed in any
Ordovician ophiuroid (though ambulacrals very rarely show the perradial surface).
Encrinaster grayae (Spencer, 1914) and Furcaster have perradial processes which appear
to enhance cross furrow articulation, and correspond to shallow perradial wells from
which muscle may have originated. Adambulacrals are truly lateral in Furcaster,
articulate via a ball-socket joint to ambulacrals and are differentiated into a head
and flange region. Each bears vertical and groove spines.
In Stenaster, longitudinal ambulacral musculature is most developed close to the
jaw. Distal muscle fields are present but confined to small aboral pockets, which
would have restricted movement primarily to within an aboral-oral plane. In contrast
to Furcaster, lateral movement would be limited due to the underdevelopment of
muscle fields. As in Furcaster, successive ambulacrals articulate via a ball-socket joint.
There is no evidence of significant cross-furrow musculature, although the aboral
contact is relatively undulatory, suggesting rudimentary dentition. Associated adambulacrals are tall, spine-bearing, lateral and slightly overlapping, although remain
blocky despite the distinction of a head and flange region. With the exception of
the blocky nature of the adambulacrals, these features resemble Ordovician ophiuroids. As in all ophiuroids, there is no ambulacral groove, however, the adambulacrals of Stenaster are unusually tall and certainly served to lift the ambulacra
from the substrate, achieving an analogous effect to an asteroid ambulacral groove.
240
J. DEAN
Jaw structure
Major jaw plates (mouth angle plates, orals,) are assumed to be ambulacral in
origin in ophiuroids (Hendler, 1978, 1988) but may be ambulacral or adambulacral
in origin in asteroids (Blake, 1994). Palaeozoic ophiuroids share the same basic jaw
plan with Recent ophiuroids of enlarged proximal jaw elements (first ambulacrald
mouth angle plates), paired interradially and associated proximally with a toothbearing ?adambulacral dental plate (Smith &Jell, 1990) and distally with an enlarged
oblique plate (second ambulacral). Bjork, Goldberg & Kesling (1968), studied the
mouth frame of three species of the Mississippian ophiuroid Onychaster and found
that each jaw plate or ‘first ambulacral’ is crossed in viva by a proximal groove,
housing the circum-oral nerve ring and a distal groove housing the circumoesophageal water canal. Adjacent second ambulacrals met radially and subsequent
arm ambulacrals were of normal morphology. Large muscle fields, developed
between proximal ambulacrals, were involved in opening the jaw.
In Furcaster the jaw arrangement is virtually identical to Onychuster, except that the
second ambulacral has a strong radial projection on its aboral surface, causing
adjacent second ambulacrals to form a radial ‘V’ which overrides the adjacent third
ambulacral.
Stenaster compares precisely to Furcaster in jaw arrangement, although the second
ambulacral is squarer and set in the same plane as the third ambulacral, a feature
observed by Smith &Jell (1990) for Archegonuster. The first ambulacral and second
ambulacral share a podia1 basin and are in direct continuity with each other,
confirming an ambulacral origin for jaw plates. Five dental plates appear present,
noted as a rather derived condition by Smith &Jell (1990). Clear muscle fields are
present between ambulacrals. The close association of interradially-adjacent first
ambulacrals indicates interradial muscles may have been well-developed, although
disarticulated first ambulacrals were not observed.
Jaw structures for Palaeozoic asteroids are difficult to observe and interpret,
primarily because the simpler, internal view is generally obscured by overlying
aboral plates. Rare views available of this surface in Hudsonaster sp. Sttirtz 1900
suggest an identical pattern of plates. The most proximal jaw elements are interradial
‘mouth angle plates’. These are associated with distally-positioned ‘circumoral
ossicles’. Each circumoral ossicle is oblique and enlarged and is itself in contact with
an unmodified ambulacral ossicle which forms the most proximal arm ambulacral.
It seems probable that the mouth angle plates of this asteroid are equivalent to the
first ambulacral ossicles of ophiuroids and circumorals to the second ambulacrals.
