Origin and the Early Evolution of the Phylum Mollusca

The evolution of molluscs

“…Irrespective of whether or not Kimberella is an early stem-line mollusc and whether or not other debated fossils such as Acaenoplax, Wiwaxia, Halkieria, or the newly described Calvapilosa belong to the molluscan stem, are members of later-branching molluscan sublineages, or do not constitute molluscs at all (Conway Morris, 1985;Butterfield, 1990;Conway Morris & Peel, 1990, 1995Steiner & Salvini-Plawen, 2001;Sutton et al, 2001Sutton et al, , 2004Scheltema & Ivanov, 2002;Vinther & Nielsen, 2005;Conway Morris & Caron, 2007;Telford & Budd, 2011;Vinther, 2014;Parkhaev, 2017;Vinther et al, 2017), a sound inference of molluscan evolutionary history involving ground pattern reconstruction requires a well-supported phylogenetic framework (Wanninger, 2015). While this has traditionally been a major gap in molluscan research, both concerning intraspecific relationships of class-level taxa but also with respect to unresolved molluscan sister group relationships (Wanninger, 2009;Haszprunar & Wanninger, 2012), recent large-scale phylogenomic analyses (Kocot et al, 2011;Smith et al, 2011) lend hope that some emerging phylogenetic patterns within the phylum will consolidate and will provide a long-sought base for addressing questions concerned with molluscan origins and phenotypic diversification.…”

“…The almost unmatched diversity of molluscan morphological phenotypes is exemplified by well‐known representatives such as the gastropods (snails, slugs), bivalves (clams, mussels), and cephalopods (nautiluses, squids, octopuses), but also includes more enigmatic groups such as spicule‐bearing, simple worms (the aplacophorans), flattened, ovoid, shell plate‐bearing polyplacophorans (chitons), circular monoplacophorans with a single, cap‐like shell, and the scaphopods (tusk shells), that owe their name to their bent, elephant tooth‐like shell in which the animal resides (Haszprunar & Wanninger, ). These dramatic variations in overall body plan morphology render molluscs an ideal group for comparative studies into how evolution has brought about phenotypic diversity from a common ancestor that roamed the oceans' seafloors at least 550 million years ago (mya) (Parkhaev, , ; Haszprunar & Wanninger, ; Vinther et al, a , b ; Vinther, , ; Wanninger & Wollesen, ) (Fig. ).…”

“…The combination of such an ancient evolutionary history together with the occurrence of mineralized exoskeletal hard parts in their body plan has resulted in a rich fossil record, at least of the shell‐bearing taxa (Parkhaev, , ). These findings together with molecular clock estimates revealed a picture according to which all major molluscan sublineages are deeply rooted in the Cambrian (Vinther, , ) (Fig.…”

“…Additional proposed shared features between Kimberella and recent molluscs mainly concerned characteristics related to the digestive system (Vinther, ), as well as extra‐corporal, brush‐like imprints that are commonly found in association with Kimberella specimens (Fedonkin, Simonetta, & Ivantsov, ; Ivantsov, , ; Gehling, Runnegar, & Droser, ; Vinther, ). These are thought to be trace fossils of a scraping‐type feeding apparatus similar to a molluscan radula (Vinther, ; disputed by Parkhaev, ). However, despite being heavily chitinized and equipped with teeth that in some cases are impregnated with mineral ions, no traces of a fossilized radula have hitherto been found in any of the dozens of Kimberella specimens reported to date (Fedonkin & Waggoner, ).…”

“…The enigmatic Ediacaran fossil Kimberella quadrata Glaessner & Wade, 1966, originally described in the late 1950s (Glaessner & Daily, 1959) and proposed to represent a Precambrian cnidarian in early studies (Glaessner & Wade, 1966;Wade, 1972), received considerable attention in the decades following its description, because re-analyses considered it the earliest known bilaterian -and likely a stem-group mollusc (Fedonkin & Waggoner, 1997;Fedonkin, Simonetta & Ivantsov, 2007;Ivantsov, 2010;Gehling, Runnegar & Droser, 2014; but see, e.g. Parkhaev, 2017 for an alternative view). The fossil specimens found were interpreted as bearing imprints of an elongated, slender foot surrounded by a mantle that is separated from the former by a circumpedal mantle cavity (Seilacher, 1999;Vinther, 2015), thus resembling the gross morphological ventral view of modern polyplacophoran molluscs.…”