TRACKS OF ORNITHOPODS PUTTING THEIR BEST FEET FORWARD IN THE FRONTIER FORMATION (CONIACIAN), MONTANA

Basal iguanodontians and hadrosaurid dinosaurs display a conservative rear foot (pes) morphology. Consequently, differentiating specific producers from their footprints solely based on morphological differences is often hampered and restricted by their stratigraphic and geographic distribution. In contrast, front foot (manus) anatomy reflects distinctive differences in phalanges, unguals, and articulations of joints. These differences may produce independent morphological features useful for discriminating among track makers. Here we provide a description of three purported ornithopod manus tracks preserved as sandstone casts from the Frontier Formation (Coniacian) of southwestern Montana. Tracks were in fluvial overbank deposits, in which tracks deeply emplaced in soft mud were filled with sand. For this study, we applied manus anatomical features observed in basal iguanodontian and hadrosaurid ornithopods to identify a possible producer. Tracks are assigned to an iguanodontian according to their steeply inclined morphology, functionally tridactyl condition, presence of hoof-like impressions, and crescentic cross-sectional shape. Kinematic markers (i.e., scale-scratch traces) indicate that digit II could be extended medially, whereas digits III and IV had limited ranges of movement. The short, spade-like ungual II impression and an absence of any lateral displacement of digit IV suggest a closer affinity of these tracks to hadrosauroids, rather than more basal iguanodontians or derived hadrosaurids. The incorporation of anatomical characters resulted in more effectively recognizing the track producer, and tracks preserved as tridimensional casts were particularly helpful for this comprehensive analysis. Our results provide the first clear evidence of hadrosauroid ornithopods in the Frontier Formation, thus expanding their stratigraphic range and paleogeographic distribution to Coniacian deposits in southwestern Montana.