The major differences in jaw structure appear to be that: (1) in Hudsonaster, proximal
ambulacrals are not linked by extensive longitudinal musculature (this extends to
the remaining ambulacrals in asteroids); (2) in Hudsonasterjaw muscules are associated
with the axillary, an enlarged inferomarginal ossicle (Schuchert, 1915; Gale, 1987).
Further work is required to refine these jaw characters and compare with those of
other Ordovician asteroids. In that Stenaster has a highly muscular set of jaw
ambulacrals, a dental plate, but lacks an axillary, it must be considered an ophiuroid.
CLADISTIC ANALYSIS
The new morphological information gained from this revision has been incorporated into a numerical cladistic analysis of six exclusively Palaeozoic taxa, to
WHAT MAKES AN OPHIUROID?
241
Figure 6. Platanaster ordovicus Spencer, Caradoc of Shropshire, England. A, B, British Geological Survey
GSM 25347 (oral surface) & 8238 (aboral surface)-part and counterpart. A, oral view of single arm
and central disc area, x 4. B, aboral view of same region, x 4. Latex casts whitened with ammonium
chloride sublimate.
determine whether it shares more synapomorphies with asteroids or with ophiuroids.
Included were in this analysis were the ophiuroid Furcaster trepiduns and the asteroid
Platanaster (Fig. 6). Although the class-level relationships of asteroids and ophiuroids
remain unresolved, the conclusions of Smith &Jell (1990),suggest that the somasteroid
Archegonaster is a reasonable choice of outgroup for this study, which is to establish
the position of Stenaster relative to the asteroids and ophiuroids only. The inclusion
of a hypothetical ancestor or an edrioasteroid as an outgroup in the analysis
consistently placed Archegonaster in a basal position. The somasteroid taxa Chiniunaster
leyi Thoral, 1935 and Ellebrunaster thorali Spencer, 1951 are in the process of restudy
and have been included provisionally in this analysis, pending full revision and
redescription.
A major question in stelleroid phylogeny concerns the homologies of the plate
series that run along the arms of ophiuroids, asteroids and somasteroids, and this
may lead to ambiguities in character designation. Scoring of several characters
depends particularly on whether the ' 1st virgals' of Archegonaster, Chiniunaster and
J. DEAN
242
0
0
0
0
0
0
0
0
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0
0
0
0
0
0
0
0
0
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s
?
0
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?
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?
0
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0
+0
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0
7
0
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0
+0
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+0
+
Figure 7. Character matrix and cladogram for six Ordovician stelleroid taxa. Characters are given in
Table 1. Key 0= plesiomorphic states, = derived states, = different character state, ? = unknown
character state.
+
Ellebrunaster, which were termed 'adambulacral ossicles' by Smith & Jell (I 990) in
their description of Archegonaster, are homologous to the adambulacral ossicles of
Furcaster, Platanaster and Stenaster. Similarly, 'terminal virgals' of somasteroids may
not be homologous to inferomarginals of asteroids. As the 1st virgals of somasteroids
are identical in position to adambulacrals of other stelleroids and those of Archegonaster
in particular show marked morphological similarity to adambulacrals, the simplest
biological interpretation is to treat these plates as homologous. Similarly marginals
of Platanaster and 'terminal virgals' of somasteroid taxa show general positional
similarity and morphological similarity (particularly the marginals of Archegonaster
and Platanaster) and were also treated as homologous.
A data matrix of six taxa and 24 characters was assembled, including 20 binary
characters and 4 multistate characters, giving 52 character states in total (Fig. 7;
Table 3). All characters were treated as unordered. The character states for
somasteroids and Furcaster were scored by examining new latex casts of type specimens
and material found subsequently from the type localities. Character states for
WHAT MAKES AN OPHIUROID?
