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1. The Oldest Gnathostome Teeth
2. Spiny Chondrichthyan from the Lower Silurian of South China
P. S. Andreev, I. J. Sansom, Q. Li, W. Zhao, J. Wang, C.-C. Wang, L. Peng, L. Jia, T. Qiao, and M. Zhu*
2022/12/14
Mandibular teeth and dentitions are features of jawed vertebrates that were first acquired by the Palaeozoic ancestors1,2,3 of living chondrichthyans and osteichthyans. The fossil record currently points to the latter part of the Silurian period4,5,6,7 (around 425 million years ago) as a minimum date for the appearance of gnathostome teeth and to the evolution of growth and replacement mechanisms of mandibular dentitions in the subsequent Devonian period2,8,9,10. Here we provide, to our knowledge, the earliest direct evidence for jawed vertebrates by describing Qianodus duplicis, a new genus and species of an early Silurian gnathostome based on isolated tooth whorls from Guizhou province, China. The whorls possess non-shedding teeth arranged in a pair of rows that demonstrate a number of features found in modern gnathostome groups. These include lingual addition of teeth in offset rows and maintenance of this patterning throughout whorl development. Our data extend the record of toothed gnathostomes by 14 million years from the late Silurian into the early Silurian (around 439 million years ago) and are important for documenting the initial diversification of vertebrates. Our analyses add to mounting fossil evidence that supports an earlier emergence of jawed vertebrates as part of the Great Ordovician Biodiversification Event (approximately 485–445 million years ago).    
Nature, 609, 964-968 (2022)

Modern representatives of chondrichthyans (cartilaginous fishes) and osteichthyans (bony fishes and tetrapods) have contrasting skeletal anatomies and developmental trajectories1,2,3,4 that underscore the distant evolutionary split5,6,7 of the two clades. Recent work on upper Silurian and Devonian jawed vertebrates7,8,9,10 has revealed similar skeletal conditions that blur the conventional distinctions between osteichthyans, chondrichthyans and their jawed gnathostome ancestors. Here we describe the remains (dermal plates, scales and fin spines) of a chondrichthyan, Fanjingshania renovata gen. et sp. nov., from the lower Silurian of China that pre-date the earliest articulated fossils of jawed vertebrates10,11,12. Fanjingshania possesses dermal shoulder girdle plates and a complement of fin spines that have a striking anatomical similarity to those recorded in a subset of stem chondrichthyans5,7,13 (climatiid ‘acanthodians’14). Uniquely among chondrichthyans, however, it demonstrates osteichthyan-like resorptive shedding of scale odontodes (dermal teeth) and an absence of odontogenic tissues in its spines. Our results identify independent acquisition of these conditions in the chondrichthyan stem group, adding Fanjingshania to an increasing number of taxa7,15 nested within conventionally defined acanthodians16. The discovery of Fanjingshania provides the strongest support yet for a proposed7 early Silurian radiation of jawed vertebrates before their widespread appearance5 in the fossil record in the Lower Devonian series.
Nature, 609, 969-973 (2022)