Comparative Anatomy – Aves and on
March 7, 2008
Class Aves -
Generally define birds as endothermic and have feathers.
A. First fossil was archaeopteryx – primitive reptile or early bird. Thought to have feathers, vertebrae extending into the tail.
Fossils from mid-Jurassic.
1. Argument about whether or not archaeopteryx could actually fly is the lack of the keel, which allows the modern pigeon
to fly. But bats don’t have it either, so not necessarily valid.
B. 2 subclasses:
– newer, modern birds, all have teeth, fossils from the early Cretaceous. No birds today have teeth, but if
look at them in the embryo stage, have teeth. By late Cenozoic, no birds have teeth in the fossils. Most bones hollow,
with a fused skull.
Origin of Birds:
Traditional view is that they have evolved from pseudosuchian thecodonts
Coelurosauria – small, biped, saurischian. chicken-sized, agile. When John Ostrum looking at a fossil of this, he found the
imprint of feathers. So possible that ceolurosaurs came from archaeopteryx, and that modern birds came from this line of
coelurosaur dinosaurs that leads to birds. Still the current view today.
Origin of Feathers:
Found many fossils in China with the imprint of feathers.
Theories of feather evolution: why and what are they good for?
Hypothesis: that they evolved for fight. But not correct, because feathers are only good for flight when they’re fully formed.
So cannot account for the intermediate stages of feather development
Evolved as thermal insulators. Possibly became pre-adaptors for gliding. In archaeopteryx, argued that as the feathers
developed more and more, gained the ability to glide.
Evolved initially to keep heat out. Many modern lizards can do this by fine-tuning their body heat by a degree or more by
how they face the sun with their scales.
Could archaeopteryx fly?
Some argue no, because no keel, and the feathers appear to be mainly for thermal insulation.
Others suggest that if that’s all the feathers were for, why were the wing and tail feathers so well developed? Another
suggestion that they used the feathers as insect traps, but again, what is the tail for?