243
TABLE
3. Inferred primitive and derived character states of characters used in cladistic analysis
(autapomorphies omitted)
~
Primitive (0)
Derived
1. Ambulacrals alternate i.e. repeated consistent
offset along arm length
2. Ambulacral transverse bar distal in position
3. Longitudinally-adjacent ambulacrals strongly
overlapping [> one third of ambulacrum width]
4. Pore @) absent - indicating external lateral
branch to radial water vessel
5. Aboral longitudinal muscle fields on ambulacrals
absent
6. Adambulacral spines present
7. Adjacent adambulacrals are not in contact
8. Adambulacrals virgal-like in shape
9. Adambulacrals are flattened in an aboral-oral
plane
10. Virgal series present
11. R r < or = 1.9 (pentagonal disc-shape)
12. Adambulacrals straight moving towards disc
centre
13. Marginal ossicles present
14. Marginal spines absent
15. Marginal ossicles homogeneous in morphology
16. Abactinals densely packed or overlapping
17. Abactinals plate-like
18.
19.
20.
21.
22.
Abactinal spines absent
Buccal slits present
Calcified stone canal/madreporite present
Mouth angle plate spines absent
Second ambulacral abuts third ambulacral directly
23. Odontophore (axillary) absent
24. Adjacent ambulacrals articulate along smooth
surface
(0)
Ambulacrals oppose along entire arm length
Transverse bar sub-central
(0) Abutting ambulacrals
(*) Weak overlap [up to one third of ambulacrum
width]
Pore @) present indicating enclosed lateral branch to
radial water vessel
Aboral muscle fields present
-
Adambulacral spines absent
( 0 )Adjacent adambulacrals abut
(*)
Adjacent adambulacrals overlap
Adambulacrals blocky
Adambulacrals are tall and extend beyond ambulacral
height in oral and aboral views i.e. ‘wrap-around the
ambulacra
Virgal series absent
(0) R: r 1.9-3.8
(*) R r>or= 3.8 (elongate arms, small disc)
Adambulacrals petalloid in arrangement moving
towards disc centre
Marginal ossicles absent
Marginal spines present
Marginal spines heterogeneous in morphology
Abactinals form reticulate network
(0)Abactinals granular
(*)Abactinals paxillate
Abactinal spines present
Buccal slits absent
Calcified stone canal and madreporite absent
Mouth angle plate spines present
Second ambulacral projects aborally and distally to
third
Odontophore developed
Distinct ball and socket articulation developed
Platanaster were drawn from Blake’s (1994) description and latex casts of the type
specimen.
RESULTS
The data were analysed using PAUP 3.1 (Swofford, 1993) and trees were rooted
on Archegonaster. A single most parsimonious tree was found with a length of 31, a
consistency index (CI) of 0.903 and retention index (RI) of 0.875. Percentage
bootstrap support figures indicate that the branches are all very well supported (Fig.
7). The morphological data divide the ingroup taxa into two separate sister groups:
Chinianaster plus Webrunaster, with a second sister group comprising the ‘true’ asteroids
plus ophiuroids. Within this clade there is strong support for pairing Furcaster and
Stenaster. The asteroid Platanaster occupies a lower node in the topology. This analysis
244
J. DEAN
clearly demonstrates that Stenaster is more closely related to Furcaster, an accepted
Palaeozoic ophiuroid, than to any of the remaining taxa including the asteroids.
Data matrices based on different hypotheses of homology for marginal and
adambulacral ossicles between taxa, when analysed, consistently produced an identical single tree, with near-identical CI, RI and bootstrap values.
DISCUSSION
Although a discussion of the detailed phylogeny of stelleroids awaits further
research, this cladistic analysis reveals several points worthy of consideration.
The clade comprising (Platanasterplus Stenaster plus Furcaster) i.e. the ‘asteroids’plus
‘ophiuroids’ are distinguished from the somasteroids by their abutting, opposite
ambulacrals, contacting adambulacrals, non-pentagonal disc and lack of a virgal
series. Two other features uniting this group (paxillate aboral ossicles and petalloid
proximal adambulacrals) have been subsequently lost in Furcaster.
