The Origins of Molluscs - Vinther (2015)

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FRONTIERS IN PALAEONTOLOGY
THE ORIGINS OF MOLLUSCS
by JAKOB VINTHER
Schools of Earth Sciences and Biological Sciences, University of Bristol, Life Sciences Building, 24 Tyndall Avenue, BS8 1TQ, Bristol, UK;
e-mail: jakob.vinther@bristol.ac.uk
Typescript received 3 September 2014; accepted in revised form 20 October 2014
Abstract: The interrelationships and evolutionary history
of molluscs have seen great advances in the last decade.
Recent phylogenetic studies have allowed alternative mor-
phology-based evolutionary scenarios to be tested and, most
significantly, shown that the aplacophorans are sister group
to polyplacophorans (chitons), corroborating palaeontologi-
cal and embryological evolutionary scenarios in which apla-
cophorans are secondarily simplified from a chiton-like
ancestor. Aplacophoran morphology therefore does not
represent the plesiomorphic condition for molluscs as a
whole. The mollusc crown group radiated in the Early Cam-
brian, and rapidly thereafter, stem lineages to the major mol-
luscan classes emerged: cephalopods, gastropods, bivalves
(= pelecypods), monoplacophorans, rostroconchs (inferred
stem scaphopods) and aculiferans. This attests to the fast,
adaptive radiation of the crown group during the Cambrian
explosion. Kimberella from the latest Ediacaran exhibits
several molluscan traits, which justifies its position as a
molluscan stem-group member, rather than as a more basal
Lophotrochozoan. The interrelationships among the
conchiferan molluscs are still a matter of contention and
require further palaeontological and molecular phylogenetic
scrutiny.
Key words: Mollusca, Aculifera, Cambrian explosion, phy-
logenomics, molecular palaeobiology, small shelly fossils.
THE overall framework for understanding fundamental
molluscan interrelationships has finally reached a stage in
which several robust molecular phylogenetic analyses con-
verge on similar topologies confirming certain hypotheses
of early molluscan evolution. Relaxed molecular clocks
provide means to test the completeness of the fossil record
and to evaluate evolutionary scenarios. Recent fossil dis-
coveries have been adding to the picture of how the mol-
luscan body plan has evolved. Particularly, illustrative cases
are highlighted in groups, such as cephalopods (Kr€oger
et al. 2011), which showcase a major transition from exter-
nally shelled orthocones to the internally shelled coleoids
through the Phanerozoic. Molecular and palaeontological
studies have also presented convincing evidence that mol-
luscs evolved planktotrophy convergently in two major
clades, the bivalves and gastropods (Peterson 2005; N€utzel
et al. 2006) linked to changes in suspension feeding inten-
sity. Another recent advance has been the identification of
the vermiform and morphologically simple aplacophorans
as a derived clade (Sutton et al. 2012; Vinther et al.
2012a), rather than a primitive grade of molluscs (Salvini-
Plawen and Steiner 1996; Haszprunar 2000), which forces
a reassessment of the ancestral morphology of the phylum.
Molluscs are ecologically important invertebrates,
which occupy a broad range of niches in marine,
freshwater and terrestrial environments. They can be sessile,
agile, infaunal, nektonic and planktonic; they can be car-
nivores, scavengers, herbivores, grazers, photosymbiotic
and suspension feeders. Several extinct clades were
important and abundant marine denizens such as the
Palaeozoic/Mesozoic ammonoids or the Cretaceous ru-
dists (Skelton 1978). The most diverse and disparate
clades are the bivalves, gastropods and cephalopods, and
the least diverse and disparate are the scaphopods, monopla-
cophorans, chitons and aplacophorans.
MOLLUSCAN PHYLOGENY AND
EVOLUTION
While molluscs are morphologically distinct and well
studied, early attempts to obtain a stable molecular phy-
logeny of the phylum (Winnepenninckx et al. 1996; Pass-
amaneck et al. 2004; Giribet et al. 2006) failed to support
established morphological hypotheses, or even molluscan
monophyly. However, the advent of phylogenomic studies
(Dunn et al. 2008) and better informed choice of molecu-
lar loci for analyses (Vinther et al. 2012a) enabled the
recovery of molluscan monophyly. Furthermore, three
independent studies in 2011 (Fig. 1) recovered the clade
© The Palaeontological Association doi: 10.1111/pala.12140 19
[Palaeontology, Vol. 58, Part 1, 2015, pp. 19–34]
Aculifera (Polyplacophora + Aplacophora) (Kocot et al.
2011; Smith et al. 2011; Vinther et al. 2012a) and there-
fore could reject the Testaria hypothesis (Salvini-Plawen
and Steiner 1996; Haszprunar 2000; Salvini-Plawen 2006),
in which aplacophorans are primitive and chitons and
conchiferan molluscs are united in a monophyletic shell
plate-bearing clade.
Conchiferan monophyly, which is justified from a mor-
phological viewpoint (e.g. Nielsen 2012), has been recov-
ered in some recent analyses (Kocot et al. 2011; Smith
et al. 2011) using extensive genetic sampling (300–1100
gene fragments), but with little coverage of each loci.
Using only seven gene loci, but gene fragments with high
phylogenetic informativity and more than 80% gene
sequence coverage, conchiferans were recovered as para-
phyletic with cephalopods as sister group to the Aculifera.
This topology was robust to a series of sensitivity tests,
such as long-branch taxon exclusion, dissection of fast-
evolving sites from the data set and heterogenous taxon
exclusion, which would be expected to eliminate or mini-
mize rooting or long-branch attraction artefacts (Vinther
et al. 2012a). Here, the Conchifera are accepted as mono-
phyletic due to the congruence with morphological
hypotheses.
Analyses generally recover a sister group relationship
between bivalves and gastropods (Kocot et al. 2011; Vin-
ther et al. 2012a) with scaphopods as a sister group to
this clade. However, Smith et al. (2011) recovered scaph-
opods as a sister group to gastropods, while in a corri-
gendum (correcting faults in the pipeline that miscoded
certain amino acids) instead recovered scaphopods as a
sister group to bivalves (Smith et al. 2013). Cephalopods
have been recovered as a sister to all other conchiferans
(Kocot et al. 2011), together with the monoplacophorans
(Smith et al. 2011) or, as mentioned before, as sister
group to the Aculifera (Vinther et al. 2012a). The differ-
ence between these topologies is mainly in terms of
where the root has been inferred in the analysis (Fig. 1).
A recent study using mitochondrial genomes also recov-
ers Aculifera and Conchifera (Osca et al. 2014). A recent
study (St€oger et al. 2013) revisiting the data set of
Giribet et al. (2006) with more taxa, could not recover
Aculifera or Conchifera, which can be ascribed to the
little phylogenetic informativity of the gene selection and
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Smith et al. 2011
Kocot et al. 2011
Vinther et al. 2012
Aculifera Conchifera
F IG . 1 . Phylogeny of molluscs, a review and consensus of three recent studies. Ambiguous taxon placements are indicated by stippled
lines and a reference to the given study.
20 PALAEONTOLOGY , VOLUME 58
resulting relationships recovered mainly driven by
long-branch attraction artefacts (i.e. clustering of the
long-branched cephalopods and aplacophorans as well as
scaphopods).
THE ACULIFERA
Aplacophorans are derived from footed, eight-plated
ancestors
Former studies, based on morphology, pointed to the sim-
ple morphology ofaplacophorans as evidence for their
primitive status in the molluscs, giving them importance
for understanding early molluscan evolution (Salvini-Plawen
and Steiner 1996; Haszprunar 2000; Salvini-Plawen 2006;
Shigeno et al. 2007). In particular, some scenarios pre-
sented aplacophorans as a paraphyletic basal grade, render-
ing their morphology plesiomorphic for the molluscs. The
chitons in such scenarios bridged the morphological gap
between aplacophorans and the remaining conchiferan
molluscs. However, the fossil record could not corroborate
this scenario (Runnegar and Pojeta 1974), which otherwise
shows the appearance of conchiferan groups in the earliest
Cambrian, while chitons appear in the Late Cambrian and
aplacophorans then lacked fossil representatives.
Others have contemplated the possibility of aplacopho-
rans being secondarily derived as a sister group to chitons
(Scheltema 1993, 1996). Furthermore, evidence suggested
that aplacophorans could have evolved from a chiton-like
ancestor (Ivanov 1996). Specifically, embryological study
of both neomeniomorphs (Solenogastres) and chae-
todermomorphs (Caudofoveata) demonstrated a sevenfold
dorsal iteration expressed as a series of naked regions in a
post-larval individual (Scheltema and Ivanov 2002) or as
transverse ridges containing calcium carbonate-secreting
cells in the chaetodermomorph Chaetoderma nitidulum
(Nielsen et al. 2007). Fossil discoveries made in the new
millennium have been of tremendous importance. A fossil
vermiform animal Acaenoplax hayae (Fig. 2D), from the
Wenlock (Silurian) Herefordshire Lagerst€atte (Fig. 2D)
with seven dorsal shell plates and a posterior ventral plate
(Sutton et al. 2001, 2004), fitted the scenario in which
aplacophorans evolved from a chiton-like ancestor
through secondary reduction and loss of the foot and
dorsal shell plates (Vinther et al. 2012a).
Additional fossils were discovered and forms inter-
preted as polyplacophorans re-examined. These were
shown to have a rounded transverse profile with a mantle
extending ventrally leaving only a shallow groove for a
foot and no ventral mantle cavity (Sutton and Sigwart
2012). The fossils are from the Late Ordovician and pre-
serve eight shell plates (Donovan et al. 2010, 2011). The
fossils therefore provide a minimum timing for the diver-
gence between aplacophorans and chitons with their
derived aplacophoran traits. As chiton-like forms, which
could subtend both chitons and aplacophorans, are
known from deposits no older than the latest Cambrian
(Runnegar et al. 1979; Vendrasco and Runnegar 2004;
Pojeta et al. 2010; but see Vendrasco et al. 2009), these
provide a putative maximum age for the divergence of
Aculifera, the common ancestor to chitons and aplacoph-
orans. Scheltema (1993) pondered that this clade would
have diversified in the Early Ordovician. The molecular
clock analyses of Vinther et al. (2012a) did indeed
recover an Early Ordovician divergence of the Aculifera
robustly, irrespective of clock and substitution models or
the inclusion of cephalopods as a contentious sister group
to Aculifera. Therefore, an emerging scenario holds that
Aculifera is an Early Ordovician clade, which evolved
from an ancestor, which has sclerites and eight dorsal
shell plates. Subsequently, an additional seven-plated stem
aplacophoran, Kulindroplax (Fig. 2E), has been unearthed
in the Herefordshire Lagerst€atte (Sutton et al. 2012). This
form resembles well-known fossils previously described as
chitons, such as Chelodes (Cherns 1998a, b), but also pre-
serves no visible foot, except for a diminutive ventral ridge.
The loss of plesiomorphic characters in aplacophorans is
chimaeric. While chaetodermomorphs have retained a pair
of gills in their posterior mantle cavity, the neo-
menimorphs have reduced gills to a series of papillae.
Meanwhile, neomenimorphs retain a narrow ciliated foot,
which is lost in chaetodermomorphs. The observations
that Kulindroplax and Acaenoplax seem to completely lack
a foot (Sutton et al. 2001; Sutton et al. 2004; Sutton et al.
2012) and have posterior structures in a mantle cavity sug-
gest that they are stem chaetodermomorphs. Furthermore,
their possession of shell plates implies convergent loss of
these in the two modern aplacophoran groups. These
results would indicate that aplacophorans diverged before
the deposition of the Herefordshire Lagerst€atte (Wenlock,
c. 426 Ma). The molecular clock analyses of Vinther et al.
(2012a) estimate a divergence of the Aplacophora of
around 450 Ma, which is congruent with this hypothesis.
Both stem-group aplacophorans and stem-group chitons
have members with highly conical shells, which harbour
one or two internal lacunae. These forms were classified as
an extinct group of chitons, Palaeoloricata, by Bergenhayn
(1955). It is apparent that this group is paraphyletic, sub-
tending aplacophorans, chitons and the aculiferan stem.
Chiton evolution
Chitons retained the plesiomorphic complement of eight
shell plates in a row. A putative early stem chiton is
the remarkable Ordovician Echinochiton (Pojeta et al.
2003; Pojeta and DuFoe 2008; Fig. 2C). Fossil evidence
VINTHER : MOLLUSC ORIG INS 21
suggests that the crown group radiated in the Carbonif-
erous (Sigwart 2009) about 350 Ma. Molecular clock
analyses are congruent with the fossil record and esti-
mate divergences at c. 340 Ma (Vinther et al. 2012a, b).
A subgroup of chitons, the Chitonida, are represented
by stem-group forms from the Early Permian around
270–275 Ma, and molecular clock analysis similarly
estimates the Chitonida to have diverged by about
268 Ma (Vinther et al. 2012a, b).
A major transition happened in the skeleton of chi-
tons during the lead up to the crown group. Anterior
projections of the medial shell layer form so-called
sutural laminae, which serve for firmer embedding of
the shell plates within the tissues. The evolution of this
feature in the Devonian or Carboniferous coincides with
an increased predation landscape and the Devonian
Nekton Revolution (Klug et al. 2010). Chitons evolved
from vermiform molluscs (still possessing a foot and
A B C
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22 PALAEONTOLOGY , VOLUME 58
elaborate mantle cavity) with conical, projecting shell
plates into more dorsoventrally flattened forms with
more closely appositioned shell plates, which are more
firmly embedded with sutural laminae. This evolutionary
trend continued in the Chitonida, which, by the
Permian, had evolved lateral projections of the lower
shell layer (articulamentum) in addition to the anterior
sutural laminae for even firmer shell attachment. Preda-
tion pressure increased in shallower waters (Vermeij
1987) and presumably drove the evolution of this clade.
In modern oceans, there is a distinct correlation with
Chitonida generally inhabiting shallower and more
exposed environments compared to the more plesiomor-
phic lepidopleurids, which now live in deeper and more
cryptic environments. As modern chitons radiated to
inhabit shallow energetic environments with less fossili-
sation potential, they in effect evolved themselves out of
the fossil record; Cenozoic and Mesozoic occurrences of
chitons are heavily biased towards deeper water lepi-
dopleurids (Sirenko 2006).
While the skeleton is extremely conserved in crown
chitons, one particular group of stem chitons took the
exploration of a multiplated scleritome to an extreme.
The multiplacophorans (Hoare and Mapes 1995) have
seven transverse shell fields, with the anterior and poster-
ior shell plates representing single valves and the five
intermediate regions divided intothree distinct sections
(Fig. 2F; Vendrasco et al. 2004). Multiplacophorans had
appeared by the Silurian (Aurivillius 1892) and disap-
peared in the Permian (Hanger et al. 2000). Some confu-
sion has existed with respect to their affinity after the first
recognition of their unequivocal affiliation with the
Polyplacophora (Vendrasco et al. 2004). This stems from
their amalgam of characters, which are shared with cer-
tain crown members of chitons. While Vendrasco et al.
(2004) assigned multiplacophorans to stem-group chitons,
arguing that these particular features were likely to have
evolved prior to the diversification of the crown, Puchal-
ski et al. (2009) chose to assign them to crown group chi-
tons based on the possession of the same features.
However, this would push the origin of the crown group
of chitons back from the Carboniferous to the Silurian,
for which there is no fossil evidence despite relatively
good sampling (Cherns 2004). The congruence between
the Carboniferous appearance of fossil crown group
chitons and molecular clock estimates rejects an older,
hidden history of crown chitons and argues for multipla-
cophorans as stem-group chitons that evolved certain
crown group characteristics convergently (Vinther et al.
2012b).
Stem aculiferans: Halkieria and the sachitids
The ancestral aculiferan arguably looked like a chiton
with eight shells in a row (Vinther et al. 2012a). Fossils
F IG . 2 . Fossil early crown group molluscs. A, Halkieria evangelista Conway Morris and Peel, 1990 (MGUH 30887), a stem-group acu-
liferan from the Early Cambrian Sirius Passet Lagerst€atte, Buen Formation, North Greenland (newly collected specimen from a 2011
expedition). B, Maikhanella multa (left, SMNH X2232) and Siphogonuchites (right, GPIN 106788) from the Early Cambrian Bayan-Gol
Formation; note that this stem aculiferan has a shell plate consisting of merged sclerites. C, Echinochiton dufoei Pojeta, Eernisse et al.,
2003 (USNM 517482), from the Ordovician Forreston Mb, Grand Detour Formation, cast. D, the total group aplacophoran Acaenoplax
hayae Sutton, Briggs et al., 2001 (virtual reconstruction) from the Silurian (Wenlock) of Herefordshire, a putative stem chae-
todermomorph (reprinted with permission from MacMillan Publishers: Nature). E, the total group aplacophoran Kulindroplax perissoko-
mos Sutton, Briggs et al., 2012 (virtual reconstruction) from the Silurian (Wenlock) of Herefordshire (reprinted with permission from
MacMillan Publishers Ltd: Nature). F, the multiplacophoran (stem polyplacophoran) Protobalanus spinicoronatus Vinther, Jell et al.,
2012b from the Devonian (Early Givetian) Silica Formation of Lucas County, Ohio; the image is a re-articulated reconstruction based
on a segmented lCT scan (Vinther et al. 2012b). G, Watsonella crosbyi, Grabau 1900, from the Early Cambrian of Siberia (SEM image)
a univalved stem bivalve. H, Pojetaia runnegari Jell, 1980, from the Early Cambrian of South Australia (SEM image), a hinged and bival-
ved stem bivalve. I, Mellopegma schizocheras Vendrasco, Kouchinsky et al., 2011b (CPC 40416) from the Middle Cambrian Gowers
Formation of Australia. J, Stenotheca drepanoida SMNH Mo167627 from the Early Cambrian Ajax limestone. These forms (I, J) have
been interpreted as early putative rostroconchs and thus stem scaphopods. K, the stem cephalopod Plectronoceras cambria (Walcott,
1905) from the Upper Cambrian of China (USNM 57819, holotype left and paratype right), one mile west of Tsi-Nan, Shantung (Yo-
chelson, Flower et al., 1973). L, the chambered helcionellacean Tannuella elinorae Brock and Paterson, 2004 (SAMP 340146, holotype),
from the Early Cambrian Mernmerna Formation of the Flinders Ranges, South Australia; this univalved mollusc possesses distinct ter-
minal chambers (but lacks a siphon), which suggest that it might be a stem cephalopod. M–O, the stem gastropod Pelagiella atlantoides
(Matthew, 1894) USNM 298724 from the Early Cambrian Hanford Brook, New Brunswick; the specimen preserves distinct dorsolateral
pedal muscle scars (black arrows) surrounded by a grooved line, suggesting the extent of the mantle cavity (white arrows); muscle scars
suggest that this form was an untorted stem gastropod, but exhibiting a c. 10 degree rotation (Runnegar 1981); M, view from the left;
N, dorsal view; O, view from right. Images courtesy of Stefan Bengtson (B), Mark D. Sutton (D, E), Michael J. Vendrasco (G–J), John
Paterson (L). Abbreviations: MGUH, Geological Museum of Copenhagen, Denmark; SMNH, Swedish Museum of Natural History;
USNM, United States National Museum of Natural History; SAMP, South Australian Museum; CPC, Commonwealth Palaeontological
Collections, Geoscience Australia, Canberra; GPIN, Nanjing Institute of Geology and Palaeontology, Academica Sinica.
