Penegephyrosaurus curtiscoppi, Whiteside & Duffin, 2017
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publication ID |
https://doi.org/10.1111/zoj.12458 |
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persistent identifier |
https://treatment.plazi.org/id/760A879E-FF9E-5462-0B89-FE14FC46FD69 |
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treatment provided by |
Plazi |
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scientific name |
Penegephyrosaurus curtiscoppi |
| status |
sp. nov. |
PENEGEPHYROSAURUS CURTISCOPPI SP. NOV.
FIG. 4A – D View Figure 4
Derivation of name: Species name from latinized Curtis and Copp, the surnames of the two researchers Mike Curtis and Charles Copp, who discovered a great deal of information on the fissure deposits of the Bristol region, and of Holwell in particular.
Type locality: Rhaetian fissure fills in Carboniferous Limestone, Holwell quarry, Somerset, UK.
Holotype: BATGM C193 : a part of a left dentary with emplacements for five teeth ( Fig. 4A – D View Figure 4 ).
Diagnosis: A rhynchocephalian with teeth of a similar shape to those of Gephyrosaurus , but with a longer mesial – distal length of tooth. As a consequence there are at least 20% fewer teeth in the equivalent mid-section of the dentary. There are no significant gaps on the lingual side between the bases of the posterior teeth; any space between the lower regions of the teeth is overgrown with bone. The dentition is pleuracrodont, with the teeth positioned on an obvious dental shelf. Unlike Gephyrosaurus , the teeth are permanently ankylosed distally, mesially, labially, and lingually with bone, and resorption pits are either not present or extremely rare in the posterior teeth.
Remarks: BATGM C193 ( Fig. 4A – D View Figure 4 ) is a section of a dentary broken into two parts, with a total of five teeth. The specimen is rounded at the anterior end and on the lower labial side from abrasion in transport. Despite this abrasion there are discernible tooth wear facets on the lingual side between the teeth and also on the labial side between two middle teeth ( Fig. 4A, B View Figure 4 ). These facets demonstrate that the specimen is from the dentary, and that the teeth occluded between maxillary and palatine teeth.
The teeth, to some extent, resemble the simple triangular forms from the posterior acrodont region of Diphydontosaurus , but become more swollen ventrally and are more than 25% wider mesial – distally than the largest acrodont tooth in D. avonis . The dental shelf is more clear-cut than the mere remnant in Diphydontosaurus . There are no significant gaps between the teeth bases, with the space between the two posterior teeth overgrown with bone; this compares with the noticeable space (up to 25% of a tooth length) in the acrodont region of Diphydontosaurus (compare Fig. 4C View Figure 4 with E, F). The two posterior teeth are almost symmetrical, whereas the middle tooth has a steeper slope on the anterior side. The teeth are not ankylosed on the crest of the dentary and the dental shelf remains obvious, so the dentition is not truly acrodont; however, similar implanted teeth are found in juvenile Planocephalosaurus but are acrodont in adults. A similar implantation can also be found in the posterior mid-region of some Diphydontosaurus specimens, but P. curtiscoppi gen. et sp. nov. does not show alternating sized teeth, or the distal trough found in Diphydontosaurus ( Fig. 4H View Figure 4 ). The dentition is best described as pleuracrodont, as the teeth are strongly ankylosed at the lingual base as well as the mesial, distal, and labial sides. No carina is developed ( Fig. 4C, D View Figure 4 ), and the mean lateral compression (longest mesial – distal length to maximum transverse width) of the posteriormost tooth is 1.6 ( Table 1), just within the range of Gephyrosaurus and below the near 1.8 found in Diphydontosaurus . Furthermore, some posterior acrodont teeth of Diphydontosaurus reach a ratio of 2.0, whereas the highest for any Penegephyrosaurus tooth is ~ 1.8. By comparison with Gephyrosaurus and Diphydontosaurus , the specimen is from the mid-posterior region of the dentary. In this respect two of the three ratios shown in Table 1 are similar to those of BRSUG 29384 from the mid region of Gephyrosaurus .
