Question

Assume the above cladogram of archosaurs (“ruling reptiles” - a potential misnomer) is accurate. Of the tips, only birds and crocodilians are still extant - all other tips are extinct.

I t to do Crocodylia Ornithischia Sauropoda Ceratosauria Camosauria Omithomimosauria Deinonychosauria Archaeopteryx Birds Aves Avialae Maniraptora Coelurosauria Tetanurae Theropoda Saurischia Dinosauria Archosauria

Q) Based on the above tree, which of the following is not considered a stem bird?

1. Archaeopteryx
2. Sauropoda
3. Carnosauria
4. Ornithischia
5. Crocodylia

Answer 1

Option (e) is the correct answer.

A stem group is composed of a pan-group, minus the crown group itself. So phylogenetic lineage leading back from Neornithes (which comes under the aves) to the point where it merges with the crocodilians, along with all side branches, constitute the pan-birds.

Neornithes which comes under the Aves are considered to be the crown group of birds.

If we follow the phylogenetic lineage leading to Neornithes(comes under the aves) in the left, all side branches belong to the stem birds except the crocodilians as they are the closest relative of birds.