NEW VERTEBRATE FAUNA FROM THE LATE TRIASSIC MESA MONTOSA MEMBER (PETRIFIED FOREST FORMATION: CHINLE GROUP), CHAMA BASIN, NORTH-CENTRAL NEW MEXICO

Abstract

The vertebrate fauna of the lower Petrified Forest Formation of the Chinle Group is generally not well known. However, recent work in the Mesa Montosa Member of the Petrified Forest Formation in the Chama basin of north-central New Mexico has greatly expanded the known fauna from this unit. Taxa include the metoposaurid amphibian cf. Buettneria , indeterminate phytosaurs (Parasuchidae), the aetosaurs cf. Typothorax coccinarum and Paratypothorax , the enigmatic archosaur Vancleavea , 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 Late Triassic. Other fossil material recovered from the Mesa Montosa Member includes numerous coprolites and unionid bivalve shells. The assemblage of vertebrates recovered thus far indicates that the Mesa Montosa Member is Revueltian in age. All 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 are much less abundant in this layer than in the overlying sandstone.