The hyomandibulae of rhizodontids (Sarcopterygii, stem-tetrapoda).

Despite its important role in the study of the evolution of tetrapods, the hyomandibular bone (the homologue of the stapes in crown-group tetrapods) is known for only a few of the fish-like members of the tetrapod stem-group. The best-known example, that of the tristichopterid Eusthenopteron, has been used as an exemplar of fish-like stem-tetrapod hyomandibula morphology, but in truth the conditions at the base of the tetrapod radiation remain obscure. We report, here, four hyomandibulae, from three separate localities, which are referable to the Rhizodontida, the most basal clade of stem-tetrapods. These specimens share a number of characteristics, and are appreciably different from the small number of hyomandibulae reported for other fishlike stem-tetrapods. While it is unclear if these characteristics represent synapomorphies or symplesiomorphies, they highlight the morphological diversity of hyomandibulae within the early evolution of the tetrapod total-group. Well-preserved muscle scarring on some of these hyomandibulae permit more robust inferences of hyoid arch musculature in stem-tetrapods.