Question

Fossilized remains were discovered in a remote Cretaceous formation from Wyoming-Montana area of the United States. After carefully examining the bones, you see that the jaws are toothed and that the anterior limbs are modified for flight. You assume it is a tetrapod's fossils.  Determine whether this tetrapod is related phylogenetically to the subclass Archaeornithes, the Neornithes of the Aves or to the subclass Eutheria of the Mammalia. Describe the features of the fossils that allows you to make your determination while eliminating other possibilities.

Answer 1

It belongs to the extinct group of Archaeornithes. This group includes birds showing some characters of repriles and some characters of birds. They are considered to be intermediates of reptiles and birds. The characters of reptiles include, teeth attached to jaws, claws in their fore limbs, reptilian rib cage , long tail with free caudal vertebra and heavy bones are some of the characters. The avian characters include, fore limbs modified in to wings as it can be seen with the bone structure of limb. The beak like structure and the fore limb bones modified to form wing like structure are avian characters.

So the fossil definitely belongs to Archaeornithes.

Neornithes are modern birds with out teeth in the beak, no claws in the fore limbs, hollow bones to make the body light, tail bones forming pygostyle. So the fossil does not belong to Neornithes because it does not show these characters.

It does not belong to Eutheria because it has a beak and teeth attached to beak indicate the reptilian character. Because mammals have thecodont teeth which means the teeth are set in sockets. Where as the reptiles have teeth attached to jaw bone. The teeth in mammals are also heterodont which means there are different types of teeth present in the jaw bone. Because of this reason the fossil does not belong to Eutheria.