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Osteichthyan-like cranial conditions in Early Devonian stem gnathostome

Ben Creisler

A new paper that may be of interest:

Sam Giles, Matt Friedman & Martin D. Brazeau (2015)
Osteichthyan-like cranial conditions in an Early Devonian stem gnathostome.
Nature (2015) (advance online publication)

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- or osteichthyan-like 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
fish, 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 gnathostomes. 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
Ramirosuarezia 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 vertebrates, 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.

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