Look at this paw:
See how flaky it is. Look at the wrinkled details. See what tremendous claws.
Let’s look now with another visual example, the leg compared to a guinea fowl with the skeleton:
Yet another paw with scaly features and large claws. Now let’s move on to a non-avian theropod dinosaur, like the king of tyrant lizards, Tyrannosaurus.
Same three fingers, same shape as unguals. In fact, if we were to analyze the anatomy of theropods, we would see many more converging features, from the bones of the tibia, fibula, parts of the pelvis. Not to mention the evolutionary convergence with ornithomimosaurine theropods in the case of paleognathous birds.
Honestly, the amount of convergence of taxa does not always represent ancestral traits. There needs to be a lot of fossil evidence to come to a conclusion like that. Therefore, in the science of phylogenetics, the concept of synapomorphy is established, which means similar traits between different taxa that establish kinship. In the past, it was thought that the dorsal sails of the genus Dimetrodon (and later several members of Pelicosauria) were a sign of ancestry with Spinosaurus.
Today this has no indication, as it is an isolated evolutionary trait, whose evolutionary convergence explains without having to establish any kinship ( Dimetrodon is an ancient relative of mammals, unrelated to dinosaurs, much less to Mr. Spinosaurus ). The dorsal sail, therefore, is just a converging trace.
But in the case of birds and non-avian theropods, the similarities are many and appear not just in an isolated taxon, but in several taxa. All birds share the same synapomorphies, despite species variations ( autapomorphies ), leg shape, fibula and tibia arrangement, among others. And that these traits are also seen in non-avian theropods. Therefore, it is too strong a common trait to be considered a mere convergence as in the case of Uncle Spino and Mister Dimetrodon.
Therefore, analyzing the most diverse traits in the skeletons of theropod dinosaurs, it is concluded today that birds are theropod dinosaurs, classified within the following large clades: Tetanurae, Coelurosauria, Tyrannoraptora, Maniraptora, Paraves and Avialae. And look, I’m cutting a shitload of other classification taxa.