Fig. 2.

Competing hypotheses of coracoid homology. (A) Among many early workers (e.g. Parker, 1868) the procoracoid of basal taxa was assumed to be lost in more deeply nested forms. Consequently, the coracoid process of therians and the coracoid element of modern reptiles were both considered homologous with the metacoracoid. (B) Howes (1887, 1893) later argued that in therians the procoracoid gave rise to the coracoid process while the metacoracoid contributed to the glenoid. (C) Alternatively, Lydekker (1893) suggested that the procoracoid formed the coracoid process of therians and the metacoracoid was the equivalent of the reptilian coracoid. Abbreviations: acr (acromion), cpr (coracoid process), gln (glenoid), met (metacoracoid [grey]), pro (procoracoid [black]), scp (scapula [white]). Sources for images: basal amniote: †Captorhinus modified from Gaffney (1990) and reproduced with permission from the American Museum of Natural History; therian: Bathyergus modified from Parker (1868); reptile: Alligator modified from Mook (1921) and reproduced with permission from the American Museum of Natural History. Not to scale.