Redescription and phylogenetic analysis of the mandible of an enigmatic Pennsylvanian (Late Carboniferous) tetrapod from Nova Scotia, and the lability of Meckelian jaw ossification.

The lower jaw of an unidentified Pennsylvanian (Late Carboniferous) tetrapod from Nova Scotia--the "Parrsboro jaw"--is redescribed in the light of recent tetrapod discoveries and work on evolution of tetrapod mandibular morphology and placed for the first time in a numerical cladistics analysis. All phylogenetic analyses place the jaw in a crownward polytomy of baphetids, temnospondyls, and embolomeres. Several features resemble baphetids and temnospondyls including dermal ornamentation, absence of coronoid teeth, and presence of coronoid shagreen. Dentary dentition is most similar to Baphetes. An adsymphysial toothplate may not preclude temnospondyl affinity. An apparent large exomeckelian fenestra, with the dorsal foraminal margins formed by an unossified element, echoes the morphology of the stem tetrapod Sigournea and is unusually primitive given the other features of the jaw. The jaw may thus provide an example of an intermediate stage in Meckelian element evolution.