18
Polyplacophoran and
Symmetrical Univalve Mollusks
Lesley Cherns, David M. Rohr, and Jiří Frýda
G
eneralized taxonomic relationships and biodiversity patterns of the Ordovician representatives of two small molluscan groups—the scleritebearing polyplacophorans (or chitons), and the
symmetrical univalves (Tryblidiida and Bellerophontida)—are discussed here.
■
Polyplacophorans (LC)
The polyplacophorans (or chitons) have a long
fossil record (Cambrian–Recent) but are rare fossils
and are mostly known from isolated intermediate
sclerites. Head and tail plates, which commonly differ markedly in morphology, size, and ornament from
the series of intermediate plates, are notably few in the
fossil record. A similar complement of eight sclerites
to Recent polyplacophorans is apparent from rare
articulated specimens (e.g., Rolfe 1981) and also a
comparable paleoecology for most in shallow marine
facies (e.g., Smith and Toomey 1964; Runnegar et al.
1979; Cherns 1999). The generally poor preservation
potential of rocky shore paleoenvironments probably
contributes to the sparse fossil record.
The pattern of Ordovician chiton diversification is
distorted by detailed records limited to only a few
faunas. Those major faunas come from the Lower
Ordovician of the United States and Australia (e.g.,
Bergenhayn 1960; Smith and Toomey 1964; Runnegar et al. 1979; Stinchcomb and Darrough 1995).
Other records are fairly sparse and isolated but include
significant Upper Ordovician genera from Scotland
(e.g., Rolfe 1981). The phosphatic, debatably polyplacophoran Cobcrephora from Australia (Bischoff
1981), from an allochthonous block of probable Late
Ordovician age within the Lower Devonian Cuga
Burga Volcanics (I. G. Percival pers. comm.), remains
a problematic taxon.
The higher taxonomy of polyplacophorans is far
from settled. In the current scheme, almost all Early
Paleozoic chitons are included in the subclass Paleoloricata (Cambrian–Upper Cretaceous), whose
members lack an articulamentum and hence sutural
laminae and insertion plates (Smith and Hoare 1987).
In the subclass Neoloricata (Carboniferous–Recent),
which includes all Recent chitons, only some Carboniferous lepidopleurine families lack insertion
plates. Carboniferous lepidopleurid species previously assigned to Helminthochiton were reassigned
to Gryphochiton because the Silurian type species
H. griffithi and also Middle Ordovician H.? aequivoca
apparently lack both sutural laminae and insertion
plates (Smith and Hoare 1987). Hoare (2000: figure 5) proposed the paleoloricate family Helminthochitonidae, including Early to Mid Paleozoic Helminthochiton species, as a possible stem group for later
neoloricates.
Discussion of the early history and origin of
polyplacophorans centers around genera of Upper
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Cambrian–Lower Ordovician multiplated organisms
that are questionably chitons, such as Matthevia,
Hemithecella, and Preacanthochiton. Lower Cambrian
microgenera from China (Yu 1987) were rejected as
polyplacophorans by Qian and Bengtson (1989; also
Sirenko 1997; Hoare 2000). Separate higher-level
molluscan taxa were proposed for Matthevia (Yochelson 1966) and Hemithecella (Stinchcomb and
Darrough 1995). Stinchcomb and Darrough (1995)
established several new genera for elongate sclerites
associated with Hemithecella but wished to exclude all
these and also Preacanthochiton (Bergenhayn 1960)
from the Polyplacophora because of what they recognized as “monoplacophoran-like multiple muscle
scars” (Stinchcomb and Darrough 1995:52) and some
valve asymmetry. However, Runnegar et al. (1979)
argued that Upper Cambrian Matthevia represented
the earliest known polyplacophoran and that its tall
conical valves became replaced by flatter, elongate,
and overlapping valves through younger Hemithecella
to Chelodes. Matthevia, Hemithecella, and Preacanthochiton are accepted here as paleoloricates (Runnegar
et al. 1979; Smith and Hoare 1987; Hoare 2000), although Sirenko (1997) proposed their exclusion from
his classification into two orders and eight families.
Hoare’s (2000) modified classification (from Sirenko
1997), including all those genera, has only four families present in the Ordovician.
Paleozoic chiton phylogeny still has much to resolve, and the recent description of a plated aplacophoran from the Silurian (Sutton et al. 2001) has
further widened the debate. The data in figure 18.1
include only published species and follow Hoare’s
(2000) scheme except that the Septemchitonidae is restored. The elongate, robust sclerites of Tremadocian
Hemithecella, Conodia, Robustum, and Calceochiton,
together with Chelodes and Eochelodes, are in the
broad grouping of the Mattheviidae (figure 18.1A–F).
