The Early Cambrian Radiation of Mollusca

Abstract

This chapter examines the radiation of the Mollusca during the early Cambrian period. It provides a brief history of the study of ancient molluscs and their systematic interpretation, describes major groups of Cambrian gastropods, and explains the concept of the so-called adaptive radiation. It highlights the major gaps in knowledge about the early evolution of the Mollusca and outlines some issues that require further studies including microstructural studies and mass phosphate accumulation.

Full text

33
multicellular animals (Fedonkin 1987). Most
of the fi nds are diffi cult to assign to any group
of the younger Phanerozoic animals. How-
ever, Kimberella quadrata was interpreted as
a primitive mollusc-like organism (Fedonkin
and Waggoner 1997; Fedonkin 1998) due to the
oval shape of the imprints (Figure 3.1A) with
parts of some impressions suggesting a foot,
a mantle edge, and a dorsal, non-mineralized
“shell.” The recent fi nds of Kimberella with
traces of crawling (Figure 3.1B) prove the ani-
mal nature of this organism and favor its mol-
luscan affi nity (Ivantsov and Fedonkin 2001).
However, to determine whether Kimberella is
really a mollusc, specimens are needed that
show a defi ned head with a ventral mouth.
Also, more data on its shell morphology is
needed. Only two Vendian fossils (i.e., tubu-
lar Cloudina and Sinotubulus) were apparently
capable of biomineralization (Signor and Lipps
1992), and all others lacked hard mineralized
tissues (Fedonkin 1992; Rozanov and Zhurav-
lev 1992: fi g. 27; Ivantsov and Fedonkin 2001).
This suggests that the radula, which is often a
mineralized structure and a diagnostic feature
of Mollusca, may not have been present or was
3
The Early Cambrian Radiation of Mollusca
Pavel Yu. Parkhaev
The Early Cambrian interval is of great inter-
est because of the explosive origin of abundant,
diverse mineralized skeletons marking the
beginning of a remarkable 500-million-year
fossil record of metazoan evolution. The Lower
Cambrian sequences contain representatives
of most extant high-level taxa (e.g., sponges,
molluscs, arthropods, brachiopods, echino-
derms), as well as various extinct groups of
organisms (archaeocyaths, trilobites, hyoliths,
and various problematic forms). Molluscs—or,
rather, the fossils that we at least believe to be
the most ancient molluscs—fi rst appeared just
below the Precambrian-Cambrian boundary as
a part of the mass skeletonization event (Run-
negar 1982; McMenamin and McMenamin
1990; Bengtson and Conway Morris 1992; Fortey
et al. 1996). For the most part they are found
as members of diverse microfaunal assem-
blages, the so-called small shelly fossils (SSF;
Matthews and Missarzhevsky 1975; for a review
see Dzik 1994) that are commonly extracted
from Cambrian rocks.
The earlier biota known from the Upper
Proterozoic is the famous Ediacaran or Vendian
fauna. It is composed of soft-bodied, possibly

34 early cambrian radiation of mollusca
unmineralized in Kimberella. Consequently,
the origin of molluscs may continue to be tied
to the skeletonization event of the Nemakit-
Daldynian–Tommotian age and with the devel-
opment of the SSF faunas.
Chemical preparation, consisting of treat-
ing the rock with weak (usually acetic) acid, is
to date the most effi cient and widely used tech-
nique for the extraction of SSF. The insoluble
residue contains original and secondary, silici-
fi ed or phosphatized fossils. This technique has
revealed numerous microscopic forms attribut-
able to Mollusca, mostly cap-shaped or variously
coiled, together with other diverse morpholo-
gies of problematic affi nities.
The SSF molluscs range from 0.5 to a few mil-
limeters. Although there have been larger speci-
mens known from the Early Cambrian of Siberia
(Valkov and Karlova 1984; Sundukov and Fedorov
1986; Dzik 1991; Gubanov and Peel 2000), Altaj,
Russia (Rozanov et al. 1969; Missarzhevsky
1989), Mongolia (coll. E. A. Zhegallo, personal
observation), and China (personal observation),
these macroscopic forms are extremely rare.
Thus, most early molluscs were probably min-
ute, rather than this distribution being simply
an artifact of preservation or of the preparation
technique, although why this is so, is unclear
(Runnegar and Pojeta 1985; Gubanov and Peel
2000, but see Haszprunar 1992 for an oppos-
ing view). The prevalence of phosphatic inter-
nal molds in the residue is likely to be a direct
consequence of the acid preparation resulting
in the dissolution of the original carbonate
shells, although, rarely, the shell is secondarily
phosphatized or silicifi ed (see Robison 1964;
Runnegar and Jell 1976) and hence available for
study.
Apart from the limpet-like and trochoid
shell morphology, these fossils share with more
modern molluscs characteristic shell micro-
structures. Thus, imprints of nacreous, pris-
matic, cross-lamellar, and other structures can
be observed on the surface of internal molds
(Runnegar 1983, 1985; Kouchinsky 1999, 2000;
Parkhaev 2002b, 2006a; Feng and Sun 2003),
and occasionally the original shell material
is found replaced by phosphate. The shape of
FIGURE 3.1. Soft-bodied mollusc-like fossil Kimberella quadrata from the Upper Vendian
of the White Sea region. (A) PIN, no. 3993/5136, imprint of body, ⫻1.1; Winter Coast,
Zimnegorsk Lighthouse, Mezen Formation. (B) PIN, no. 4853/9, 11, 12, imprints of bodies
of three specimens with traces of crawling inside the sediment that buried the organisms,
⫻1.3; Summer Coast, Solza River, Ust-Pinega Formation. Photos courtesy of Andrey Yu.
Ivantsov (Paleontological Institute, Russian Academy of Sciences, Moscow).

early cambrian radiation of mollusca 35
the shells and their microstructure are the only
convincing evidence for their assignment to
Mollusca, and some workers have disputed the
molluscan affi nities of several Early Cambrian
forms (e.g., Yochelson 1975, 1978).
The most ancient probable molluscs are
known from the terminal Precambrian of
Siberia (uppermost Nemakit-Daldynian;1 see
Khomentovsky et al. 1990; Khomentovsky
and Karlova 1993, 2002) and China (Lower
Meishucunian) (Yu 1987; Qian and Bengtson
1989). These are the cap-shaped forms, Purella,
Anabarella, and Canopoconus, and the spirally
coiled Latouchella, and Barskovia. A range of uni-
valved forms had evolved by the Tommotian, the
basal stage of the Cambrian system (Figure 3.2).
As the precise systematic position of these early
forms remains controversial, a brief review of
the main hypotheses follows.
Some “worm”-like organisms from the
Burgess Shale (i.e., soft-bodied Odontogriphus
and sclerite-bearing Wiwaxia) have been inter-
preted as stem-group molluscs on the basis of
the interpretation of fossil structures, notably
radula and ctenidia (Caron et al. 2006). How-
ever, interpretation of these putative structures
seems doubtful, and possibly they represent
other lophotrochozoans (Butterfi eld 2006).
A very rough estimate suggests that there
are over 600 named species of Early-Middle
Cambrian molluscs (personal observation). The
most diverse faunas are found in China (about
250 nominal species, although over half of them
are synonyms; see Parkhaev and Demidenko
2005) and Siberia (about 150 nominal species).
The Australian and Mongolian faunas include
over 50 nominal species each. The rest of the
species are distributed among such areas as
North America, Morocco, Europe, Kazakhstan,
and Iran. An estimate based on a current taxo-
nomic revision of Chinese and Siberian mate-
rial suggests that the number of valid species
and genera is at least half as many as recognized
in the literature, indicating an urgent need for
revision of all Cambrian molluscs at the genus
and species level.
BRIEF HISTORY OF THE STUDY OF
ANCIENT MOLLUSCS AND THEIR
SYSTEMATIC INTERPRETATION
PLACOPHORAN MOLLUSCS
The oldest fossil fi nds, more or less gener-
ally accepted as polyplacophoran molluscs,
come from the Late Cambrian deposits (e.g.,
Matthevia, see Runnegar et al. 1979; or Hemithe-
cella and Elongata, see Stinchcomb and Dar-
rough 1995). However, a number of Early
Cambrian forms have been repeatedly assigned
to chitons by some authors. An especially
diverse polyplacophoran fauna was reported
from the Lower Cambrian (Meishucunian) of
China (Yu 1987, 1989, 1990, 2001). These fos-
sils are minute, isolated shell-like plates of dif-
ferent morphology. Strong doubts as to their
polyplacophoran affi nities have been expessed
(Qian and Bengtson 1989; Runnegar 1996;
Qian et al. 1999). These “microchitons” (e.g.,
Stoliconus, Yangtzechiton, Luyanhaochiton, Mei-
shucunchiton, Runnegarochiton, Tchangsichiton)
most likely represent different types of dorsal
sclerites of problematic animals, similar to
pairs of dorsal plates of the remarkable taxon
Halkieria evangelista discovered from the Sirius
Passet Lagerstätte of the Atdabanian of Greenland
(Conway Morris and Peel 1990) and interpreted
as a stem-group brachiopod (Conway Morris
and Peel 1995). However, the latest comparative
study of Halkieria and Recent chitons (Vinther
and Nielsen 2005) favored a molluscan affi nity
of this enigmatic Cambrian fossil. The authors
1. The position of the Lower Cambrian boundary in
the Fortune Head section, Burin Peninsula, Newfound-
land, Canada (Landing 1994), cannot be reliably rec-
ognized within any other sections except the type one
(Rozanov et al. 1997; Khomentovsky and Karlova 2005).
In practice, geologists still use the regional schemes for
the Precambrian-Cambrian interval (Peng et al. 2005) or
use the Siberian stage standard (e.g., Gravestock et al.
2001; Khomentovsky and Karlova 2002, 2005). On the
Siberian Platform, the base of the Cambrian system cor-
responds to the base of the Tommotian Stage (Rozanov
and Sokolov 1984; Shergold et al. 1991; Khomentovsky
and Karlova 2005).

36 early cambrian radiation of mollusca
concluded that Halkieria may be a sister group
for chitons and established a new class, Dipla-
cophora, distinguished from Polyplacophora
by the “posterior and anterior shell separated
by elongate zone of scale-like sclerites, together
surrounded by zones with other types of scler-
ites” (Vinther and Nielsen 2005: 87). While I
generally support their opinion, separation of
the halkieriids from polyplacophorans at a lower
level (e.g., as a distinct subclass) may be advis-
able. By doing this, the general conception of
polyplacophorans as molluscs with dorsal armor-
ing composed of plate-like elements, varying in
number, is retained. Finding a Carboniferous
chiton, Polysacos, with 17 shell plates (Vendrasco
et al. 2004), considerably extended the range
FIGURE 3.2. Phylogeny of the Cambrian univalved molluscs and ranges of the main higher taxa. Numbers indicate the origin
of the following key features on each branch. 1, torsion; 2, anterior buttress; 3, posterior siphonal groove; 4, deep siphonal
groove or snorkel; 5, strong lateral compression; 6, infaunal adaptations (internal plates, nonplanar aperture); 7, planispiral
shell, spire whorls fl attened, aperture elongated; 8, asymmetric shell; 9, turbospiral coiling with elevated spire; 10,
planispiral shell, spire whorls and aperture circular, pallial cecum; 11, hyperstrophic shell (modifi ed from Parkhaev 2002a;
ages of stratigraphic boundaries from Geological Time Scale 2004). Note: Dextrobranchia and Divasibranchia are names
introduced by Minichev and Starobogatov (1979) for groups of heterobranchs.

FIGURE 3.3. Sclerites of the Early Cambrian halkieriids, possible polyplacophoran molluscs.
(A–H) Shell-like sclerites that can be interpreted as anterior and posterior dorsal shells of
halkieriids. (I–V) blade-like sclerites of halkieriids (photographs from I through V courtesy
of Yuliya E. Demidenko, Paleontological Institute of the Russian Academy of Sciences).
(A, B, H) Ocruranus fi nial, Lower Cambrian, Meishucunian, Zhujiaqing Formation;
Xiaowaitoushan, Meishucun, Yunnan, China: (A, B) PIN, no. 4552/1483, internal mold, ⫻21,
Dahai Member: (A) dorsal view, (B) oblique lateral view. H, PIN, no. 4552/2123, shell, oblique
apical view, ⫻20, Zhongyicun Member. (C, D) Ocruranus trulliformis, Lower Cambrian,
Meishucunian, Zhujiaqing Formation, Zhongyicun Member; Xiaowaitoushan, Meishucun,
Yunnan, China: C, PIN, no. 4552/2551, shell, dorsal view, ⫻23; D, PIN, no. 4552/2747,
shell, lateral view, ⫻46. (E–G) Eohalobia diandongensis, Lower Cambrian, Meishucunian,
Zhujiaqing Formation, Dahai Member; Xiaowaitoushan, Meishucun, Yunnan, China; PIN,
no. 4552/1514, internal mold, ⫻20: E, dorsal view, F, oblique lateral view, G, oblique apical
view. (I – L) Siphogonuchites triangularis, Lower Cambrian, Meishucunian, Zhujiaqing
Formation, Zhongyicun Member; Xiaowaitoushan, Meishucun, Yunnan, China: I, PIN, no.
4552/2910, right sclerite, dorsal view, ⫻34; J, PIN, no. 4552/2741, left sclerite, ventral view,
⫻29; K, L, PIN, no. 4552/2200, left sclerite, ⫻38: K, lateral view, L, oblique ventral view.
(M, N) Halkieria parva, Lower Cambrian, Botomian, Sellick Hill Formation; Sellick Hill,
Fleurie Peninsula, South Australia: M, PIN, no. 4664/3104, palmate sclerite, ventral view,
⫻29; N, PIN, no. 4664/3176, cultrate sclerite, ventral view, ⫻18. (O–V) Thambetolepis delicata,
Lower Cambrian, Botomian, Parara Limestone; Yorke Peninsula, South Australia: O, PIN, no.
4664/4230, cultrate sclerite, ventral view, ⫻31, bore-hole CD-2 (depth 12.56 m); P, PIN, no.
4664/4216, intermediate sclerite, between palmate and cultrate types, dorsal view, ⫻33,
bore-hole CD-2 (depth 52.26 m); Q, PIN, no. 4664/4269, siculate sclerite, dorsal view,
⫻26, bore-hole CD-2 (depth 28.26 m); R, PIN, no. 4664/5338, siculate sclerite, dorsal view,
⫻33, bore-hole Minlaton-1 (depth 534.9 m); S, PIN, no. 4664/4006, palmate sclerite, dorsal
view, ⫻26, bore-hole CD-2 (depth 28.82 m); T, PIN, no. 4664/4682, palmate sclerite,
ventral view, ⫻26, bore-hole SYC-101 (depth 198.5 m); U, PIN, no. 4664/3013, palmate
sclerite, ventral view, ⫻22, Horse Gully (HG0); V, PIN, no. 4664/3416, palmate sclerite,
ventral view, ⫻23, Horse Gully (HG0).

38 early cambrian radiation of mollusca
from the normal eight. A reexamination of the
Early Cambrian shell-like plates and sclerites
(Figure 3.3) and reevaluation of their taxonomic
affi nities should provide useful information
on early placophoran evolution (see Todt et al.,
Chapter 4, for more discussion on this topic).
BIVALVED MOLLUSCS
The fi rst bivalves, Fordilla troyensis, F. sibirica,
and Bulluniella borealis, appear in the second
half of the Tommotian and are only locally
distributed (Barrande 1881; Pojeta et al. 1973;
Pojeta 1975; Krasilova 1977; Jermak 1986). Dur-
ing the next Early Cambrian stage, the Atdaba-
nian, the bivalve Pojetaia runnegari (Figure 3.4)
was widely distributed geographically, being
recorded from Australia, China, Mongolia,
Transbaikalia, and Europe (Jell 1980; Runnegar
and Bentley 1983; Gravestock et al. 2001;
Parkhaev 2004b). Further bivalve genera (Tau-
rangia MacKinnon, 1982, Pseudomyonia Run-
negar, 1983, Camya Hinz-Schallreuter, 1995, and
Arhouriella Geyer and Streng, 1998) have been
described from the latest Early Cambrian and
Middle Cambrian (see MacKinnon 1982, 1985;
Runnegar 1983; Jermak 1986; Berg-Madsen
1987; Hinz-Schallreuter 1995; Geyer and Streng
1998). However, Early-Middle Cambrian taxa
are diffi cult to link to the next bivalves that
appear in the fossil record (in the Ordovician
and later) because of a major gap in the Late
Cambrian molluscan fossil record (Budd and
FIGURE 3.4. Early Cambrian bivalve Pojetaia runnegari from the Lower Cambrian, Botomian,
Parara Limestone, Yorke Peninsula, South Australia. (A–C) Internal molds, views on a
hinge composed of posteriorly placed ligament, two right and single left teeth: A, PIN, no.
4664/0396, ⫻42; B, PIN, no. 4664/0447, ⫻39; C, PIN, no. 4664/0473, ⫻35. (D) Internal
mold viewed ventrally, PIN, no. 4664/0494, ⫻40. (E) internal mold viewed anteriorly, PIN,
no. 4664/0451, ⫻39. (F) Internal mold viewed posteriorly, PIN, no. 4664/0450, ⫻33. (G) PIN,
no. 4664/0417, internal mold, left lateral view, ⫻41. (H) PIN, no. 4664/0475, internal mold,
left lateral view, ⫻41. (I, J) PIN, no. 4664/1613, right valve viewed interiorly: I, ⫻34, J, ⫻87.

early cambrian radiation of mollusca 39
Jensen 2000). This hiatus casts some doubt
on the bivalve affi nity of Fordilla and Pojetaia
(e.g., Yochelson 1978, 1981). Nevertheless, the
presence of truly separate valves, a primitive
hinge, adductor scars, and, most importantly,
housing for what is assumed to be a ligament
(Figure 3.4A–C), favor their bivalve nature.
Hinz-Schallreuter (2000) and Pojeta (2000)
recently reviewed all known bivalves reported
from the Cambrian. In the latter publication,
many bivalved taxa are reinterpreted as bra-
chiopods or forms with doubtful affi nities, but
Fordilla, Pojetaia, Taurangia, and several others
are confi rmed as ancient Bivalvia.
Apart from bivalves, several Early Cambrian
problematic groups, such as siphonoconchs
(Figure 3.5A–K) and stenothecoids, are also charac-
terized by their bivalved shells. Based on a peculiar
type of bilateral symmetry (with a dorsal and a ven-
tral valve) and some minor features of shell mor-
phology, these groups were given class rank: the
Siphonoconcha Parkhaev, 1998 and Stenothecoida
FIGURE 3.5. Early Cambrian problematic bivalved fossils. (A–K) Siphonoconchs. (L, M)
Aroonia. A–D, Apistoconcha siphonalis: A, PIN, no. 4664/0019, valve of B-morphotype,
exterior view, ⫻24; B, PIN, no. 4664/0094, valve of B-morphotype, interior view,
⫻43; C, PIN, no. 4664/0005, valve of A-morphotype, exterior view, ⫻36; D, PIN, no.
4664/0018, valve of A-morphotype, interior view, ⫻29; E, F, H, Apistoconcha presiphonalis,
Lower Cambrian, Botomian, Parara Limestone; Horse Gully, Yorke Peninsula, South
Australia: E, PIN, no. 4664/0065, valve of B-morphotype, exterior view, ⫻33; F, PIN,
no. 4664/0058, valve of B-morphotype, interior view, ⫻47; H, PIN, no. 4664/0074,
valve of A-morphotype, interior view, ⫻47; G, I–K, Apistoconcha apheles, Lower
Cambrian, Botomian, Parara Limestone; Curramulka Quarry, Yorke Peninsula, South
Australia: G, PIN, no. 4664/0042, valve of B-morphotype, exterior view, ⫻15; I, PIN, no.
4664/0053, valve of A-morphotype, interior view, ⫻36; J, K, PIN, no. 4664/0074, valve
of B-morphotype, interior views, ⫻47; L, M, Aroonia seposita L, PIN, no. 4664/0035,
tooth-valve, interior view, ⫻33; M, PIN, no. 4664/0036, pit-valve, interior view, ⫻38.

40 early cambrian radiation of mollusca
Yochelson, 1968 (⫽ Probivalvia Aksarina, 1968)
(see Aksarina 1968; Yochelson 1968, 1969;
Pelman 1985; Parkhaev 1998). Their relationship
to Mollusca is, however, questionable.
UNIVALVED MOLLUSCS
Following the fi rst description of a Cambrian
mollusc in 1847 (Metoptoma? rugosa Hall, 1847
⫽ Helcion subrugosus d’Orbigny, 1850), the num-
ber of taxa had signifi cantly increased by the end
of the nineteenth century, as more publications
appeared dealing with Cambrian fauna (e.g.,
Barrande 1867, 1881; Billings 1872; Shaler and
Foerste 1888; Tate 1892; Matthew 1895, 1899).
New species and genera of univalved molluscs
were affi liated with extant families and higher
taxa of gastropods; for example, cap-like forms
were grouped together with patelloids, whereas
those spirally coiled were grouped with trochids
or other primitive groups. Such an approach to
the systematics of Cambrian molluscs domi-
nated studies until the middle of the twentieth
century (e.g., Cobbold 1921, 1935; Cobbold and
Pocock 1934; Kobayashi 1933, 1935, 1937, 1939,
1958; Resser 1938) when the situation was
markedly changed because: (1) the introduc-
tion of Monoplacophora, initially as an order
of fossil gastropods (Odhner in Wenz 1940:5
Wenz in Knight 1952: 46), and later as a class
of molluscs following the discovery of living
forms (Lemche 1957; Knight et al. 1960); and
(2) the application of the chemical extraction
of fossils from Cambrian rocks (also see previ-
ous discussion), resulting in the recognition of
diverse, well-preserved molluscs (Rozanov and
Missarzhevsky 1966; Rozanov et al. 1969).
Following the pioneer work by Rozanov and
Missarzhevsky on the excellently preserved Cam-
brian fossils of Siberia, diverse molluscan assem-
blages from the Cambrian of Australia were
interpreted in terms of functional morphology
(Runnegar and Pojeta 1974, 1985; Runnegar and
Jell 1976, 1980; Pojeta and Runnegar 1976; Run-
negar 1981, 1983, 1985). Since then, the systematic
position of the Cambrian univalved molluscs has
been divided between three competing opinions
(Table 3.1): (1) They share gastropod affi nities;
(2) they share monoplacophoran affi nities; (3) most
of them represent a separate molluscan class.
GASTROPOD AFFINITY OF CAMBRIAN UNIVALVES
The placement of the ancient univalved mol-
luscs in Gastropoda was accepted by almost all
malacologists (e.g., Knight et al. 1960; Rozanov
and Missarzhevsky 1966; Rozanov et al. 1969;
but see Knight 19522) before the discovery
of extant untorted exogastric univalves, the
monoplacophorans (Lemche 1957; Lemche
and Wingstrand 1959). The limpet-like fossils
Scenella and Helcionella were thought to be the
oldest patelloids, whereas the spirally coiled
forms Pelagiella and Coreospira were assigned to
trochoids or pleurotomarioids. After the estab-
lishment of the class Monoplacophora, the cap-
shaped taxa were excluded from Gastropoda
(Runnegar and Pojeta 1974, 1985; Runnegar
and Jell 1976; 1980; Pojeta and Runnegar 1976;
Runnegar 1981, 1983; Missarzhevsky 1989). The
most signifi cant contributions during that period
were the studies by Runnegar et al. (Runnegar
and Pojeta 1974, 1985; Runnegar and Jell 1976;
1980; Pojeta and Runnegar 1976; Runnegar
1981, 1983) that supported the untorted nature of
helcionelloideans3 (see following).
Beginning from the late 1960s to early 1970s,
the Russian school of malacologists undertook
a morphological and taxonomic study of higher
gastropod taxa, together with an evaluation of
all Mollusca (Starobogatov 1970, 1976, 1977;
Golikov and Starobogatov 1975; Minichev and
2. Knight (1952) proposed a classifi cation of Gastropoda
composed of two subclasses, that is, Isopleura (includes
orders Monoplacophora, Polyplacophora, and Aplacoph-
ora) and Anisopleura (includes superorders Prosobran-
chia, Opisthobranchia, and Pulmonata). The Cambrian
genera Scenella and Helcionella were treated in the family
Triblidiidae of the order Monoplacophora, that is, they for-
mally belong to gastropods in this classifi cation, but were
considered to be untorted forms.
3. Helcionelloideans, a group of Cambrian univalved
molluscs with a bilaterally symmetric, cap-shaped, or coiled
shell. In different publications its rank varies greatly, that
is, from the subfamily Helcionellinae (Wenz 1938) to the
class Helcionelloida (Peel 1991a, b). Here helcionelloide-
ans are considered as the order Helcionelliformes within
the gastropod subclass Archaeobranchia (Figure 3.2).

early cambrian radiation of mollusca 41
Starobogatov 1979; Starobogatov and Moskalev
1987). This study resulted in a new classifi ca-
tion with Gastropoda being divided into eight
subclasses (Golikov and Starobogatov 1988).
The systematic position of some taxa, including
several groups of Early Paleozoic molluscs, was
revised, and the majority of Cambrian univalved
molluscs were interpreted as gastropods. Argu-
ments supporting helcionelloids as gastropods,
however, were not entirely convincing, because
these studies focused mainly on Recent taxa and
barely touched the Cambrian forms.
TABLE 3.1
Alternative Systematic Positions of Main Groups of Cambrian Univalved Molluscs
Helcionellids Gastropods Knight et al. 1960; Rozanov and Missarzhevsky 1966;
Rozanov et al. 1969; Golikov and Starobogatov 1975;
Starobogatov 1976; Minichev and Starobogatov 1979;
Missarzhevsky and Mambetov 1981; Golikov and
Starobogatov 1988; Parkhaev 1998, 2000, 2002a;
Gravestock et al. 2001
Monoplacophorans Knight 1952; Runnegar and Pojeta 1974, 1985; Pojeta
and Runnegar 1976; Runnegar and Jell 1976; 1980;
Qian et al. 1979; Yu 1979, 1987; Runnegar 1981, 1983,
1985; Missarzhevsky 1989
Separate molluscan class Yochelson 1978; Peel and Yochelson 1987; Peel
1991a,b; Gubanov and Peel 2000; Yu Wen 1984, 1987;
Geyer 1986, 1994
Pelagiellids Gastropods Rozanov and Missarzhevsky 1966; Rozanov et al. 1969;
Runnegar and Pojeta 1974; Runnegar and Jell 1976,
1980; Qian et al. 1979; Yu 1979, 1987; Golikov and
Starobogatov 1975; Starobogatov 1976; Minichev and
Starobogatov 1979; Golikov and Starobogatov 1988;
Missarzhevsky 1989; Parkhaev 1998, 2000, 2002a;
Gravestock et al. 2001
Advanced Runnegar 1981
monoplacophorans
Separate molluscan class Yochelson 1978; Linsley and Kier 1984; Geyer 1994
Aldanellids Gastropods Rozanov and Missarzhevsky 1966; Rozanov et al. 1969;
Runnegar and Pojeta 1974, 1985; Golikov and
Starobogatov 1975; Starobogatov 1976; Pojeta and
Runnegar 1976, 1985; Runnegar and Jell 1976; 1980;
Qian et al. 1979; Yu 1979, 1987; Runnegar 1981, 1983,
1985; Golikov and Starobogatov 1988; Missarzhevsky
1989; Parkhaev 1998, 2000, 2002a; Gravestock et al. 2001
Separate molluscan class Linsley and Kier 1984
Non-molluscs Yochelson 1975, 1978; Bockelie and Yochelson 1979

42 early cambrian radiation of mollusca
Additional support for the hypothesis that
helcionelloids are gastropods was provided by
recent insights into possible functional aspects
of the shell structures of Cambrian univalved
molluscs (Parkhaev 2000, 2001, 2002a, b). It
is possible to reconstruct details of the internal
anatomy and paleoecology of Cambrian mol-
luscs and to begin to resolve uncertainties relat-
ing to their systematics. Particular attention was
given to the features determining water current
circulation inside the shell (grooves, siphons,
sinuses) and the patterns of shell muscle
arrangement (Parkhaev 2000, 2002b, 2004a).
Arguably, torsion appears to be the main apo-
morphy distinguishing gastropods from their
monoplacophoran ancestors, but this character
cannot be directly recognized from shell mor-
phology (Harper and Rollins 1982; Haszprunar
1988). Shell orientation in helcionelloids, there-
fore, can only be established from indirect evi-
dence. Recently discovered muscle attachment
scars in the columellar area of coiled helcionel-
loids (Parkhaev 2002b, 2004a), coupled with
earlier suggestions that the internal columellar
folds functioned to divide and support muscles
in some helcionelloid shells (Parkhaev 2000),
strongly argues for the endogastric4 nature of
these molluscs (Figure 3.6).
The position of the mantle cavity in helcio-
nelloids is of great signifi cance in interpreting
whether or not the animal was torted. A lateral
position is highly unlikely because their shells
are strongly compressed laterally (Yochelson
1978). The posterior subapical region in various
helcionelloids is signifi cantly narrower than the
opposite anterior side, so the most likely posi-
tion of the mantle cavity is within the wider
contra-apical area of the shell, that is, above the
head. In such a position, the mantle cavity is
unaffected by pressure exerted from the spire,
while the shell muscles are housed in the pos-
terior (parietal) side of the last whorl and do not
fi ll a space in the area occupied by the mantle
cavity (Parkhaev 2000). One can suggest two
alternative reconstructions of helcionelloids
corresponding to a torted (Figure 3.7A) and an
untorted condition (Figure 3.7B), with water in
these alternative schemes circulating in opposite
4. An endogastric shell has a posteriorly directed spire
(Figure 3.6B), in contrast to an exogastric shell, which has
an anteriorly directed spire (Figure 3.6A).
FIGURE 3.6. Shell muscles of helcionelloids. (A, B) Position
of muscle attachment area in respect to the type of shell
coiling (Parkhaev 2000, modifi ed from Starobogatov 1970):
A, exogastric shell with, shell muscles attached to the
peripheral wall; B, shell endogastric, with shell muscles
attached to the columellar wall. (C, D) Shell of Latouchella
merino with internal parietal folds (pf ) supporting and
dividing muscle threads (Parkhaev 2000, redrawn from
photos in Peel, 1991a,b): C, lateral view, D, apertural
view. (E, F) Latouchella korobkovi, internal mold, PIN, no.
4386/1411, Lower Cambrian, Tommotian, Kotui River,
Anabar Region, Siberian Platform: E, left lateral view, ⫻15,
F, “columellar” area with imprints of prismatic pallial
myostracum, ⫻66. (G, H) Anhuiconus microtuberus, Lower
Cambrian, Botomian, Yorke Peninsula, South Australia
(Parkhaev 2002b), internal mold: G, PIN, no. 4664/1867,
left lateral view, ⫻9, H, PIN, no. 4664/1738, “columellar”
area with imprints of prismatic pallial myostracum, ⫻55.
FIGURE 3.7. Two possible schemes of internal organization
of the helcionelloid molluscs. (A) Torted. (B) Untorted.
Arrows indicate supposed water current circulation in the
mantle cavity (Parkhaev 2000).

early cambrian radiation of mollusca 43
directions. To either validate or disprove a par-
ticular reconstruction, parallels can be drawn
from other molluscs, particularly those with
special structures associated with inhalant and
exhalant currents. The most striking form is
Yochelcionella, with a tubular projection (called a
“snorkel”) on the subapical side of the shell (see
Hinz-Schallreuter 1997 and Parkhaev 2004b for
the latest revisions of the genus). The snorkel’s
morphology suggests that its function was inhal-
ant (Parkhaev 2001). A similar and probably
phylogenetically related form, Eotebenna, with a
suggested semi-infaunal mode of life (Peel 1991a,
b; Parkhaev 2002a), has a deep subapical sinus
with a similar assumed inhalant function. More-
over, this genus morphologically links Yochelcio-
nella to forms with shallow parietal sinuses or
grooves. Finally, the helcionelloids with an ante-
rior buttress (Figure 3.8) suggest the exhalation
of water through the anterior sector of the aper-
ture (Parkhaev 2000). Thus, the torted variant of
helcionelloid organization (Figure 3.7A) appears
to be a plausible hypothesis. In addition, the
origin of asymmetry among ancient gastropods
(Figure 3.9) can be explained in terms of water
circulation pattern (Parkhaev 2001).
Finally, the recent discoveries (Parkhaev
2006b) of columellar muscles in turbospiral
helcionelloids-aldanellids and the morphologi-
cal similarity of their protoconch with larval
shells of primitive modern gastropods support
the position of helcionelloid molluscs within
the Gastropoda.
MONOPLACOPHORAN AFFINITY OF
CAMBRIAN UNIVALVES
Several Early Cambrian limpet-like forms were
grouped with Monoplacophora (e.g., Scenella
and Helcionella, originally assigned by Knight
[1952] to an untorted gastropod family Tryblidi-
idae). In a subsequent publication (Knight et al.
1960), the monoplacophorans were treated as a
separate class, and Scenella alone shared mono-
placophoran affi nities.
In the 1970s, a rich molluscan assemblage
from the basal Middle Cambrian of Australia
(Coonigan Formation, New South Wales, and Cur-
rant Bush Limestone, Queensland) was described
(Runnegar and Pojeta 1974; Runnegar and Jell
1976, 1980). The excellent preservation of silici-
fi ed shells from this locality and their taxonomic
diversity provided the opportunity to study their
functional morphology and to make assump-
tions about their ecology and systematic posi-
tion. Helcionelloid molluscs with cap-like shells
were thought to be monoplacophorans based on
the interpretation of Yochelcionella with a snorkel
on the subapical part of the shell by Runnegar
et al. (Runnegar and Pojeta 1974; Runnegar and
Jell 1976, 1980). These authors believed that the
snorkel lay anteriorly and water fl owed through it
into the pallial cavity. With this interpretation, the
shell would be exogastrically coiled, conforming
to a monoplacophoran placement. This recon-
struction of the general morphology of helcionel-
loids placed it at the base of the new phylogenetic
scheme (Figure 3.10B) for Mollusca (Runnegar
and Pojeta 1974, 1985; Runnegar and Jell 1976,
1980; Pojeta and Runnegar 1976).
Based on the study of Early Paleozoic
bivalves and rostroconchs, the Mollusca were
divided into two subphyla: the Cyrtosoma and
Diasoma (Pojeta 1971; Pojeta and Runnegar
a
b
FIGURE 3.8. Suggested water circulation pattern inside the
shell of the Igarkiellidae. b ⫽ anterior buttress; a ⫽ anus
(after Parkhaev 2000).

