Vertebrate fauna of the Upper Triassic Mesa Montosa Member (Petrified Forest Formation, Chinle Group), Chama Basin, northern New Mexicio

Abstract

—The vertebrate fauna of the Mesa Montosa Member of the Petrified Forest Formation of the Chinle Group in the Chama Basin of northern New Mexico is generally not well known. However, recent work has greatly expanded the known vertebrate fauna from this unit. Taxa include the metoposaurid amphibian cf. Buettneria sp., the phytosaur Pseudopalatus buceros (Cope) as well as indeterminate phytosaurs (Parasuchidae), the aetosaurs Typothorax coccinarum and Paratypothorax sp., the enigmatic archosaur Vancleavea sp. and theropod dinosaurs. An unusual vertebra and a distinctive shell(?) fragment may pertain to a pterosaur and a turtle, respectively, both of which are rare in the Chinle Group. Other fossil material recovered from the Mesa Montosa Member includes numerous vertebrate coprolites, charcoal and unionid bivalve shells. The assemblage of fossil vertebrates indicates that the Mesa Montosa Member is Revueltian in age. Most of the fossils were collected from a coarse brown sandstone that contains some pebbles and calcrete nodules and is less than a meter below the contact between the Mesa Montosa Member and the overlying Painted Desert Member. These fossils are disarticulated and fragmentary, very few of the fossils are unweathered and many are abraded to the point where identification is impossible. Thus, these fossils represent a time-averaged attritional assemblage that is most likely derived from the floodplain near the channel system that deposited the sandstone. More complete skeletal elements have been recovered from a green shaley siltstone underlying the sandstone, but fossils from this layer are much less abundant.