Some dinosaurs flew as dinosaurs, not as proto-birds, part 1

Flapping flight has evolved only thrice in the entire history of the vertebrates: in

• pterosaurs,
  -bats, and

• birds.

Because pterosaurs were not dinosaurs, the only dinosaurs that flew were avian or paravian, i.e. birds and their transitional ancestors. Any feathers grown by other dinosaurs – in clades separate from the origin of birds – served thermoregulation and perhaps gliding, but not flapping, flight.

No dinosaur converged with pterosaurs, bats or birds in the abbreviation of the bony tail. However, certain true dinosaurs – including Archaeopteryx itself – used flapping flight despite being neither birds nor on the evolutionary path to birds.

These persistently long-tailed dinosaurs add a new, fourth Class to the list of vertebrates that evolved flapping flight in their own right.

Biologists have underplayed flight in ‘non-avian’ dinosaurs for two reasons.

Firstly, Archaeopteryx was initially interpreted as a ‘bird’1 when Darwinists needed to show a missing link; misinterpretation turned into dogma as Neo-Darwinists continued to shoehorn what was more likely a dromaeosaur into an avian lineage.

This remains the (false) consensus to this day.

Secondly, an extremely reduced bony tail defines all true Aves. This tail

• was later incorporated partly into a synsacrum, which comprises sacral vertebrae, pelvis, and caudal vertebrae excluding pygostyle,
• crucially distinguishes birds from feathered dinosaurs, and
• has been brushed aside in the emphasis on wing feathers in the fossil record.

Feathered wings have been assumed to signify ‘bird’, without sober scrutiny of whether the tails of winged dinosaurs qualify them as avian.

We now know that the clade, including (in chronological order) Xiaotingia, Archaeopteryx, Microraptor, and Rahonavis, lasted 90 million years. The best-recorded fossil of this clade is Microraptor, which lived after Archaeopteryx but before Rahonavis and belongs to the family Dromaeosauridae of the theropod dinosaurs.

This means that the flying dinosaurs – contrary to being merely transitional – actually resisted the revolution in other dinosaurs during the Jurassic and the Cretaceous. Furthermore, their tenure far exceeds the entire time since the extinction of all dinosaurs

Once we put aside previous assumptions, the evidence suggests that ‘non-avian’ dinosaurs evolved true flight, and would have persisted in their flapping even if the birds had become extinct in the early Cretaceous.

In summary:
Some dinosaurs evolved into birds, but others flew in parallel right until all dinosaurs died. We should no longer conflate the two lineages.

to be continued in https://www.inaturalist.org/journal/milewski/98661-plumosaurs-flew-as-dinosaurs-not-as-proto-birds#...

Posted on June 14, 2022 05:43 AM by milewski