44 early cambrian radiation of mollusca
1976; Runnegar 1996). In this model, the
untorted, exogastric helcionelloids are regarded
as ancestral to initially laterally compressed Dia-
soma (Rostroconchia ⫹ Bivalvia ⫹ Scaphopoda)
and initially dorsoventrally elongated Cyrto-
soma (Cephalopoda ⫹ Gastropoda). The origin
of gastropods, with the advent of torsion, has
been directly linked to the evolution of turbo-
spiral coiling (Runnegar 1981, 1996). However,
the recent discovery of columellar attachment
areas of shell muscles, suggesting that coiled
helcionelloid shells are endogastric (Parkhaev
2002b, 2004a), signifi cantly weakened this
hypothesis.
FIGURE 3.9. Suggested asymmetry formation among helcionelloids. (A) With symmetric
shell and mantle cavity. (B) With slightly asymmetric (dextral) shell, mantle cavity still almost
symmetric. (C) Same condition in slightly sinistral shell. (D) With considerably asymmetric
(dextral shell), mantle cavity asymmetric. (E) Trochoideans and caenogastropods (ct ⫽ ctenidia;
r ⫽ rectum; w ⫽ water currents). Condition A is proposed for most of helcionelloids; conditions
B and C can be found among the Coreospiridae, and D in Pelagielliformes (see Parkhaev 2001
for detailed explanation).

early cambrian radiation of mollusca 45
SEPARATE CLASS-LEVEL LINEAGES
Yochelson controversially suggested that several
taxa should be removed from Mollusca; in par-
ticular, the spirally coiled Aldanella Vostokova,
1962 was considered to be a worm tube
(Yochelson 1975, 1978; Bockelie and Yochelson
1979), while the cap-shaped shells of Scenella
or Palaeacmaea were interpreted as casts of a
medusoid organism (Yochelson and Gil-Cid
1984; Webers and Yochelson 1999; Babcock
and Robison 1988). Also, the morphology of
Yochelcionella was reinterpreted as endogastric
and untorted with a posterior exhalant snor-
kel (Yochelson 1978). All other helcionelloids
were reconstructed similarly with a posteriorly
directed apex. Thus, Yochelson (1978) was the
fi rst to argue for a separate taxonomic posi-
tion for helcionelloids, as embodied in his
“two waves model” of molluscan evolution (see
Runnegar 1996) with the fi rst “blind” wave
of Cambrian molluscan lineages and the sec-
ond wave of modern classes appearing in the
Ordovician.
Yochelson (1978) regarded Pelagiella as an
asymmetric, endogastric but untorted mollusc,
hence separating it from both the Gastropoda
and Monoplacophora. A similar conclusion was
reached later by Runnegar (1981), who argued
that Pelagiella evolved an asymmetric shell but
failed to complete torsion, and proposed it to
be intermediate between Monoplacophora and
Gastropoda. This line of thought was followed by
Linsley and Kier (1984) in their study of the the
functional aspects of the morphology of Paleo-
zoic hyperstrophic shells, and they also con-
cluded that they were untorted. They removed
several taxa from Gastropoda, including the
Cambrian Onychochilidae, Pelagiellidae, and
Aldanellidae, and established a new class, the
Paragastropoda. Later, Peel further developed
Yochelson’s ideas (Peel and Yochelson 1987; Peel
1991a, b; Gubanov and Peel 1999, 2000, 2001)
and described helcionelloid molluscs as endo-
gastric but untorted, and separated them into
a new class, the Helcionelloida (Peel 1991a, b),
a move supported by some (Berg-Madsen and
Peel 1987: fi g. 1b; Geyer 1986, 1994). In addi-
tion, Geyer (1994) reconstructed the pelagiellids
as exogastric, untorted molluscs and affi liated
them into the class Amphigastropoda Simroth
in Wenz (1940:9) (⫽ Galeroconcha Salvini-
Plaven, 1980, ⫽ Tergomya Peel, 1991b) along
with the bellerophonts and monoplacophorans,
including triblidiids.5
FIGURE 3.10. Alternative models of phylogenetic
relationships of major mollusc taxa (from Runnegar 1996):
(A) Based on Yochelson 1978. (B) Based on Runnegar and
Pojeta 1974, 1975; Runnegar et al. 1975, 1979. (C) based on
Peel 1991a.
5. Triblidiida Lemche, 1957 is an order of the Early
Paleozoic Monoplacophora with cap-shaped shell, anterior
apex, and paired muscle scars.

46 early cambrian radiation of mollusca
Runnegar (1996) noted that the “viability
of Peel’s model depends ultimately on his
interpretation of the snorkel of Yochelcionella
and its possible homologues.” Peel’s (1991a, b)
interpretation of this structure as exhalant
was strongly criticized by Runnegar (1996)
and Parkhaev (2001). The most convincing
argument favoring the inhalant functioning
of the snorkel comes from its morphology
(Parkhaev 2001). The snorkel has a character-
istic funnel-like fl aring distally that is clearly
visible on well-preserved specimens (Pei 1985:
pl. 1, fi g. 1; see also Runnegar and Jell 1976:
fi g. 11a-5, 1980: fi g. 1; Missarzhevsky and
Mambetov 1981: pl. 15, fi g. 10; Bengtson et al.
1990: fi g. 162A). Such a shape is diagnostic of
“entry” structures (e.g., siphons of bivalves or
funnel-shaped pores in archaeocyathian walls)
because it increases the effi ciency of water
movement; that is, the velocity of water inside
the tube is increased by the combining of
velocity vectors in the inhalant funnel (Vogel
1988). In addition, the funnel-like shape of
inhalant structures prevents the effect of jet
contraction, which occurs in cylindric tubes
(Butikov et al. 1989). If this interpretation is
correct, the endogastric, untorted helcionel-
loid model is considerably weakened.
Chinese malacologists generally followed the
earlier treatments of Cambrian molluscs, with
coiled forms as gastropods and cap-shaped ones as
monoplacophorans (e.g., Yu 1974, 1979, 1981, 1987;
Luo et al. 1982; Xing et al. 1984; Qian et al. 1999).
Yu (1984) introduced a new class-rank group,
the Merismoconcha, for a few molluscan genera
found in the Meishucunian of China. According
to Yu, the univalved shell of these organisms sug-
gests a metameric organization inherited from
polyplacophorans, a group also identifi ed among
the vast amount of Early Cambrian Chinese SSF
taxa (see previous discussion).
In summary, consideration of the hypoth-
eses on the nature and systematic position of
the Cambrian univalved molluscs suggests that
the interpretation of helcionelloids as ancient
gastropods (Parkhaev 2002a, b, 2004c, 2005)
is the most plausible. However, comparison of
their shell morphology and reconstructed fea-
tures of their internal anatomy suggests that
helcionelloids differed from other gastropods.
Here they are thought to be endogastric torted
molluscs with a primarily symmetrical shell
(cap-shaped or planispiral), having a symmet-
rical pallial complex with a primitive postero-
anterior water circulation in the mantle cavity.
Such a diagnosis justifi es their placement in a
separate subclass of gastropods, the Archaeo-
branchia (Gravestock et al. 2001; Parkhaev
2002a).
A number of new high-level taxa, ranging
from class to order, have been introduced to
accommodate Early Cambrian univalved mol-
luscs. The concepts of those taxa are contrasted
in Table 3.2 with the current concept of the
Archaeobranchia. As shown in the table, no pre-
viously introduced name exactly corresponds to
the Archaeobranchia, and no one has previously
considered helcionelloid molluscs as the earli-
est gastropods.
Below is a synopsis of the families included
in Archaeobranchia (Figure 3.2), with a sum-
mary of their main characteristics and place in
early gastropod evolution. A phylogenetic sce-
nario of Early Cambrian gastropod evolution
follows this next section.
MAJOR GROUPS OF CAMBRIAN
GASTROPODS
helcionellidae Wenz, 1938 (Figures 3.11,
3.12). This family comprises numerous, diverse
forms having the simplest morphology. The
conical shell varies from low to high with a cen-
tral, subcentral, or posterior apex. The aperture
is simple, devoid of any grooves or deep sinuses.
Posterior or anterior shallow notches occur
in some advanced genera. The fi rst members
appeared in the terminal Precambrian (Nemakit-
Daldynian) and were present until the Ordovi-
cian (Tremadocian). Forms with a compara-
tively low shell and a marginally placed apex are
very similar to monoplacophorans, which could
be ancestral to helcionelloids. For some genera
treated here such as Helcionellidae (marked

early cambrian radiation of mollusca 47
by *), there is no information on internal or
other shell morphology that distinguishes them
from monoplacophorans. A thorough study of
their protoconch morphology and microstruc-
ture could shed light on their affi nities.
Valid genera included: Absidaticonus, Aequi-
conus, Anuliconus, Asperconella, Bemella, Calyp-
troconus*, Ceratoconus, Chabaktiella, Chu iliella,
Codonoconus, Daedalia, Emarginoconus, Fenqiar-
onia, Hampilina, Helcionella, Igorella, Ilsanella,
Lenoconus, Marocella*, Miroconulus, Obtusoco-
nus, Pararaconus, Prosinuites, Pseudoscenella*,
Randomia, Salanyella, Scenella*, Securiconus,
Tannuella, Tichkaella, Truncatoconus*, Tuoraco-
nus, and Yangtzeconus.
igarkiellidae Parkhaev, 2001 (Figure 3.13).
Overall shell morphology is very similar to the
Helcionellidae, except for one peculiar apomor-
phic feature—a buttress running along the ante-
rior fi eld of a cap-like shell from the apex toward
the anterior margin of the aperture and form-
ing a groove on the inner surface of the shell
(Figure 3.13). This structure is interpreted as a
drainage sump in the mantle cavity, where
TABLE 3.2
Relation of the Subclass Archaeobranchia with Earlier Introduced High-Level Groups of the
Early Cambrian Molluscs
HIGH-LEVEL TAXON CONCEPT WITH REFERENCE RELATIONSHIP WITH ARCHAEOBRANCHIA
Class Paragastropoda Univalved untorted The families Aldanellidae and
anisostrophically coiled Pelagiellidae (i.e., order, Orthostrophina
molluscs (Linsley and Kier, 1984) Linsley and Kier, 1984) considered as
torted, i.e., gastropods, and assigned
to the subclass Archaeobranchia
as the order Pelagielliformes. The
rest of the paragastropods (order
Hyperstrophina Linsley and Kier, 1984)
are placed in other gastropod
subclasses.
Order Eomonoplacophora Untorted exogastric molluscs All valid families are included in
(Missarzhevsky 1989) the order Helcionelliformes of
the subclass Archaeobranchia, except
the family Khairkhaniidae, which
comprises the order Khairkhaniiformes
included in the Heterobranchia.
Class Helcionelloida Univalved untorted endogastric Peel declined to give the exact volume
molluscs (Peel 1991a, b) and hierarchical subdivision of the
class, but the composition of assigned
genera approximately corresponds to
that of the order Helcionelliformes,
subclass Archaeobranchia.
Class Rostroconchia Bivalved molluscs with The genera Watsonella and
single-valved protoconch Eurkcapegma are considered univalved
(Pojeta et al. 1972; Runnegar and assigned to Archaeobranchia.
and Pojeta 1974; Pojeta and Most of the remaining are Class
Runnegar 1976) Rostroconchia (Pojeta and
Runnegar 1976), and some are
Bivalvia (Starobogatov 1977).