Moving from Platanaster to the ‘ophiuroids’ involved changes in the water vascular
system and an apparent trend towards increasing arm and jaw flexibility. The radial
water vessel and lateral canals were enclosed and a calcified stone canal and
madreporite lost. Ambulacrals developed ball and socket-like articulations and strong
longitudinal aboral musculature. Adambulacrals became overlapping and ‘wrapped
around’ the ambulacrals; marginals were lost. In the jaw, circum-oral ossicles (second
ambulacrals) moved aborally over the adjacent ambulacrals (third ambulacrals).
Furcaster displays several autapomorphies suggesting an ordered morphological
trend moving from the somasteroid taxa to asteroids to ophiuroids and involving
relative reduction in disc diameter and increase in arm length. Adambulacrals were
non-articulating in the somasteroids, abutting in the asteroids and overlapping in
the ophiuroids.
The position and status of the somasteroids Chinianaster and Ellebrunaster remain
problematic. Although they appear to form a monophyletic group, they display
mixed characteristics, sharing features with the ophiuroids, with Platanaster and with
Archegonaster. This analysis suggests that the somasteroids and ophiuroids have
independently enclosed their water vascular systems, however in common with
Archegonaster, which has an open radial water vessel, Chinianaster and Ellebmnaster have
a virgal ossicle series. Buccal slits occur in both Platanaster and the clade comprising
(Chinianaster plus Ellebrunaster), again suggesting independent acquisition of these
characters and non-homology.
Palaeobiological implications
Evidence from the water vascular system in Stenaster (lack of a calcified madreporite
and stone canal), jaw structure (five dental plates per jaw), arm structure (opposite
ambulacrals, articulated by longitudinal musculature), and general morphology (axial
support to arms), suggest that Stenaster is a relatively derived member of the ophiuroid
stem group. However, large podia1 basins and general lack of arm mobility point
to a behaviour and ecology very different to modern ophiuroids and convergent on
that more typical of the asteroids.
WHAT MAKES AN OPHIUROID?
245
The arms of Stenaster obtusus were relatively inflexible in the lateral plane, making
arm coiling impossible. Aboral/oral movement was severely limited in comparison
to extant ophiuroids and probably relied on both ambulacral and dorsal integument
muscles (c.f. Heddle, 1967). Lack of ampullar pores and presence of large, rounded
podia1 basins suggest the podia were substantial and operated by external headbulbs, perhaps with some isolation of opposite ambulacral segments. In addition,
the podia were evidently lifted aborally with respect to the adambulacral oral edges,
producing a pseudo-ambulacral groove and arms are short and broad. These features
all suggest that the tube-feet in Stenaster took the primary role in feeding and
locomotion. Extra-oral feeding can be ruled out due to lack of extensive crossfurrow musculature and absence of suckered tube-feet (Gale, 1987).
Extant ophiuroids are predominantly epifaunal and adopt either a carnivorous
or microphagous (sediment or suspension-feeding) trophic strategy (for a review see
Warner, 1982). Carnivory generally involves arm-sweeping foraging behaviour and
arm-loop method of food transport (Reimer & Reimer, 1975), or less commonly
the tube-feet or buccal apparatus alone. Infaunal microphagous deposit-feeding
amphiurid ophiuroids use similar sweeping arm movements and mucus to forage
and sinusoidal arm movements to burrow; food is transferred to the mouth by the
tube-feet. Epifaunal ophiuroids rarely feed solely by deposit-feeding. Suspension
feeding is via several mechanisms, generally using arm-spines and mucus or tubefeet for food capture. Lack of arm mobility in Stenaster suggests simple deposit
feeding, chance whole-prey ingestion (rather than involving specific foraging) and
possibly tube-foot suspension feeding represent possible trophic strategies for Stenaster.
