Novel features and morphological variation in the lower jaw of the Permian temnospondyl amphibian Trimerorhachis

<p> The dvinosaur temnospondyl <em>Trimerorhachis</em> is one of the best-known tetrapod genera from the Early Permian of the continental United States. Although the cranial and postcranial anatomy of this taxon is known in considerable detail, no comprehensive appraisal of its lower jaw is available. The present study aims to fill this gap. We surveyed numerous, previously unfigured, mechanically prepared jaw rami of <em>Trimerorhachis</em>, most attributed to <em>T. insignis</em>, from several localities in North America. This material demonstrates remarkable phenotypic variation, clarifies aspects of the jaw construction that have remained elusive or poorly documented until now, and reveals new information, including: the course of sutures around the symphyseal region; the construction of the precoronoid fossa (an excavation situated posterior or posterolingual to the dentary fangs); the proportions and distribution of foramina on the medial surface of the ramus; the presence of a conspicuous adductor process on the angular; the extent of Meckelian ossifications inside the adductor fossa; and the morphology of the coronoids, including differences in the size, shape, and arrangement of coronoid denticles. We present a revised diagnosis of <em>Trimerorhachis</em> based upon jaw characters, compare its jaw with those of other dvinosaurs, and comment upon the presumed diagnostic jaw traits in two other <em>Trimerorhachis</em> species, namely <em>T. rogersi</em> and <em>T. mesops</em>. </p>