Application of micro-computed tomography towards a taxonomic and palaeobiological reassessment of Huxley and Wright's tetrapods from the jarrow assemblage (langsettian, pennsylvanian) co. Killkenny

Abstract

The Jarrow assemblage records a diverse collection of fish and tetrapods from the Leinster Coalfield (Langsettian, Pennsylvanian), south east Ireland. Specimens are preserved in an area where the Jarrow Seam thickens locally, called the Jarrow Channel. The Jarrow Channel and assemblage are considered to have been deposited in an abandoned channel or oxbow lake. Brackish invertebrates and fish indicate that the assemblage represents an estuarine community which was been washed into the abandoned channel and preserved, contra to the previous idea that the assemblage was autochthonous. Despite the Jarrow assemblage representing the chronologically oldest Westphalian coal swamp assemblage, it has been relatively understudied compared to the contemporary assemblages of Linton, Ohio and Nýřany, Czech Republic. This is likely a result of the poor appearance of the fossils. Sectioning and cathode luminesce (Cl) imaging of body fossils reveal that bone morphology has been replaced with bituminous material and elongated apatite crystals. These thesis places the Jarrow assemblage in a geological context, looks at bone preservation of fossils from the assemblage and uses micro-computed tomography (μCT) to redescribe five of the original tetrapods described by Huxley and Wright (1867). The μCT forms the main body of the thesis. Huxley and Wright, 1867 described two aïstopods, Ophiderpeton brownriggi and Dolichosoma emersoni from the assemblage. μCT identifies previous unobserved characters in O. brownriggi, including the absence of a postfrontal and the presence of K-shaped ribs. Additionally, a new ophiderpetontid is describes as Peisterpeton milnerorum from one of the syntypes of O. brownriggi. The second aïstopod, D. emersoni, has characteristic more in tune with the basal aïstopods, and does not appear to be related to the derived phlegethontids as previously thought. The problematic taxonomic history of D. emersoni is resolved and it is formally placed in the genus Dolichosomatites. Two urocordylid nectrideans, Urocordylus wandesfordii and Lepterpeton dobbsii are here redescribed, with previously unidentified cranial elements being described. This will help tease out inter-nectridean relationships. The last of the tetrapods described using μCT is Ichthyerpeton bradleyae, which only preserves the trunk, pelvic and caudal regions. I. bradleyae has previously been considered to be a member of the embolomeres or the colosteids. μCT results show that the vertebral arrangement matches neither grouping and that vertebrae of Ichthyerpeton bradleyae are composed of a single centrum. The replaced nature of the body fossils from the Jarrow means that μCT has limitations. Yet this thesis shows that it can useful in resolving anatomical issues from the problematic Jarrow taxa