All of these strategies occur in asteroids (seeJangoux, 1982).In contrast, the elongate,
highly flexible arms of Furcaster and muscular jaw frame are adaptations for selective
carnivory on macroprey.
Increased flexibility in the Stenaster jaw due to the second/third ambulacral
arrangement, parallels the jaw of Recent goniopectinid and porcellanasterid asteroids
(Madsen, 1961). Shick, Edwards & Dearborn (1981) found that the extant Goniopectinid Ctenodiscus crispatus (Retzius, 1805), ‘shovels substrate’ into the mouth
indiscriminately during burrowing using the proximal podia. Increased sediment
loads are accommodated in the stomach by aboral paxillae. Feeding on infaunal
and epifaunal macroprey has been reported for Astropecten irregularis (Pennant, 17 7 7)
(Jangoux, 1982). Epifaunal prey are engulfed by the starfish and subsequently
digested (Christensen, 1970).Stenaster has aboral paxillae and proximal jaw ambulacral
musculature, the proximal podia are swung into the jaw and the dental plates are
stout, suggesting a similar feeding mechanisms to these taxa. Extant asteroid depositfeeders generally feed on a wide variety of macroprey and microphagous particles
(Jangoux, 1982) and trophic strategy in any fossil species to this level is highly
speculative, although both strategies appear appropriate to Stenaster.
Life-position determination in fossil stelleroids is also conjectural, but has been
discussed for Stenaster (Spencer, 1925). Morphology in Stenaster supporting a semiinfaunal life habit include presence of aboral paxillae (possible respiratory and
sediment-guarding function, however, see Blake (1994)),adambulacral-adambulacral
fascioles and lateral arm channels (possible cribiform and excurrent channel analogues). Stenaster is found commonly in fine sandstones, for example the Ladyburn
Starfish Beds at Girvan, Scotland, U.K. thought to be formed through downslope
slumping and burial (Harper, 1982). An infaunal life position would facilitate
preservation and is one explanation for the relative abundance of Stenaster in
246
J. DEAN
comparison to most other stelleroids at this locality. However Stenaster lacks complex
muscular and skeletal digging characteristics described by Heddle (1967) for Astropecten.
The arm is not flexible enough to permit an Amphiura-style digging behaviour and
a digging mechanism involving tube feet and the jaw would need to be invoked. It
seems likely that any burrowing in Stenaster was shallow and temporary, perhaps
relying on strong pericoelomic muscles to open the groove and using lateral sediment
movement by tube feet aided by sediment ingestion as a digging mechanism.
Epifaunal activity was likely dominant, although in contrast to Furcaster which has
whip-like arms and small tube-feet, suggesting a highly mobile, epifaunal lifestyle,
mobility in Stenaster was undoubtedly limited.
Intermittent suspension-feedinghas been suggested, together with deposit-feeding,
for Stenaster (Spencer, 1925; Spencer & Wright, 1966)but seems unlikely. Suspensionfeeding has been suggested exclusively for some Ordovician asteroids including
Platanaster (Blake, 1994). Stenaster shares petalloid arm shape, aboral paxillae and
strong ‘interadambulacral’ and lateral adambulacral fascioles with Platanaster. However, Stenaster does not have flat arms, and petalloid arms are shared by many extant
asteroids which are non-suspension-feeding (e.g. Echinaster sp. which feed primarily
on encrusters, algal films and sediment [Tangoux, 19821). Aboral paxillae are small
and irregular in distribution in Stenaster, the aboral surface is densely packed and
probably would have inhibited pronounced aboral movement of the arms. Fascioles
may have functioned in ciliary feeding but a respiratory function is equally valid. Blake
interpreted pads between adambulacrals in Platanaster as sites of interadambulacral
musculature which facilitate raising and twisting of the arm into a suspension-feeding
posture. However, although Stenaster possesses pads between adambulacrals, these
bear no resemblence to muscle fields developed on ambulacrals of Stenaster. In my
view, these structures are more likely to indicate close connective tissue association
(i.e. abutment) of adjacent adambulacrals to provide strength to the skeleton rather
than to increase flexibility.
