The oldest complete jawed vertebrates from the early Silurian of China

Molecular studies suggest that the origin of jawed vertebrates was no later than the Late Ordovician period (around 450 million years ago (Ma))(1,2). Together with disarticulated micro-remains of putative chondrichthyans from the Ordovician and early Silurian period(3-8), these analyses suggest an evolutionary proliferation of jawed vertebrates before, and immediately after, the end-Ordovician mass extinction. However, until now, the earliest complete fossils of jawed fishes for which a detailed reconstruction of their morphology was possible came from late Silurian assemblages (about 425 Ma)(9-13). The dearth of articulated, whole-body fossils from before the late Silurian has long rendered the earliest history of jawed vertebrates obscure. Here we report a newly discovered Konservat-Lagerstatte, which is marked by the presence of diverse, well-preserved jawed fishes with complete bodies, from the early Silurian (Telychian age, around 436 Ma) of Chongqing, South China. The dominant species, a `placoderm' or jawed stem gnathostome, which we name Xiushanosteus mirabilis gen. et sp. nov., combines characters from major placoderm subgroups(14-17) and foreshadows the transformation of the skull roof pattern from the placoderm to the osteichthyan condition(10). The chondrichthyan Shenacanthus uermiformis gen. et sp. nov. exhibits extensive thoracic armour plates that were previously unknown in this lineage, and include a large median dorsal plate as in placoderms(14-16), combined with a conventional chondrichthyan bauplan(18,19). Together, these species reveal a previously unseen diversification of jawed vertebrates in the early Silurian, and provide detailed insights into the whole-body morphology of the jawed vertebrates of this period.

Automated summary

Molecular studies suggest that jawed vertebrates originated no later than the Late Ordovician period. Newly discovered fossils from the early Silurian provide important insights into the early diversification of jawed vertebrates.