Chelodes is long-ranging and occurs widely in Ordovician and later Silurian rocks. Tremadocian Preacanthochiton (figure 18.1G), which has small conical
plates with a pustular ornament, in the Preacanthochitonidae, was placed by Runnegar et al. (1979) outside its Matthevia-Chelodes lineage. In the Gotlandochitonidae, sclerites are wider than long, with weak
to clearly defined shell areas. Gotlandochiton, Paleochiton, and Ivoechiton (figure 18.1H–J) occur in an
important chiton assemblage from the early Arenig
(Smith and Toomey 1964). The Helminthochitonidae
(Sirenko 1997; figure 18.1K, L) includes Kindbladochiton from that same Arenig assemblage (Smith
and Toomey 1964) and Helminthochiton?. The Septemchitonidae includes Upper Ordovician Septemchiton and Solenocaris (figure 18.1M, N), which have
very elongate, narrow sclerites. Priscochiton (figure
18.1O), based on very limited material, lacks family
assignment.
The pattern of Ordovician diversification in figure 18.1 shows that the Early Ordovician was an
important period of polyplacophoran radiation, particularly on the low-latitude Laurentian margin.
Upper Cambrian–lower Tremadocian assemblages of
Matthevia, Hemithecella, Preacanthochiton, Conodia,
and Robustum occur in stromatolitic, shallow marine
carbonates and dolomites (Yochelson 1966; Runnegar et al. 1979; Stinchcomb and Darrough 1995).
Similar paleoenvironments were associated with Smith
and Toomey’s (1964) diverse lower Arenig Laurentian
assemblage and on the Gondwanan margin for Chelodes whitehousei from Australia (Runnegar et al.
1979). The Lower Ordovician shows the highest
diversity of chitons with 10 genera and 20 species.
This peak of diversity compares with the initial radiation of bivalves, except that chitons were diversifying on the low-latitude Laurentian carbonate margin,
whereas bivalves were confined to higher latitudes
and siliciclastic-dominated settings on Gondwana and
Avalonia (e.g., chapter 20).
The data from the Middle Ordovician are currently restricted to very few, isolated records. Chelodes?
mirabilis from Alabama represents Laurentia, and
Helminthochiton? aequivoca from Bohemia represents
a peri-Gondwanan terrane, Perunica (chapter 5).
The Upper Ordovician shows more diversity, with
five genera and nine species. Solenocaris includes the
first record from Baltica. Laurentian chitons are represented by Solenocaris, Chelodes, and Septemchiton
from the Midland Valley of Scotland, the last two also
from the United States. Eochelodes from Bohemia represents Perunica. Isolated and localized occurrences
preclude useful paleoenvironmental interpretation.
There are no records of polyplacophorans from late
Ashgill rocks.
The Silurian record of polyplacophorans shows
diverse assemblages particularly from shallow carbonates on Gotland (Bergenhayn 1955; Cherns 1998a,
Polyplacophoran and Symmetrical Univalve Mollusks
6c
6b
6a
5d
Mohawkian
Caradoc
UPPER
ORDOVICIAN
Cincinnatian
443
Ma
Ashgill
TIME
SLICES
GLOBAL
SERIES &
STAGES
North American
British
Reg.
series
5c
5b
5a
Whiterockian
Llanvirn
DARRIWILIAN
MIDDLE ORDOVICIAN
460.5
Ma
4b
4a
3b
3a
2c
2b
2a
Ibexian
1d
Tremadoc
LOWER
TREMADOCIAN
ORDOVICIAN
Arenig
472
Ma
4c
FIGURE 18.1. Range chart for genera
of Ordovician polyplacophorans. Information on ranges is given as accurately
as possible, for most genera to one time
slice (TS ), and ranges are shown to
time-slice boundaries. Numbers against
each line indicate species recognized for
Lower, Middle, and Upper Ordovician
parts of ranges, as appropriate. Dotted
lines signify gaps within ranges through
which a genus can be assumed to continue. Family assignments: A–F, Mattheviidae; G, Preacanthochitonidae; H–J,
Gotlandochitonidae, K–L, Helminthochitonidae; M–N, Septemchitonidae;
O, indeterminate family.