In summary, the morphology of Stenaster obtusus can best be explained by a nonselective deposit-feeding strategy, predominantly on microphagous material, although
opportunist macrophagy (although not through extra-oral feeding) was likely possible.
Tube-feet play the major role in feeding and movement, in common with most
extant asteroid taxa but few extant ophiuroids, none of which restrict themselves to
this feeding behaviour alone. Life position was likely primarily epifaunal, although
some infaunal activity may have occurred, in common to some extant paxillosids.
Lack of an epiproctal cone and derived digging musculature suggest any burrows
were temporary and shallow, perhaps covering the proximal disc only. So, whereas
other primitive ophiuroids were adapting towards a highly mobile, epifaunal lifestyle
and selective carnivory, Stenaster has converged towards a lifestyle more similar to
that of the intra-oral-feeding neoasteroids; life position was likely semi-infaunal and
trophic strategy non-selective deposit-feeding.
ACKNOWLEDGEMENTS
This work forms part of a PhD project funded by The Mary Anne Ewart Research
Studentship of Newnham College, Cambridge. I would like to thank Drs Andrew
B. Smith, Daniel B. Blake and Peter Forey, and Professors Chris Paul and Andy
iZIHAr MAKES AN OPHIUROID?
247
Gale for helpful comments which improved the manuscript. This is Cambridge
Earth Sciences Publication 5 165.
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APPENDIX 1: Synonymy list for Stenaster obtusus
Subclass Ophiuroidea
Family Stenasteridae Schuchert, 1914
Stenaster Billings, 1858
q p e species
Uraster obtusus Forbes. 1848
Stenaster obtusus (Forbes, 1848)
Astm’as primaeva, Salter & Sowerby, 1845 pp. 8, 20 (nom. nud.).
Uraster obtusus, Forbes, 1848, p. 463.
Uraster obtusus, Forbes, 1849, vol. i, p. 2, pl. i, fig. 3.
Uraster obtusus, Murchison, 1854, p. 182, fig. 17.
Palaeaster obtusus (Forbes), Salter, 1857, p. 326.
Stenaster salta”, Billings, 1858, p. 78, pl. x, fig. la.
Stenaster obtusus (Forbes), Wright, 1862, p. 24.
Palaeaster obtusus (Forbes), Salter, 1866, pp. 479, 480, pl. xxxiii, fig. 1 .
letraster wyuille-thomsoni, Nicholson & Etheridge, 1880, p. 324, pl. xxi, figs 1-8.
Stenaster obtusus (Forbes), Stiirtz, 1886, p. 153.
Stenaster obtusus (Forbes), Sturtz, 1893, pp. 41-56.
Ztraster wyville-thomsoni Nicholson & Etheridge, Etheridge, 1899, p. 129.
Stenaster obtusus (Forbes), Spencer, 1914, pp. 22, 31, 49, text-figs 21, 28, pl. i, figs 6,7.
Stenaster obtusus (Forbes), Schuchert, 1914, pp. 12, 29, 39-44.
Stenaster salteri Billings, Schuchert, 1914, p. 40.
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Ztraster wyville-thomsoni, Nicholson & Etheridge, Schuchert, 1914, pp. 5, 7, 42.
Stmaster obtusus (Forbes), Schuchert, 1915, pp. 165, 167.
Stenaster salta. Billings, Schuchert, 1915, pp. 164, 165, pl. xxxii, fig. 1.
Hudsonaster bathen, Schuchert, 1915, pp. 55, 65, 167, pl. iii, fig. 3.
Ztraster wyuille-thomsoni Nicholson & Etheridge, Schuchert, 1915, p. 168, pl. xxxiii, fig. 4.
Stenaster salten' Billings, Ruedemann, 1916, pp. 52-54, pl. ii, figs 1, 2.
Stenaster obtusus (Forbes), Ruedemann, 1916, p. 52.