Comparison of the Anapra prints with illustrated prints (Lockley and Hunt 1995; Lucas et al. 2000; Lockley et al. 2001; Thulborn 1990) from middle-Cretaceous strata, as well as other Dakota-age (early Cenomanian) rocks in New Mexico suggests that the Anapra track-makers were theropods, and that the undertracks are attributable to the ichnogenus Magnoavipes. This ichnogenus was first described by Lee (1997) who described the tracks as large bird tracks, and hence the name, which means "big bird track." Our assignment of the tracks described here to this particular ichnogenus is based on morphological similarity with the systematic ichnology of Magnoavipes in Lockley et al. (2001), as well as similarity in age to Magnoavipes tracks described by Lee (1997) and Lockley et al. (2001). The tracks described here, like all tracks attributable to the ichnogenus Magnoavipes, have long, slender toes, which gently taper to a sharp point or claw impression. The total angles of digit divarication are wide (60º-85º), with observable angles between digits II and III being commonly smaller than between digits III and IV. The average footprint length to width ratio is 1.07 (range of 0.87-1.43), which is also characteristic of tracks attributed Magnoavipes Lockley et al. (2001).
The Magnoavipes undertracks described herein differ in several details from published data. For example, several of the tracks are much larger than previously described Magnoavipes tracks of the Dinosaur Freeway. Previously studied tracks are up to 38 cm in length Lockley et al. (2001), whereas the tracks at Cristo Rey reach lengths of 53 cm. Also, at the outcrop mapped in this study, undertracks of Magnoavipes are the only ichnogenus present.
Tracks of the ichnogenus Caririchnium (and possibly Amblydactylus) are also found in great abundance at several other sites around Cristo Rey in association with additional tracks attributed to the ichnogenus Magnoavipes (Kappus et al. in press). Caririchnium tracks have bilobate heels, whereas similar Amblydactylus tracks have rounded heels. This is presently the principal characteristic used to distinguish them from each other, although shape and orientation of the pads of the foot (Brown 1999) may prove to be valuable in the future. The presence of crescent-shaped manus prints is also associated with Caririchnium, but not Amblydactylus (Lockley and Hunt 1995).
Caririchnium is distinguishable from Magnoavipes by the presence of wider, blunt-ended toes with smaller digit divarication angles (Thulborn 1990). Digit III of the Caririchnium tracks is also shorter in comparison to digits II and IV, whereas in Magnoavipes digit III is longer (Thulborn 1990). Also, Magnoavipes tracks are elevated digitigrade (see Figure 3.1), while Caririchnium is plantigrade, showing the bilobate heel impressions mentioned above.
The presence of Caririchnium along with Magnoavipes has biostratigraphic significance and supports the value of the ichnofacies concept described by Lockley et al. (1994). More research on the Cristo Rey tracksite is needed to assess whether or not this tracksite can be included in the "Caririchnium ichnofacies" of Lockley et al. (1994).