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:
Alvarazsauridae. (Late Cretaceous): E.G.: Shuvuuia -slender chicken - turkey sized runners with:
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:
Basal members of the major eumaniraptoran groups were broadly similar:
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:
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:
Thus, it now seems likely that the initial evolutionary impetus toward flight was the adaptation of the forelimb for WAIR. Of course once theropods could get into the trees, the door was opened to using wing-generated lift for:
Oddly, of the two major Eunamiraptoran groups, only Avialae stepped across the threshold of proper flight while deinonychosaurs backed away from it.
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. 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:
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:
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
AVES (Late Cretaceous - Recent): The last common ancestor of all living birds and all of its descendants. Distinguished by: