BSCI392
11-30-07
Flying Theropods

Adapted from notes for GEOL104 by Tom Holtz

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Coelurosauria (Middle Jurassic - Recent) includes a bewildering diversity of forms, each with ecological specializations of interest. Taken as a whole, their evolution reflects a trend toward increasing "birdiness." Some specific trends include:

Coelurosaur diversity: We discussed tyrannosaurs and ornithomimosaurs in the previous lecture. A couple of other landmarks in the evolution of coelurosaur trends include:

Oviraptorosauria ("oviraptors"). (Early - Latest Cretaceous) North America and East Asia: Distinctive, with boxy skulls and a tendency to lose teeth develop impressive crests. They share with more birdlike theropods:
  • Proper contour feathers. (Well preserved in Caudipteryx)
  • Relatively long forelimbs.
  • A large bony sternum
  • Reorientation of the shoulder socket to face laterally
  • A semilunate carpal (=half-moon shaped): Enabling the hand to fold back in the manner of a bird, rather than going "limp-wristed."
  • Brooding behavior on nests.
Despite this proto-birdiness, some oviraptors lived distinctly non-birdy lives. Indeed, the largest oviraptorosaur, Gigantoraptor was the size of a medium-sized tyrannosaur.


Caudipteryx from Dinosaurs in Art: The Paleo Art of Michael Skrepnick

Alvarazsauridae. (Late Cretaceous): E.G.: Shuvuuia -slender chicken - turkey sized runners with:


Shuvuuia from Julius T. Csotonyi/

Eumanirpatora: (Middle Jurassic - Recent) The coelurosaur flight-plan

Archaeopteryx - "The first 'bird'" (Late Jurassic)

When first discovered, in the 1860s, this creature was the only known feathered fossil. Thus, for over a century it has been a fundamental benchmark in paleontology - "the first bird." As more specimens of Archaeopteryx have been found and studied, and as OTHER feathered theropods have come to light over the last ten years, Archaeopteryx has come to seem less and less birdy. Indeed, if we were to see a living one, it would seem immediately weird with its:

To us, it represents something like the basal condition of Eumaniraptora, the monophyletic group of coelurosaurs in which feathers became functionally coupled with aerodynamics. Among their derived features:
    Longer arms and larger hands
  • Small size (crow)
  • The tail is very mobile at its base and very stiff along its length
  • Backwards facing pubis (pelvic bone)
  • Long leg feathers (seen on all basal members in which they might be preserved)

Basal members of the major eumaniraptoran groups were broadly similar:

  • Deinonychosauria (including "Raptors", Middle Jurassic - Latest Cretaceous): Microraptor. From such beginnings, deinonychosaurs, evolved into larger terrestrial predators like Velociraptor.
  • Avialae (including birds, Early Cretaceous - Recent): Jeholornis - a chicken - turkey sized omnivore found with both seeds and fish in its gut.

The Berlin Archaeopteryx 1881, from Linda Hall Library of Science, Engineering, and Technology

But could it fly?: Biomechanically, Archaeopteryx is considered to be at or near the threshold of flight. Its wings are very low-aspect ratio, its sternum is not large enough to serve as the origin for powerful flight muscles, and its skeleton was not strengthened and fused in the manner of modern birds. If it could fly, it did so weakly and over short distances. So why did it waste metabolic energy growing and supporting them? Indeed, what did Caudipteryx do with its dinky "wings?"

Traditionally, paleontologists have considered two hypotheses for the origin of bird flight:

Wing assisted incline running (WAIR):
In the early 2000s research by Ken Dial of the University of Montana's Flight Laboratory revealed a locomotory behavior in modern birds not previously realized. Birds (in this case chukar patridges) were discovered to run vertically up surfaces, aided by flapping their wings back and forth in order to generate traction against the surface. They called this behavior Wing Assisted Incline Running, or WAIR for short.

Note that these birds are NOT climbing in the typical sense: they are literally running up the sides of trees. Dial and his team studied the ontogenetic (growth) changes in the ability for birds to use this behavior, and also experimented by trimming the feathers of birds to different lengths. They found:

A rogue's-gallery of Cretaceous avialians

Confusciusornis (Early Cretaceous): A crow-sized theropod whose tail vertebrae were fused into a short solid rod, the pygostyle (Note, some oviraptorosaurs independently evolved this feature) and a toothless beak (independently evolved from proper birds.) Unlike in proper birds and their kin, there was no tail fan in the Confuciusornithidae. Instead, most specimens show no major tail feathers, while others show two very long ones. Perhaps these were sexual signals, or growth indicators, or simply lost in the tail-featherless ones.
Confusciusornis from Biology-Online.org

Enantiornithes (Early - Latest Cretaceous): The most diverse and abundant group of birds of their time. Most were toothed. Some may have been insect eaters, some fish eaters, some fruit or seed eaters, and possibly even some meat eaters. The smallest were sparrow-sized; the largest, eagle-sized (and thus the largest flying birds of the Mesozoic). They have been recovered from all over the world, and environments from deserts to shores.

Enantiornithes retained small finger claws, but these were greatly reduced compared to earlier avialians. Despite many depitctions to the contrary, enantiornithines do not seem to have the tail fan of feathers which characterize modern birds, and at most of a pair of long tail feathers (though not as long as those of Confusciusornis).

Enantiornithes share with proper birds, synapomorphies including:

  • Carpometacarpus: fusion of the wrist bones to the metacarpals, and the metacarpals to each other.
  • Development of the alulua: digit I supports feathers separate from the rest of the wing, for better control in flight
  • Increased sternum size
  • Tarsometatarsus: fusion of the distal tarsals to the metatarsals, and the metatarsals to each other
  • Backwards facing pedal digit I facilitating the grasping of branches
Based on the evidence of the forelimb, powered flight was almost certainly present in the last common ancestor of Enantiornithes and proper birds.

Enantiornis from Pavel Riha

Hesperornithes (Early - Late Cretaceous): Diving sea birds. Early hesperornithines (such as Early Cretaceous Enaliornis) may have still had the power of flight, but later ones greatly reduced their wings. In fact, the most specialized forms lost their forearms altogher, with only stumps of humeri for wings. These latter include Hesperornis of the Late Cretaceous of the inland seas of midwestern North America. Even these advanced birds still retained teeth.

Despite their unique derivations, they share with proper birds the following synapomorphies that Enantiornithes lacks:

  • The tail fan: many broad feathers coming out of a bulb surrounding a shortened pygostyle
      This allows for the tail to act as a separate surface, controlling flight and landing to a much greater degree
  • Loss of long leg feathers. Indeed, these changes could be correlated. Perhaps the tail fan took over a flight control function previously exercised by leg feathers.

Hesperornis from Palaeos

Ichthyornis (Late Cretaceous): A sea "bird" ecologically similar to living sea birds. This creature shared with proper birds, a keeled sternum to support more powerful wing muscles.

Even so, it retained primitive features such as teeth


Ichthyornis from U C Davis

AVES (Late Cretaceous - Recent): The last common ancestor of all living birds and all of its descendants. Distinguished by:
  • Toothless beaks (Evolved convergently with some ornithomimosaurs, some oviraptors, Confusciusornis, and some enantiornithines)
For truly enigmatic reasons, these were the only theropods to escape the Cretaceous-Tertiary extinction.


Prespyornis from Critters Pixel Shack