V INTHER : MOLLUSC ORIG INS 23
fitting this description can be traced back to the Late
Cambrian (Runnegar et al. 1979; Vendrasco and Runne-
gar 2004; Pojeta et al. 2010). However, the aculiferan fos-
sil record should extend back to at least the Early
Cambrian when their sister group, the conchiferans, radi-
ated. A plethora of lepidote molluscs (sachitids) are
known from the Cambrian and include forms such as the
halkieriids (Fig. 2A) and siphogonuchitids (Fig. 2B)
(Bengtson and Missarzhevsky 1981). Initially, the exact
nature of these forms was difficult to decipher due to the
lack of articulated specimens in the small shelly deposits
that they are typically extracted from (Bengtson and Con-
way Morris 1984). However, the discovery of articulated
halkieriids in the Early Cambrian Sirius Passet (Conway
Morris and Peel 1990, 1995) demonstrating a body plan
with several morphologically distinct sclerites in zones
and a large shell growing by marginal accretion at the
front and back put an end to this palaeontological mys-
tery. Other forms have been discovered since then with a
different number of shell plates (Conway Morris and Ca-
ron 2007). Some forms have shell plates with sclerites
embedded into the shell plates (Bengtson 1992; Fig. 2B).
Several characteristics in these forms are shared with acu-
liferans, such as their sclerite growth and replacement,
and the presence of a complex internal canal system in
either the sclerites or the shell plates, which are derived
from mantle papillae (Vinther and Nielsen 2005; Vinther
2009). A number of shell plates described in isolation
likely belong to the sachitids and indicate a highly vari-
able scleritome (Conway Morris and Caron 2007; Pater-
son et al. 2009; Vendrasco et al. 2009). The sachitids
extend the aculiferan fossil record back to the earliest
Cambrian (Terreneuvian) (Maloof et al. 2010) and thus
fill the gap between the earliest conchiferan fossils and
the appearance of aculiferans.
Two other fossils worth mentioning are the wiwaxiids
and Odontogriphus. Wiwaxia is now known as body fos-
sils from a range of localities other than the Burgess shale
(Conway Morris 1985; Smith 2014) including China
(Zhao et al. 1994; Sun et al. 2014; Yang et al. 2014), Sibe-
ria (Ivantsov et al. 2005) and Europe (Fatka et al. 2011).
Wiwaxiid sclerites are also well known as ‘small organic
bits’ (Butterfield 1990, 1994; Butterfield and Harvey
2012). Wiwaxia was early on recognized as a likely rela-
tive of Halkieria (Bengtson and Conway Morris 1984).
The radula in Wiwaxia was originally thought to consist
of two bars (Conway Morris 1985), similar to a disti-
chous aplacophoran radula (Scheltema et al. 2003). How-
ever, recent studies using back-scattered electron imaging
have provided a more detailed understanding of the radu-
lar morphology, showing that both Wiwaxia and the
unarmoured Odontogriphus had isolated denticles with a
median tooth flankedby shoehorn-shaped lateral denti-
cles (Smith 2012).
CONCHIFERAN EVOLUTION
Conchiferans have generally been assumed to be mono-
phyletic from a morphological point of view. However,
only recently have molecular phylogenies recovered con-
chiferan monophyly (Kocot et al. 2011; Smith et al.
2011), although the relationships among its members
remain in flux. Most conchiferan groups recognized as
classes in the Linnean system can be traced back to the
Cambrian small shelly fossil assemblages (Runnegar and
Pojeta 1974). These in turn can be traced back to unival-
ved forms, resembling monoplacophoran limpets, which
arguably is the plesiomorphic condition for the group.
These fossils are usually found as secondarily phospha-
tized small shelly fossils (SSFs; Kouchinsky et al. 2012).
This taphonomic window records a polyphyletic assem-
blage of skeletal organisms ranging from submillimetric
to a couple of millimetres in size. Based on these fossils,
it has been speculated that early molluscs were generally
microscopic (Haszprunar 1992), with implications for the
evolution of the molluscan body plan. However, discover-
ies of large, centimetre-sized halkieriids (Conway Morris
and Peel 1990) and helcionellids (Mart�ı Mus et al. 2008)
as crack-out specimens in shales demonstrate that early
molluscs could be large and that the SSF taphonomic
window discriminates towards smaller fossils (Creveling
et al. 2014). There is still evidence that many early mol-
luscs were relatively minute (B. N. Runnegar pers. comm.
and pers. obs.) compared to Late Cambrian and Ordovi-
cian forms. The intricate replacement of internal moulds
(steinkerns) and partial replacements of the shell material
make it possible to characterize their mineralogical ultra-
structure, which provides additional information of
potential phylogenetic relevance (Runnegar 1985; Kou-
chinsky 1999, 2000; Feng and Sun 2003; Vendrasco et al.
2010, 2011a). An interesting study was performed using
phosphatized steinkerns to characterize the larval shell
(protoconch) through the Cambrian and Ordovician
(N€utzel et al. 2006; N€utzel 2014). As the size of the pro-
toconch reflects the size of the embryo, which is tightly
correlated with either a prolonged planktotrophic devel-
opment (small embryonic shell) or a short-lived, yolk
feeding (lecithotrophic) stage (large embryonic shell),
N€utzel et al. (2006) demonstrated that early molluscan
groups were all yolk feeding during early ontogeny and
only evolved planktotrophy in parallel at the onset of the
Ordovician. This contrasts with ideas in which the ances-
tor of bilaterians and animals should have primitively
been planktotrophic (J€agersten 1972; Peterson et al. 1997;
Nielsen 2013), which seems firmly refuted now (Haszpr-
unar et al. 1995; Rouse 2000; Peterson 2005; Degnan and
Degnan 2006). The parallel onset of planktotrophy in the
Early Ordovician correlates with the Great Ordovician
Biodiversification Event (GOBE; Harper 2006), which saw
24 PALAEONTOLOGY , VOLUME 58
the radiation and wide geographical dispersal of several
molluscan classes. The onset of planktotrophy and the
GOBE is correlated with a great radiation of sessile sus-
pension feeders, which would have predated on larvae,
and the evolution of planktotrophy thus could be viewed
as driven by predator–prey feedback.
Monoplacophoran roots
Monoplacophorans were originally thought to be an
extinct group of molluscs, but are now frequently
recorded in deep-sea samples from across the globe
(Lemche 1957; Lemche and Wingstrand 1959; War�en and
Gofas 1996; Schr€odl et al. 2006). Monoplacophorans are
unique among conchiferans in having a serialized body
plan (Lemche 1957; Wingstrand 1985) with particular
similarities to chitons in their muscle anatomy and serial-
ization of the gills, while also possessing serialized kid-
neys. Unequivocal monoplacophorans, identified by serial
muscle scars in the shell, are known from the Late
Cambrian (Stinchcomb 1986) and are present in the Bur-
gess shale mollusc Scenella (Rasetti 1954; Runnegar and
Pojeta 1974). Several monoplacophoran-like forms are
known from older deposits. However, as all conchiferan
groups can be traced back to univalved ancestors resem-
bling monoplacophorans, many of these can be referred
to stem lineages of specific classes or merely to the con-
chiferan total group. There has been much discussion
concerning the interpretation of these forms, as the fossils
provide few characters, which provide for inferences
about muscle scars, position of mantle cavity, direction of
coiling relative to the body axis and whether a fossil form
is torted or not (Runnegar 1981; Peel 1991; Parkhaev
2008).
Bivalve evolution
Conchiferan molluscs with a bivalved scleritome con-
nected by a dorsal hinge are represented in the Early
Cambrian by forms such as Fordilla troyensis (Pojeta et al.
1973; Pojeta and Runnegar 1974) and the slightly older
Pojetaia runnegari (Fig. 2H; Jell 1980; Runnegar and
Bentley 1983). These forms were likely to be shallow bur-
rowers (Runnegar and Bentley 1983), but the crown
group radiated in the Early Ordovician and quickly
evolved practically all the main modes of life exhibited by
modern bivalves (Pojeta et al. 1978; Cope 2000). While
molecular studies still are in some disagreement (Plazzi
et al. 2011; Sharma et al. 2012), protobranchs are gener-
ally recovered as a sister to all other bivalves (autolamelli-
branchiata/autobranchiata), which is corroborated by the
fossil record. Bivalves were generally less diverse and
abundant compared to the ecologically similar brachio-
pods in the Palaeozoic, with their relative abundances
through the Phanerozoic being roughly inversely corre-
lated (Clapham et al. 2006). Comparing bivalves and bra-
chiopods from the perspective of their metabolic activity
demonstrates bivalve dominance throughout most of the
Phanerozoic (Payne et al. 2014).
Bivalves can be traced back to laterally compressed
univalved conchiferan forms, such as Watsonella (Fig. 2G;
Pojeta and Runnegar 1974), and similarly, there are mor-
phological and shell ultrastructural evidence to link
Watsonella with the potentially more plesiomorphic An-
abarella (Kouchinsky 1999). Further studies of shell
microstructure in Fordilla, Pojetaia, Anabarella and
Watsonella demonstrate that these early bivalves did not
possess nacre in their shells, but rather foliated aragonite
(Vendrasco et al. 2011a), nacreous mother of pearl
appears to have evolved multiple times convergently in
molluscs in the Ordovician period (Vendrasco et al.
2011a, 2013) as a response to the increased predation
pressure of the GOBE.
Scaphopods and the rostroconchs
Scaphopods, or tusk shells, are a small group of conchif-
erans with limited diversity and disparity. They possess a
distinct, elongate conical shell and live infaunally as selec-
tive detritus feeders. The fossil record of unequivocal
scaphopods extends back to the Devonian or Carbonifer-
ous (Peel 2004, 2006). Scaphopods probably evolved from
the extinct clade of rostroconchs, consisting of two
groups, the ribeirioids and the conocardioids. It was orig-
inally proposed that scaphopods evolved from Ordovician
ribeirioids (Runnegar and Pojeta 1974; Pojeta and Runne-
gar 1976, 1979), but Peel (2004, 2006) demonstrated by
close examination of the protoconch morphology and
growth direction (exogastric vs. endogastric) of these
forms that a derivation from younger conocardioid ro-
stroconchs is more justifiable. Rostroconchs can be traced
back to the Late Cambrian and potentially even back to
the Early Cambrian with forms such as Mellopegma(Fig. 2L; Pojeta and Runnegar 1976; Runnegar and Jell
1976; Vendrasco et al. 2011b). However, much confusion
exists with respect to the affinity of several laterally com-
pressed Early Cambrian conchiferans and their potential
affinity to either bivalves or rostroconchs (and thus
scaphopods; see discussion in Vendrasco 2012). Some of
this confusion might be resolved if scaphopods and bival-
ves were to prove to be sister groups (Diasoma) as seen
in the corrected analyses by Smith et al. (2013) as well as
in some analyses of Osca et al. (2014), with all these later-
ally compressed forms representing total group diaso-
mans.
V INTHER : MOLLUSC ORIG INS 25
Cephalopod evolution
Cephalopods are nektonic predators, which are important
constituents of the pelagic food chain. Modern cephalo-
pod diversity is dominated by the coleoids (i.e. squids,
cuttlefish and octopods), which have internalized or even
secondarily lost their shell. The plesiomorphic, chambered
external shell is retained in nautilids. The oldest unequiv-
ocal cephalopod with a chambered phragmocone con-
nected by a siphon is the Late Cambrian Plectronoceras
(Yochelson et al. 1973; Chen and Teichert 1983; Fig. 2K).
Several older, chambered shells have been proposed as
candidates to subtend the cephalopod stem, such as Tan-
nuella (Fig. 2L; reviewed in Kr€oger et al. 2011), but firm
evidence remains elusive. A long-standing scenario holds
that the two main extant lineages of cephalopods, the
coleoids and nautilids, can be traced back to different
Early Palaeozoic lineages, the orthocerids and oncocerids,
respectively, which in turn diversified from each other in
the Early Ordovician. However, molecular clock studies
(Kr€oger et al. 2011; Warnke et al. 2011) consistently
recover a divergence of the cephalopod crown at the
Silurian–Devonian boundary. This is in fact remarkably
congruent with the cephalopod fossil record, which sees
the appearance of stem coleoids (bactritids and ammo-
noids) and bona fide nautilids in the Devonian (Dzik and
Korn 1992; Kr€oger and Mapes 2007) as well as the first
fossil cephalopod beaks, a crown cephalopod apomorphy
(Kr€oger et al. 2011). A general trend in cephalopod evo-
lution is towards the loss of the shell and more efficient
swimming by mantle pumping rather than shell pumping
(Wells and O’Dor 1991). The timing of these transitions
is closely matched by the evolutionary innovations and
diversification of osteichthyan fishes (Packard 1972),
which can be explained by evolutionary arms races among
the two groups of pelagic predators.
Note that the recent description of the Cambrian
Nectocaris as a cephalopod (Smith and Caron 2010; Smith
2013) has been firmly refuted (Kr€oger et al. 2011;
Mazurek and Zaton 2011; Runnegar 2011).
Gastropod evolution
Gastropods are characterized by the 180 degree torsion of
the visceral mass and shell field relative to the head–foot.
This means that the anus is above the head, whose little
practicality has not gone unnoticed (Peel 1987, p. 306).
The earliest forms, ascribable to the gastropod stem, are
the Early to Mid-Cambrian pelagiellids and aldanellids
(but see Dzik and Mazurek 2013). Muscle scars and pal-
lial line imprints in a large specimen of Pelagiella
(Fig. 2M) demonstrate an asymmetric arrangement of
dorsolaterally inserted muscles, which suggests about
10 degrees of torsion (Runnegar 1981). Crown gastropods
seem to have radiated in the latest Cambrian. Current
molecular studies suggest that the patellogastropods and
the vetigastropods form a basal clade (Smith et al. 2011;
Vinther et al. 2012a; Smith et al. 2013), which is sister to
the remaining gastropods. Within this latter clade, the
following topology is seen (Neritimorpha (Caenogastro-
poda + Heterobranchia)) (Zapata et al. 2014).
EDIACARAN ROOTS AND KIMBERELLA
Molluscs all have biomineralized skeletons, but some
argue against homologizing the shell plates of conchifer-
ans and aculiferans (Haas 1981; Scheltema 1988). It has
been speculated that the shell plates of aculiferans could
have evolved from amalgamation of sclerites as is
evidenced in Maikhanella, which consists of sclerites of
the genus Siphogonuchites (Bengtson 1992). This observa-
tion also highlights the likely origin of the sensory aes-
thete canal system in chitons. These structures are derived
from papillae, which also secrete sclerites in the mantle
(Bengtson 1992; Vinther 2009). Siphogonuchites and the
other Cambrian sachitids exhibit hollow sclerites, often
with a branching canal system within, and it has been
argued that this canal system is similar in dimensions to
modern chiton aesthetes (Vinther 2009). A case can still
be made, however, for homology of conchiferan and
aculiferan shell plates: both exhibit crossed lamellar
ultrastructural fabric, which is a unique molluscan
characteristic, suggesting that the latest common ancestor
to crown molluscs had a biomineralized shell.
While the molluscan crown group had diversified by
the Terreneuvian (Stage 1) in the earliest Cambrian, there
is little evidence to suggest that crown group molluscs
existed in the Precambrian, as no skeletal fossils have
been found even in the Nama group, which yields the
first abundant material of presumed skeletized metazoans
(Grotzinger et al. 2000; Wood et al. 2002). This provides
a tight bracket for molluscan diversification of less than
15 million years (but see Runnegar 1987, for an alterna-
tive explanation).
A noteworthy fossil from the Ediacaran is Kimberella
(Fig. 3A), which has been interpreted to be molluscan
(Fedonkin and Waggoner 1997). While this was initially
met with scepticism (Nielsen 2001), further studies have
strengthened this hypothesis (Fedonkin et al. 2007; Ivant-
sov 2009, 2010; Gehling et al. 2014). Kimberella is a cru-
cial Ediacaran organism in that it exhibits a series of
taphomorphs, which reveal a complex organism with a
differentiated ventral and dorsal surface and internal anat-
omy; often, these structures are superimposed. From these
fossils, it is clear that Kimberella exhibits an anterior and
posterior polarity and bilateral symmetry. The ventral
26 PALAEONTOLOGY , VOLUME 58
surface shows that there is a large ventral surface, which
is smooth or transversely wrinkled, surrounded by con-
centric body units, which can be interpreted as a foot and
a mantle separated by a groove. These impressions resem-
ble a molluscan body plan in having a distinct creeping
sole (foot) and a surrounding mantle separated by a
D
BA
C
E
F
F IG . 3 . Kimberella and Kimberichnus. A, Kimberella quadrata from Erginskaya Formation, Zimnie Gory, Local Cluster Z1, ‘Kimberella
lenses’ (PIN 3993/5136); specimen preserves the presumed ventral surface, compared to experiments in Seilacher (1999, fig. 7). B, Kim-
berichnus teruzzi Ivantsov, 2013, holotype PIN 3993/5619; specimen preserves radial, paired scrape marks; notice the relief beyond the
paired markings, demonstrating that the feeding trace extends beyond the activity of the paired tooth marks; rounded pellets on bed-
ding planes have been interpreted as sand pellets from their feeding activity (Gehling et al. 2014). C, Kimberella quadrata (bf) associ-
ated with Kimberichnus (tf) at the presumed anterior end (PIN 3493/5137), also figured in Fedonkin (2003). D, Kimberella quadrata,
associated with Kimberichnus at the presumed anterior end (PIN 4853/333). E, Kimberella quadrata preserving internal anatomy; notice
the preserved pharynx with paired pouches, arrowed (PIN 4853/326). F, Kimberella quadrata associated with Kimberichnus at the pre-
sumed anterior end (PIN 4853/379). Abbreviations: PIN, PalaeontologicalInstitute, Moscow; bf, body fossil; tf, trace fossil.
