TIEMPOS DE LA CRÍTICA IDEOLÓGICA (1975-1984)

The finer grained sediments recovered from the fossiliferous fissure at Ruthin consist of buff-brown fissile shales, yellow-grey inorganic limestones, and harder, red, marly calcareous sandstones. The calcareous sandstone is the dominant lithology, containing mud clasts, small cavities with calcite crystals and an abundance of disassociated bone. The rock is only moderately sorted, three grain size classes are present:

approximately 15% pebble-grade, clasts (limestone fragments, mud clasts and bone fragments) are tabular, subangular to rounded with generally low sphericity. The maximum clast size is approximately 1cm

(inorganic) to 2.5cm (bone),

approximately 80% coarse sand-grade component including equant, angular- subangular calcite crystals with generally low sphericity, remaining 5% silty red matrix

The rock fabric shows no obvious preferred orientation of clasts, be they inorganic or organic.

The bone is entirely disassociated, no articulated elements have ever been found. It is typically white to light brown in colour and is generally easy to distinguish from the surrounding matrix. Unlike the preservation at other fissure localities (e.g. Cromhall, Pant-y-fiynnon) the disassociated bone at Ruthin is also highly fragmented, with most fragments being in the 2-5mm size range. The preservation of bone fragments varies from tiny specimens that preserve the most delicate teeth (figures 3.5.12, 3.6.2) to the large majority of fragments that are highly abraded and have lost almost all recognisable anatomical features.

The poor size-sorting and lack of any preferred clast orientation of the Ruthin sediments suggests the bone has not been concentrated as a result of in situ winnowing.

Ruthin assemblage: introduction 85

but was introduced into the fissure and deposited quickly (allocthonous deposition).The nature of the fossil assemblage supports this interpretation, preserving an apparently wholly terrestrial fauna. The fossil assemblage does not appear to have been accumulated by predator activity. The assemblage is not dominated by shed archosaurian teeth, and scanning electron microscopy of bone surfaces reveals no evidence for acid etching.

The similar sedimentology in which Cromhall Faunal Association B has been recovered (section 1.4.2a) has suggested to Walkden & Fraser (1993) that the Cromhall bone originally accumulated as surface lag deposits that were introduced into the fissures rapidly, perhaps as storm surges. A similar interpretation seems reasonable for the Ruthin accumulation. Interestingly, the bone preservation differs significantly between Ruthin and Cromhall Faunal Association B. Although disarticulated, the Cromhall bones are generally well-preserved, maintaining all facets and delicate processes; by contrast, the Ruthin elements are typically highly abraded but may be moderately well-preserved. The difference may relate to greater transport time for the Ruthin specimens, or a greater length of time accumulating on the land surface before they were redeposited in the fissure. A greater length of time between deposition events may be suggestive of a more arid local climate.

There remain further unprepared samples of bone-rich rock from Ruthin in the collections of Aberdeen University and the National Museum of Wales. These may be of interest to anyone who wishes to study the taphonomy and sedimentology of the site in more detail.

86

3.2 TRICUSPISAURUS THOMASI ROBINSON. 19571>

3.2.1 INTRODUCTION

The Ruthin fauna is best known for the presence of Tricuspisaurus thomasi Robinson, 1957b. Tricuspisaurus was originally described as related to the unusual archosauromorph reptile Trilophosaurus buettneri Case, 1928 from the Chinle Group (Dockum Formation) of Texas (Gregory, 1945).

Trilophosaurus buettneri has a peculiarly derived dental and cranial anatomy (Gregory, 1945; Parks, 1969; figures 3.2.1A-C, 3.2.2). The premaxilla and the anterior portion of dentary and maxilla are edentulous, suggesting the presence of a keratinous beak. Posterior to this, a dentition of around twelve transversely widened teeth is developed, each tooth being fused into its socket. The teeth (figure 3.2.2) are tricuspid, the three equal sized cusps being arranged in a straight, transverse line across the tooth; sharp, concave ridges link up the cusps. Vertical wear facets, found on the anterior and posterior surfaces of the teeth, show that Trilophosaurus masticated with a simple, vertical motion and lacked any horizontal component of movement when the teeth were in contact (Parks, 1969). The cranial anatomy is notable for the absence of laterotemporal fenestrae, and the presence of wide supratemporal fenestrae. Both may be seen to be adaptations linked to the unusual dentition, through skull strengthening and increasing the area for adductor muscle attachment.

