Diversification and biogeography of bivalves during the Ordovician Period

Abstract

Bivalves have a wide distribution in the Lower and to a lesser extent Middle Cambrian rocks, but they have not yet been certainly identified in the Upper Cambrian. Recent discoveries have significantly increased our knowledge of Lower Ordovician bivalve faunas and their explosive radiation from the Early Ordovician apparently coincides with the evolution of the feeding gill. Early Ordovician faunas were confined to the siliciclastic facies of Gondwanan shelf seas; most genera were clearly latitudinally constrained, but others apparently migrated over wide latitudes. By the Mid-Ordovician, bivalves had begun to escape the confines of Gondwana and marked latitudinal differences in the composition of the faunas became apparent, with pteriomorphians showing clear preference for low latitudes, whilst heteroconchs clearly preferred median to high latitudes; surprisingly, nuculoids were both most diverse in terms of species and most abundant as a percentage of individuals within the bivalve populations at low latitudes. It was in the Late Ordovician that bivalves colonized the low-latitude carbonate platforms of Laurentia and Baltica, leading to a second major diversification particularly within the pteriomorphian bivalves, which developed semi-infaunal and epifaunal habits; they became the dominant low-latitude bivalve group. The latest-Ordovician eustatic regression which exposed the low-latitude carbonate platforms resulted in a major reduction in the epifaunal and semi-infaunal bivalves involving extinction of many genera, including the only Ordovician boring bivalves.

The early bivalve radiation in the Ordovician Gondwanan basins of Argentina

“…Of further note is that the dominance of higher taxa during the Tremadocian-lower Darriwilian interval of both the NWA and Famatina basins deviates from patterns recognized in previous studies (e.g. Cope & Babin 1999, Babin 2000.…”

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

“…runnegar and benTley (1983) suggested that Pojetaia and Fordilla were anatomically like a young Nucula; but in Fang's (2006a) opinion, they were at a proto-palpal evolutionary stage. In any case, it is improper to place Pojetaia and Fordilla in Nuculoida (Protobranchia) (runnegar & PojeTa, 1992;coPe, 1997;geyer & sTreng, 1998;coPe & babin, 1999;hinZschallreuTer, 2000), because they probably lacked palp proboscides, which are unique to Protobranchia (Waller, 1998), as well as paleotaxodont hinge teeth, which are characteristic of the earliest Protobranchia (carTer, caMPbell, & caMPbell, 2000).…”

“…It has long been acknowledged that the Ordovician is characterized by two diversification peaks, one in the early Ordovician and another in the late Ordovician (e.g., babin, 1993Ordovician (e.g., babin, , 1995Ordovician (e.g., babin, , 2001coPe & babin, 1999;coPe, 2002coPe, , 2004. The first one is recorded mainly on the clastic platforms of northwestern Gondwana and peripheral microplates, whereas the second one is recorded mainly in carbonate and mixed platforms located at mid-to low-paleolatitudes in Laurentia, Baltica, Siberia, and Kazakhstan.…”

“…It is likely that the Ordovician bivalve clades originated from small isolated refugia of Cambrian populations in cold to temperate waters of Gondwanan and peri-Gondwanan regions. This would account for the fact that most of the oldest Ordovician bivalves are found along Gondwanan margins and not elsewhere (coPe, 1997;coPe & babin, 1999;sáncheZ, 2003;Fang, 2006aFang, , 2006c. As Fang (2006a) stated, early Ordovician bivalves could have diversified from a founder population surviving the Cambrian biotic crisis, during which time the dichotomy between protobranchs and autobranchs was accomplished.…”

“…ForTey and cocks (2003) divided this supercontinent into three latitudinal belts: high latitude West Gondwana (North Africa, Armorica, Perunica, Avalonia), low latitude East Gondwana (mainly Australia and South China), and intermediate latitude Gondwana (mainly South America). The geographic distribution of Ordovician bivalve higher taxa seems to have been controlled largely by water temperature (coPe & babin, 1999;babin, 2001;coPe, 2002;Fang & coPe, 2004, 2008. Ordovician high-latitude bivalve faunas were dominated by heteroconchs, whereas low latitude faunas were dominated by pteriomorphians and nuculoids.…”