V INTHER : MOLLUSC ORIG INS 27
mantle cavity (Seilacher 1999; Seilacher et al. 2003;
Fig. 3A). The dorsal surface exhibits a cuticular shield
with tubercular nodes, which are particularly resistant to
decay. Specimens are found in great abundance on sur-
faces that also preserve distinct scraping traces named
Kimberichnus Ivantsov, 2013 (Fig. 3B), which is also well
known from Australia (Gehling et al. 2014). These traces
suggest a mode of feeding on the microbial mats involv-
ing two larger teeth (forming paired grooves), and a ser-
ies of smaller denticles (as evidenced by the presence of
relief outside the tooth marks indicating feeding over a
larger area by a more elaborate feeding apparatus than
550 500 450 400 350 300
Cambrian Ordovician Silurian Devonian Carboniferous
250
PermianEdiacaran
Ribeirioida
Conocardioida
Scaphopoda
Cephalopoda
Bivalvia
Rostroconcha
Gastropoda
Conchifera
Aculifera
Aplacophora
Polyplacophora
Lepidopleurida
Chitonida
Chaetodermomorpha
Neomenimorpha
Monoplacophora
Mollusca
F IG . 4 . A time tree of molluscs summarizing discoveries from recent state-of-the-art molecular phylogenetic studies, clocks and the
fossil record.
28 PALAEONTOLOGY , VOLUME 58
the visible paired teeth marks; Fig. 3B). Many specimens
(Fedonkin et al. 2007; Ivantsov 2009, 2010, 2013; unpub.
obs.) show the body fossil Kimberella in direct association
with the Kimberichnus scrape marks (Fig. 3C, D, F). Sur-
ficial locomotion trails are almost never observed in this
mat ground environment, and observed traces are all
feeding traces from organisms that are actively removing
the mat (Sperling and Vinther 2010; Ivantsov 2013).
However, some creeping trails are known connecting a
feeding trace with a nearby body fossil (Fedonkin et al.
2007; Ivantsov 2009, 2010). Other features of Kimberella
are molluscan, such as the presence of a structure resem-
bling a digestive tract and a pharynx with a set of paired
pouches that resembles oesophageal pouches (Fig. 3E;
Vinther et al. 2012a); similar pouches are unknown in
any lophotrochozoan other than molluscs.
Unfortunately, Kimberella does not offer any characters
for establishing character polarity in molluscan evolution
as it does not possess a biomineralized skeleton and thus
cannot establish whether the ancestor had multiple scle-
rites and shell plates, or a single shell field, as in conchif-
erans (Vinther et al. 2012a).
Chitinous elements secreted by microvilli are present in
a number of lophotrochozoans (annelids, brachiopods
and some molluscs). Wiwaxia is a mollusc (Conway Mor-
ris 1985), which possesses apparently unmineralized, likely
chitinous, sclerites with a microvillar microstructure (But-
terfield 1990) as well as a radula (Smith 2012). The lack
of a mineralized skeleton and the presence of chitinous
elements suggest that this form is a stem-group mollusc,
which would polarize the aculiferan trait of having scle-
rites in multiple distinct longitudinal/concentric zones, a
plesiomorphy of the phylum. However, nonmineralic
sclerites are observed in both chitons and aplacophorans
and a microvillar microstructure is also observed in the
organic pellicle covering aculiferan sclerites (Fischer et al.
1980, fig. 9). Furthermore, Wiwaxia is only known from
Burgess Shale-type Lagerst€atten (Sun et al. 2014; Yang
et al. 2014) and from small organic bits (Butterfield and
Harvey 2012), which generally do not preserve any miner-
alized components, complicating detailed inference of its
skeletal nature as biomineralized or not. Odontogriphus is
another ‘naked’ mollusc, whose morphology might per-
tain to questions about the ancestral mollusc (Caron et al.
2006) or the evolution along the molluscan stem. In par-
ticular, the radula is of interest and is comparable to the
radula preserved in Wiwaxia (Smith 2012).
CONCLUSIONS
A general consensus has finally been reached concerning
the fundamental relationships among molluscs. Aplacoph-
orans were thought to be important extant members for
understanding early molluscan evolution, but are highly
derived and secondarily reduced (Vinther et al. 2012a).
The fossil record in conjunction with molecular clock
analyses shows the molluscs radiated rapidly during the
Cambrian explosion (Fig. 4), while bivalves and gastro-
pod crown groups radiated in the Ordovician Biodiversi-
fication Event. Aplacophorans, chitons, scaphopods and
cephalopod crown groups radiated later in the Palaeozoic.
Some stem-group lineages were still remarkably disparate
and diverse in the Early Palaeozoic, for example, cephalo-
pods (Kr€oger et al. 2011). Monoplacophorans appear to
have experienced a Late Cretaceous bottleneck (Kano
et al. 2012), which can be ascribed to their colonization
of the deep sea as a refugium, and the series of
Cretaceous ocean anoxic events (Jenkyns 1980), which
would have limited their distribution episodically.
Several aspects of molluscan evolution still need scru-
tiny: in particular, the sequence of character evolution,
the nature of the transitions between major molluscan
classes, especially within the Conchifera, and the
evolution within them would benefit from more palaeon-
tological discoveries and novel integrative studies in the
light of the new phylogeny of molluscs.
Acknowledgements. I am thankful to the editor Andrew Smith for
inviting me to write this review and for his patient editorial sug-
gestions. The reviewers provided invaluable feedback and made
helpful corrections and suggestions, but any omission or mistake
would be the responsibility of the author. I appreciate the hospi-
tality of Andrei Yu Ivantsov and John Pojeta and for letting me
study their collections. I thank Mike Vendrasco, John Pojeta,
Bruce Runnegar, Stefan Bengtson, Christiane Todt, Chris Schand-
er, Amelie Scheltema, Derek E. G. Briggs, Andreas Wanninger,
Claus Nielsen, Kevin J. Peterson, Andrei Yu Ivantsov, Erik A. Sper-
ling and many others for discussions during the years. Sten Lenn-
art Jakobsen is thanked for collections assistance in Copenhagen.
Editor. Andrew Smith
REFERENCES
AURIVILLIUS , C. W. S. 1892. Ueber einige Ober-Silurische
Cirripeden aus Gotland. Kungliga Svenska Vetenskapsakadem-
iens Handlingar, 18, 1–24.
BENGTSON, S. 1992. The cap-shaped Cambrian fossil Mai-
khanella and the relationship between coeloscleritophorans
and molluscs. Lethaia, 25, 401–420.
-and CONWAY MORRIS , S. 1984. A comparative study
of Lower Cambrian Halkieria and Middle Cambrian Wiwaxia.
Lethaia, 17, 307–329.
-and MISSARZHEVSKY, V. 1981. Coeloscleritophora –
a major group of enigmatic Cambrian metazoans. US Geologi-
cal Survey Open-File Report, 81, 19–21.
BERGENHAYN, J. R. M. 1955. Die fossilen Schwedische Lo-
ricaten nebst einer vorlaeufigen Revision der ganzen Klasse
V INTHER : MOLLUSC ORIG INS 29
Loricata. Kungliga Fysiografiska Sallskapets Handlingar (Lund
Universitets �Arskrift, NF), 66, 1–44.
BROCK, G. A. and PATERSON, J. R. 2004. A new species
of Tannuella (Helcionellida, Mollusca) from the Early Cam-
brian of South Australia. Memoirs of the Association of Austral-
asian Palaeontologists, 30, 133–143.
BUTTERFIELD, N. J. 1990. A reassessment of the enigmatic
Burgess Shale fossil Wiwaxia corrugata (Matthew) and its rela-
tionship to the polychaete Canadia spinosa Walcott. Paleobiol-
ogy, 16, 287–303.
-1994. Burgess Shale-type fossils from a Lower Cambrian
shallow-shelf sequence in Northwestern Canada. Nature, 369,
477–479.
- and HARVEY, T. 2012. Small carbonaceous fossils
(SCFs): a new measure of early Paleozoic paleobiology. Geol-ogy, 40, 71–74.
CARON, J. B., SCHELTEMA, A., SCHANDER, C. and
RUDKIN, D. 2006. A soft-bodied mollusc with radula
from the Middle Cambrian Burgess Shale. Nature, 442,
159–163.
CHEN, J.-Y. and TEICHERT, C. 1983. Cambrian cephalo-
pods. Geology, 11, 647–650.
CHERNS, L. 1998a. Chelodes and closely related Polyplaco-
phora (Mollusca) from the Silurian of Gotland, Sweden.
Palaeontology, 41, 545–573.
-1998b. Silurian polyplacophoran molluscs from Gotland,
Sweden. Palaeontology, 41, 939–974.
-2004. Early Palaeozoic diversification of chitons (Polyplaco-
phora, Mollusca) based on new data from the Silurian of Got-
land, Sweden. Lethaia, 37, 445–456.
CLAPHAM, M. E., BOTTJER, D. J., POWERS, C. M.,
BONUSO, N., FRAISER, M. L., MARENCO, P. J.,
DORNBOS, S. Q. and PRUSS , S. B. 2006. Assessing the
ecological dominance of Phanerozoic marine invertebrates.
Palaios, 21, 431–441.
CONWAY MORRIS , S. 1985. The Middle Cambrian meta-
zoan Wiwaxia corrugata (Matthew) from the Burgess Shale
and Ogygopsis Shale, British Columbia, Canada. Philosophical
Transactions of the Royal Society of London, Series B, Biological
Sciences, 307, 507–582.
-and CARON, J.-B. 2007. Halwaxiids and the early evolu-
tion of the lophotrochozoans. Science, 315, 1255–1258.
-and PEEL, J. S. 1990. Articulated halkieriids from the
Lower Cambrian of north Greenland. Nature, 345, 802–805.
--1995. Articulated halkieriids from the Lower Cam-
brian of North Greenland and their role in early protostome
evolution. Philosophical Transactions of the Royal Society of
London, Series B, Biological Sciences, 347, 305–358.
