Global Ordovician vertebrate biogeography

Abstract

Cambrian–Ordovician vertebrate and supposed vertebrate occurrences have been repeatedly claimed during recent decades, with confirmed taxa bearing mineralized tissues with a vertebrate histomorphology still relatively rare. The only biogeographic province that we can presently recognize is the Gondwana Endemic Assemblage (GEA) with possible Late Cambrian fragmentary remains from Australia but more definite Early Ordovician (Arenigian) to early Late Ordovician (Caradocian) arandaspids (i.e., Sacabambaspis, Arandaspis) and other taxa known from South America and Australia. Certain chondrichthyans (‘sharks’, at first without teeth and which might not constitute a monophyletic group) might have originated in East Gondwana province and then are found in the Late Ordovician and Early Silurian of Mongolia, Tarim, and South China. The GEA fauna proper disappears by middle–late Caradocian and vertebrates do not reappear in Gondwana until mid Late Silurian. Late Ordovician (−455 Myr or earlier) vertebrates are also known with certainty from Laurentia, viz., North America (pteraspidomorphs Astraspis, Eriptychius, and various gnathostome-like taxa including chondrichthyan-, placoderm- and acanthodian-like remains), and Siberia (astraspid-like microremains with an unusual histology, which might correspond to a new group of lower vertebrates) as well as scales from putative loganiid and thelodontidid thelodonts from North America and Russia (Timan–Pechora, the Severnaya Zemlya archipelago and Siberia). This is defined as the Laurentia–Baltica–Siberia Assemblage (LBSA). We also mention one enigmatic reference to a Late Ordovician anaspid in South Africa. There is no clear association of taxa between the GEA and LBSA despite a small overlap in time. Various recent palaeogeographic models published for the Ordovician are critically analyzed and considered within four groups: the archetypal palaeogeographic reconstructions, two alternative solutions, and a compact version. Habitats of vertebrates in mostly BA1 (marine intertidal) to BA3 (shallow subtidal) environments, and their dispersal capabilities are evaluated with regard to those models. The main feature of Ordovician vertebrate biogeography is endemism. Furthermore, the present lack of complete descriptions of most taxa, which are often represented only by isolated microremains, and the need for a thorough phylogenetic analysis preclude any phylogenetic palaeobiogeographic study. In such a framework, we also evaluate possible links between external, physical factors and the Ordovician radiation of vertebrates. The Late Proterozoic deposition of oceanic phosphate and the Early Cambrian increase in oxygen on Earth might have been the spur for vertebrate evolution before the phase when hard tissues appeared. The sharp decline of the marine strontium isotope ratio during the Middle to Late Ordovician transition, interpreted as having been controlled primarily by continental collisional tectonics and its associated erosion and weathering, has been proposed as the consequence of a possible mantle superplume event which could have caused the prominent Caradocian transgressive phase. This might have been a factor in the changeover from a Gondwanan to a Laurentian focus for vertebrates.