Ichnofossils (or ichnites) are especially important for clarifying theevolution and prehistoric diversity oftaxa. These cannot usually be associated with a particulargenus, let alonespecies of bird, as hardly ever they are associated with fossil bones. But it is possible to group them intoichnotaxa based on their morphology (form). In practice, the details of shape that reveal the birds' behavior or biologic affinity are generally given more weight in ichnologic classification.
Bird footprints typically have a wider angle between the toes. These goose tracks show that webs do not necessarily leave an impression.Grallator are the footprints of aCoelophysis-liketheropod, initially mistaken for those of aratite bird.Footprints of a largemoa found in 1911
These fossil traces of birds are sometimes hard to interpret correctly, especially when they are from theMesozoic when the birds'dinosaurian relatives were still in existence. Nests at least ofNeornithes 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 certainMesozoic 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 ichnogenusGrallator were initially attributed toratites 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 thecreationist dogma, scientists would believe that e.g.rheas had been around for all eternity. In theJurassic andEarly 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 byGargantuavis; if theGastornithidae were indeed close toAnseriformes, their lineage must also have been distinct by then. Such footprints may resemble those of non-avian theropod or evenornithopod dinosaurs. Among the former, theOrnithomimiformes (= "Arctometatarsalia"sensu stricto) wereconvergent 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.[1]
There exist documented tracks that appear avian since theLate Triassic, by some 55 million years predating the first proper evidence that very birdlike theropods were present. The Late Triassic and early-midJurassic tracks have been assigned to the ichnogeneraTrisauropodiscus andAquatilavipes. Few scientists would go as far though to consider these traces evidence that birds evolved much earlier than generally believed, andperhaps not from theropod dinosaurs as per today's mainstream opinion. In fact, it seems that the initial dating of these very ancient bird-like tracks was in error, and they seem to date from a much later time when modern birds were already known from bone fossils.
Footprints of at leastNeornithes can be distinguished by several features:
if ahallux is present, it is directed straight backwards or nearly so.
the second to fourth (front) toes have a wide angle between them (generally 90–180° or so)
due to Neornithes having a completely fusedtarsometatarsus (the "lower leg", actually the ankle and midfoot bones) they have no heel pads (except large terrestrial birds)
It is notable thatHeterodontosauridae are known from the localities and times when the first avian-looking footprints started to appear. These small ornithopod dinosaurs were entirely unbirdlike, except for theirornithischianpelvis and a tarsometatarsus stronglyconvergent to that ofEnantiornithes. Though some details remain unresolved, it is far more plausible thatTrisauropodiscus etc. were made by aHeterodontosaurus-like animal rather than some sort of bird.
Fossil eggshells are not actually ichnofossils. As they preserve direct evidence of an organism'sphysiology, their shape, size and the structure of the eggshell give more robust clues to their origin than do footprints. Typically, fossil eggs can be quite unequivocally assigned to a specific group of organisms, e.g.chelonians,squamates,dinosaurs,crocodiles,pterosaurs or (modern)birds.
Still, egg fossils rarely are identifiable even tofamily, let alone tospecies. Thus, they are assigned toootaxa, which are much like ichnotaxa but form a distinct group (Veterovata) inparataxonomy. For the time being however, ootaxa assigned to prehistoric birds at least tentatively are listed here:
†Oolithus (Late Jurassic of England) – avian?
†Dispersituberoolithus (Oldman Late Cretaceous of S Alberta, Canada) – neornithine?
†Gobioolithus (Late Cretaceous) – paleognath?
†Subtiliolithus (Late Cretaceous of Mongolia)
†Tristraguloolithus (Oldman Late Cretaceous of S Alberta, Canada) – galliform (cracid)?
†Ornitholithus (Late Paleocene of Spain – Early Eocene of France) – presumably fromGastornis
†Incognitoolithus (Eocene of North America) – ratite?
† Type A ("aepyornithoid") eggs (Tsondab Early Miocene of Namibia – Pliocene of Asia) – ratite?
†Namornis (Middle Miocene of Namibia – Late Miocene of Kenya) – ratite?
†Diamantornis (Middle Miocene of Namibia – Late Miocene of UAE and Kenya) – ratite?
^Wright, Joanna L. (2004): Bird-Like Features of Dinosaur Footprints.In: Currie, Philip J.; Koppelhus, Eva B.; Shugar, Martin A. & Wright, Joanna L. (eds.):Feathered Dragons: Studies on the Transition from Dinosaurs to Birds: 167–184. Indiana University Press.ISBN0-253-34373-9
^Payros, Aitor; Astibia, Humberto; Cearreta, Alejandro; Pereda-Suberbiola, Xabier; Murelaga, Xabier & Badiola, Ainara (1930). "The Upper Eocene South Pyrenean Coastal Deposits (Liedena Sandstone, Navarre): Sedimentary Facies, Benthic Foraminifera and Avian Ichnology".Facies.42 (1):19–23.doi:10.1007/BF02562569.
^Similar footprints, now lost, were reported from roughly contemporary strata in France in the 19th century:Buffetaut, Eric (2004). "Footprints of Giant Birds from the Upper Eocene of the Paris Basin: An Ichnological Enigma".Ichnos.11 (3–4): 357.doi:10.1080/10420940490442287.
^Patterson, John & Lockley, Martin (2004). "A ProbableDiatryma Track from the Eocene of Washington: An Intriguing Case of Controversy and Skepticism".Ichnos.11 (3–4): 341.doi:10.1080/10420940490442278.
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Bühler, Paul & Bock, Walter J. (2002). "Zur Archaeopteryx-Nomenklatur: Missverständnisse und Lösung".Journal für Ornithologie.143 (3): 269.doi:10.1046/j.1439-0361.2002.02006.x.