COPE, J. C. 2000. A new look at early bivalve phylogeny.
81–95. In HARPER, E. M., TAYLOR, J. D. and
CRAME, J. A. (eds). The evolutionary biology of the Bival-
via. Geological Society, London, Special Publication, 177,
494 pp.
CREVELING, J. R., KNOLL, A. H. and JOHNSTON,
D. T. 2014. Taphonomy of cambrian phosphatic small shelly
fossils. Palaios, 29, 295–308.
DEGNAN, S. M. and DEGNAN, B. M. 2006. The origin
of the pelagobenthic metazoan life cycle: what’s sex got
to do with it? Integrative & Comparative Biology, 46,
683–690.
DONOVAN, S. K., SUTTON, M. D. and SIGWART, J. D.
2010. Crinoids for lunch? An unexpected biotic interaction
from the Upper Ordovician of Scotland. Geology, 38, 935–938.
---2011. The last meal of the Late Ordovician mol-
lusc ‘Helminthochiton’ thraivensis Reed 1911, from the Lady
Burn Starfish Beds, southwest Scotland. Geological Journal, 46,
451–463.
DUNN, C., HEJNOL, A., MATUS, D., PANG, K.,
BROWNE, W., SMITH, S., SEAVER, E., ROUSE, G.,
OBST, M. and EDGECOMBE, G. 2008. Broad phyloge-
nomic sampling improves resolution of the animal tree of life.
Nature, 452, 745–749.
DZIK, J. and KORN, D. 1992. Devonian ancestors of Nauti-
lus. Pal€aontologische Zeitschrift, 66, 81–98.
-and MAZUREK, D. 2013. Affinities of the alleged earliest
Cambrian gastropod Aldanella. Canadian Journal of Zoology,
91, 914–923.
FATKA, O., KRAFT, P. and SZABAD, M. 2011. Shallow-
water occurrence of Wiwaxia in the middle Cambrian of the
Barrandian area, Czech Republic. Acta Palaeontologica Polo-
nica, 56, 871–875.
FEDONKIN, M. A. 2003. The origin of the Metazoa in the
light of the Proterozoic fossil record. Paleontological Research,
7, 9–41.
-and WAGGONER, B. M. 1997. The Late Precambrian
fossil Kimberella is a mollusc-like bilaterian organism. Nature,
388, 868–871.
-SIMONETTA, A. and IVANTSOV, A. Y. 2007. New
data on Kimberella, the Vendian mollusc-like organism (White
Sea region, Russia): palaeoecological and evolutionary implica-
tions. 157–179. In VICKERS-RICH, P. and KOMA-
ROWER, P. (eds). The rise and fall of the Ediacaran biota.
Geological Society, London, Special Publication, 286, 456 pp.
FENG, W. and SUN, W. 2003. Phosphate replicated and
replaced microstructure of molluscan shells from the earliest
Cambrian of China. Acta Palaeontologica Polonica, 48,
21–30.
FISCHER, F. P., MAILE, W. and RENNER, M. 1980. Die
Mantelpapillen und Stacheln von Acanthochiton fascicularis L.
(Mollusca, Polyplacophora). Zoomorphologie, 94, 121–131.
GEHLING, J. G., RUNNEGAR, B. N. and DROSER,
M. L. 2014. Scratch traces of large Ediacara bilaterian animals.
Journal of Paleontology, 88, 284–298.
GIRIBET, G., OKUSU, A., LINDGREN, A., HUFF, S.,
SCHROEDL, M. and NISHIGUCHI, M. 2006.
Evidence for a clade composed of molluscs with serially
repeated structures: Monoplacophorans are related to chi-
tons. Proceedings of the National Academy of Sciences, 103,
7723–7728.
GRABAU, A. W. 1900. Palaeontology of the Cambrian terranes
of the Boston Basin. Occasional Papers of the Boston Society of
Natural History, 4, 606–694.
GROTZINGER, J. P., WATTERS, W. A. and KNOLL, A.
H. 2000. Calcified metazoans in thrombolite-stromatolite reefs
of the terminal Proterozoic Nama Group, Namibia. Paleobiol-
ogy, 26, 334–359.
30 PALAEONTOLOGY , VOLUME 58
HAAS, W. 1981. Evolution of calcareous hard-parts in primi-
tive mollusks. Malacologia, 21, 403–418.
HANGER, R. A., HOARE, R. D. and STRONG, E. E. 2000.
Permian Polyplacophora, Rostroconchia, and problematica
from Oregon. Journal of Paleontology, 74, 192–198.
HARPER, D. A. 2006. The Ordovician biodiversification: set-
ting an agenda for marine life. Palaeogeography, Palaeoclima-
tology, Palaeoecology, 232, 148–166.
HASZPRUNAR, G. 1992. The first molluscs-small animals.
Italian Journal of Zoology, 59, 1–16.
-2000. Is the Aplacophora monophyletic? A cladistic point
of view. American Malacological Bulletin, 15, 115–130.
-SALVINI-PLAWEN, L. V. and RIEGER, R. M. 1995.
Larval planktotrophy – a primitive trait in the Bilateria? Acta
Zoologica, 76, 141–154.
HOARE, R. D. and MAPES , R. H. 1995. Relationships of the
Devonian Strobilepis and related Pennsylvanian problematica.
Acta Palaeontologica Polonica, 40, 111–128.
IVANOV, D. L. 1996. Origin of Aculifera and problems of
monophyly of higher taxa in molluscs. 59–65. In TAYLOR,
J. D. (ed.). Origin and evolutionary radiation of the Mollusca.
Oxford University Press, Oxford, xiv + 392 pp.
IVANTSOV, A. Y. 2009. New reconstruction of Kimberella,
problematic Vendian metazoan. Paleontological Journal, 43,
601–611.
-2010. Paleontological evidence for the supposed Precam-
brian occurrence of mollusks. Paleontological Journal, 44,
1552–1559.
-2013. Trace Fossils of Precambrian Metazoans “Vend-
obionta” and “Mollusks”. Stratigraphy & Geological Correla-
tion, 21, 8–21.
-ZHURAVLEV, A. Y., LEGUTA, A. V., KRASSILOV,
V. A., MELNIKOVA, L. M. and USHATINSKAYA,
G. T. 2005. Palaeoecology of the Early Cambrian Sinsk biota
from the Siberian platform. Palaeogeography, Palaeoclimatolo-
gy, Palaeoecology, 220, 69–88.
J €AGERSTEN, G. 1972. Evolution of the metazoan life cycle: a
comprehensive theory. Academic Press, New York.
JELL, P. A. 1980. Earliest known pelecypod on Earth – a new
Early Cambrian genus from South Australia. Alcheringa, 4,
233–239.
JENKYNS, H. 1980. Cretaceous anoxic events: from continents
to oceans. Journal of the Geological Society, 137, 171–188.
KANO, Y., KIMURA, S., KIMURA, T. and WAR�EN, A.
2012. Living Monoplacophora: morphological conservatism or
recent diversification? Zoologica Scripta, 41, 471–488.
KLUG, C., KR €OGER, B., KIESSLING, W., MULLINS,
G. L., SERVAIS , T., FR �YDA, J., KORN, D. and
TURNER, S. 2010. The Devonian nekton revolution. Le-
thaia, 43, 465–477.
KOCOT, K. M., CANNON, J. T., TODT, C., CITAREL-
LA, M. R., KOHN, A. B., MEYER, A., SANTOS, S. R.,
SCHANDER, C., MOROZ, L. L., L IEB, B. and HALA-
NYCH, K. M. 2011. Phylogenomics reveals deep molluscan
relationships. Nature, 477, 452–456.
KOUCHINSKY, A. V. 1999. Shell microstructures of the
Early Cambrian Anabarella and Watsonella as new evidence
on the origin of theRostroconchia. Lethaia, 32, 173–180.
-2000. Shell microstructures in Early Cambrian molluscs.
Acta Palaeontologica Polonica, 45, 119–150.
-BENGTSON, S., RUNNEGAR, B., SKOVSTED, C.,
STEINER, M. and VENDRASCO, M. 2012. Chronology
of early Cambrian biomineralization. Geological Magazine,
149, 221–251.
KR €OGER, B. and MAPES, R. H. 2007. On the origin of bac-
tritoids (Cephalopoda). Pal€aontologische Zeitschrift, 81, 316–
327.
-VINTHER, J. and FUCHS, D. 2011. Cephalopod origin
and evolution: a congruent picture emerging from fossils,
development and molecules. BioEssays, 33, 602–613.
LEMCHE, H. 1957. A new living deep-sea mollusc of the
Cambro-Devonian class Monoplacophora. Nature, 179, 413–
416.
-and WINGSTRAND, K. G. 1959. The anatomy of
Neopilina galatheae Lemche, 1957. Galathea Reports, 3, 9–71.
MALOOF, A. C., PORTER, S. M., MOORE, J. L., DU-
DAS, F. O., BOWRING, S. A., HIGGINS, J. A., F IKE,
D. A. and EDDY, M. P. 2010. The earliest Cambrian record
of animals and ocean geochemical change. Geological Society of
America Bulletin, 122, 1731–1774.
MART�I MUS, M., PALACIOS, T. and JENSEN, S. 2008.
Size of the earliest mollusks: did small helcionellids grow to
become large adults? Geology, 36, 175–178.
MATTHEW, G. F. 1894. The Protolenus fauna. Transactions of
the New York Academy of Sciences, 14 (1–8 Series I), 101–153.
MAZUREK, D. and ZATON, M. 2011. Is Nectocaris pteryx a
cephalopod? Lethaia, 44, 2–4.