1c
1b
1a
490
Ma
1998b). Though still rare, these chiton-rich assemblages emphasize the bias thrown on analyses of diversity where the fossil record otherwise is very sparse.
■
Tryblidiids and Bellerophontids
(DMR, JF)
Paleozoic symmetrical univalved mollusks (Tryblidiida and Bellerophontida) are one of the most
discussed groups of the phylum Mollusca. Correct
evaluation of phylogenetic relations of these ancient
mollusks to other molluscan groups is crucial for
our understanding of the evolution of all molluscan
classes of the subphylum Cyrtosoma. Opinions on
their phylogenetic relations were recently analyzed in
several papers (e.g., Starobogatov 1970; Peel 1991a,
1999b; Wahlman 1992; Geyer 1994). However, these
analyses resulted in quite different phylogenetic
models and caused the erection of many new orderand class-level names. In addition, their subsequent
usage and generic content have often been very different, and they have disregarded their original diagnoses (e.g., Monoplacophora and Tergomya). Tryblidiida with cap-shaped shells and isostrophically coiled
Bellerophontida belong to the largest groups among
the Ordovician symmetrical univalved mollusks. In
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addition, several minor molluscan groups such as
Helcionelloida (= Eomonoplacophora), Cyrtolitida,
and Pelagiellida are known also from Ordovician strata
and probably played a significant role in early molluscan phylogeny (see Starobogatov 1970; Geyer 1994;
and Gubanov and Peel 2000 for discussion).
Because of the absence of a generally accepted
classification of the Paleozoic symmetrical univalved
mollusks, we analyzed only the two largest Ordovician groups, Tryblidiida and Bellerophontida, which
can be easily distinguished by their shell morphologies. We (DMR, JF) feel confident that both groups
are probably polyphyletic as suggested by many authors (see later in this chapter and chapter 19). Nevertheless, their more detailed analysis is impossible
without future reevaluation of all available data as
well as reconsideration of the significance of some
characters (e.g., muscle scars). The main goal here is
to illustrate the Ordovician biodiversity patterns of
Tryblidiida and Bellerophontida and to point out
some problems in the evaluation of their phylogenetic relationships. Refer to the next chapter in this
volume on the Gastropoda for the definitions of
diversity parameters.
Tryblidiida (= “Monoplacophora”)
The Ordovician groups placed in Tryblidiida by
Geyer (1994) as well as Achinacelloidea are united
here in one group. The systematic position of the latter group is still uncertain, and it has been considered
to belong to the class Gastropoda (e.g., Knight and
Yochelson 1958; Peel and Horný 1999) or to the
“Monoplacophora” (Horný 1963; Runnegar and Jell
1976). At present there is no reliable evidence about
whether members of both these groups, Tryblidiida
and Achinacelloidea, are torted or untorted. As correctly noted by Harper and Rollins (1982, 2000) and
Wanninger et al. (2000), the usage of muscle scar patterns as a key for determination of torsion in fossil
mollusks has no basis in fact. In addition, the belief
that symmetrically arranged multiple muscle scars
are evidence for symmetrical, segmented anatomic
arrangement of the shell body is false. These two
speculations were evoked by discoveries of living
“monoplacophorans” (Neopilinidae), which seem to
confirm a theoretical concept of a stem group for
the conchiferan mollusks as expressed by Odhner (in
Wenz 1940). These facts also opened a controversial
debate over which characters of neopilinid anatomy
are derived and which are primary (see Haszprunar
and Schaefer 1997 and references therein). Anatomy
of the modern Neopilinidae seems to be a result of
ancient molluscan organization as well as specialization. Interpretation of the body plan in the Paleozoic
Tryblidioidae as being closely similar to that of the
living Neopilinidae is thus problematic. Moreover,
the integration of both groups into one natural group
is uncertain because there is no reliable evidence that
they are really closely related. As shown earlier in this
chapter, we are very far from being able to determine
natural groups within the Ordovician mollusks with
cap-shaped shells.
The Ordovician Tryblidiida are known from equatorial as well as high-latitude regions. The majority of
genera were described from the paleotropical realm
(Laurentia, Baltica, and Siberia), and they had very
limited geographic ranges.
The quantitative diversity pattern of the Ordovician Tryblidiida cannot be described in detail because
of a lack of detailed stratigraphic data for the majority (mainly Siberia and Baltica). Their genera are
often based on limited material (sometimes even on a
single shell with well-preserved muscle scars). The
Ordovician Tryblidiida reached their highest diversity
during the Early Ordovician (18 genera), and most
genera were restricted to this time interval. During
the Mid and Late Ordovician their diversity is slightly
lower (14 and 13 genera, respectively). Only about
one-quarter of the genera crossed the Ordovician/
Silurian boundary.
