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Received: 30 August 2021
Accepted: 11 August 2022
Published online: 28 September 2022
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Modern representatives of chondrichthyans (cartilaginous fishes) and osteichthyans (bony fishes and tetrapods) have contrasting skeletal anatomies and developmental trajectories1-4 that underscore the distant evolutionary split5-7 of the two clades. Recent work on upper Silurian and Devonian jawed vertebrates7-10 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 ofjawed vertebrates10-12. 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 ofjawed vertebrates before their widespread appearance5 in the fossil record in the Lower Devonian series.
Hypotheses17,18 of skeletal evolution within the principal divisions of early jawed vertebrates (osteichthyans, chondrichthyans and stem-gnathostome 'placoderms') have recently come under scrutiny after conventionally recognized crown-group conditions (such as an endochondral bone and marginal jaw bones) were reported in the stem lineage among placoderms8-10,19 as well as the description of placoderm-like dermal scales in chondrichthyans7. Together with these data from upper Silurian and Devonian specimens, phylogenies5-7 of earlyjawed vertebrates expose gaps in the fossil record that extend well into the Upper Ordovician series and predict considerable range extensions for a number of important lineages. Upper Ordovician (Sandbian) and lower Silurian (Llandovery) remains of isolated dermal scales and spines attributed to acanthodian-grade stem chondrichthyans20,21 and enigmatic groups such mongolepids22-24, sinacanthids23,25 and elegestolepids20,26 provide further evidence of early diversity. Nevertheless, owing to the absence of articulated fossils and characters unequivocally linking them to established groups, the incorporation of these specimens into existing phylogenetic schemes of early vertebrates remains problematic5,7.
Here we...