AbstractThalattosaurians are a cosmopolitan clade of secondarily aquatic tetrapods that inhabited low-latitude, nearshore environments during the Triassic. Despite their low taxic diversity, thalattosaurians exhibit remarkable morphological disparity, particularly with respect to rostral and dental morphology. However, a paucity of well-preserved material, especially leading up to their extinction, has hampered efforts to develop a robust picture of their evolutionary trajectories during a time of profound marine ecological change. Here, we describe a new taxon based on an articulated and nearly complete skeleton from Norian sediments of southeastern Alaska, USA. The holotype is the most complete North American thalattosaurian yet described and one of the youngest occurrences of the clade worldwide. We present a new hypothesis of interrelationships for Thalattosauria and investigate potential feeding modes in the Alaskan taxon. An integrated view suggests that the absence of pelagic lifestyles and restricted ecological roles may have contributed to thalattosaurs’ eventual extinction.