The teeth are close in structure to G. bridensis , and the similarity of the upper part of the tooth shape is striking (cf. Fig. 4A, B and I, J View Figure 4 ), particularly the change from the cusp to a lower swollen region. In the G. bridensis example from Pontalun used for comparison, the specimen comes from the mid region of the dentary above a closed Meckelian canal, and the cusps of some teeth have been worn during life so that they look as though they sit on the ‘shoulder’ of the lower part of the tooth. The teeth bases, similar to those in G. bridensis , seem almost aligned to the jaw axis (anterior labial offset of 0 – 4 °). The dental implantation has some similarities to Gephyrosaurus , but the teeth bases, particularly of the anterior teeth, lie closer to the dental shelf and there are no discernible resorption pits (unlike some large pits present in Gephyrosaurus ; Fig. 4G View Figure 4 ), suggesting an even lower frequency (perhaps none) of tooth replacement. In the Gephyrosaurus specimens used for comparative purposes the gaps between the teeth are negligible in the posteriormost dentary dentition ( Fig. 4G View Figure 4 ), but can rise to about 10% of the tooth length in the mid region; there are, however, gaps in Evans’ (1980) illustration of the posteriormost teeth of holotype dentary UCL T.1503. Gephyrosaurus has mesial – distally shorter teeth and further differs in having a carina on the posterior dentary teeth ( Fig. 4G View Figure 4 ). The Penegephyrosaurus posterior teeth lingual apical – basal height to basal width mean ratio of 1.0 falls between the equivalent ratios of Diphydontosaurus (mean of 0.6) and Gephyrosaurus (mean of 1.6). Despite specimen BATGM C193 lacking the radial ridges present on the lingual side (any present in life may have been abraded during post-mortem transport) of both Diphydontosaurus and Gephyrosaurus ( Fig. 4F, G, J View Figure 4 ) it does have characteristics in common with both genera, but shows a much greater affinity with the latter genus; however, it is over 25% larger than Gephyrosaurus in the same jaw region, and the teeth more closely approach an acrodont condition. It is probable that like Diphydontosaurus the variation in mid-posterior dentary teeth of Gephyrosaurus is substantial, but we believe that there is sufficient difference to describe C193 as a new genus and species.
GEPHYROSAURIDAE GEN. ET SP. INDET. 1
Remarks: BATGM C126 consists of a jaw fragment ( Fig. 5A – C), which we assign to the posterior region of a left dentary. Viewed dorsally, the bone is narrow and there is little labial expansion, which is greater in maxillae (compared with the same tooth region of the dentaries) of all species of rhynchocephalians found in the fissure deposits (e.g. compare the larger labial area in occlusal views of Fig. 5C, D). There are discernible wear facets on both the lingual and labial sides ( Fig. 5A, B), tending to confirm the identification of the dentary; the smaller tooth base set slightly away from the lingual side than the others is also in accord with the suggestion of dentary. There are also no obvious facets for a jugal. We believe that our identification of the specimen as a dentary fragment (rather than a maxilla) is reasonable, but that the bone has clearly been worn and polished in post-mortem transport, as the fractured edge is rounded, so it is possible that other wear marks have been made indistinct. There are emplacements for four teeth ( Fig. 5C), three of which are mostly intact with bone of attachment surrounding and between the lower parts of the teeth; however, the dental shelf is obvious, particularly below the three anterior teeth emplacements, and the implantation is best described as pleuracrodont. The posteriormost tooth is more ankylosed than the others, with bone around the base in lingual and occlusal views.
In occlusal view the bone around the tooth base appears similar to that of the posteriormost Diphydontosaurus dentary tooth (cf. Fig. 5C with 4E, F). The base of the penultimate tooth is smaller and extends less medially than its mesial neighbour (which has a broken cusp), possibly indicating a mid-dentary alternating tooth pattern like Diphydontosaurus ( Fig. 4H View Figure 4 ), or simply an erupting tooth; however, the teeth more closely resemble the simple triangular tooth shape, bearing lingual radiating ridges with a similar number of teeth per unit length, found posteriorly on Diphydontosaurus dentaries. The posteriormost tooth is much more laterally compressed (ratio of ~ 2.7; Table 1), and about 20% longer in mesial – distal length than the posteriormost Diphydontosaurus dentary acrodont tooth. The teeth display a carina along the long axis and through the apex of the tooth, and there are wear facets that truncate the apices; these features are also seen in the Diphydontosaurus dentary tooth row ( Fig. 4E, F View Figure 4 ). Diphydontosaurus dentaries at the ‘transition’ between the pleurodont and acrodont dentition do have a distinct shelf, but the teeth are significantly smaller than those in the posteriormost acrodont region. This specimen, albeit with a higher lingual apicobasal height to base width ratio (approximately 1.1; Table 1), is therefore similar in many respects to Diphydontosaurus ; however, the gaps between the teeth are relatively smaller and do not have true acrodonty in the same region of the dentary as that genus. Also, the teeth bases seem more aligned with the jaw axis (offset by up to 6 °) compared with the anterior labial offset of about 7 – 10 ° in Diphydontosaurus (see Fig. 2D). Therefore we cannot assign the specimen to Diphydontosaurus but realise that it has close affinities with that genus.