48 early cambrian radiation of mollusca
water accumulated before being exhaled with
waste. The earliest members of the family are
known from the Nemakit-Daldynian and basal
Tommotian, and the group persisted until the
Botomian or possibly later. The presence of the
buttress suggests a water circulation pattern
(Figure 3.8) similar to that of the Paleozoic bel-
lerophontids, to which the igarkiellids could be
ancestral. However, testing this latter assump-
tion requires the study of material of late Middle
Cambrian–early Late Cambrian age, which has
not been done to date because of an apparent
gap in the fossil record.
Valid genera included: Gonamella, Igarki-
ella, Mastakhella, and Protoconus.
coreospiridae Knight, 1947 (Figure 3.14).
The family includes genera characterized by a
planispiral shell with a compressed last whorl
and an elongate aperture. It appeared at the base
of the Tommotian and persisted until the Middle
FIGURE 3.11. Representatives of the family Helcionellidae. (A, B) Aegitellus placus, Lower
Cambrian, Tommotian, Zhujiaqing Formation, Zhongyicun Member; Xiaowaitoushan,
Meishucun, Yunnan, China; PIN, no. 4552/1373, shell, ⫻30: A, left oblique view, B, dorsal
view. (C, D) Ilsanella atdabanica?, Lower Cambrian, Tommotian; Selinde River, Uchur-Maya
Region, Siberian Platform; PIN, no. 5083/0628, internal mold of immature (?) specimen,
⫻15: C, left oblique view, D, dorsal view. (E, F) Truncatoconus campylurus, Lower Cambrian,
Tommotian, Zhujiaqing Formation, Dahai Member; Xiaowaitoushan, Meishucun, Yunnan,
China; PIN, no. 4552/1506, internal mold, ⫻26: E, oblique right view, F, dorsal view.
(G, H) Bemella septata, Lower Cambrian, Tommotian; Aldan River, Siberian Platform; PIN,
no. 5083/0436, shell, ⫻12: G, left view, H, oblique dorsal view. (I) Auricullina papulosa,
Lower Cambrian, Tommotian; Tiktirikteekh Creek, middle reaches of Lena River, Siberian
Platform; PIN, no. 5083/0038, internal mold, dorsal view, ⫻32. (J) Fenqiaronia proboscis,
Lower Cambrian, Botomian, Parara Limestone; bore-hole SYC-101 (depth 169.30 m), Yorke
Peninsula, South Australia; PIN, no. 4664/1730, internal mold with shell fragments,
⫻13, (K, L) Emarginoconus mirus, Lower Cambrian, Tommotian, Zhujiaqing Formation,
Zhongyicun Member; Xiaowaitoushan, Meishucun, Yunnan, China; PIN, no. 4552/1341,
shell, ⫻34: K, dorsal view, L, oblique posterior view.

early cambrian radiation of mollusca 49
Cambrian. This group probably originated from
helcionelloid ancestors, which include some
forms with a strongly hooked apex and which
appear to be intermediate between typical cap-
shaped helcionelloids and coiled coreospirids.
A slight deviation from symmetrical planispiral
coiling occurs in some taxa. Such slight dex-
trality or sinistrality can occur simultaneously
within the same genus and even species (along
with normal, bilaterally symmetrical speci-
mens). The variation in coiling in this group
suggests that dextral or sinistral asymmetry of
FIGURE 3.12. Representatives of the family Helcionellidae. (A, B) Igorella emeiensis, Lower
Cambrian, Tommotian, Zhujiaqing Formation, Dahai Member; Xiaowaitoushan, Meishucun,
Yunnan, China specimen PIN, no. 4552/0136, internal mold, ⫻10: A, oblique right view,
B, dorsal view. (C, D) Igorellina monstrosa, Lower Cambrian, Tommotian; Rassokha River,
West Anabar Region, Siberian Platform; PIN, no. 5083/0053, internal mold, ⫻26: C, oblique
apertural view, D, right view. (E, F) Igorella maidipingensis, Lower Cambrian, Tommotian,
Zhujiaqing Formation, Dahai Member; Xiaowaitoushan, Meishucun, Yunnan, China;
holotype PIN, no. 4552/0144, internal mold, ⫻14: E, left view; F, dorsal view.
(G, H) Ceratoconus striatus, Lower Cambrian, Tommotian; Kengede River, East Anabar
Region, Siberian Platform; PIN, no. 5083/0091, internal mold, ⫻29: G, oblique posterior
view, H, right view; (I) Lenoconus sulcatus, Lower Cambrian, Tommotian; Selinde River,
Uchur-Maya Region, Siberian Platform; PIN, no. 5083/0514, internal mold, left view, ⫻31.
(J) Obtusoconus honorabilis, Lower Cambrian, Tommotian, Zhujiaqing Formation, Dahai
Member; Xiaowaitoushan, Meishucun, Yunnan, China; PIN, no. 4552/1167, internal mold,
left view, ⫻29. (K) Anuliconus magnifi cus, Lower Cambrian, Botomian, Parara Limestone;
Horse Gully, Yorke Peninsula, South Australia; holotype PIN, no. 4664/0544, internal mold,
right view, ⫻24. (L) Obtusoconus brevis, Lower Cambrian, Botomian, Sellick Hill Formation;
Myponga Beach, Fleurieu Peninsula, South Australia; PIN, no. 4664/1337, internal mold,
left view, ⫻29. (M) Daedalia daedala, Lower Cambrian, Botomian, Parara Limestone; Horse
Gully, Yorke Peninsula, South Australia; holotype PIN, no. 4664/0511, internal mold, left view,
⫻40. (N) Aequiconus zigzac, Lower Cambrian, Botomian, Parara Limestone; Horse Gully,
Yorke Peninsula, South Australia; holotype PIN, no. 4664/1507, internal mold, left view, ⫻29.

50 early cambrian radiation of mollusca
the body was not yet fi xed (Minichev and Staro-
bogatov, 1979).
Valid genera included: Cambrospira, Coreo-
spira, Kutanjia, Latouchella, Pseudoyangtzespira,
and Tichkaella.
trenellidae Parkhaev, 2001 (Figure 3.15).
Members of the family are characterized by a
shell with a distinct groove or arch developed at
the posterior end of the aperture (Figure 3.15B,
C, G, H), presumably to facilitate water intake
(Parkhaev 2000). This apomorphic character,
which separates them from their helcionel-
loid ancestors, appeared in taxa from the Early
Tommotian and persisted until the mid-Middle
Cambrian. Further development of this struc-
ture into a tubular projection gave rise to the
Yochelcionellidae.
Valid genera included: Figurina, Horsegullia,
Mackinnonia, Obscurella, Oelandia, Parailsanella,
Perssuakiella, Prosinuites, Rugaeconus, Trenella, Tuba-
toconus, and Xianfengella.
yochelcionellidae Runnegar and Jell,
1976 (Figure 3.16K–N). Among the Cambrian
molluscs, representatives of this family have
the most remarkable shell morphology. Its
diagnostic feature is a tubular projection, the
FIGURE 3.13. Representatives of the family Igarkiellidae. (A–C) Protoconus crestatus,
Lower Cambrian, Tommotian, Zhujiaqing Formation, Dahai Member; Xiaowaitoushan,
Meishucun, Yunnan, China; PIN, no. 4552/1530, internal mold, ⫻26: A, dorsal view, B, left
oblique view, C, oblique anterior view. (D, E) Purella cristata, Lower Cambrian, Tommotian;
Selinde River, Uchur-Maya Region, Siberian Platform; PIN, no. 5083/0625, internal mold,
⫻33: D, oblique right view, E, dorsal view. (F–H) Protoconus elegans, Lower Cambrian,
Tommotian, Zhujiaqing Formation, Dahai Member; Xiaowaitoushan, Meishucun, Yunnan,
China; PIN, no. 4552/1467, internal mold, ⫻25: F, dorsal view, G, left oblique view,
H, oblique anterior view. (I, J) Igarkiella levis, Lower Cambrian, Tommotian; Selinde River,
Uchur-Maya Region, Siberian Platform; PIN, no. 5083/0144, internal mold, ⫻38: I, oblique
anterior view, J, dorsal view. (K, L) Gonamella rostrata, Lower Cambrian, Tommotian;
Fomich River West Anabar Region, Siberian Platform; PIN, no. 5083/0186, internal mold,
⫻35: K, oblique right view, L, dorsal view.

early cambrian radiation of mollusca 51
snorkel, on the posterior shell surface, which is
thought to be homologous to the parietal groove
of the trenellids (Peel 1991a, b; Parkhaev 2001).
A transitional phase from a groove to a snorkel
is seen in Eotebenna, members of which pos-
sess a very deep groove with converging lower
edges separated by a narrow slit. The earliest
yochelcionellids are known from the Middle
Tommotian, but they are most diverse from the
Botomian through the early Middle Cambrian.
Valid genera included: Eotebenna, Run-
negarella, Yochelcionella, and possibly, Enigmaconus.
stenothecidae Runnegar and Jell, 1980
(Figure 3.16A–J). Representatives have a trenellid-
like shell characterized by very strong lateral
compression. The family fi rst appeared in the
uppermost Nemakit-Daldynian and Early Tom-
motian, achieved its maximum diversity in the
late Atdabanian–Botomian, and persisted until
the the mid-Middle Cambrian. Strong lateral
compression of the shell is assumed (Parkhaev
2006c) to be an adaptation to new types of
habitats—dense algal fi elds (subfamily Stenoth-
ecinae) and soft- sediment environments (sub-
family Watsonellinae). The lateral compression
is also thought to be an adaptation to an infaunal
habit; this view is also favored by an increased
curvature of the aperture and the appearance
FIGURE 3.14. Representatives of the family Coreospiridae. (A, B) Pseudoyangtzespira
selindeica, Upper Vendian, Nemakit-Daldynian; Selinde River, Uchur-Maya Region,
Siberian Platform; PIN, no. 5083/0604, internal mold, ⫻23: A, right view, B, apertural
view. (C, D) Latouchella korobkovi, Lower Cambrian, Tommotian, Zhujiaqing Formation;
Xiaowaitoushan, Meishucun, Yunnan, China; internal molds: C, PIN, no. 4552/0147, right
view, ⫻12, Zhongyicun Member; D, PIN, no. 4552/1554, dorsal view, ⫻21, Dahai Member.
(E, F) Latouchella memorabilis, Lower Cambrian, Tommotian; West Anabar Region, Siberian
Platform: E, PIN, no. 5083/0148, internal mold right view, ⫻17, Rassokha River; F, PIN,
no. 5083/0182, internal mold, oblique dorsal view, ⫻24, Fomich River. (G, H) Anhuiconus
microtuberus, Lower Cambrian, Botomian, Parara Limestone; Yorke Peninsula, South
Australia; PIN, no. 4664/1867, internal mold, ⫻13: G, apertural view, H, left view.