NIELSEN, C. 2001. Animal evolution. Oxford University Press,
Oxford, 563 pp.
-2012. Animal evolution: interrelationships of the living phyla.
Third edition. Oxford University Press, 402 pp.
-2013. Life cycle evolution: was the eumetazoan ancestor a
holopelagic, planktotrophic gastraea. BMC Evolutionary Biol-
ogy, 13, 1–18.
-HASZPRUNAR, G., RUTHENSTEINER, B. and
WANNINGER, A. 2007. Early development of the
aplacophoran mollusc Chaetoderma. Acta Zoologica, 88,
231–247.
N €UTZEL, A. 2014. Larval ecology and morphology in fossil
gastropods. Palaeontology, 57, 479–503.
-LEHNERT, O. and FR �YDA, J. �I. 2006. Origin of plank-
totrophy – evidence from early molluscs. Evolution & Develop-
ment, 8, 325–330.
OSCA, D., IRISARRI , I., TODT, C., GRANDE, C. and
ZARDOYA, R. 2014. The complete mitochondrial genome
of Scutopus ventrolineatus (Mollusca: Chaetodermomorpha)
supports the Aculifera hypothesis. BMC Evolutionary Biology,
14, 197.
PACKARD, A. 1972. Cephalopods and fish: the limits of con-
vergence. Biological Reviews, 47, 241–307.
PARKHAEV, P. Y. 2008. The early Cambrian radiation of
Mollusca. 33–70. In PONDER, W. F. and LINDBERG,
D. R. (eds). Phylogeny and evolution of the Mollusca. University
of California Press, Berkeley, 480 pp.
PASSAMANECK, Y., SCHANDER, C. and HALANYCH,
K. 2004. Investigation of molluscan phylogeny using large-sub-
V INTHER : MOLLUSC ORIG INS 31
unit and small-subunit nuclear rRNA sequences. Molecular
Phylogenetics & Evolution, 32, 25–38.
PATERSON, J. R., BROCK, G. A. and SKOVSTED, C. B.
2009. Oikozetetes from the early Cambrian of South Australia:
implications for halkieriid affinities and functional morphol-
ogy. Lethaia, 42, 199–203.
PAYNE, J. L., HEIM, N. A., KNOPE, M. L. and MCC-
LAIN, C. R. 2014. Metabolic dominance of bivalves predates
brachiopod diversity decline by more than 150 million years.
Proceedings of the Royal Society of London, Series B: Biological
Sciences, 281, 20133122.
PEEL, J. S. 1987. Class Gastropoda. 304–329. In BOARD-
MAN, R. S., CHEETHAM, A. H. and ROWELL, A. J.
(eds). Fossil invertebrates. Blackwell, San Fransisco, 713 pp.
-1991. The classes Tergomya and Helcionelloida, and early
molluscan evolution. Bulletin Grønlands Geologiske Un-
dersøgelse, 161, 11–65.
-2004. Pinnocaris and the origin of scaphopods. Acta Palae-
ontologica Polonica, 49, 543–550.
-2006. Scaphopodization in Palaeozoic molluscs. Palaeontol-
ogy, 49, 1357–1364.
PETERSON, K. J. 2005. Macroevolutionary interplay between
planktic larvae and benthic predators. Geology, 33, 929–932.
-CAMERON, R. A. and DAVIDSON, E. H. 1997. Set–
aside cells in maximal indirect development: evolutionary and
developmental significance. BioEssays, 19, 623–631.
PLAZZI , F., CEREGATO, A., TAVIANI , M. and PASSA-
MONTI, M. 2011. A molecular phylogeny of bivalve mol-
lusks: ancient radiations and divergences as revealed by
mitochondrial genes. PLoS ONE, 6, e27147.
POJETA, J. Jr and DUFOE, J. 2008. New information about
Echinochiton dufoei, the Ordovician spiny chiton. American
Malacological Bulletin, 25, 25–34.
-and RUNNEGAR, B. 1974. Fordilla troyensis and the
Early History of Pelecypod Mollusks: Early Cambrian fossils
from New York State provide important clues to the evolution
of the class. American Scientist, 62, 706–711.
--1976. The paleontology of rostroconch mollusks and
the early history of the phylum Mollusca. USGS Professional
Paper, 968, 1–88.
--1979. Rhytiodentalium kentuckyensis, a new genus and
new species of Ordovician scaphopod, and the early history of
scaphopod mollusks. Journal of Paleontology, 53 (3), 530–541.
--and KRIZ J. 1973. Fordilla troyensis Barrande: the
oldest known pelecypod. Science, 180, 866–868.
-YOCHELSON, E. and BRASIER, M. 1978. The origin
and early taxonomic diversification of pelecypods [and discus-
sion]. Philosophical Transactions of the Royal Society of London,
Series B, Biological Sciences, 284, 225–246.
-EERNISSE, D. J., HOARE, R. D. and HENDERSON,
M. D. 2003. Echinochiton dufoei: a new spiny Ordovician chi-
ton. Journal of Paleontology, 77, 646–654.
-VENDRASCO, M. J. and DARROUGH, G. 2010.
Upper Cambrian chitons (Mollusca: Polyplacophora) from Mis-
souri, USA. Bulletins of American Paleontology, 379, 1–79.
PUCHALSKI , S. S., JOHNSON, C. C., KAUFFMAN, E.
G. and EERNISSE, D. J. 2009. A new genus and two new
species of multiplacophorans (Mollusca, Polyplacophora,
Neoloricata), Mississippian (Chesterian), Indiana. Journal of
Paleontology, 83, 422–430.
RASETTI , F. 1954. Internal shell structures in the Middle
Cambrian gastropod Scenella and the problematic genus
Stenothecoides. Journal of Paleontology, 28, 59–66.
ROUSE, G. W. 2000. The epitome of hand waving? Larval
feeding and hypotheses of metazoan phylogeny. Evolution &
Development, 2, 222–233.
RUNNEGAR, B. 1981. Muscle scars, shell form and torsion in
Cambrian and Ordovician univalved molluscs. Lethaia, 14,
311–322.
-1985. Shell microstructures of Cambrian molluscs repli-
cated by phosphate. Alcheringa, 9, 245–257.
-1987. Rates and modes of evolution in the Mollusca.
39–60. In CAMPBELL, K. S. W. and DAY, M. F. (eds).
Rates of evolution. Allen & Unwin, London, 314 pp.
-2011. Once again: is Nectocaris pteryx a stem-group cepha-
lopod? Lethaia, 44, 373.
-and BENTLEY, C. 1983. Anatomy, ecology and affinities
of the Australian Early Cambrian bivalve Pojetaia runnegari
Jell. Journal of Paleontology, 57, 73–92.
-and JELL, P. A. 1976. Australian Middle Cambrian mol-
luscs and their bearing on early molluscan evolution. Alcherin-
ga, 1, 109–138.
-and POJETA, J. JR 1974. Molluscan phylogeny: the pale-
ontological viewpoint. Science, 186, 311–317.
--TAYLOR, M. E. and COLLINS, D. 1979. New
species of the Cambrian and Ordovician chitons Matthevia
and Chelodes from Wisconsin and Queensland – evidence for
the early history of polyplacophoran mollusks. Journal of Pale-
ontology, 53, 1374–1394.
SALVINI-PLAWEN, L. 2006. The significance of the Placo-
phora for molluscan phylogeny. Venus, 65, 1–17.
- and STEINER, G. 1996. Synapomorphies and
plesiomorphies in higher classification of Mollusca. 29–52. In
TAYLOR, J. D. (ed.). Origin and evolutionary radiation of
the Mollusca. Oxford University Press, New York, 392 pp.
SCHELTEMA, A. H. 1988. Ancestors and descendant relation-ships and the Aplcaophora and Polyplacophora. American
Malacological Bulletin, 6, 57–68.
-1993. Aplacophora as progenetic aculiferans and the coelo-
mate origin of mollusks as the sister taxon of Sipuncula.
Biological Bulletin, 184, 57–78.
-1996. Phylogenetic position of Sipuncula, Mollusca and the
progenetic Aplacophora. 53–58. In TAYLOR, J. D. (ed.).
Origin and evolutionary radiation of the Mollusca. Oxford Uni-
versity Press, Oxford, xiv + 392 pp.
-and IVANOV, D. L. 2002. An aplacophoran postlarva
with iterated dorsal groups of spicules and skeletal similarities
to Paleozoic fossils. Invertebrate Biology, 121, 1–10.
-KERTH, K. and KUZIRIAN, A. M. 2003. Original mol-
luscan radula: comparisons among Aplacophora, Polyplaco-
phora, Gastropoda, and the Cambrian fossil Wiwaxia
corrugata. Journal of Morphology, 257, 219–245.
SCHR €ODL, M., LINSE , K. and SCHWABE, E. 2006.
Review on the distribution and biology of Antarctic Monopla-
cophora, with first abyssal record of Laevipilina antarctica.
Polar Biology, 29, 721–727.
32 PALAEONTOLOGY , VOLUME 58
SEILACHER, A. 1999. Biomat-related lifestyles in the Precam-
brian. Palaios, 14, 86–93.
-GRAZHDANKIN, D. and LEGOUTA, A. 2003. Ediac-
aran biota: the dawn of animal life in the shadow of giant
protists. Paleontological Research, 7, 43–54.