GEPHYROSAURIDAE GEN. ET SP. INDET. 2
Remarks: Other jaw fragments include BATGM CD7, a right maxilla with six pleurodont teeth ( Fig. 5D, E, G – I), three of which are intact on the mid and anterior sections; the anteriormost of these has a large resorption pit and there are incipient replacement pits on the other three anterior teeth ( Fig. 5E, G). This indicates frequent replacement in the anterior region. The intact teeth display simple distinctive ridges on the lingual side radiating from their apices ( Fig. 5E, G, I), and two of them have a marked trough (or groove) that runs mesial – distally across the cusp ( Fig. 5D) in this ridged region (there is also a faint trace of this trough in the anteriormost tooth). This specimen differs from Diphydontosaurus where the teeth in the equivalent maxillary region are acrodont. Moreover, the teeth of Diphydontosaurus have more complex bifurcating and sometimes trifurcating ridges just below the tooth apex ( Fig. 5J). A trough is not well developed in the maxillary teeth of that genus, or generally in Gephyrosaurus , but G. bridensis displays a slight groove on the mesial side of the cusp ( Fig. 5K); however, as the SEM image makes clear ( Fig. 5D, E), the final two teeth of the specimen appear to be more ankylosed to the dental shelf, with thick bone of attachment. There is also a significant difference from the equivalent maxillary region of Gephyrosaurus , where replacement pits are infrequent. The simple radiating ridges ( Fig. 5I) are reminiscent of similar ridges found on the lingual side of Gephyrosaurus maxillary teeth ( Fig. 5F) but the ridges can run to the base in some G. bridensis specimens, unlike in BATGM CD7, where they appear to end mid-tooth. One interesting feature is the slightly lower base of the posteriormost teeth ( Fig. 5E, G, I), which is reminiscent but less pronounced than the more ventral position of the bases of posterior acrodont teeth found in the maxilla of Diphydontosaurus ( Whiteside, 1986) .
Overall, there are greater affinities of the specimen to Gephyrosaurus rather than to Diphydontosaurus , but the specimen probably belongs to a new, undescribed species of rhynchocephalian. Without the current knowledge about the diversity of rhynchocephalian tooth implantation, an assignment to an early squamate would have been likely. In this respect BATGM CD7 is similar to a fragment of a maxilla with pleurodont teeth bearing radially ridged cusps or ‘striae’ from the early – mid Jurassic of India, attributed to Squamata by Evans, Prasad & Manhas (2002: fig. 9C). Pleurodonty alone is equivocal evidence as it is found in rhynchocephalians, and therefore without any additional evidence of the presence of squamates, and considering the radial ridges in G. bridensis ( Fig. 5F), we attribute the specimen to an indeterminate gephyrosaurid.
LEPIDOSAURIA ? GEN. ET SP. INDET
Remarks: BATGM CD9 ( Fig. 5L, M) consists of a small, worn, jaw fragment with two robust triangular teeth that are damaged on the lingual side. We tentatively identify the fragment as the posterior part of a dentary, as the largest tooth appears to be at the rear terminal end of the dental row. The teeth fuse anteriorly, and are reminiscent of, and are of similar anteroposterior width, some damaged specimens of Clevosaurus at Tytherington. However, the dentition appears to be pleuracrodont (rather than acrodont) with the teeth firmly ankylosed to the labial wall but sitting much lower on the dental shelf of the lingual side. It does not appear that the teeth are ankylosed in a socket, so that it is unlikely that the specimen has procolophonid affinities. There is insufficient detail in the specimen to make a confirmed assignment except to reptilia in general, and most probably to Lepidosauria .
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