52 early cambrian radiation of mollusca
of internal plates for the attachment of strong
pedal musculature, suggestive of a digging foot
(genera Eurekapegma and Watsonella), as sup-
posed by some authors (MacKinnon 1985; Land-
ing 1989; Peel 1991a, b).
Valid genera included: Anabarella, Eureka-
pegma, Mellopegma, Stenotheca, and Watsonella.
pelagiellidae Knight, 1952 (Figure 3.17).
This family is characterized by a turbospiral,
dextrally coiled shell with a somewhat trian-
gular aperture having a drawn basal part. The
basal angulation of the aperture (Figure 3.17B,
D, F) is assumed to be an inhalant area homolo-
gous to the posterior groove or arch in coreo-
spirids (Parkhaev 2001), which are thought to
be the pelagiellid ancestors (Parkhaev 2002a).
The fi rst members of the family appeared in the
earliest Atdabanian, were most diverse from the
Late Atdabanian–Toyonian to the Middle Cam-
brian and persisted until the Middle Cambrian.
Valid genera included: Costipelagiella,
Pelagiella, and Tannuspira.
FIGURE 3.15. Representatives of the family Trenellidae. (A) Parailsanella murenica,
Lower Cambrian, Botomian, Parara Limestone; bore-hole SYC-101 (depth 205.60 m),
Yorke Peninsula, South Australia; PIN, no. 4664/1656, internal mold, right view, ⫻33.
(B, C) Trenella bifrons, Lower Cambrian, Botomian, Parara Limestone; Horse Gully, Yorke
Peninsula, South Australia; holotype PIN, no. 4664/0665, internal mold, ⫻33: B, right
view, C, oblique dorsal view. (D, E) “Securiconus” costulatus, Lower Cambrian, Tommotian;
Kotui River, West Anabar Region, Siberian Platform; PIN, no. 5083/0007, internal
mold, ⫻22: D, oblique posterior view, E, right view. (F) Horsegullia horsegulliensis, Lower
Cambrian, Botomian, Parara Limestone; Horse Gully, Yorke Peninsula, South Australia;
holotype PIN, no. 4664/1499, internal mold, left view, ⫻19. (G, H) Mackinnonia rostrata,
Lower Cambrian, Botomian, Parara Limestone; Horse Gully, Yorke Peninsula, South
Australia; G, PIN, no. 4664/0233, internal mold right view, ⫻20; H, PIN, no. 4664/0274,
internal mold, posterior view, ⫻39. (I, J) Leptostega hyperborea, Lower Cambrian, Botomian;
Anabar River basin, Siberian Platform; holotype PIN, no. 5083/0092, internal mold, ⫻33:
I, left view, J, dorsal view.

early cambrian radiation of mollusca 53
aldanellidae Linsley and Kier, 1984
(Figure 3.18). The members of this family
have a turbospiral, mostly dextral shell with
a protruding spire and an elliptical aperture.
Although this type of shell could be derived
from a pelagiellid with the development of
greater asymmetry through the extension of
the spire, aldanellids fi rst appear in the basal
Tommotian, before the fi rst appearance of
pelagiellids, suggesting an independent ori-
gin from coreospirids. They persisted until the
mid-Atdabanian.
Valid genera included: Aldanella and Nom-
goliella.
FIGURE 3.16. Representatives of the families Stenothecidae (A–J) and Yochelcionellidae
(K - N). (A, B) Stenotheca drepanoida, Lower Cambrian, Botomian, Parara Limestone; Yorke
Peninsula, South Australia; A, PIN, no. 4664/1731, internal mold, left view, ⫻26, bore-hole
SYC-101 (depth 168.80 m); B, PIN, no. 4664/0608, internal mold, dorsal view, ⫻30, Horse
Gully. (C, D) Watsonella crosbyi, Lower Cambrian, Tommotian; Rassokha River, West Anabar
Region, Siberian Platform; PIN, no. 5083/0047, internal mold, ⫻33: C, oblique posterior
view, D, right view. (E, F) Anabarella australis, Lower Cambrian, Yorke Peninsula, South
Australia: E, PIN, no. 4664/1780, shell, right view, ⫻29; Atdabanian, Kulpara Formation,
bore-hole CD-2 (depth 32.66 m); F, PIN, no. 4664/0945, shell, apertural view, x54;
Atdabanian, Kulpara Formation, Horse Gully. (G, H) Mellopegma uslonica, Lower Cambrian,
Botomian, Bystraya Formation; Georgievka, East Transbaikalia; G, PIN, no. 2019/1047,
internal mold, right view, ⫻34; H, PIN, no. 2019/1049, internal mold, dorsal view, ⫻31.
(I, J) Anabarella plana, Lower Cambrian, Tommotian; Selinde River, Uchur-Maya Region,
Siberian Platform; PIN, no. 5083/0634, internal mold, ⫻31: I, left oblique view; J, dorsal
view. (K–M) Yochelcionella crassa, Lower Cambrian: K, PIN, no. 3302/5001, apical fragment
of internal mold, right view, ⫻22, Botomian, Shingein-Nuruu, West Mongolia; L, PIN,
no. 2019/1086, apical fragment of internal mold, oblique dorsal view, ⫻40; Atdabanian,
Bystraya Formation, Georgievka, East Transbaikalia; M, PIN, no. 2019/1112, apical fragment
of internal mold, left view, ⫻29; Atdabanian, Bystraya Formation, Georgievka, East
Transbaikalia. (N) Runnegarella americana, Lower Cambrian, Newfoundland, Canada;
specimen ex J.S. Peel, internal mold, left view, ⫻26.

54 early cambrian radiation of mollusca
khairkhaniidae Missarzhevsky, 1989 (Fig-
ure 3.19 A–I). This family is characterized
by a spirally coiled shell and includes both
planispiral and slightly dextral and sinistral
forms. Compared to the elongated cross sec-
tion of the whorls in coreospirids, the whorls
of khairkhaniid shells are almost circular in
cross section. This family could represent
another lineage derived from helcionelloids
that appeared during the latest Nemakit-Dal-
dynian–earliest Tommotian and persisted
until the Middle Cambrian. If this interpreta-
tion is correct, the spirally coiled shell seen
in this group has been independently derived
from that of the Coreospiridae. The coiling
of the relatively narrow tube into a tight spi-
ral was accompanied by major transforma-
tions inside the mantle cavity and resulted
in the origin of the pallial cecum (Parkhaev
2002a: text-fi g. 4). Thus, this family com-
poses a monotypic order Khairkhaniiformes
and may be the most ancient member of the
Heterobranchia (Parkhaev, in press).
Valid genera included: Ardrossania, Bar-
skovia, Khairkhania, Michniakia, Philoxenella,
Protowenella, and Xinjispira.
FIGURE 3.17. Representatives of the family Pelagiellidae. (A, B) Pelagiella adunca, Lower
Cambrian, Atdabanian; Chekurovka, lower reaches of Lena River, Siberian Platform;
A, PIN, no. 5083/0300, internal mold, spire view, ⫻37; B, PIN, no. 5083/0298, internal
mold, apertural view, ⫻35. (C, D) Pelagiella subangulata, Lower Cambrian, Botomian, Parara
Limestone; Yorke Peninsula, South Australia: C, PIN, no. 4664/1708, internal mold, spire
view, ⫻31, bore-hole SYC-101 (depth 194.45 m); D, PIN, no. 4664/1556, internal mold,
apertural view, ⫻18, bore-hole SYC-101 (depth 216.35 m). (E, F) Pelagiella madianensis, Lower
Cambrian, Botomian, Parara Limestone; Horse Gully, Yorke Peninsula, South Australia;
E, PIN, no. 4664/1253, internal mold, spire view, ⫻20; F, PIN, no. 4664/1143, shell,
apertural view, ⫻37. (G, H) Tannuspira magnifi ca, Lower Cambrian, Botomian, Sanashtyk-
Gol Horizon, Eastern Tannu-Ola Range, Altai-Sayanian Folded Belt; holotype GIN, no.
3593/505, shell, ⫻3.3: G, spire view, H, apertural view.

early cambrian radiation of mollusca 55
onychochilidae Koken, 1925 (Figure 3.19J,
K). This family accommodates many of the Early
Paleozoic taxa with hyperstrophic shells, along
with a few earliest Cambrian forms. They are
not included in the Archaeobranchia and are
instead placed within extinct order Onychochili-
formes (Figure 3.2). Its members are character-
ized by possessing a hyperstrophic, turbospiral
shell, a mantle cavity with a pallial cecum, and
they possibly retain only the right pallial organs
(Starobogatov 1976; Golikov and Starobogatov
1988). This family may have originated from
khairkhaniids, as suggested by a gradual transi-
tion from the almost planispiral Khairkhaniidae
to the hyperstrophic Onychochilidae within the
lineage ProtowenellaAXinjispiraABeshtashella.
Main genera included: Apart from numer-
ous Ordovician and younger genera, a single,
variable genus occurs in the Early Cambrian:
Beshtashella (⫽ Yuwenia, ⫽ Kistasella).
FIGURE 3.18. Representatives of the family Aldanellidae. (A, B) Aldanella crassa, Lower
Cambrian, Tommotian; Kotui River, Anabar Region, Siberian Platform; A, PIN, no.
4386/1096, internal mold, spire view, ⫻14; B, PIN, no. 4386/1208, internal mold,
apertural view, ⫻15. (C, D) Aldanella attleborensis, Lower Cambrian, Tommotian; Selinde
River, Uchur-Maya Region, Siberian Platform; PIN, no. 5083/0479, internal mold, ⫻19:
C, spire view, D, oblique apertural view. (E, F) Aldanella utchurica, Lower Cambrian,
Tommotian; Maimakan River, Uchur-Maya Region, Siberian Platform; PIN, no.
5083/0463, internal mold, ⫻15: E, spire view, F, oblique apertural view. (G, H) Aldanella
operosa, Lower Cambrian, Atdabanian; Achchagyi-Kyyry-Taas Creek, middle reaches of
Lena River, Siberian Platform; G, PIN, no. 5083/0264, shell, spire view, ⫻31; H, PIN,
no. 5083/0262, shell, apertural view, ⫻31. (I, J) Aldanella rozanovi, Lower Cambrian,
Tommotian; Olenek River, Olenek Uplift, Siberian Platform; PIN, no. 5083/0460,
internal mold, ⫻26: I, spire view, J, oblique apertural view. (K, L) Aldanella golubevi,
Lower Cambrian, Tommotian; Kotui River, West Anabar Region, Siberian Platform;
K, PIN, no. 4386/1523, internal mold, spire view, ⫻19; L, PIN, no. 4386/1520, internal
mold, apertural view, ⫻16.

56 early cambrian radiation of mollusca
THE CAMBRIAN RADIATION OF
GASTROPODS
According to the interpretation presented
above, gastropods appeared during the latest
Precambrian (Nemakit-Daldynian) to Earliest
Cambrian (Tommotian) (for an alternative
interpretation see Fr´yda et al., Chapter 10). They
formed a paraphyletic group, the Archaeobran-
chia, which can be positioned at the base of the
gastropod phylogenetic tree, and comprised
eight family-level taxa arranged in two orders
(Parkhaev 2002a, in press) (Figure 3.2). The
position of two other families (Khairkhaniidae
and Onychochilidae), are thought to be derived
from the Archaeobranchia, is questionable.
Most likely, they can be placed somewhere at the
root of the Heterobranchia. The Archaeobran-
chia reached its maximum diversity during the
Early to Middle Cambrian and extended into
FIGURE 3.19. Representatives of the families Khairkhaniidae (A–I) and Onychochilidae
(J, K). (A, B) Khairkhania rotata, Lower Cambrian, Tommotian; Rassokha River, West
Anabar Region, Siberian Platform; PIN, no. 5083/0153, internal mold with shell fragments,
⫻33: A, left view, B, oblique apertural view. (C, D) Khairkhaniidae gen. et sp. indet., Lower
Cambrian, Tommotian; Chekurovka, lower reaches of Lena River, Siberian Platform; PIN,
no. 5083/0279, internal mold, ⫻34: C, spire view, D, apertural view. (E, F) Philoxenella
spirallis, Lower Cambrian, Tommotian; Rassokha River, West Anabar Region, Siberian
Platform; PIN, no. 5083/0639, internal mold, ⫻17: E, spire view, F, oblique apertural
view. (G, H) Barskovia hemisymmetrica, Lower Cambrian, Tommotian; Rassokha River,
West Anabar Region, Siberian Platform; PIN, no. 5083/0132, internal mold, ⫻27: G, spire
view, H, oblique apertural view. (I) Ardrossania pavei, Lower Cambrian, Botomian, Parara
Limestone; bore-hole Cur-D1B (depth 278.35 m), Yorke Peninsula, South Australia; PIN,
no. 4664/1535, internal mold with shell fragments, right view, ⫻29. (J, K) Beshtashella
tortilis, Lower Cambrian, Botomian, Parara Limestone; Horse Gully, Yorke Peninsula, South
Australia; J, PIN, no. 4664/1008, internal mold, dorsal view, ⫻39; K, PIN, no. 4664/1817,
internal mold, apertural view, ⫻45.

