Spiny chondrichthyan from the lower Silurian of South China

Abstract

Modern representatives of chondrichthyans (cartilaginous fishes) and osteichthyans (bony fishes and tetrapods) exhibit contrasting skeletal anatomy and development [1][2][3][4] that underscore the distant evolutionary split 5-7 of the two clades. Recent work on upper Silurian and Devonian jawed vertebrates 7-10 has exposed similar skeletal conditions that blur 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 predates the earliest articulated fossils of jawed vertebrates [10][11][12] . Fanjingshania possesses dermal shoulder girdle plates and a complement of fin spines of striking anatomical similarity to those recorded in a subset of stem chondrichthyans 5,7,13 (climatiid 'acanthodians' 14 ). Uniquely among chondrichthyans, however, it demonstrates osteichthyan-like resorptive shedding of scale odontodes and absence of odontogenic tissues in spines. Our results identify independent acquisition of these conditions in the chondrichthyan stem group, adding Fanjingshania to an increasing number of taxa 7,15 nested within traditionally-defined 'acanthodians' 16 . The discovery of Fanjingshania provides the strongest support yet for a proposed 7 early Silurian radiation of jawed vertebrates prior to their widespread appearance 5 in the fossil record in the Lower Devonian. sequence of evolutionary changes leading up to the emergence of crown chondrichthyans.

Broad snouted cladoselachian with sensory specialization at the base of modern chondrichthyans

“…13 shows 63% of tips and 76% of mean divergence time estimates predating the end-Devonian mass extinction event. Thus, most branching events of interest (in the present analysis) are embedded within the Devonian, which corresponds well with molecular clock data (Inoue et al, 2010 ) and a recently presented tree including earliest chondrichthyans (Andreev et al, 2022 ). In particular, the divergence of the chondrichthyan crown clade is estimated between 393.9 (Emsian-Eifelian boundary) and 377.1 mya (middle Frasnian), with the mean estimate rooting elasmobranchs and holocephalans into the Givetian (Middle Devonian) some twelve million years earlier than Maghriboselache .…”

“…Timing: clade origins. As suggested by Zhu et al ( 2022 ) and Andreev et al ( 2022 ), the roots of the total clade Chondrichthyes date back to the early Silurian or even late Ordovician (Fig. 13 ).…”

“…Emerging consensuses include the wholesale movement of acanthodians on to the chondrichthyan stem (Brazeau, 2009 ; Davis et al, 2012 ; Dearden et al, 2019 ; Maisey et al, 2017 ; Miller et al, 2003 ; Zhu et al, 2013 ), and, more contentiously, recognition of numerous Paleozoic clades as stem holocephalans (Coates et al, 2017 , 2018 ; Dearden et al, 2019 ; Frey et al, 2020a , b ). With this change has come increasing recognition that total group Chondrichthyes (Andreev et al 2022 ; Coates et al, 2018 ), and thus all crown gnathostomes, originated at least as far back as the early Silurian (Andreev et al, 2022 ; Zhu et al, 2022 ). Among the stem holocephalans, the Symmoriiformes are an important group, comprising a series of bizarre species, several of which bear unusual fin spines (Coates & Sequeira, 1998 , 2001 ; Lund, 1985 , 1986 ; Williams, 1985 ; Zangerl, 1990 ; Zangerl & Case, 1976 ).…”

The skeletal completeness of the Palaeozoic chondrichthyan fossil record

“…[10,26–30]). These hypotheses of range extension have now been confirmed with body fossils and further diagnostic fragments from the Early Silurian of China [3,31,32]. However, how the quality of these various fossils and their differing degrees of completeness influence estimated patterns of diversity and phylogenetic relationship remains uncertain.…”

“…Chondrichthyans (elasmobranchs and chimaeroids) are a highly successful class of predominantly predatory fishes. They originated and first diversified in the Palaeozoic and persist until the present day [ 1 – 3 ]. Extinct and extant chondrichthyans show adaptations to different habitats in both freshwater and marine environments, occupying various ecological niches (e.g.…”