THE LAKES-MARSH ATLANTOSAURUS BEDS: A UNIQUE DINOSAUR ECOSYSTEM FROM THE UPPER JURASSIC MORRISON FORMATION OF COLORADO

Fossils collected at Dinosaur Ridge (Morrison, CO) in the 1870s for the Yale Peabody Museum hail from six sites in the Upper Jurassic Morrison Formation. Morrison Natural History Museum diversified the census. Dinosaur Ridge quarries are ecologically and taxonomically unique.

Sauropods dominate; Apatosaurus are more common than Camarasaurus, while Atlantosaurus and a novel sauropod are rare. Diplodocus is absent. The fossiliferous zone consists of quartz arenite and claystone which represent fluvial and lacustrine depositional environments. Freshwater biotics are rare indicating ephemeral water bodies during deposition. Sauropods and allosaurs are found in all local environments while stegosaurs are restricted to fluvial settings. Allosaurus are overly sampled due to the taphonomic bias of a resident population.

Quarry 10 contains the type Apatosaurus ajax. Also present are testudine Dinochelys, unionid mollusks and a gastropod. The absence of other freshwater vertebrates indicates this was a vernal pool. Shed allosaur teeth with corresponding dental striations on A. ajax dorsal ribs indicate feeding. Stegosaur and ornithopod tracks are present in the fluvial sandstone capping this site.

Quarries 1, 5, and XYZ are fluvial sandstones. Fine heterolithic sands with clay rip-up clasts entomb an allosaur and two historical holotypes at Quarry 1. The type of Atlantosaurus montanus is limited to a distinctive sacrum. Diplosaurus felix is a sufficient type sample for the crocodilian. An unknown sauropod is also present and is being described by MNHM staff. At XYZ, fluvial channel lags contain fragmentary teeth of Camarasaurus and the first documented Ceratosaurus from the area.

Quarry 5 is a grain-supported quartz arenite and entombs the historic type of Stegosaurus armatus, which is adequate for the taxon. This site contains Apatosaurus and Camarasaurus cranial elements, a partial cranium of Diplosaurus, dipnoan (c.f. Potamoceratodus), and a heilochyledrid turtle. Allosaurus is represented by a jugal and teeth. Stegosaur, sauropod, theropod, and ornithopod tracks are known from Quarry 5.

Quarry 8 contains postcranial material from A.ajax in alluvial mudrock and fluvial sandstone. Nugent’s Quarry is an overbank environment that contains a glyptopsid with fragmentary remains of Allosaurus and A.ajax.