Abstract
Discoveries of putative stem sarcopterygians from the late Silurian and Early Devonian of South China have increased our knowledge of the initial diversification of osteichthyans while also highlighting incongruities in character evolution in this major jawed vertebrate group. Character-rich endocrania are incompletely preserved for early bony fishes, limiting a detailed understanding of complex internal morphology and evolutionary changes in the cranium. Here we report a new sarcopterygian (Ptyctolepis brachynotus gen. et sp. nov.) from the Pragian (Early Devonian) of South China, which preserves a unique example of a completely ossified otoccipital division of the braincase in a stem lobe-finned fish. The hyomandibular facets are paired but lie dorsal to the jugular canal, representing a hitherto unobserved combination of derived and primitive character states. This new taxon prompts a reassessment of early osteichthyan interrelationships, including the phylogenetic placement of psarolepids, which might branch from the osteichthyan—rather than sarcopterygian—stem.
Highlights
Discoveries of putative stem sarcopterygians from the late Silurian and Early Devonian of South China have increased our knowledge of the initial diversification of osteichthyans while highlighting incongruities in character evolution in this major jawed vertebrate group
Osteichthyans are well represented in the Early Devonian, but intact braincases for members of the group are rarely preserved intact
The posterior dorsal fontanel is a median opening on the dorsal surface of the endocranium, and in taxa with a macromeric dermal skeleton it lies close to the posterior margin of the skull roof (Fig. 3). It is absent in stem gnathostomes (e.g. Dicksonosteus[50] and Janusiscus16), but present in chondrichthyans inclusive of acanthodians (Acanthodes[15]; Cladodoides[45], and Pucapampella47) and likely stem osteichthyans ('Ligulalepis'), and is a probable synapomorphy of crown gnathostomes (Supplementary Data 1)
Summary
Discoveries of putative stem sarcopterygians from the late Silurian and Early Devonian of South China have increased our knowledge of the initial diversification of osteichthyans while highlighting incongruities in character evolution in this major jawed vertebrate group. While most recent work on the early diversification of bony fishes has centered on material from deposits flanking the Silurian–Devonian boundary, including those yielding psarolepids, slightly younger strata have provided important new fossils Key among these is the Posongchong Formation of South China, which contains an abundance of osteichthyans including a diverse array of crown sarcopterygians, such as the earliest tetrapodomorphs[20], anatomically modern coelacanths[21], and early onychodonts[22,23]. We report a new sarcopterygian from the Posongchong Formation (~409 mya, Pragian, Early Devonian) of Yunnan, China This taxon is represented by a single well preserved and completely ossified otoccipital division of the skull measuring 4.4 cm in width, 1.8 cm in length, and 2.7 cm in height, which suggests an individual considerably larger than other known members of the co-occurring sarcopterygian fauna (e.g. Tungsenia[20] and Euporosteus yunnanensis[21]). Based on these and other observations, we include this new taxon in a c ppa mp a b pdf ppa pr.pam pp hyd d.ot.n d pr.ad c.ju hyd hyv pr.ad gr.ju e oatm hyv nc c.a.dl gr.a.dl pdf c.X pr.pam fm gr.ju f.occ.lat hyd nc pr.pam nc sup.art gr.ju gr.a.dl a b cer ot.n rec.utr ant.amp lat.amp pre.amp sac asc lsc post.amp psc f.occ.lat c.X c.c pdf c d pdf psc c.c s.su asc cer f.occ.lat ot.n psc rec.utr c.VII c.X post.amp lsc lat.amp sac
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