Squamation and scale morphology at the root of jawed vertebrates

Placoderms, as the earliest branching jawed vertebrates, are crucial to
understanding how the characters of crown gnathostomes comprising
Chondrichthyes and Osteichthyes evolved from their stem relatives. Despite
the growing knowledge of the anatomy and diversity of placoderms over the
past decade, the dermal scales of placoderms are predominantly known from
isolated material, either morphologically or histologically, resulting in
their squamation being poorly understood. Here we provide a comprehensive
description of the squamation and scale morphology of a primitive taxon of
Antiarcha (a clade at the root of jawed vertebrates), Parayunnanolepis
xitunensis, based on the virtual restoration of an articulated specimen by
using X-ray computed tomography. Thirteen morphotypes of scales are
classified to exhibit how the morphology changes with their position on
the body in primitive antiarchs, based on which nine areas of the
post-thoracic body are distinguished to show their scale variations in the
dorsal, flank, ventral, and caudal lobe regions. In this study, the
histological structure of yunnanolepidoid scales is described for the
first time based on disarticulated scales from the type locality and
horizon of P. xitunensis. The results demonstrate that yunnanolepidoid
scales are remarkably different from their dermal plates as well as
euantiarch scales in lack of a well-developed middle layer. Together, our
study reveals that the high regionalization of squamation and the
bipartite histological structure of scales might be plesiomorphic for
antiarchs, and jawed vertebrates in general.