L imusaurus inextricabilis ( T heropoda: C eratosauria) gives a hand to evolutionary teratology: a complementary view on avian manual digits identities

It is widely accepted that birds are rooted within theropod dinosaurs. However, there is controversy between palaeontological and developmental data regarding manual digit identities of birds and their tetanuran ancestors ( I , II and III vs. II , III and IV ). To resolve this conflict, the principle of a frame‐shift has been considered. Identities of digits I – III would develop on condensations 2–4. Nevertheless, the discovery of the basal C eratosauria L imusaurus inextricabilis has been used as a reference to define the digital identity of Tetanurae as II – IV . The new concept of evolutionary teratology states that certain anatomical structures identified in evolutionary lineages are viable developmental anomalies (‘adaptive’ or not), becoming part of the considered groups. The features of L imusaurus ' forelimb match teratological characterization. This diagnosis, associated with the variations previously identified in derived C eratosauria taxa ( C arnotaurinae), underline an anatomical and developmental independence (regarding evolutionary conserved mechanisms) compared with T etanurae and therefore birds. Consequently, L imusaurus should not be used as a reference concerning the identity of avian manuals digits. Evolutionary teratology supports identities I , II and III of the tetanuran manus via a frame‐shift that did not occur in the C eratosauria lineage. © 2016 The Linnean Society of London