A new antarctaspid (Placodermi, Arthrodira) from the Lower Devonian of Yunnan, China

Antarctaspids, a basal subgroup within the Arthrodira, display a mosaic of arthrodire and petalichthyid features. However, their morphology has remained poorly understood due to the paucity of well-preserved specimens. Here, we describe a new antarctaspid arthrodire, Panjiangosteus eurycephala gen. et sp. nov., from the Posongchong Formation (Pragian, Lower Devonian) of Wenshan Prefecture, Yunnan, China. Using X-ray computed tomography, we reveal previously undocumented anatomical details of antarctaspids, including the neck joint and the dorsal surface of the neurocranium. In addition to diagnostic antarctaspid characters, Panjiangosteus also exhibits some features characteristic of more derived arthrodires, notably the convergence of middle and posterior pit-lines onto the central plate and the subdivision of the posterior pit-line into two discrete segments. Crucially, Panjiangosteus is characterized by its broad skull roof and the vertical sliding neck joints reminiscent of petalichthyids. Phylogenetic analysis recovered antarctaspids as a paraphyletic grade at the base of Arthrodira, with Panjiangosteus placed more derived than the other antarctaspids. These findings refine our knowledge of antarctaspid cranial morphology, shed new light on character transitions in early arthrodire evolution, and support a close relationship between petalichthyids and arthrodires.https://zoobank.org/urn:lsid:zoobank.org:pub:F07656F3-1537-48DD-8133-D67B44B9FE31