NEW EVIDENCE FOR HATCHLING AND JUVENILE HADROSAUROIDS IN THE SAN JUAN BASIN, NEW MEXICO

The record of dinosaurs and specifically, ornithopods, in the San Juan Basin of New Mexico is limited. In contrast to the record in the Northern part of the United States and Southern Canada, represented by well preserved specimens, the hadrosauroid record of New Mexico is comparatively sparse. Also limited in the San Juan Basin record, are growth series and evidence of juvenile and subadult hadrosauroid individuals. The previous record of hatchling hadrosauroids in the San Juan Basin has been restricted to a single humerus, referred to Parasaurolophus tubicen, from the De-na-zin Member of the Kirtland Formation.

A reexamination of specimens collected from the San Juan Basin in 1922 by Charles H. Sternberg has revealed new evidence of the presence of hatchling and juvenile hadrosauroids in the region. The collection, housed at the Field Museum of Natural History, is composed of disarticulated elements from the Kirtland Formation of McKinley County, New Mexico, approximately 85 miles northeast of Thoreau, New Mexico.

The juvenile elements consist of postcrania including ribs, femora, and scapulae, and fragmentary skull elements, such as a dentary, partial quadrate, and quadratojugal. The smallest element, a scapula (PR 1295) that is approximately 66 mm in length, is comparable in size to those of hatchling individuals of Maiasaura and Hypacrosaurus. The lateral profile of the dorsal margin of this scapula is craniocaudally straight, suggesting that the hatchling represents a basal hadrosauroid taxon. The scapula is well preserved and lacks abrasions that would signify transport, suggesting that the hatchling elements were buried near their origin, possibly near a nesting site. The presence of these specimens in the San Juan Basin indicates that hadrosauroid growth series may be preserved in the region.