Bellerophontida (Amphigastropoda)
The class-level assignment of bellerophontoidean
mollusks has often been discussed during the past 50
years. Considerations about the morphology of soft
parts of these coiled, bilaterally symmetrical mollusks
divided paleontologists into several groups. The question essentially resolves to whether all or part or none
of the bellerophontiform mollusks were untorted,
exogastrically oriented monoplacophorans or torted,
endogastrically oriented gastropods (see Yochelson
1967; Harper and Rollins 1982, 2000; Peel 1991a,
1991b; Wahlman 1992; Frýda 1999b, for review).
Recently Frýda (1999b) described a multiwhorled
Polyplacophoran and Symmetrical Univalve Mollusks
Amphigastropoda
dtot 3
dnorm
di 0
dtot
oi
e
0 i
dnorm
1
di
1a 1b 1c 1d 2a 2b 2c 3a 3b 4a 4b 4c 5a 5b 5c 5d 6a 6b 6c
TREMADOCIAN
LOWER ORDOVICIAN
489 Ma
DARRIWILIAN
M ORDOVICIAN
472
UPPER ORDOVICIAN
460.5
443
FIGURE 18.2. Plot illustrating generic biodiversity and turnover
rates of Amphigastropoda (bellerophontiform mollusks). Lines
show the biodiversity (dtot = total diversity; dnorm = normalized
biodiversity, and di = number of genera per m.y.). Bars show
turnover rates (white = rate of originations per m.y. [oi ], black =
rate of extinctions per m.y. [ei ]). For definitions of the parameters
used, see chapter 19.
protoconch in the type genus Bellerophon and interpreted it as a true larval shell. This fact suggests that
this genus (and thus, also the Bellerophontoidea,
Amphigastropoda) does not belong to the Archaeogastropoda (Frýda 1999c, 2001). On the other hand,
some new data on the protoconch morphology indicate that the Paleozoic bellerophontiform mollusks
represent a polyphyletic group (Frýda 1999c). Recently Harper and Rollins (2000) concluded that the
bellerophontoideans and the coiled and high-domed
“monoplacophorans” were gastropods.
The Early Paleozoic bellerophontoidean mollusks
were a very successful group and occupied a wide range
of depositional environments (Peel 1977, 1978; Wahlman 1992). In his excellent monograph, Wahlman
(1992) summarized knowledge of the paleoecology
of Ordovician bellerophontoidean mollusks and
clearly showed that they led a variety of modes of
life. This fact probably explains their cosmopolitan
distribution.
The oldest bellerophontoidean mollusks (family
Sinuitidae) are known from Upper Cambrian strata.
Their generic diversity was relatively low until the
early Mid Ordovician. From approximately the beginning of the Darriwilian (TS.3b and 4a; see time
slices in chapter 2), the number of genera per million years (m.y.) (di ) continually increased to the end
of the Ordovician (figure 18.2). The mid Late Ordovician (TS.5c–6a) was the time of the highest
diversity of the Ordovician bellerophontoidean mollusks (dnorm close to 14). The rate of originations per
m.y. (oi ) was variable through the Ordovician with
distinct peaks at the early Tremadocian (TS.1a), early
Mid Ordovician (TS.3b), and early (TS.5a) and mid
(TS.5c) Late Ordovician (figure 18.2). The rate of
extinctions per m.y. (ei ) reached the highest values at
the beginning of Arenig (TS.2a) and during the Ashgill
(TS.6a and 6b). At present it is difficult to make a
more detailed analysis of the turnover rates because
the bellerophontoidean mollusks probably represent
a polyphyletic group (see earlier in this chapter).
It is concluded that the Ordovician symmetrical
univalved mollusks form a relatively large group
(about 75 genera), even though the majority of them
are imperfectly known. They include several morphological groups (see Wahlman 1992 and Geyer
1994 for review) for which neither the phylogenetic
relationships nor their relationships to extant molluscan groups are known. Biodiversity of the Tryblidiida
seems to have decreased slightly during the Ordovician, and it was strongly affected by the extinction
event close to the Ordovician/Silurian boundary. On
the other hand, the number of the genera belonging
to the Bellerophontida increased through the Ordovician. Their diversity pattern shows similarities with
some gastropod groups (see chapter 19).
183