DISTRIBUTION OF PTEROSAUR SPECIMENS IN THE MORRISON FORMATION (LATE JURASSIC) OF NORTH AMERICA

Pterosaurs are extremely rare in the continental sediments of the Morrison Formation with only about a dozen sites known and most of these yielding single pterosaur specimens. Five of the localities are clustered near the Uncompahgre Plateau in western Colorado, and four are near Como Bluff, Wyoming. Single occurrences of pterosaur remains have been collected near Cañon City, Colorado; Moab, Utah; and possibly Shell, Wyoming. Half of the localities contain abundant microvertebrate remains. Stratigraphically, most of the sites (seven) occur in the middle third of the formation, and just four are in the upper third. Pterosaurs known from the Morrison Formation include several individuals of the pterodactyloid Mesadactylus from two sites, the pterodactyloids Kepodactylus and Dermodactylus, the “rhamphorhynchoids” Harpactognathus, Comodactylus, and Utahdactylus, and indeterminate specimens from five sites. However, the genera Dermodactylus and Comodactylus may be nomina dubia, and Utahdactylus is based on an unprepared partial skeleton. Only Mesadactylus is well known, but Kepodactylus and Harpactognathus, although fragmentary, appear diagnostic. Among the pterosaur occurrences, 40% are in channel or splay sands, 30% are in overbank ponds, 20% are in floodplain muds, and 10% are in lacustrine/wetland limestones. The rarity of pterosaurs in the Morrison Formation is likely related to destruction in the continental environment and not because of their scarcity in the vertebrate populations. World-wide, pterosaurs are found most commonly in marine sediments and appear to have been piscivorous. Morrison pterosaurs are also believed to be piscivorous and would have fed on the abundant fish in perennial rivers and ponds supplied by a high water table, seasonal rain, and surface runoff from western mountains. Fish remains are more abundant in the Morrison Formation than is commonly believed. Pterosaurs may have also fed partly on insects and scavenged dinosaur carcasses.