The hyomandibulae of rhizodontids (Sarcopterygii, stem‐tetrapoda)

Abstract

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 fish-like 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.

A reappraisal of the origin and basal radiation of the Osteichthyes

“…In rhizodonts, these nerves extend through a deep sulcus that Brazeau and Jeffery (2008) interpret as a possible ontogenetic precursor of an enclosed canal. We are convinced that these canals and sulci are derived based on their absence in Acanthodes (Miles, 1973b), chondrichthyans (Hotton, 1952;Maisey, 1989;Coates and Sequeira, 2001), and, apparently, 'placoderms' (Stensiö, 1963;Forey and Gardiner, 1986; there is uncertainty surrounding the identity of the 'hyomandibular' of Jagorina: Goujet, 1984).…”

“…Continuous, but variously bilobate, facets are present in other coelacanths (Forey, 1998) and lungfishes (Miles, 1977). Clues to the shape of the hyomandibular articulation come only from the hyomandibular itself in porolepiforms (Jarvik, 1972) and rhizdonts (Brazeau and Jeffery, 2008), where the proximal surface is bilobate but lacks distinctly separated articular facets. The stem sarcopts Onychodus (hyomandibular only; Andrews et al, 2006) and Psarolepis (otic capsule only; Yu, 1998) appear to have a single facet, lacking the derived arrangement found in crown members.…”