early cambrian radiation of mollusca 57
the Ordovician (Gubanov and Peel 2001). The
most primitive members of this group had a
simple, cap-shaped shell with a central or sub-
central apex and an apertural margin devoid
of any notches. The two major morphogical
trends occurred at the base of the archaeobran-
chian radiation. The fi rst was the development
of structures enhancing the effi ciency of water
circulation, such as special sinuses, grooves,
buttresses, or tubes. The second trend is the
formation of a spirally coiled shell; this, as dis-
cussed previously, appeared independently in at
least two lineages and led to the origin of coreo-
spirids and khairkhaniids. The most advanced
coiled archaeobranchians (pelagiellids and
aldanellids) developed an asymmetrical, turbo-
spiral shell.
The phylogenetic relationships of major
Recent gastropod taxa have been disputed (e.g.,
contrast Golikov and Starobogatov 1975, 1988;
Starobogatov 1970; Minichev and Staroboga-
tov 1979; Salvini-Plawen 1980; Graham 1985;
Haszprunar 1988; Bieler 1992; Ponder and Lind-
berg 1997), although there is now an increas-
ingly accepted arrangement (Haszprunar 1988;
Bieler 1992; Ponder and Lindberg 1997). Most of
the major living taxa probably represent crown
groups of different stems and possess rather
advanced shell characters that are diffi cult to
trace through the long history of gastropod evo-
lution. In the earliest group, the archaeobran-
chians, a number of lineages can be identifi ed,
although these results are tentative because our
knowledge of Cambrian molluscs remains very
incomplete. The considerable gap in the fossil
record between Cambrian and Ordovician gas-
tropods, the latter more readily fi tting within the
commonly accepted groups, makes it especially
diffi cult to establish relationships between the
Cambrian and later taxa.
Possibly, the internally asymmetrical forms
(Parkhaev 2000: text-fi g. 3) with a turbospiral
shell from the order Palegielliformes, gave
rise to the Vetigastropoda. The position of the
bellerophonts is of special interest. Golikov and
Starobogatov (1988) included them as the most
primitive order within their subclass Scutibran-
chia (⫽ Vetigastropoda in part) because some
possess an exhalant antero-median slit. Their
supposed basal position is questionable, because
shell slits in Recent gastropods can be conver-
gent. Hence, it can be argued that the “shell slit,
used alone, is of little phylogenetic importance”
(Haszprunar 1988: 372). The order Bellerophon-
tiformes is probably best kept separate from
“Scutibranchia” and placed in Archaeobranchia
on the basis of their planispiral shell coiling.
As already suggested, the bellerophonts could
arise from Igarkiellids, which have similar but-
tresses. Runnegar and Jell (1976) associated the
Early to Middle Cambrian symmetrically coiled
genus Prowenella with bellerophonts, allowing
Dzik (1981: fi g. 6) to hypothesize a relationship
between bellerophonts and some of the Early
Cambrian helcionelloids.
ADAPTIVE RADIATIONS
Adaptive radiation is usually considered as the
acquisition of a number of new morphological,
physiological, or behavioral features that allow
the members of a group to occupy new ecologi-
cal niches. Because the interpretation of hard-
part morphology refl ects only hints of some
of these features, a study of adaptive radiation
based on fossils is challenging and speculative.
Such studies are facilitated by the presence of
modern relatives of a group, or unrelated but
convergent analogs, but are frustrated when
there are no obvious living analogs.
The Cambrian molluscs are morphologically
and phylogenetically very distant from extant
groups, so hypotheses about their biology and
structure are highly speculative and based on
ambiguous interpretations. Consequently, such
studies are virtually absent, with only a few pub-
lications that have touched upon this problem
in a general way (Gubanov et al. 1999; Gubanov
and Peel 1999; Kouchinsky 2001). Many special-
ists do not share the opinion espoused here, that
most of the Early Cambrian univalved molluscs
compose a phylogenetically uniform group (see

58 early cambrian radiation of mollusca
Table 3.1). However, the following discussion
is based on the assumption that the helcionel-
loid molluscs do form a monophyletic group
(Figure 3.2), with all families direct or indirect
descendants of the root (paraphyletic) group,
family Helcionellidae, which is, in turn, derived
from monoplacophorans.
The most simple type of radula (docoglos-
sate) is present among the most primitive Recent
gastropods, the patellogastropods. It is similar
in structure to the radula of chitons and mono-
placophorans (i.e., stereoglossate; Golikov and
Starobogatov 1988). We can suppose that archaeo-
branchians possessed a docoglossate radula
adapted for grazing biofi lms on relatively hard
substrates. It is unlikely that suspension feeding
or, especially, predation originated among the
Early Cambrian univalve forms, as has been sug-
gested in some publications (Burzin et al. 2001;
Kouchinsky 2001), although these feeding types
are present among more advanced gastropods
(see Ponder and Lindberg 1997: table 4).
The prevalent shell form of the archaeo-
branchian basal family, Helcionellidae, is
limpet-like, and this is considered plesiomor-
phic, inherited from monoplacophoran ances-
tors (Haszprunar 1988), although limpets have
secondarily and independently evolved in vari-
ous gastropod groups from coiled ancestors
(Ponder and Lindberg 1997: table 6). Mod-
ern limpets inhabit a great variety of habitats,
from intertidal rocks (most patellogastropods,
Siphonariidae) to hard (hot-vent groups) or
biogenic (Cocculinidae, Pseudococculinidae)
substrates in abyssal depths or in fresh water
(Acroloxidae, some Planorbidae [⫽ Ancylidae]),
where they occupy a wide range of habitats.
Similarly, the Early Cambrian limpet-like mol-
luscs may have occurred in a wide range of
marine habitats. Several forms, such as large
Bemella (Figure 3.11G, H), Ilsanella, Tannuella,
and Randomia, appear to have inhabited shal-
low water shores, because they are sometimes
associated with archaeocyath-algal bioherms
(Sundukov and Fedorov 1986; Dzik 1991; Land-
ing 1992). The smaller forms with depressed
shells (Figure 3.11E, F, K, L) may have grazed
algae. Some members of the Helcionellidae
have laterally compressed shells (Figure 3.12A–
F, N), a possible adaptation for crevice or cave
dwelling, life among dense algae, or even life on
narrow stem-like algae, in much the same way
as some modern Lottiidae live on seagrasses.
Some helcionellids had a highly conical shell
(Figure 3.12G–L), unlike those of modern gastro-
pods. An approximate analogous form is seen in
caecids (e.g., Caecum, Fartulum, and Brochina),
which live in a wide variety of shallow marine
habitats including rock crevices, under stones,
interstitially in gravel, or among Zostera fi elds
(Golikov and Kusakin 1978; Rehder 1994).
The formation of a coiled shell may have led
to increasing mobility. A compact, coiled shell
not only allows a decrease in shell size while
retaining the same volume, but also results in a
smaller aperture, allowing a thin neck between
the visceral mass and cephalopodium (Pon-
der and Lindberg 1997). This fl exible neck and
narrowed foot enabled more active behavior.
The formation of spirally coiled shells among
archaeobranchians probably occurred at least
twice (see earlier discussion). It happened fi rst
in Coreospiridae, characterized by a large, longi-
tudinally elongated aperture and relatively small
spire (Figure 3.14). This group possibly arose
from laterally compressed helcionellids, whose
shell underwent gradual bending and coiling of
the apex ( Figure 3.12A–F). A similar, or possibly a
little more active, lifestyle is suggested for coreo-
spirids: that is, dwelling in fi elds of macroalgae or
on their narrow stems. Possibly, the most similar
modern analogs are certain (freshwater) Planor-
bidae usually associated with dense weed beds.
The second lineage in which spirally coiled
shells formed, Khairkhaniidae, is characterized
by the aperture diameter being considerably
less than that of the shell (Figure 3.19A–I). This
shell form probably originated from the ances-
tral helcionellids with highly conical shells
(Figure 3.12G–I). Some are symmetric, others
slightly dextral or slightly sinistral. Numerous
Recent gastropods have shells of similar shape
and size, such as Skeneopsidae (Skeneopsis)
and Tornidae (Circulus), as well as freshwater

early cambrian radiation of mollusca 59
Valvatidae (Valvata) and many Planorbidae.
Those forms inhabit a very wide range of habi-
tats, ranging from fi lamentous algae to inter-
stitial, and the marine taxa occur from the
intertidal to the deep sea. Based on these analo-
gies, it is not possible to predict the habitat of
kairkhaniids with any certainty.
The formation of a hyperstrophic6 shell of
the family Onychochilidae (Figure 19J, K), which
may have evolved from khairkhaniids, was prob-
ably accompanied by an increase in mobility,
with the shell elongated along the animal’s body
axis. There are few species with hyperstrophic
shells among Recent gastropods, but similar
bulliform shells with long, narrow apertures are
typical of many bulliform Opisthobranchia and
some freshwater Pulmonata (e.g., Physidae,
some Planorbidae). The former group includes
mainly infaunal active predators as well as some
herbivores, whereas the latter families inhabit
various substrates and are herbivorous. Both
epifaunal or infaunal lifestyles are possible for
Cambrian onychochilids, and in the latter case
they may have been detritus feeders.
The families Pelagiellidae and Aldanellidae
developed a turbospiral shell, which probably
enhanced mobility compared with its assumed
ancestor Coreospiridae. Asymmetry of the shell
was achieved by different transformations of
shell shape: by the spire projection in aldanel-
lids (Figure 3.18) and by the protrusion of the
left (basal) part of the last whorl in pelagiellids
(Figure 3.17). The posterior rotation of the spire
was enabled in the ancestral Coreospiridae
(see Parkhaev 2001: text-fi g. 3c), resulting in
the angle between the axis of coiling and the
axis of the cephalopodium being less than 90°
(see reconstructions in Parkhaev 2001: text-
fi g. 3g; Parkhaev, 2006b: text-fi g. 1). Based on
aldanellid shell morphometry, analogous shells
among Recent gastropods are Tornidae (e.g.,
Tornus, Pseudoliotia, and Pygmaeorota), some
Trochidae (Margarites), and Skeneidae (Skenea),
whereas analogs for Pelagiellidae can be found
among some members of the Trichotropidae
(Lepistes), Littorinidae (Lacuna), and Vanikori-
dae (Vanikoro). Considering the variety of habi-
tats occupied by living analogs, the Cambrian
forms could have occupied habitats ranging
from algal thalli to crevices or caves. Species of
Aldanella and Pelagiella are commonly encoun-
tered in Cambrian rocks, possibly because they
had rather high population densities, analogous
to some Recent Lacuna and Margarites, with
population densities reaching 1,500 individuals
per square meter (Skarlato 1987).
The formation of structures controlling
water currents inside the pallial cavity is seen
in two lineages of helcionelloids that arose from
helcionellids. The fi rst lineage, Igarkiellidae, has
a groove (which looks like a buttress externally)
inside the limpet-like shell, and it runs from
the apex toward the anterior edge of the aper-
ture (Figure 3.13). In spite of the obvious adaptive
signifi cance of the internal groove as a structure
for the localization of the exhalant water current,
and hence the increased effi ciency of water cir-
culation inside the pallial cavity (Figure 3.8), we
have no data suggesting that igarkiellids devel-
oped new adaptive zones compared with their
helcionellid ancestors. Judging from the low or
even depressed shell, igarkiellids may have been
slowly moving epifaunal dwellers, grazing bio-
fi lms from various substrates.
The shell was modifi ed to facilitate water
intake in Trenellidae and in its offshoot
Yochelcionellidae. In these groups the poste-
rior groove of some ancestral helcionellids was
modifi ed to form an arch (Figure 3.15B, D, H)
or even a siphon (snorkel) (Figure 3.16K–N). In
spite of those innovations, the general helcio-
nellid shell shape was retained in trenellids and
yochelcionellids, suggesting that these derived
families may have remained in the same adap-
tive zone as their ancestors. A few members of
the Yochelcionellidae (e.g., Eotebena and Run-
negarella) are characterized by extreme lateral
compression of the shell and a strongly arched
apertural margin. These taxa may have been
semi-infaunal (Peel 1991b: fi g. 32), with the snor-
kel of Runnegarella (Figure 3.16N) and the deep
posterior sinus of Eotebenna (Runnegar 1996:
6. See Fr´yda et al., Chapter 10, for an alternative view.

