BSCI392
12-3-07
Flightless Flying Theropods

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Aves (Sometimes called Neornithes). (Late Cretaceous - Recent) The most recent common ancestor of modern birds and all of its descendants. In broad strokes, the phylogeny of Aves is very clear:

Many of the most significant avian biomechanical issues have been dealt with in other lectures, so in this survey we concentrate on one thing:

Flightlessness: On many occasions, birds have secondarily lost the ability to fly. How strange is that, given the degree to which their bodies have been transformed for flight? At least as strange as the idea of terrestrial amniotes plopping back into the oceans.

General observations:

For example: Cormorants are foot-propelled diving sea-birds, but also powerful fliers. At some point in the last seven million years, however, a flock got blown to the remote Galápagos Islands. there they encountered a situation in which good fishing could be had right off shore, but once out of range of the islands, the next reliable source of food as 500 miles away on the South American mainland. The paradoxical results:

Thus, growing wings was both a waste of energy and an actual hazard, since they were more likely to get you killed than to save you. The result was the rapid evolution of the flightless Galápagos cormorant, Nannopterus harrisi.

The ecological signal: We distinguish three general settings in which flightlessness evolves:

And yet, there have been special times and places when birds have become established as major parts of the terrestrial biota. Here is a survey. It's broken down taxonomically, but for each, be aware of the ecological setting.

Paleognathae

Flying paleognaths: The only extant flying members of this group are the tinamous of South America. Quail - chicken size. They live on the ground and prefer to run but fly clumsily in emergencies.

Tinamous are committed land birds. To the best of our knowledge, there has never been a paleognath, flightless or otherwise, on a small oceanic island.

The remaining paleognaths belong to the ratites - the great adaptive radiation of flightless birds of the southern continents.

Large island ratites:

Continental Ratities: Although flying and flightless paleognaths are present in the fossil record from the Paleogene onward, they are well represented by living species.

Neognathae - Galloanserae

Galloanserae are made up of two monophyletic groups:

Of these, only the Anseriformes turn up as flightless birds, but their record is interesting.

Oceanic island Galloanserae:

Continental flightless Galloanserae:

  • Diatryma (Paleogene) of Eurasia and North America. A large biped, usually interpreted as carnivore in the spirit of the old-fashioned theropods, but lacking arms and teeth. Diatryma was a rough contemporary of the hoofed crocodylian Pristichampsus, and if it was a carnivore, was also attempting to take over the terrestrial top predator niche.: Neognathae - Neoaves

    Oceanic flightless Neoaves: Flightless Neoaves are almost common on oceanic islands, with many Pacific islands boasting their own species of flightless rail (small ground running marsh birds, including the Phillipine rail and the takahe mentioned before). Other neoavian groups have contributed flightless ground birds, including the pigeons, notatbly:

    Continental flightless Neoaves: One neoavian group has joined the ranks of large continental flightless birds, the Phorusrachidae of Neogene South America:

    General observations

    The birds most likely to evolve flightlessness:

    The places in which flightlessness is most likely to evolve:

    So how do we explain the exceptional cases of continental flightless birds:

    Extinction: