The origins of molluscs

Abstract

The interrelationships and evolutionary history of molluscs have seen great advances in the last decade. Recent phylogenetic studies have allowed alternative morphology-based evolutionary scenarios to be tested and, most significantly, shown that the aplacophorans are sister group to polyplacophorans (chitons), corroborating palaeontological and embryological evolutionary scenarios in which aplacophorans 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 Cambrian, and rapidly thereafter, stem lineages to the major molluscan 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.

The evolution of molluscs

“…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 Ecological Physiology of Earth's Second Oxygen Revolution

“…By contrast, large triploblastic Ediacaran organisms, such as Kimberella (commonly but not definitively thought to be related to the mollusks; Fedonkin et al 2007, Vinther 2015, moved across the seafloor, making extensive scratch marks on the sediment surface (Gehling et al 2014). The combination of Kimberella's size, locomotion, and feeding mode would have required both a blood vascular system and much higher minimum oxygen levels than those of rangeomorphs.…”

Early metazoan life: divergence, environment and ecology

“…As discussed further in §4, this does not require that such lineages had acquired the morphological characters of the crown groups (see also discussion in reference [11]). The only generally accepted fossil representative of any of these clades identified from the Ediacaran to date is Kimberella, from 555 Ma rocks in the White Sea of Russia, which could be a stem mollusc but is certainly a stem lophotrochozoan [20,21]. This molecular clock analysis should be re-done to assess the impact of a possible basal position for ctenophores, but such a topology would seem to require an earlier Cryogenian date for the origin of Metazoa.…”