Pseudoolenoides aspinosus Fortey and Droser, 1996

Adrain, Jonathan M. & Karim, Talia S., 2012, 3467, Zootaxa 3467, pp. 1-97 : 18-19

Pseudoolenoides aspinosus Fortey and Droser, 1996

Plate 8

1996 Pseudoolenoides aspinosus Fortey and Droser , p. 87, fig. 12.1, 12.3, 12.5, 12.7 (only; fig. 12.2, 12.4, 12.6 = Pseudoolenoides pogonipensis n. sp.?).

1999 Pseudoolenoides aspinosus Fortey and Droser ; Fortey and Droser, p. 189.

Material. Holotype, cranidium, USNM 481341 (Pl. 8, figs 1, 4, 5), and paratypes USNM 481344–481346, along with an unnumbered specimen, all from a single bedding plane, 36 m above Hesperonomiella minor bed ("K"), Juab Formation (Dapingian; Psephosthenaspis pseudobathyurus Zone ), Thomas Range section, Juab County, western Utah, USA.

Diagnosis. Cranidial anterior border slightly nasute and pointed; librigena with tubercles on anterior part of field and raised lines on posterior part; raised lines prominent on upper surface of genal spine; pygidium with well impressed border furrow.

Discussion. Fortey and Droser (1996, p. 87) described the species on the basis of six specimens. Four of the specimens, including the holotype cranidium, were from a single bedding surface in the Psephosthenaspis pseudobathyurus Zone in their Thomas Range section. The other two specimens, both cranidia, were from the Psephosthenaspis glabrior Zone at Ibex Section J. The Ibex specimens, of younger age than the Thomas Range material, also show clear morphological differences. The glabella and occipital ring are more coarsely tuberculate, the glabella does not expand anteriorly to nearly the same extent as the holotype, the anterior border is not bowed forward medially nearly as much, and SO is considerably shorter both sagittally and exsagittally. Although the Ibex cranidia are smaller than the Thomas Range holotype, the silicified samples of Pseudoolenoides species documented herein (as well as many unpublished species of the Acidiphorus group) demonstrate that changes of this nature and magnitude do not occur through ontogeny.

In a later paper, Fortey and Droser (1999) named Acidiphorus ? lineotuberculatus from basal Whiterockian strata at Whiterock Canyon, Nevada. They recognized that the small Section J cranidia they had previously assigned to P. aspinosus were unlikely to belong, illustrated a third example ( Fortey and Droser, 1999, fig. 4.20), and assigned these specimens to A.? lineotuberculatus . We agree that the specimens do not belong to P. aspinosus , but they do appear to represent a plesiomorphic species of Pseudoolenoides , and they do not represent A.? lineotuberculatus . Among the obvious differences are a coarsely tuberculate glabella and occipital ring versus sparsely tuberculate with fine raised lines, and the absence of a preglabellar field, with the frontal lobe of the glabella intruding on the anterior border, versus the unambiguous presence of a field in all (three) known specimens. We have discovered a similar species, P. pogonipensis , at Section K-South 1.5T m. The Ibex cranidia are tentatively referred to this new species (see discussion below).

The hypostome assigned to P. aspinosus (Pl. 8, figs 2, 3, 8) is unlike those definitely belonging to species of Pseudoolenoides , which have their lateral and posterior borders developed into spines or broad shelves. In its simple morphology, this hypostome closely resembles those firmly associated with species of Acidiphorus in our collections. Fortey and Droser (1996) assigned the hypostome on the basis of its association on a bedding plane with the other Thomas Range specimens. The possibility that it belongs to a cooccurring species of Acidiphorus is difficult to exclude. On the other hand, Pseudoolenoides aspinosus is a stratigraphically low and plesiomorphic species and could plausibly lack the modifications to the hypostome. Pseudoolenoides ludificatus is also generally plesiomorphic, however, and a close comparison for P. aspinosus in the morphology of other sclerites, yet it has a spinose hypostome.

With the younger Ibex specimens excluded and the association of the hypostome at least somewhat tentative, P. aspinosus is known from only four definitely assigned specimens (one newly illustrated herein), which limits the degree to which it can be interpreted. Nevertheless, the single known cranidium, while incomplete, is nearly as large as the typical large holaspid specimens of P. ludificatus , P. acicaudus , and P. fossilmountainensis . The pygidium, however, is small (though holaspid), and less than half the size of the largest P. ludificatus specimens.

Fortey and Droser (1996, p. 87) diagnosed P. aspinosus on the basis of what it lacked (features of the derived group such as unreleased spines in the pygidium and a deep S1) and on its tuberculate sculpture. All of the features listed in their diagnosis are symplesiomorphies shared with all species of Acidiphorus . The species, insofar as it is known, does appear to have a few autapomorphies associated with the librigenal sculpture and the pygidial border furrow, and these are what constitute the present diagnosis. Pseudoolenoides aspinosus was compared with P. ludificatus above and is compared with P. pogonipensis below.