SHARMA, P. P., GONZ �ALEZ, V. L., KAWAUCHI, G. Y.,
ANDRADE, S., GUZM �AN, A., COLLINS, T. M., GLO-
VER, E. A., HARPER, E. M., HEALY, J. M. and MIK-
KELSEN, P. M. 2012. Phylogenetic analysis of four nuclear
protein-encoding genes largely corroborates the traditional
classification of Bivalvia (Mollusca). Molecular Phylogenetics &
Evolution, 65, 64–74.
SHIGENO, S., SASAKI , T. and HASZPRUNAR, G. 2007.
Central nervous system of Chaetoderma japonicum (Caudofove-
ata, Aplacophora): implications for diversified ganglionic plans
in early molluscan evolution. Biological Bulletin, 213, 122–134.
S IGWART, J. D. 2009. Morphological cladistic analysis as a
model for character evaluation in primitive living chitons
(Polyplacophora, Lepidopleurina). American Malacological
Bulletin, 27, 95–104.
S IRENKO, B. I. 2006. New outlook on the system of chitons
(Mollusca: Polyplacophora). Venus, 65, 27–49.
SKELTON, P. W. 1978. The evolution of functional design in
rudists (Hippuritacea) and its taxonomic implications. Philo-
sophical Transactions of the Royal Society of London B, Biologi-
cal Sciences, 284 (1001), 305–318.
SMITH, M. R. 2012. Mouthparts of the Burgess Shale fossils
Odontogriphus and Wiwaxia: implications for the ancestral
molluscan radula. Proceedings of the Royal Society of London,
Series B: Biological Sciences, 279, 4287–4295.
-2013. Nectocaridid ecology, diversity, and affinity: early ori-
gin of a cephalopod-like body plan. Paleobiology, 39, 297–321.
-2014. Ontogeny, morphology and taxonomy of the soft-
bodied Cambrian mollusc Wiwaxia. Palaeontology, 57, 215–
229.
-and CARON, J.-B. 2010. Primitive soft-bodied cephalo-
pods from the Cambrian. Nature, 465, 469–472.
SMITH, S. A., WILSON, N. G., GOETZ, F. E., FEEHERY,
C., ANDRADE, S. C. S., ROUSE, G. W., GIRIBET, G. and
DUNN, C. W. 2011. Resolving the evolutionary relationships
of molluscs with phylogenomic tools. Nature, 480, 364–367.
--------2013. Corrigendum:
resolving the evolutionary relationships of molluscs with phy-
logenomic tools. Nature 493, 708.
SPERLING, E. A. and VINTHER, J. 2010. A placozoan affin-
ity for Dickinsonia and the evolution of late Proterozoic meta-
zoan feeding modes. Evolution & Development, 12, 201–209.
STINCHCOMB, B. L. 1986. New Monoplacophora (Mollus-
ca) from Late Cambrian and Early Ordovician of Missouri.
Journal of Paleontology, 60, 606–626.
ST €OGER, I., S IGWART, J. D., KANO, Y., KNEBELS-
BERGER, T., MARSHALL, B. A., SCHWABE, E. and
SCHR €ODL, M. 2013. The continuing debate on deep mol-
luscan phylogeny: evidence for Serialia (Mollusca, Monoplaco-
phora + Polyplacophora). BioMed Research International,
407072, 18 pp. doi: 10.1155/2013/407072
SUN, H.-J., ZHAO, Y.-L., PENG, J. and YANG, Y.-N.
2014. New Wiwaxia material from the Tsinghsutung Forma-
tion (Cambrian Series 2) of Eastern Guizhou, China. Geologi-
cal Magazine, 151, 339–348.
SUTTON, M. D. and SIGWART, J. D. 2012. A chiton with-
out a foot. Palaeontology, 55, 401–411.
-BRIGGS, D. E. G., S IVETER, D. J. and SIVETER, D. J.
2001. An exceptionally preserved vermiform mollusc from the
Silurian of England. Nature, 410, 461–463.
----2004. Computer reconstruction and analy-
sis of the vermiform mollusc Acaenoplax hayae from the Here-
fordshire Lagerstatte (Silurian England), and implications for
molluscan phylogeny. Palaeontology, 47, 293–318.
----and SIGWART J. D. 2012. A Silurian
armoured aplacophoran and implications for molluscan phy-
logeny. Nature, 490, 94–97.
VENDRASCO, M. J. 2012. Early evolution of molluscs. 1–43.
In FYODOROV, A. and YAKOVLEV, H. (eds). Mollusks:
morphology, behavior and ecology. Nova Science Publishers,
Hauppauge, NY, 285 pp.
-and RUNNEGAR, B. 2004. Late Cambrian and Early
Ordovician stem group chitons (Mollusca: Polyplacophora)
from Utah and Missouri. Journal of Paleontology, 78, 675–689.
-WOOD, T. E. and RUNNEGAR, B. N. 2004. Articu-
lated Palaeozoic fossil with 17 plates greatly expands disparity
of early chitons. Nature, 429, 288–291.
-LI, G. X., PORTER, S. M. and FERNANDEZ, C. Z.
2009. New data on the enigmatic Ocruranus-Eohalobia group
of Early Cambrian small skeletal fossils. Palaeontology, 52,
1373–1396.
-PORTER, S. M., KOUCHINSKY, A. V., GUOXI-
ANG, L. and FERNAND, C. 2010. Shell microstructures in
early mollusks. Festivus, 42, 43–53.
-CHECA, A. and KOUCHINSKY, A. V. 2011a. Shell
microstructure of the early bivalve Pojetaia and the indepen-
dent origin of nacre within the Mollusca. Palaeontology, 54,
825–850.
-KOUCHINSKY, A. V., PORTER, S. M. and FER-
NANDEZ, C. Z. 2011b. Phylogeny and escalation in
Mellopegma and other Cambrian molluscs. Palaeontologia
Electronica, 14, 1–44.
-CHECA, A., HEIMBROCK, W. P. and BAUMANN,
S. D. 2013. Nacre in Molluscs from the Ordovician of the
Midwestern United States. Geosciences, 3, 1–29.
VERMEIJ , G. J. 1987. Evolution and escalation. Princeton Uni-
versity Press, New Jersey, 537 pp.
VINTHER, J. 2009. The canal system in sclerites of Lower Cam-
brian Sinosachites (Halkieriidae: Sachitida): significance for the
molluscan affinities of the sachitids. Palaeontology, 52, 689–712.
-and NIELSEN, C. 2005. The Early Cambrian Halkieria is
a mollusc. Zoologica Scripta, 34, 81–89.
-SPERLING, E. A., BRIGGS, D. E. G. and PETER-
SON, K. J. 2012a. A molecular palaeobiological hypothesis
for the origin of aplacophoran molluscs and their derivation
from chiton-like ancestors. Proceedings of the Royal Society of
London, Series B: Biological Sciences, 279, 1259–1268.
-JELL, P., KAMPOURIS , G., CARNEY, R., RACI-
COT, R. A. and BRIGGS, D. E. 2012b. The origin of multi-
placophorans – convergent evolution in Aculiferan molluscs.
Palaeontology, 55, 1007–1019.
V INTHER : MOLLUSC ORIG INS 33
WALCOTT, C. E. 1905. Cambrian Faunas of China. – No.
1415, September 1905. Proceedings of the US National
Museum, 29 (1906), 1–106.
WAR�EN, A. and GOFAS, S. 1996. A new species of Monop-
lacophora, redescription of the genera Veleropilina and Roko-
pella, and new information on three species of the class.
Zoologica Scripta, 25, 215–232.
WARNKE, K. M., MEYER, A., EBNER, B. and LIEB, B.
2011. Assessing divergence time of Spirulida and Sepiida
(Cephalopoda) based on hemocyanin sequences. Molecular
Phylogenetics & Evolution, 58, 390–394.
WELLS, M. and O’ DOR, R. 1991. Jet propulsion and the
evolution of the cephalopods. Bulletin of Marine Science, 49,
419–432.
WINGSTRAND, K. G. 1985. On theanatomy and relation-
ships of Recent Monoplacophora. Galathea Reports, 16, 7–94.
WINNEPENNINCKX, B., BACKELJAU, T. and DE
WACHTER, R. 1996. Investigation of molluscan phylogeny
on the basis of 18S rRNA sequences. Molecular Biology & Evo-
lution, 13, 1306–1317.
WOOD, R. A., GROTZINGER, J. P. and DICKSON,
J. A. D. 2002. Proterozoic modular biomineralized meta-
zoan from the Name Group, Namibia. Science, 296, 2383–2386.
YANG, J., SMITH, M. R., LAN, T., HOU, J.-B. and
ZHANG, X.-G. 2014. Articulated Wiwaxia from the Cam-
brian Stage 3 Xiaoshiba Lagerst€atte. Scientific Reports, 4, 4643.
doi: 10.1038/srep04643
YOCHELSON, E., FLOWER, R. H. and WEBERS, G. F.
1973. Bearing of new Late Cambrian monoplacophoran genus
Knightoconus upon origin of Cephalopoda. Lethaia, 6,
275–309.
ZAPATA, F., WILSON, N. G., HOWISON, M., KATHA-
RINA, M., GOETZ, F. E., GIRIBET, G. and DUNN, C.
W. 2014. Phylogenomic analyses of deep gastropod relation-
ships reject Orthogastropoda. Proceedings of the Royal Society
of London, Series B: Biological Sciences, 281, 1794. doi:
10.1098/rspb.2014.1739
ZHAO, Y., QIAN, Y. and LI , X. 1994. Wiwaxia From Early–
Middle Cambrian Kaili Formation in Taijiang, Guizhou. Acta
Palaeontologica Sinica, 33, 359–366.
34 PALAEONTOLOGY , VOLUME 58