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

摘要

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. Copyright (c) 2006 John Wiley & Sons, Ltd.