A PROBABLE DEINOSUCHUS TRACK: A FIRST RECORD FROM THE UPPER CRETACEOUS MENEFEE FORMATION, SAN JUAN BASIN, NORTHERN NEW MEXICO

The early Campanian Menefee Formation, San Juan Basin, northwestern New Mexico, preserves fossils from a poorly understood interval in Earth’s history. A brief survey in 2017 recovered new remains of turtles and dinosaurs and a variety of ichnofossils including a large, well-preserved track attributed to a crocodylian. Crocodylian tracks have not yet been reported for the Menefee Formation, and few crocodylian tracks are documented in Upper Cretaceous deposits of the Western Interior Basin relative to the abundance of skeletal remains. We describe the newly discovered track, including integumentary impressions, create a 3D photogrammetric model of the track, and use measurements to estimate the body length of the trackmaker.

The crocodylian track, NM-17-05-21-K6, is a natural cast subsequently overlain by a fine to medium-grained interbedded sandstone with some traces of organic material. The track shape is a well-preserved manus print bearing evidence of each of the five digits. The track exhibits a heel-drag mark, which makes the perimeter of the heel ambiguous.

Approximately 50 photographs of the fossil were taken to render the three-dimensional photogrammetric model. The manus length was calculated by measuring a straight line from the terminus of the digit III impression through the long axis of the same digit to the caudal edge of the track. Since the natural caudal most extent of the manus heel mark was unclear, we measured from the cranialmost to caudalmost aspect of the heel drag and derived three point estimates (max, min, mean). Body length was estimated by measuring from the tip of the snout to the tip of the tail of 12 extant species. Our regressions of manus length to body length derived from a database of extant crocodylians suggests that the track was made by a crocodylian between ~10 meters and ~18 meters in size. Currently four taxa have been identified from the Menefee Formation based on body fossils including isolated teeth, osteoderms, and other skeletal materials: Brachychampsa, Leidyosuchus, Denazinosuchus kirtlandicus, and Deinosuchus rugosus. Of these, only Deinosuchus reaches body lengths consistent with our trackmaker estimates. We therefore conclude that the track from the Menefee Formation is likely attributable to Deinosuchus, one of the largest crocodylians to have inhabited North America.