Birds are generally thought to have evolved from small theropod dinosaurs in the mid-late Jurassic, over 150 million years ago.
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However, it has also been suggested that they evolved from thecodonts, which are primitive archosaurs, or crocodylomorphs. All three theories will be discussed here (Padian et al., 1998).
A cladogram showing the systematic position of birds according to all three hypotheses is illustrated in figure two.
Figure 2: Possible systematic positions of birds. Redrawn from Padian et al (1998).
The Thecodont Hypothesis
The thecodont hypothesis for the origin of birds is characterised by being a default option. This is because it is not due to positive correlation of characters and taxa but due to the negative association with other taxa. It was originally thought that theropods shared more features with birds than any other group. However, at the time there was no evidence for clavicles in theropods, which are the equivalent of the furcula or wishbone in birds. It was thought that clavicles could not have been lost and then re-evolved into furcula and so a more ancient ancestor for birds was sought for.
The candidate suggested was the thecodonts from which all other archosaurs are thought to have evolved. A problem is that the archosaurs are a "wastebasket" group containing all archosaurian reptiles that do not fit into dinosaurs, crocodiles or pterosaurs. Hence, they have no diagnostic characters of their own and are not a "good" phylogenetic group. This makes it difficult to compare them with other taxonomic groups (Padian et al., 1998).