New interpretations of Ignotornis, the first-reported Mesozoic avian footprints: implications for the paleoecology and behavior of an enigmatic Cretaceous bird

Abstract

The type material of Ignotornis mcconnelli, the first reported Mesozoic bird track, consists of a large, monospecific sample of ∼70 footprints comprising at least 15 recognizable trackways. However, the exact type horizon and locality, discovered in 1930 in the Cretaceous Dakota Group near Golden, Colorado, was not indicated in the original 1931 description by Maurice Mehl, and only the single holotype trackway was illustrated. In 1988, the known sample was doubled by the discovery of ∼70 additional tracks representing at least seven trackways, but again the exact type horizon and locality remained uncertain. In 2007, we discovered what we infer to be the original type locality, and identified approximately 150 additional footprints comprising at least 17 additional trackways. During the study we also located three more specimens, in other collections, comprising at least 60 tracks and 10 trackways. Thus, the type (holotype, paratype, and topotype) sample now consists of ∼360 footprints comprising about 50 trackways, of which 41 have been measured. Although most footprints from the 1930, 1988, and 2007 finds all appear to originate from the same “type” horizon associated with a volcanic ash, a few tracks were found in 2007 at two additional levels. The relatively long, reversed hallux and the incipient semi-palmate webbing in the hypex between digits III and IV make Ignotornis distinct from any other Cretaceous bird tracks known from North America. These features, used to infer the extant forms with which Ignotornis is most convergent, are reminiscent of small herons, and unlike the typical tracks of most Cretaceous shorebird-like species which resemble those of plovers and sandpipers. Clearly defined parallel and sub parallel trackways indicate gregarious behavior, while some trackways indicate unusual “shuffling” and “stop-start” progression, probably related to some type of “foot-stirring” foraging activity. The concentration of abundant Ignotornis tracks at a single locality within the ichnologically-famous “Dinosaur Freeway” (represented by more than 60 dinosaur and crocodile dominated track assemblages) suggests that the Ignotornis track maker was a “rare-bird” in the region during the Cretaceous.