Bird Ichnology - Bird Ichnofossils

Bird Ichnofossils

These fossil traces of birds are sometimes hard to interpret correctly, especially when they are from the Mesozoic when the birds' dinosaurian relatives were still in existence. Nests at least of Neornithes are usually quite easy to identify as such due to the unique structures of their eggshells; there is some uncertainty as regards the origin of certain Mesozoic eggshells, which makes nests of this age problematic.

Mesozoic fossil footprints are hardest to attribute. "Proto-bird" and related theropod feet were very much alike; non-avian theropod tracks such as the ichnogenus Grallator were initially attributed to ratites because in the early 19th century when these were described, the knowledge about dinosaurian diversity was marginal compared to today, whereas ratites were well-known. Also, under the creationist dogma, scientists would believe that e.g. rheas had been around for all eternity. In the Jurassic and Early Cretaceous, juvenile non-avian theropods left very birdlike footprints. Towards the end of the Cretaceous, the tracks of aquatic birds are usually recognizable due to the presence of webbing between the toes; indeed, most avian ichnotaxa fall into this group. However, giant flightless birds also existed by that time, as evidenced by Gargantuavis; if the Gastornithidae were indeed close to Anseriformes, their lineage must also have been distinct by then. Such footprints may resemble those of non-avian theropod or even ornithopod dinosaurs. Among the former, the Ornithomimiformes (= "Arctometatarsalia" sensu stricto) were convergent to ratites in many respects, including the feet, and it is impossible to tell if some large bird-like footprints from the Late Cretaceous are from an ornithomimiform or a giant bird, without associated bone material.