IDENTIFICATION AND INTERPRETATION

Put simply, the purpose of this study has been to discover as much as possible about the animals living on the limestone islands of South Wales, in the vicinity of the current Ruthin Quarry, two hundred million years ago. Since these animals lived at a key time in the evolutionary history of many terrestrial vertebrate groups, their anatomy may have great bearing in determining the course of that evolution. At the outset, the fossil material available from Ruthin Quarry offered great potential to achieve the study’s objectives. In particular, the number of different taxa found in the site is high (this thesis identifies at least nine distinct taxa as present), and the sample size of many skeletal elements is large. The fossil material is easy to prepare, and unlike larger vertebrate remains, requires only limited storage space, and is relatively easy to manipulate. However, because the fossil material is disarticulated, and individual bones are typically also fragmented, a clear and objective means of working must be set out, so that any conclusions reached are founded in a degree of rigour.

A rigorous means of working with disarticulated material has only recently been introduced into the study of microvertebrate remains (e.g. Brinkman, 1990; Blob & Fiorjtllo, 1996). The widespread use of synapomorphy-based phylogenies has led microvertebrate workers to seek to identify derived characters in their specimens (e.g. Kaye & Padian, 1994) and resist the temptation to name taxa on the basis of undiagnostic fossils, i.e. those that show only plesiomorphic characters.

The practice used in this study has been firstly to approximate the number of taxa in the sample (all vertebrate fossils recovered from Ruthin Quarry) on the basis of easily identifiable elements. If preservation of all bones in the site were perfect, and all parts of each animal were preserved in equal frequency, then any element showing a range of morphology across taxa could be used for this task. Because this was obviously not the case, tooth-bearing elements were preferred. Tooth-bearing elements have the advantage of being easily identifiable, even when fragmentary, and of having a broad range of discrete morphologies that can reasonably be assumed (in most cases) to represent taxonomic diversity. Ideally, only one type of tooth-bearing element would be used (i.e. premaxillae, or dentaries, or maxillae) to discount dental variation along the tooth row. Since this method might exclude taxa with a low sample size, or those for which tooth-

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bearing elements were too fragmentary to assign to a specific bone type, a more catholic approach was adopted. Sensible assumptions (based on comparative studies with more completely known taxa) were made on how to group elements (premaxillae with maxillae etc.) so that as many taxa as possible were identified while ensuring that taxa were not counted twice. Various methods were used to document and discriminate between the morphologies of tooth-bearing elements. These included anatomical descriptions using light and scanning-electron microscopy, to document such features as tooth morphology (transversely broad, transversely narrow, presence of cusps, serrations etc.) and implantation (pleurodont, thecodont, ‘protothecodonf etc.); and the morphology of the bone away from the tooth row (presence and distribution of facets, foramina etc.). These are discussed fully in chapter 3.

Once the number of taxa present was approximated on the basis of tooth-bearing elements, attempts were made to identify the relationships of the taxa, so that further material might be associated with these tooth-bearing bones. The processes of determining the relationships of the taxa, and associating any other material with tooth- bearing elements were intimately related.

In a more typical palaeontological study, associating elements is relatively simple. Skeletons may still be associated, or may be reconstructed from various partly articulated skeletons; or may be assembled from monotypic assemblages, or assemblages of limited taxonomic diversity. In the Ruthin assemblage all bone is dissociated, and there is a high taxonomic diversity. Associating bones is thus difficult. Reliable associations may only be made when a population of elements shares a discrete morphology. This method has been used to associate tooth-bearing elements as described above, and has also allowed two populations of dermal bones to be identified (one procolophonid, one archosaurian) due to shared patterns of sculpture (an ornamented external surface to the bone).

The relationships of the taxa so far identified were based on comparison with other fossil and extant taxa. An early assumption, based on a sift through a sample of the Ruthin fossils, and previous reports of the locality (Robinson, 1957b; Fraser, 1986a) was that all vertebrate remains could be attributed to the Reptilia sensu Laurin & Reisz, 1995 (see figure 3.3.2). I have uncovered no evidence of the presence of non-reptilian

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vertebrates (e.g. ‘fish’, amphibians, synapsids) and this assumption appears valid.

I have attempted to identify characters that are derived within the Reptilia, since these provide support for more detailed hypotheses of relationship, while plesiomorphic characters do not. Thus, as with Kaye & Padian’s (1994) study of the microvertebrates of the Triassic Placerias Quarry I have attempted, wherever possible, to classify the Ruthin taxa down to the smallest grouping evidenced by synapomorphic characters. Where relationships can only be proposed on more general similarities in morphology, and the presence of similar taxa in other early Mesozoic localities, the uncertainties in the assignment have been explicitly stated.