Evidence of evolution

All modern birds are descended from a group of dinosaurs called theropods. Theropods first appear in the fossil record during the late Triassic to early Jurassic period, roughly 200 million years ago. In contrast, fossils of taxa similar to modern birds can be found in the fossil record by the end of the Cretaceous period, roughly 66 million years ago.

The following table provides information about the presence of certain traits in theropods and modern birds. The presence of a trait is indicated by a checkmark.

Trait	Theropods	Modern birds
Bipedal	✔	✔
Hollow bones	✔	✔
Feathers		✔
Long, bony tail	✔
Fused clavicle		✔
Jaws with sharp teeth	✔
Reversed first toe		✔

Each answer choice below describes a specific fossil. Which fossil provides the strongest evidence for the discovery of a transitional species between theropods and modern birds?

(Choice A)
the fossil of an organism with fused clavicles and a reversed front toe, found in a 60-million-year-old rock layer
(Choice B)
the fossil of an organism that has jaws with sharp teeth and a long, bony tail, found in a 175-million-year-old rock layer
(Choice C)
the fossil of an organism with feathers and a long, bony tail, found in a 145-million-year-old rock layer
(Choice D)
the fossil of a bipedal organism with hollow bones, found in a 90-million-year-old rock layer

AP®︎/College Biology

Course: AP®︎/College Biology > Unit 7

Problem