Osteichthyan-like cranial conditions in an Early Devonian stem gnathostome

Autor: Matt Friedman, Sam Giles, Martin D. Brazeau
Rok vydání: 2015
Předmět:
Zdroj: U175
Nature
ISSN: 1476-4687
0028-0836
DOI: 10.1038/nature14065
Popis: A new analysis of a 415-million-year-old fossil fish head originally described as from an early osteichthyan (bony fish) puts it instead as the sister group of the gnathosomes (jawed vertebrates), and suggests that the extinct acanthodians were relatives of cartilaginous fishes. The early evolution of the jawed vertebrates (gnathostomes) is a hot topic in palaeontology. Here Brazeau and colleagues use computed tomography scanning to take a new look at a 415-million-year-old braincase and skull roof from the Early Devonian of Siberia, originally described in 1992 by Hans-Peter Schultze as coming from an early osteichthyan (bony fish). They find that the underlying braincase shows a mixture of features seen separately in osteichthyans, chondrichthyans or in neither. Phylogenetic analysis places the fish at base of the gnathostomes and suggests that the enigmatic acanthodians — a wholly extinct group of fossil fishes — were relatives of cartilaginous fishes. The phylogeny of Silurian and Devonian (443–358 million years (Myr) ago) fishes remains the foremost problem in the study of the origin of modern gnathostomes (jawed vertebrates). A central question concerns the morphology of the last common ancestor of living jawed vertebrates, with competing hypotheses advancing either a chondrichthyan-1,2,3 or osteichthyan-like4,5 model. Here we present Janusiscus schultzei gen. et sp. nov., an Early Devonian (approximately 415 Myr ago) gnathostome from Siberia previously interpreted as a ray-finned fish6, which provides important new information about cranial anatomy near the last common ancestor of chondrichthyans and osteichthyans. The skull roof of Janusiscus resembles that of early osteichthyans, with large plates bearing vermiform ridges and partially enclosed sensory canals. High-resolution computed tomography (CT) reveals a braincase bearing characters typically associated with either chondrichthyans (large hypophyseal opening accommodating the internal carotid arteries) or osteichthyans (facial nerve exiting through jugular canal, endolymphatic ducts exiting posterior to the skull roof) but lacking a ventral cranial fissure, the presence of which is considered a derived feature of crown gnathostomes7,8. A conjunction of well-developed cranial processes in Janusiscus helps unify the comparative anatomy of early jawed vertebrate neurocrania, clarifying primary homologies in ‘placoderms’, osteichthyans and chondrichthyans. Phylogenetic analysis further supports the chondrichthyan affinities of ‘acanthodians’, and places Janusiscus and the enigmatic Ramirosuarezia9 in a polytomy with crown gnathostomes. The close correspondence between the skull roof of Janusiscus and that of osteichthyans suggests that an extensive dermal skeleton was present in the last common ancestor of jawed vertebrates4, but ambiguities arise from uncertainties in the anatomy of Ramirosuarezia. The unexpected contrast between endoskeletal structure in Janusiscus and its superficially osteichthyan-like dermal skeleton highlights the potential importance of other incompletely known Siluro-Devonian ‘bony fishes’ for reconstructing patterns of trait evolution near the origin of modern gnathostomes.
Databáze: OpenAIRE
Externí odkaz: