Aponileus aasei, Adrain, 2012

Adrain, Jonathan M., 2012, The Lower Ordovician (upper Floian) bathyurid trilobite Aponileus Hu, with species from Utah, Texas, and Greenland 3293, Zootaxa 3293 (1), pp. 1-67 : 11-12

Aponileus aasei n. sp.

Plate 2

Material. Holotype, dorsal exoskeleton, BYU 19975 (Pl. 2), from float high in Section H, Fillmore Formation (upper Floian; Blackhillsian; probably Pseudocybele paranasuta Zone ), southern Confusion Range , Ibex area , Millard County, western Utah.

Etymology. After Arvid Aase, who collected and donated the holotype specimen.

Diagnosis. Dorsal tuberculation restricted to median and rear of glabella; genal spines lost, genal angle rounded, librigena long, especially posteriorly; most dorsal surfaces lacking tubercles but with fine scrobiculate line sculpture; pygidium long, with post-axial region longer than in all tuberculate species but not as long as in A. latus or A. glaber .

Description. The species is similar enough to A. belkaae that extended written description is redundant. This differential description makes note of all observed differences between the species. Cranidium with glabella more strongly waisted posteriorly, axial furrows subparallel posteriorly, then divergent anteriorly, versus more nearly straight along course but obliquely set, and only slightly more strongly divergent anteriorly; scrobiculate raised line sculpture generally finer and more subdued; tuberculate sculpture on glabella more subdued, more finely scattered, and more posteromedially concentrated; frontal areas slightly wider; sculpture on LO much more effaced; librigenae with finer, more closely spaced, and more subdued raised line sculpture, longer posterior facial suture and rear part of field, more subdued lateral border behind midpoint of eye, and completely rounded genal angle versus robust, blade-like spine; pygidium much longer relative to width, with much longer post-axial region, much more effaced, apparently entirely lacking tuberculate sculpture on axial rings and generally with very subdued and fine raised line sculpture, versus robust and relatively coarse raised lines and strong smattering of small tubercles on axis.

Discussion. The unique holotype of A. aasei was collected in the top ¼ of the highest shale unit in Hintze's (1973) informal Calathium member, which is the highest member of the Fillmore Formation, at Section H. Although the specimen was collected in 1994, prior to our measurement of the section in metres, this level falls within the narrow Pseudocybele paranasuta Zone of Adrain et al. (2009). Silicified material from this zone is dominated by only a handful of species, including Pseudocybele paranasuta McAdams and Adrain, 2010 , and species of Isoteloides Raymond, 1910 , Ptyocephalus Whittington, 1948 , and a new genus of asaphid. Very rare sclerites belonging to a species of Aponileus occur, but are insufficient to identify (they include a few thoracic segments and a very small pygidium). It is possible these represent A. aasei , but more material would be required to be certain. We have thoroughly sampled Section H and there are no other intervals in the vicinity of the level at which the holotype was collected that yield unidentified Aponileus .

Aponileus aasei was compared above with A. latus . Aponileus aasei differs from the Greenland species A. glaber , to the extent the latter species can be assessed, in the possession of a more waisted glabella which expands more strongly anteriorly, an apparently shorter SO, and a pygidium which is much wider relative to its length, has a slightly shorter post-axial region, and has less effaced pleural regions.

Aponileus aasei is resolved as the basal species of the effaced group. It has the widest pygidium relative to length, which more closely matches the wide condition seen in most of the tuberculate species. In particular, its cranidium is very close to that of A. belkaae . The species share a scattering of small tubercles concentrated on the median and particularly rear parts of the glabella, though they are less well expressed in A. aasei . Similarly, there is a background pattern of raised, scrobiculate lines, again somewhat finer in A. aasei . In other respects, the cranidia differ only in minor proportions. Hence A. aasei is described above via comparison with A. belkaae , although the points of close similarity are likely symplesiomorphic as compared with the other effaced species and the younger tuberculate species.