Psephosthenaspis glabrior Fortey and Droser 1996

Psephosthenaspis glabrior Fortey and Droser, 1996

Plates 19, 20

1996 Psephosthenaspis glabrior Fortey and Droser, p. 87, fig. 11.1–11.11.

Material. Assigned specimens SUI 129338–129357, from Locality K-1, Kanosh Formation (Dapingian), base of Section K-South, Fossil Mountain, Confusion Range, Ibex area, Millard County, western Utah, USA.

Discussion. Fortey and Droser (1996) described P. glabrior on the basis of two cranidia, a librigena, and three pygidia from a single horizon high in the Juab Formation at Ibex Section J. They indicated (Fortey and Droser, 1996, fig. 2, left part) that it occurred at three higher horizons but did not illustrate material from these. The highest occurrence was about five metres below the contact with the overlying Kanosh Formation. At Section K-South (see Hintze, 1973) a silicified fauna occurs immediately above this contact. This is Hintze's (1953, p. 41) sample K-1 which he listed as occurring at five feet (1.5 m) above the contact. We have made new collections from this level. There is no outcrop of the bed containing the silicified fossils, but fossiliferous talus occurs as small lumps clearly arranged along strike, so there is little question the fossils are essentially in situ. The fauna is dominated by species of the bathyurid Pseudoolenoides Hintze, 1953, but rare sclerites belonging to Psephosthenaspis also occur. Following extensive sampling, enough are available to adequately treat the taxon.

The sclerites appear to represent P. glabrior, and the material is important in supplementing knowledge of the species, in particular showing ventral and internal surfaces and providing insight into earlier holaspid ontogeny. The silicified pygidia are shorter (sag.) relative to their length than the holotype, but the holotype is very large, more than double the size of any other known specimen. When the largest silicified pygidium (Pl. 20, fig. 23) is compared with the second largest calcareous pygidium (Fortey and Droser, 1996, fig. 11. 7), which is only slightly larger, the proportions are almost the same. Smaller silicified pygidia (e.g., Pl. 20, fig. 31) suggest that the pygidium becomes increasingly longer with respect to its width through ontogeny. Differences that are less easy to dismiss involve the expression of furrows on the pygidium. The first pleural furrow is incised in the smaller silicified pygidia (Pl. 20, figs 16, 22) but it is weak on the largest (Pl. 20, fig. 23). The second pleural furrow is not deeply impressed on any specimens. Further, only the first and, more weakly, the second ring furrows are well expressed on the silicified specimens, and only abaxially. The type material shows a very deeply impressed first pleural furrow, a similarly deep second pleural furrow, and all of the ring furrows deep and well impressed. These differences cannot be due to either preservation (both sets of material are very well preserved) or ontogeny (as the pygidium of Fortey and Droser, 1996, fig. 11.7 is similar in size to that of Pl. 20, fig. 23). Hence an argument could be made that the Kanosh species is diagnosibly distinct from P. glabrior and should be named as a separate taxon. Although information is somewhat limited, there are no apparent differences between the cranidia and librigenae in either sample. We prefer to assign all of the material to one species, but the question could be further investigated were a more extensive sample of cranidial and librigenal material of the Juab P. glabrior available.

Cranidia appear to change very little, despite the fact that the largest available specimen (Pl. 19, fig. 3) is nearly three times the size of the smallest (Pl. 19, fig. 1). Dense but relatively subdued tuberculate sculpture covers the glabella (the large specimen of Pl. 19, fig. 3 has clearly been effaced, with the tubercles cut across their bases). The relative size of the palpebral lobe also seems stable through ontogeny. Neither of the two calcareous cranidia have the posterior projections exposed or preserved. The largest silicified cranidium (Pl. 19, fig. 3) shows that it was very slender.

Librigenae reveal that the Panderian notch is small and located near the posterior facial suture (Pl. 20, figs 5, 7). Across the size range represented, there is no obvious trend in the length of the genal spine. However, the librigenal field is markedly broader anteriorly in the largest specimens (Pl. 20, fig. 15) versus the smallest (Pl. 20, fig. 3). The silicified librigenae show that the genal spine, including its swollen base, is densely covered with subparallel raised lines (the single calcareous specimen is exfoliated in this region).

The silicified librigenal material also confirms that Psephosthenaspis, despite the superficial similarity, is morphologically very different in detail to Aponileus. The librigena of P. glabrior retains a strong and complete socle, as does that of P. microspinosa. This feature is only faintly retained in the phylogenetically basal species of Aponileus and lost in all later species. The Panderian notch of P. glabrior is small and crowded near the posterior facial suture. That of species of Aponileus (e.g., Pl. 4, figs 2, 3, Pl. 8, figs 16, 20) is much larger and set further forward. The librigenal doublure of P. glabrior is very wide posteriorly, more than double its anterior width. That of species of Aponileus is of similar width along the cheek, expanded only slightly posteriorly. The librigenal lateral and posterior border furrows of P. glabrior are complete and connected in front of the genal spine. These furrows are effaced posteriorly in most species of Aponileus.