60 early cambrian radiation of mollusca
fi g. 6.3) probably being adaptations for water
intake above the sediment surface.
Members of the Stenothecidae, possibly
derived from trenellids, also had laterally com-
pressed shells. Typical stenothecids (Stenotheca
and Anabarella) are characterized by an almost
planar aperture (Figure 3.16A, B, E, F), whereas
the members of the subfamily Watsonellinae
(Watsonella and Eurekapegma) have strongly
arched anterior and posterior apertural margins,
possibly implying an infaunal mode of life in
soft sediments. This assumption is supported
by the fi nding of Watsonella in situ with the shell
perpendicular to the sediment bedding (Land-
ing 1989). The genus Eurekapegma has folds on
the inner sides of the shell (MacKinnon 1985:
fi g. 6A; Runnegar 1996: fi g. 6.3). which could
be interpreted as additional attachment surfaces
for a highly developed muscular system of a foot
adapted for digging.
The morphological analogs for the Cam-
brian Stenotheca and Anabarella are absent from
the modern malacofauna. The extent of shell
compression of these fossil genera is high; that
is, length/width ratio is 5–6 or even higher. A
planar aperture suggests a mode of life crawl-
ing over rather fi rm substrate, possibly among
macro or fi lamentous algae, similar to some
highly compressed modern planorbids, many
of which live in bushy macrophytes and fi la-
mentous algae.
The main trends of adaptive radiation of the
Cambrian univalved molluscs surmised from
the foregoing discussion are presented in Figure
3.20. Thus, the predominant ecological type at
the beginning of the Cambrian were epifaunal
grazers, feeding on biofi lms, inhabiting mainly
macroalgae, often in the shelter provided by
rocks (under stones, or in caves, crevices, etc.),
while the number and diversity of reef and open
FIGURE 3.20. Hypthetical adaptive radiation of Cambrian univalved molluscs. All shells
shown with anterior to the right. Arrows indicate general trends in shell morphogenesis
and are not intended to indicate phylogenetic relationships (Parkhaev 2006c).

early cambrian radiation of mollusca 61
rock dwellers was relatively low. Infaunal or
semi-infaunal taxa inhabited soft bottom environ-
ments (Eotebenna, Runnegarella, Watsonella, and
Eurekapegma), and at least some of these taxa may
have been detritus feeders. It should be empha-
sized that the majority of families, and hence,
the main ecotypes, originate almost simultane-
ously, near the Precambrian-Cambrian boundary
(Figure 3.2). This rapid radiation may possibly
have resulted from new opportunities to explore
a wider trophic space than previously available.
Early Cambrian bivalves (see previous dis-
cussion) are few, very small, and almost exclu-
sively found with the valves closed (Kouchinsky
2001; personal observation), so isolated valves
(Figure 3.4I, J) are the rare exceptions. These
articulated fi nds have been interpreted as evi-
dence for an infaunal lifestyle of Pojetaia and
Fordilla (Runnegar and Bentley 1983; Ermak
1986; Kouchinsky 2001). However, absence
of siphons and a weak ligament do not favor
the burrowing ability of those clams. Possibly,
the rapid postmortem phosphatization of the
organic tissues of bivalves caused bonding of
the valves and prevented them from separa-
tion. Rapid fossilization was common in the
Cambrian and in some peculiar cases pro-
duced Lagerstätten with perfectly preserved tiny
arthropods, known as “Orsten”-type localities
(Walossek 2003).
In addition, numerous Pojetaia runnegari
commonly co-occur with gastropods of the genera
Pelagiella and Anabarella in the Late Atdabanian–
Early Botomian of Australia (personal observa-
tion) and were possibly members of the same
biocenosis. As suggested above, Pelagiella and
Anabarella may have been epifaunal dwellers,
inhabiting mainly algal substrates. Possibly
Pojetaia and similar forms lived in dense algal
mats as some minute marine and freshwater
bivalves do today. Judging from the small size
of Early Cambrian bivalves, they were probably
deposit feeders, collecting particles with foot or
labial palps. Filter-feeding, active burrowing, and
the corresponding conquest of the infaunal soft
bottom environment by later bivalves were a Late
Cambrian–Early Ordovician phenomenon.
There are few data on the paleoecology of
the Cambrian placophorans. The Late Cambrian
forms described by Stinchcomb and Darrough
(1995) were found in deposits with stromatolite
remains. Late Cambrian Matthevia was also inter-
preted as a stromatolitic dweller (Runnegar et al.
1979). Articulated scleritomes of Halkieria evan-
gelista were found in silts, which are assumed
to accumulate in rather deep water and anoxic
conditions (Conway Morris and Peel 1995). How-
ever, Halkieria fi nds may not be autochthonous
and animals may have been transported from
shallower environments. The numerous disar-
ticulated sclerites of halkieriids in shallow-water
assemblages of the Early Cambrian SSF suggest
that they lived in those habitats, and may have
had much the same lifestyle as most living poly-
placophorans; grazing various types of algal and
colonial animal substrates in sublittoral to inter-
tidal habitats.
GAPS IN KNOWLEDGE
In spite of considerable progress during the
last decades, our knowledge on the earliest
molluscs, and hence the early evolution of the
phylum, is still far from complete. The major
gaps in knowledge and some directions for fur-
ther studies are briefl y outlined in the following
paragraphs.
NEW DATA SETS
The global event of mass phosphate accumulation
has a peak at the beginning of the Early Cambrian
(Luvsandanzan and Rozanov 1984; Rozanov 1992),
and consequently the phosphatized small shelly
fauna comes mainly from that interval.
While there have been many studies on faunas
from the Precambrian–Cambrian bound ary and
Lower Cambrian, there has been compara tively
much less coverage of younger formations. To
date, molluscan faunas from the latest Middle
to Upper Cambrian are still poorly known, and
this considerable gap is a serious impediment
to our understanding of the early evolution
of Mollusca. Thus, it is important that future
research resolves the phylogenetic linkages

62 early cambrian radiation of mollusca
between the bizarre Early Cambrian forms and
the more familiar molluscs from the Ordovician
and younger Paleozoic strata.
Another impotant problem is the accurate
recognition of monoplacophorans among the
early univalved molluscs. As it is commonly sup-
posed that gastropods originated from untorted
monoplacophoran ancestors, the remains of the
latter should be present in the Cambrian fossil
record. However, in practice it is very diffi cult
to distinguish between a torted and untorted
condition in Early Cambrian univalves (Harper
and Rollins 1982; Parkhaev 2000a). Possibly,
a study of the apical area of these fossils can
assist, since the embryonic shells of monopla-
cophorans differ signifi cantly from those of gas-
tropods (Ponder and Lindberg 1997).
NEW APPROACHES
Microstructural studies of the shell and the
micro-ornamentation of internal molds of ancient
molluscs are probably the most promising
avenues for future studies. Recent investigations
in this area resulted in the discovery of muscle
attachments in shells belonging to Cambrian
helcionelloids (Parkhaev 2002b, 2004a, 2006b;
Ushatinskaya and Parkhaev 2005). Additional
studies on suitable material would certainly
provide new data that would be of value in the
resolution of ancient molluscan morphology and
refi ning the existing models and reconstructions.
Studies of the protoconch and early ontog-
eny of ancient molluscs have been undeservedly
defi cient. The history of many molluscan lin-
eages is inferred largely from analyses of early
ontogeny (see Chapters 9–13), and Cambrian
molluscs are arguably no exception. The latest
investigations in that fi eld (Parkhaev 2006b)
discovered a protoconch of the genus Aldanella
with a pair of septa, dividing it from the teleo-
conch (Figure 3.21C, D).
The presence of septation in the initial part
of the shell is a common phenomenon among
Recent and fossil gastropods (Yochelson 1971).
FIGURE 3.21. Recent discoveries in the morphology of ancient gastropods. (A, B) Aldanella
rozanovi, PIN, no. 5083/1446, internal mold, Lower Cambrian, Tommotian; 0.5 km above
mouth of Ary-Mas-Yuryakh Creek, Kotui River, West Anabar Region, Siberian Platform:
A, apertural view, ⫻23, B, magnifi ed fragment of columellar area showing polygonal
microornamentation (replica from pallial myostracum). (C, D) Aldanella operosa, PIN,
no. 5083/0264, shell, Lower Cambrian, Atdabanian; Achchagyi-Kyyry-Taas Creek, middle
reaches of Lena River, Siberian Platform: C, oblique spire view, ⫻54; D, magnifi ed
fragment of apical area showing two septa.

early cambrian radiation of mollusca 63
The septation maintains the watertightness of
the apical part of the shell, which, being the old-
est shell part, is subjected to the most prolonged
corrosive effect of seawater, and being the most
distal part of the shell, suffers from mechanical
damage.
The presence of a protoconch and septa
in the initial part of the aldanellid shell
(Figure 3.21C, D) is characteristic of gastropods
and rejects Yochelson’s interpretation of aldanel-
lids as sedentary polychaetes (Yochelson 1975,
1978; Bockelie and Yochelson 1979). In all 14
available specimens the formation of septa is
very regular: the fi rst septum is always 100 µm
long, with a second one 150 µm long. The angle
between the two septa is about 90º. The forma-
tion of the second septum occurred when the
shell diameter reached 700–740 µm. The pres-
ence of columellar muscle scars in aldanellids
(Figure 3.21B) and the morphological similarity
of their protoconch with larval shells of primi-
tive modern gastropods support the position
of the family within one of the basal gastro-
pod groups (Parkhaev 2006b). The discovery
of additional protoconchs among the earliest
univalve molluscs, and their comparative study
with embryonic shells of younger fossil and
modern molluscs, will be an extremely impor-
tant and promising fi eld for future researchers.
The recent consideration of Halkieria as a
mollusc (Vinter and Nielsen 2005; see previ-
ous discussion) necessitates reconsideration
of the whole group of sclerite-bearing fos-
sils, including halkieriids and allied groups of
coeloscleritophorans.
Bruce Runnegar (1996) wrote that his life-
long desire was to fi nd a micromolluscan Lager-
stätte that could provide data on the soft-body
anatomy of ancient molluscs. In fact, all the
known Cambrian localities of extraordinary pres-
ervation (such as Burgess Shale, Chengjiang,
Sirius Passet, and Sinsk) yield representatives
of various metazoan lineages (e.g., arthropods,
hyoliths, brachiopods, sponges, problematic
groups, and soft-bodied organisms), but, until
recently, not conchiferan molluscs. The extraor-
dinary preservation of a gastropod from the
Silurian (Sutton et al. 2006) brings new hope
to this endeavor.
It is no less necessary, and it is also the main
objective of the present review, to accentuate the
importance of collaborative studies in under-
standing the systematics and biology of the fi rst
molluscs.
ACKNOWLEDGMENTS
This paper has been supported by the Russian Foun-
dation for Basic Research (project 03-04-48367),
Grants of the President of the Russian Federation
to Support Young Russian Scientists and Leading
Scientifi c Schools (projects nos. NSh-974.2003.5,
MK-723.2004.4 and MK-2836.2007.4, and the
Programme of the Presidium of the Russian
Academy of Sciences “Origin and Evolution of
Biosphere.” The author attendance on the 15th
World Congress of Malacology was sponsored
by the Russian Foundation for Basic Research
(project 03-04-48367) and by the Museum of
Paleontology, University of California, Berkeley,
California, United States. I am thankful to D. R.
Lindberg, W. F. Ponder, E. L. Yochelson, and
another anonymous reviewer for valuable com-
ments. I am greatly indebted to W. F. Ponder for
language improvements.
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