A new rhynchosaur maxillary tooth plate morphotype expands the disparity of the group in the Ischigualasto Formation (Late Triassic) of Northwestern Argentina

Rhynchosaurs are a clade of quadruped, herbivorous stem-archosaur diapsids restricted to the Triassic Period. The group became globally distributed and the numerically dominant tetrapods of several terrestrial ecosystems before their extinction. Derived rhynchosaurs are characterized by a specialized masticatory apparatus, composed of a blade-and-groove occlusion with multiple longitudinal maxillary tooth rows. The morphology of the maxillary tooth plate has shown to be taxonomically and phylogenetically informative. So far, two rhynchosaur maxillary tooth plate morphotypes are known in Argentina, one belonging to an unnamed stenaulorhynchine from the Chañares Formation and the other to the hyperodapedontine <i>Hyperodapedon sanjuanensis</i>, the single rhynchosaur species currently name for the Ischigualasto Formation. Here we describe a new rhynchosaur maxillary tooth plate morphotype based on an indeterminate hyperodapedontine specimen from the Ischigualasto Formation. This maxillary tooth plate (PVL 2728) possesses a single longitudinal groove that divides symmetric lateral and medial tooth-bearing areas with relatively large tooth crowns, which is an uncommon combination of features among hyperodapedontines. These qualitative observations in addition to quantitative analyses show that the morphology of PVL 2728 differs from that of, at least, other sampled South American rhynchosaurs. Therefore, this specimen expands the morphological disparity of rhynchosaurs in northwest Argentina and southwestern Pangaea.

喙头龙类（Rhynchosaurs）是一类四足、植食性的基干主龙型双孔类（stem-archosaur diapsids）演化支，仅分布于三叠纪（Triassic Period）。该类群曾实现全球广布，在灭绝前曾是多个陆地生态系统中数量占绝对优势的四足动物类群。特化的喙头龙类以特化的咀嚼器官为典型鉴别特征，该器官由带有多条纵向上颌齿列的刃沟式咬合结构组成。上颌齿板的形态特征已被证实具备分类学与系统发育信息价值。目前在阿根廷已发现两种喙头龙类上颌齿板形态型：其一产自查尼亚雷斯组（Chañares Formation），隶属于一个未命名的狭鼻喙龙亚科（stenaulorhynchine）类群；其二隶属于超齿龙亚科（hyperodapedontine）的圣胡安超齿龙（*Hyperodapedon sanjuanensis*），该种也是当前伊斯基瓜拉斯托组（Ischigualasto Formation）唯一已定名的喙头龙类物种。本文基于一件采自伊斯基瓜拉斯托组的未定属种超齿龙亚科标本，记述了一种全新的喙头龙类上颌齿板形态型。该上颌齿板标本（编号PVL 2728）具有单条纵向沟槽，可将齿着生区域对称划分为外侧与内侧两部分，且齿冠相对较大——这种特征组合在超齿龙亚科类群中较为罕见。上述定性观察结合定量分析结果显示，PVL 2728的形态特征至少与其他已采样的南美喙头龙类存在显著差异。因此，该标本拓展了阿根廷西北部及泛大陆西南部喙头龙类的形态分异度。