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Early Jurassic birds?

August 18, 2012

Bird tracks from the Santa Domingo Formation. They have infamously been dated to the late Triassic, though an Early Jurassic date is more plausible.

Back in 2002, an unique set of footprints was found on the Santa Domingo Formation from Argentina. These footprints are ostensibly those of derived birds: they show a clearly reversed hallux, and the tracks appear to show the animals taking off and landing.

There’s one problem, though: this formation dates from a time period that is speculated to be around the Early Jurassic (NOT the Late Triassic, as previously assumed). Birds, and other maniraptor dinosaurs, are not known from the fossil reccord anywhere before the Late Jurassic.

Predictably, BANDits (people who think birds aren’t dinosaurs) and creationists devoted themselves with full fervor to these fossils, believing that they showed popular opinion wrong.

As always, they failed. Epically.

While revolutionary, an avian identity for these tracks is not out of place.

How birds diversified has been one of the great mysteries of paleontology. Already by the Early Cretaceous, avifauna was ridiculously diverse,and possibly even represented by early members of recognisable taxa like waterfowl. Meanwhile, the distinctly “poorer-in-fossil-reccord” late Jurassic already witnessed a well established diversity of non-avian maniraptors, with Dromaeosauridae and Troodontidae already well defined. This, combined with controversial new ideas about avian evolution (like enantiornithes being actually closer to scansiopterygids than modern birds. SRSLY), seems to suggest that avian radiation took place much earlier, and that it simply went unreccorded thanks to the poor fossil reccord of the mid-Jurassic.

This, combined with the possible Early Jurassic therizinosaurs from China, seems to suggest that maniraptors were already around by the Early Jurassic, which seems to coincide with the rapid diversification of theropod dinosaurs in this time period, as their competitors had become extinct in the Triassic extinctions.

The only issue seems to be the reversed hallux, as only derived birds are known to possess this. However, this has been disproved, as other theropod clades seem to have had a reversed hallux as well:

A tyrannosaur footprint. Note the semi-reversed hallux

Indeed, many theropods previously thought to have small, vestigial first-toes might actually have had longer ones, as cartiladge structures seem to have elongated it on many taxa. It might also explain why the hallux was preserved in most theropods in spite of previously assumed vestigiality.

Indeed, rather than derived birds, the birds from Santa Domingo were probably rather basal, volant maniraptors, a viewpoint certainly not out of place, as maniraptors are often thought to have been basally volant.

Finally, these findings further negate whatever notions of birds outcompeting pterosaurs. At the early Jurassic, pterosaurs were still small and conservative in body planes, and with birds turning out to have evolved alongside pterosaurs all along, any notions of one group outcompeting the other become nonsensical.

 

EDIT: In Dinosaur Mailing List, some people have also suggested that these tracks belong to heterodontosaurs. Since it’s been established that the tracks show the animals taking off and landing, this could hint at a lineage of flying heterodontosaurs not preserved in the fossi reccord.

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