Redescription and Phylogenetic Analysis of the Mandible of an Enigmatic Pennsylvanian (Late Carboniferous) Tetrapod from Nova Scotia, and the Lability of Meckelian Jaw Ossification

Roland B Sookias,Christine Böhmer,Jennifer A Clack,Andrew A Farke

doi:10.1371/journal.pone.0109717

Abstract

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.

Highlights

The lower jaw of primitive tetrapods and tetrapodomorph fishes is composed of a large number of separate ossified elements, some endochondral and some dermal
The review of Ahlberg and Clack [2] identified many of the key changes seen in the lower jaws of tetrapods that distinguish them from tetrapodomorph fishes, and has been augmented by the work of Bolt and Lombard [5] and Lombard and Bolt [6]
The coronoids wholly lack dentition. This is the case in all Carboniferous tetrapods except the whatcheeriids [6] and Occidens [21]; coronoid dentition is a primitive feature of stem tetrapods [2] and is lacked by baphetids [22], [23], temnospondyls [22], [24] and Caerorhachis [25] among others

Summary

Introduction

The lower jaw of primitive tetrapods and tetrapodomorph fishes is composed of a large number of separate ossified elements, some endochondral and some dermal. Many tetrapod apomorphies had been previously identified [4], additional key characters (e.g. degree of exposure of Meckelian bone; form of coronoid and adsymphysial dentition) differentiating tetrapod and tetrapodomorph fish ( = ‘‘osteolepiform’’) jaws have been recognized subsequently. This has allowed the identification of several specimens previously identified as fishes as stem tetrapods The review of Ahlberg and Clack [2] identified many of the key changes seen in the lower jaws of tetrapods that distinguish them from tetrapodomorph fishes, and has been augmented by the work of Bolt and Lombard [5] and Lombard and Bolt [6]. Use of mandibular characters in particular has been successful in improving resolution of tetrapod phylogeny, and because mandibles are relatively durable and often remain articulated [5], further mandibular analyses are likely to bear fruit

Methods

Results

Conclusion