Despite the unusual skull morphology, the postcranium of Trilophosaurus (Gregory, 1945; figure 3.2. ID) is typically archosauromorphan.

Dilkes (1998) lists five (of six) archosauromorph synapomorphies that are shown by Trilophosaurus: presence of posterodorsal process of the premaxilla; presence of a sagittal crest; presence of slender and tapering ribs that lie at low angle to the vertebrae; dorsal margin of ilium composed of small anterior process and a larger posterior process; absence of a medial centrale of carpus.

Trilophosaurus may range in size from a skull length of 70-150mm (equivalent to a snout to vent length of 0.5-1.2m). It forms a widespread and locally abundant

Ruthin assemblage: Tricuspisaurus thomasi 87

p,

Figure 3.2.1 Trilophosaurus buettneri. Cranial anatomy in A) lateral, B) dorsal and 0 ) occlusal views (after Parks, 1969). D) Postcranial reconstruction, excluding distal caudal anatomy (from Gregory, 1945).

component of the Chinle Group fauna.

This study supports Robinson’s (1957b) assertion that the Ruthin form Tricuspisaurus is closely related to Trilophosaurus. In addition, on the basis of a uniquely similar dentition, I suggest that several other enigmatic taxa may also be attributable to the Trilophosauridae. These are: Variodens from the Triassic fissures of Emborough Quarry in the U.K. (Robinson, op. cit.), Trilophosaurus Jacobsi (=Chinleogomphius) from the Chinle Formation of Arizona (Murry, 1987; Sues & Olsen, 1993) and Xenodiphyodon from the Turkey Branch Formation of Virginia (Sues & Olsen, 1993). Two further unusual forms, Coelodontognathus ricovi from the Early

Ruthin assemblage: Tricuspisaurus thomasi 88

B

II

0

2mm

0

2mm

Figure 3.2.2 The tricuspid teeth of Trilophosaurus buettneri (after DeMar & Bolt, 1981). A) tooth from a mature individual in i) occlusal and ii) mesial views. B) tooth from a juvenile, note the presence of anterior and posterior cingula, in i) occlusal and ii) mesial views.

Triassic of Russia (Otschev, 1967) and Anisodontosaurus from the Moenkoepi Formation of Arizona (Welles, 1947; Hunt et al., 1998), although not preserving a tricuspid dentition, may also be related.

The recognition that the Trilophosauridae are not a monotypic grouping, expands both the stratigraphie and geographic range of the clade. In his recent phylogenetic analysis, Dilkes (1998) identified a ghost lineage of c.30 Ma preceding the first appearance of Trilophosaurus in the fossil record. Assigning the enigmatic taxa listed above to the Trilophosauridae reduces the length of this ghost lineage by 20Ma.

Fraser (1986^) has suggested that Tricuspisaurus is a procolophonid. In his study of the Ruthin fauna he identified several procolophonid jaw elements and sought to associate these with Tricuspisaurus. The single specimen on which this association is based has been misinterpreted. The procolophonid jaw elements Fraser identified are attributable to a separate taxon, Procolophonid A, described in section 3.3. Tricuspisaurus and other trilophosaurids (e.g. Xenodiphyodon, named by Sues & Olsen, 1993 as an aberrant procolophonid) share no derived features with any definite

Ruthin assemblage: Tricuspisaurus thomasi 89

procolophonids. Features that are shared, such as the similar ‘protothecodont’ dentition, are common to a number of disparate fossil taxa and may be plesiomorphic for Eureptilia.