Final answer:
The statement that thecodonts form a paraphyletic grade of archosaurs is true, as they include organisms that share a common ancestor but not all descendants.
Step-by-step explanation:
The statement that thecodonts form a paraphyletic grade, not a clade, of archosaurs is true. Paraphyletic groups include organisms that share a common ancestor but do not include all the descendants of that ancestor. Considering Archosauria, which comprises modern reptiles and birds, along with several extinct groups like dinosaurs and pterosaurs, thecodonts are seen as ancestral to this group but do not comprise all descendants, hence making them a paraphyletic grade. A clade, on the other hand, would include a common ancestor and all of its descendants, which thecodonts do not.
Considering the relationship and evolutionary aspects, Archaeopteryx, known as a transitional species between dinosaurs and birds, supports the evolutionary theory aspect that modern species share a common ancestor. Additionally, the Enantiornithes represents a separate evolutionary lineage from modern birds and did not survive past the Cretaceous period. The evolution of organisms like the archosaur reflects the complex branching and divergence that has occurred throughout the evolutionary history of life on Earth.