Aponileus latus Hu, 1963

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 : 10-11

Aponileus latus Hu, 1963

Plate 1

1963 Aponileus latus Hu , p. 87, pl. 13, figs 27–31.

1967 Aponileus latus Hu ; Lochman-Balk and Wilson, p. 905.

1975 Aponileus latus Hu ; Derby, p. 24.

2003 Psephosthenaspis lata (Hu) ; Adrain in Jell and Adrain, p. 343.

Material. Holotype, cranidium, USNM 143342 (Pl. 1, figs 1, 3, 5), and paratypes USNM 143343 a–d, from the Padre Formation (El Paso Group; upper Floian ; Blackhillsian), Scenic Drive Section, Franklin Mountains, El Paso County, western Texas, USA.

Diagnosis. Similar to Aponileus aasei , but with apparently smaller frontal areas; librigena with much shorter posterior suture and posterolateral region; pygidium and pygidial axis longer relative to width.

Discussion. Hu's (1963) type material is a scarcely adequate basis for a species and is only rendered fully interpretable by new Ibex material belonging to related species. Nevertheless, with the discovery in particular of Aponileus aasei n. sp., A. latus can be meaningfully compared. Although A. latus is known from inadequate material and A. aasei from a single specimen, the species, while each other's closest comparison, are definitely distinct. The most obvious difference is the morphology of the librigena. The species share the lack of a genal spine, but the librigena of A. aasei is considerably more elongate, particularly in the region behind the eye. Cranidia of A. latus are very poorly preserved, but the anterior border and anterior border furrow are less prominent medially and the frontal area seems smaller. The pygidium of the unique specimen of A. aasei is partially fragmented, but its general dimensions remain obvious and it is considerably wider relative to its length than those of A. latus . One of the pygidia of A. latus (Pl. 1, fig. 10) has a definite post-axial ridge (the other has the area where the ridge would be obscured by adhering matrix). There does not seem to be such a ridge in A. aasei , and it is definitely absent from other species. In addition to lacking genal spines, the species share a generally effaced morphology, particularly of the pygidial rings, furrows, and pleural regions. The dorsal surfaces of sclerites of both species are covered with a fine, subdued raised line sculpture much less obvious than that of other congenerics. Aponileus latus is also similar to the poorly known A. glaber from northwestern Greenland. Librigenae of the latter species are not known, but it is clearly another generally effaced species similar to A. latus and A. aasei . Cranidia of A. glaber are not much better preserved than those of A. latus , but appear to differ in the possession of straight, obliquely set axial furrows versus strongly anteriorly divergent furrows with the glabella obviously posteriorly waisted, and probably a deeper and longer (sag., exsag.) SO. Pygidia of both species are quite well preserved and that of A. glaber differs in being longer relative to its width, so that the posterior margin is nearly semicircular. In addition, the first pleural furrow, though apparent, is not as well impressed as in A. latus , and the posterior pleural regions are nearly completely effaced, whereas the pleural regions of the second and third segments are at least faintly expressed on the pygidia of A. latus .