An introduction to Ordovician bivalves of southern China, with a discussion of the early evolution of the Bivalvia

Abstract

This paper introduces the Ordovician bivalves of southern China to non-Chinese colleagues, and then incorporates them into the global Ordovician bivalve database compiled by Cope (2004). This will provide a firmer basis for the discussion of the initial adaptive radiation of the Bivalvia. A new scenario for the early evolution of the class Bivalvia is advanced herein based on the integration of a wide variety of evidence concerning bivalves, including morphological features, ontogenetic trajectory, benthic environment and lithologic setting, so as to explain reasonably why the initial explosive radiation of bivalves in the Early Ordovician followed a long-term macroevolutionary lag since their first appearance during the Cambrian explosion. The early evolution of the Bivalvia can be divided into three phases: (a) the origination and macroevolutionary lag phase in the Early and Middle Cambrian; (b) the crisis phase in the Late Cambrian; and (c) the Ordovician radiation phase, which can be subdivided into two radiation pulses or intervals (the Arenig and the Caradoc radiation intervals). It is the Cambrian substrate revolution that caused a transformation of shallow subtidal seafloors from more coherent Neoproterozoic-style matgrounds to soupier Phanerozoic-style mixgrounds, thus compelling Cambrian 'archetype' bivalves to sink into the infaunal realm and to adapt themselves to the development of the mixed layer in siliciclastic soft substrate environments. As a result, Cambrian 'archetype' bivalves entered a tight bottleneck and became extinct while the latest common ancestor of 'modern-type' bivalves originated from a certain founder population somewhere in the Gondwanan shelf seas. The rapidly evolving founder population may be the place of origin of evolutionary novelties and it became the starting point of the evolution of 'modern-type' bivalves. The macroevolutionary lag of the Bivalvia ended in the earliest Ordovician as a result of the successful invasion of the infaunal adaptive zone. The bivalve die of 'modern-type' was cast during the Ordovician, when all principal clades and all four principal life-habit groups underwent initial development. A series of morphological innovations, especially the evolution of the muscular hydraulic foot with burrowing adaptations, byssus, and feeding gill, are responsible for the initial adaptive radiation. It is these morphological innovations that provide evolutionary access to the new adaptive zone for bivalves.

Treatise Online no. 43: Part N, Revised, Volume 1, Chapter 16: Origin and early evolution of the Bivalvia

“…The Cambrian archetype bivalves Fordilla, Pojetaia, and Tuarangia are stem-group taxa that predate the latest common ancestor of the crown-group Bivalvia (Morris, 1990;Waller, 1990Waller, , 1998runnegar & PojeTa, 1992;carTer, caMPbell, & caMPbell, 2000;Fang, 2006a). Both Fordilla and Pojetaia possess a bivalved shell, a simple, opisthodetic pseudoligament, simple dentition (pretaxodont, sensu carTer, caMPbell, & caMPbell, 2000;carTer, 2001), and a series of very well-impressed muscle scars, the number and size of which are variable (geyer & sTreng, 1998).…”

“…Although Pojetaia had a small anterior adductor scar, runnegar and benTley (1983) emphasized the possibility that the pallial muscles in this position may not yet have been cross fused. In brief, the organs of Pojetaia and Fordilla are still at an incipient bivalve evolutionary stage (Fang, 2006a).…”

“…The interpretation of infaunal habits for Cambrian bivalves has long been influenced by an actualistic approach, which assumes that early bivalve evolution can be inferred from its crown group (Fang, 2006a). However, the Cambrian was not just a time of transition from matgrounds to mixgrounds but also a transition from cohesive, fine-grained sediments to softer, soupier substrates, characteristic of the rest of the Phanerozoic (seilacher, 1999;boTTjer, hagadorn, & dornbos, 2000; dornbos, boT T j e r , & chen, 2004; seil a c h e r , buaTois, & Màngano, 2005;caron & others, 2006).…”

“…Thus, the paleoecological model for pre-Ordovician archetype bivalves and paleoenvironments is, to a large extent, distinct from modern marine benthic environments. Consequently, the early evolution of archetype bivalves must have been accomplished on matgrounds, the cradle for the early evolution of metazoans (Fang, 2006a).…”

“…As independent food-gathering organs, the initial palps probably received food from the foot. That is, stem-group bivalves must have used pedal-palp feeding with a ventrally emergent foot, since there is as yet no evidence for trophic differentiation between filter-feeding and deposit-feeding 4 Treatise Online, number 43 among Cambrian archetype bivalves (Fang, 2006a).…”