Aponileus glaber ( Poulsen, 1927 )

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 : 12-13

Aponileus glaber ( Poulsen, 1927)

Figure 4

1927 Bolbocephalus glaber Poulsen , p. 304, pl. 20, figs 10, 27.

1927 Genus et sp. ind.; Poulsen, p. 299, pl. 20, fig. 11.

? 1946 Bolbocephalus glaber Poulsen ; Poulsen, p. 326.

1953 Bolbocephalus glaber Poulsen ; Whittington, p. 657.

1973 Bolbocephalus ? glaber Poulsen ; Fortey and Bruton, p. 2238.

2000 Illaenus / Presbynileus glaber (Poulsen) ; Boyce et al., p. 123.

2005 Bolbocephalus glaber Poulsen ; Adrain and Westrop, p. 1538.

Material. Lectotype (selected here), pygidium, MGUH 2406 ( Fig. 4.3, 4.6, 4.8 ); original of Poulsen (1927, pl. 20, fig. 27), Nunatami Formation ( upper Floian ), ostracod limestone member, Nunatami, Washington Land, northwestern Greenland.

Diagnosis. Effaced species with faint median keel on glabella; pygidium narrow relative to length; posterior pygidial border almost semicircular in outline.

Discussion. This species has been very difficult to interpret owing to the tiny, poorly lit photographs of only two specimens, a cranidium and a pygidium, provided by Poulsen (1927). Poulsen illustrated a second cranidium, but assigned it to "Genus et sp. ind." The material has not previously been revised. Poulsen later (1946, p. 326) assigned a pygidium from Ellesmere Island (Nunavut, Canada) to the species, opining that it was "a little wider than the Greenland pygidia; it is probable that we have before us a sex dimorphism...." As he did not illustrate the specimen, its affinity is impossible to evaluate. Whittington (1953, p. 657) interpreted the species as an effaced member of Bolbocephalus , but Fortey and Bruton (1973) noted that it lacks the inflated glabella of Bolbocephalus , and thought that it "may be more correctly referred to the Scutelluidae ." Boyce et al. (2000, p. 123) did not discuss the species, but their listing of its genus assignment as " Illaenus / Ptyocephalus " demonstrates similar doubt about Poulsen's assignment.

New photographs of the available three specimens show that while they are generally poorly preserved, there is little doubt that an effaced member of Aponileus similar to the type species is represented. The cranidium illustrated in open nomenclature by Poulsen (1927, pl. 20, fig. 11; Fig. 4.1, 4.4, 4.7 herein) agrees in all available detail with the cranidium assigned to the species by Poulsen (1927, pl. 20, fig. 10; Fig. 4.2, 4.5, 4.9 herein), including the faint median glabellar keel, upturned anterior border, broad, forwardly expanding glabella, and long SO, and there is no reason to doubt that it belongs to the species.

Aponileus glaber was compared with the most similar species, A. latus and A. aasei , under discussion of those species above. While poorly known, the median glabellar keel has not been observed in any other species, and the narrow, long pygidium with nearly semicircular outline also clearly distinguishes the taxon.

Figures

FIGURE 4. Aponileus glaber (Poulsen, 1927), from the Nunatami Formation (upper Floian), gastropod limestone member (figs 1, 4, 7) and ostracod limestone member (figs 2, 3, 5, 6, 8, 9), Nunatami, Washington Land, northwestern Greenland. 1, 4, 7. Cranidium, MGUH 2391, dorsal, right lateral, and anterior views, x6 (original of Poulsen, 1927, pl. 20, fig. 11). 2, 5, 9. Cranidium, MGUH 2390, dorsal, anterior, and oblique views, x3 (original of Poulsen, 1927, pl. 20, fig. 10). 3, 6, 8. Pygidium, lectotype, MGUH 2406, dorsal, posterior, and left lateral views, x3 (original of Poulsen, 1927, pl. 20, fig. 27).