''Xiushanosteus'' is a genus of

placoderm Placoderms (from Ancient Greek πλάξ 'plax'', ''plakos'''Plate (animal anatomy), plate' and δέρμα 'derma'''skin') are vertebrate animals of the class (biology), class Placodermi, an extinct group of prehistoric fish known from Pal ...

from the Huixingshao Formation of China. It contains one species, ''Xiushanosteus mirabilis''. It is one of the earliest known jawed vertebrates, at roughly 435 million years old. ''Xiushanosteus'' is known from around 20 specimens, most of which are complete.

Description

''Xiushanosteus'' is roughly long. Its body is dorsoventrally compressed with small, diamond-shaped scales covering it. Along its midline are scales or scutes with a linear ornament, alongside two dorsal fins of similar size, both bearing spines. The caudal fin is epicercal with a round ventral lobe. The head has long, posterolaterally directed head spines, alongside its dermoskeleton having an unusual structure with features from numerous placoderm clades.

Dermoskeleton

The margins of the skull roof are similar to acanthothoracids, however the plate pattern is closer to actinolepidoids with only one pair of

postorbital The ''postorbital'' is one of the bones in vertebrate skulls which forms a portion of the dermal skull roof and, sometimes, a ring about the orbit. Generally, it is located behind the postfrontal and posteriorly to the orbital fenestra. In some ve ...

and paranuchal plates. The skull roof's lateral lines are gently curving like those of "maxillate placoderms" and acanthothoracids. The nuchal plate does not separate the two central plates unlike in most other placoderms. The head shield resembles placoderms such as ''

Lunaspis ''Lunaspis'' is an extinct genus of armor-plated petalichthyid placoderm fish that lived in shallow marine environments of the Early Devonian period, from approximately 409.1 to 402.5 million year ago.Fossilworks: Gateway to the Paleobiology dat ...

'' and '' Romundina'' closest in general shape. The trunk has two median dorsal plates and a row of scutes, with the first of the former tightly fitting into the nuchal plate therefore reducing head mobility. A trait of ''Xiushanosteus'' which no other placoderm shares is the fact that its occipital dermal plates are separated from the otic plates by a fissure, and connected to the relatively immobile trunk. This loose connection within the skull likely compensated for the low mobility between the trunk and skull.

Etymology

''Xiushanosteus'' derives from Xiushan County where the fossils were found, alongside ''osteus'' "bone", a common suffix for placoderm names. The species name ''mirabilis'' translates to "miracle" due to how unexpected the find was.

References

{{Taxonbar, from=Q123458124 Placoderm genera Placoderms of Asia Silurian fish of Asia Enigmatic placoderm taxa Prehistoric animals of China Fossil taxa described in 2022