A new quasipetalichthyid placoderm from the Middle Devonian of China

Abstract

We describe Yipetalichthys maxiongensis gen. et sp. nov., a quasipetalichthyid placoderm from the Eifelian Shangshuanghe Formation (Qujing, Yunnan, China). High‐resolution micro‐CT reveals an endocranium bearing cranial nerve grooves and inner ear structures. The dermal neck joint permits a quantitative character, the angle between the ventral laminar ridge (v.l.r) and the skull‐roof midline. Current specimens yield two non‐overlapping bins (state 0: ≤15°; state 1: ≥28°), separating Macropetalichthyidae and Quasipetalichthyidae; the 15–28° interval is unsampled and the threshold remains testable with new material. Expanded parsimony analysis, adding Yipetalichthys and six additional ptyctodont taxa, recovers Petalichthyida as sister to Ptyctodontida and places Yipetalichthys within Quasipetalichthyidae. The diagnosis includes large unenclosed elliptical orbits, slender unbifid postorbital processes, an elongate occipital region, and a shallow posterolateral vagus canal. Yipetalichthys shows a high v.l.r angle, supporting its quasipetalichthyid placement and providing a reproducible metric for future comparisons across articulated specimens. Our analysis of the neck joint helps to clarify petalichthyid phylogeny and identifies distinct, recurring architectures among major placoderm groups: in Antiarcha, a reverse sliding joint; in Petalichthyida, an elaborate sliding joint guided by the ventral laminar ridge with laminar cranial articular facets forming a longitudinal sliding track opposing the articular flanges of the anterior dorsolateral plate (ADL); in Ptyctodontida, a sliding joint with spoon‐shaped cranial articular facets opposing ADL articular flanges; in Actinolepidoidei, a simple sliding condition; in Brachythoraci, a ginglymoid joint. These distinct neck joint architectures demonstrate the high evolutionary plasticity of this structure and its multiple, independent pathways toward cranial mobility in placoderms.