The first occurrence of Pliosauridae from the Aptian (Lower Cretaceous) of North America (Chihuahua, Mexico) and their ecological significance

Pliosauridae includes large sauropterygians, which were dominant predators from the middle Jurassic to the Early Cretaceous. The fossil record of Early Cretaceous plesiosaurians has improved in recent years; however, aspects such as their biogeographic patterns and extinction remain understudied due to several gaps in the Cretaceous record. Here, we report the first occurrence of pliosaurids in Cretaceous outcrops of Mexico, collected in two sites in the Coyame region of Chihuahua, northern Mexico. The first specimen is a disarticulated vertebral series, and the second is an isolated tooth. Both specimens belong to the clade Thalassophonea. In the first specimen, a combination of features, such as the proportions of length to height of the pectoral centra and the positions of the foramina on the ventral and dorsal surfaces, is consistent with those observed in Cretaceous thalassophonean pliosaurids. The second specimen is identified as a Thalassophonea based on its robust crown and the conspicuous apicobasal ridges on the enamel. These fossils represent the first known pliosaurids from the Aptian of North America, enhancing our knowledge of the group during the Early Cretaceous in this region.