Ibexaspis Přibyl and Vaněk in Přibyl et al., 1985

E. B, Neo, Adrain, Jonathan M. & Karim, Talia S., 2018, The pliomerid trilobite Ibexaspis and related new genera, with species from the Early Ordovician (Floian; Tulean, Blackhillsian) of the Great Basin, western USA, Zootaxa 4525 (1), pp. 1-152 : 10-11

Ibexaspis Přibyl and Vaněk in Přibyl et al., 1985

Type species. Protopliomerops quattuor brevis Young, 1973 , from the Fillmore Formation (Floian; Blackhillsian; Bathyurina plicolabeona Zone ), southern Confusion Range, Ibex area, Millard County, western Utah, USA.

Other species. Ibexaspis coadyi n. sp., Fillmore Formation (Blackhillsian; Carolinites nevadensis Zone ), western Utah; I. leuppi n. sp., Fillmore Formation (Blackhillsian; Presbynileus ibexensis Zone ), western Utah; I. rupauli n. sp., Wah Wah Formation (Blackhillsian; " Pseudocybele nasuta Zone "), western Utah, and Yellow Hill Limestone, eastern Nevada.

Diagnosis. Anterior border furrow M-shaped in anterior view; sagittal glabellar height in anterior view 22.3– 25.8% width across posterior projections (excluding genal spines); median furrow in LF present; S4 expressed; glabellar furrows usually reduced to lateral notches; L1 small, describing oblique ellipsoid in dorsal view; presence of corona-like cluster of tubercles on lateral occipital lobes, lateral glabellar lobes, adaxial end of posterior border, and adaxial edge of fixigena in large holaspids; transverse tubercle row near rear of LO aligned immediately along posterior margin; strongly expressed median occipital tubercle; anterior course of axial furrows subparallel to very slightly anteriorly divergent at L3; axial furrows usually very narrow; eye ridge clearly expressed; fixigena opposite anterior portion of palpebral lobe moderately wide; long genal spine; middle body of hypostome narrow relative to sagittal length; posterior lobe of middle body very short; ventral sculpture of robust tubercles on hypostome; caecal pitting on librigenal field weak or absent; preannulus usually present on thoracic segments; distinct median nodes present on axial rings of large pygidial specimens.

Discussion. When they proposed the genus, Přibyl and Vaněk (in Přibyl et al., 1985, p. 170) spelled it " Ybexaspis ". This is certainly a misspelling, as everywhere else in their paper the name was either spelled Ibexaspis or abbreviated " I. " and the derivation of the name was given as the Ibex area in Utah. All subsequent workers ( Peng, 1990; Jell and Adrain, 2003; Adrain et al., 2009; McAdams and Adrain, 2011a, 2011b) have used the Ibexaspis spelling.

Přibyl and Vaněk in Přibyl et al. (1985, p. 170–171) diagnosed Ibexaspis with the following characteristics: "Glabella spherical, widening anteriorly, with 3 pairs of very short glabellar furrows (1S–3S) and minute glabellar lobes. 3S and 2S run almost transversely; 1S is inclined diagonally backwards to the occipital ring. 1L incompletely separated. Eyes large, situated opposite 3S and 2S. Fixigenae vaulted with short genal spines. Number of thoracic segments unknown. Pygidium with 3–4 rings and terminal piece on the axis, and 4 pairs of pleural spines, which are arranged radially, gradually decrease towards the sag. axis of the pygidium".

This diagnosis now consists mainly of symplesiomorphic characters, either shared among the Ibexaspis group or more widely with other pliomerid genera. The glabella is not "spherical" and widens only slightly anteriorly, and the strong glabellar inflation is shared by other members of the Ibexaspis group. The glabellar furrows are similar in course to those of species of Deltapliomera , Tuleaspis , and Millardaspis , as well as Panisaspis, but they are indeed "shorter" (narrower). It is unclear which pygidial specimens with three axial rings that Přibyl and Vaněk examined; all members of Ibexaspis possess four rings, which is shared with other members of the Ibexaspis group, and also Panisaspis. The radial arrangement of the spines is common in the Ibexaspis group, and similar to that seen in species of Panisaspis, except that the latter taxon has a characteristically longer third pygidial spine. Large palpebral lobes are common in many pliomerid taxa. Those of Ibexaspis are located further anteriorly than in other members of the Ibexaspis group, although not so far forward as in Protopliomerella Harrington, 1957 , and Pseudocybele Ross, 1951 .

Přibyl and Vaněk (in Přibyl et al., 1985) assigned Protopliomerops quattuor Hintze, 1953 , to Ibexaspis , but McAdams and Adrain (2011b) assigned this species to their new Panisaspis. Přibyl and Vaněk (in Přibyl et al., 1985, p. 119) did not consider any of the pygidia assigned to Panisaspis quattuor to belong to that species, but did not give any indication of what they considered their identity to be. McAdams and Adrain (2011b, p. 21) demonstrated that both of Hintze's pygidia, as well as the pygidium that Demeter (1973, pl. 3, fig. 6) assigned to Protopliomerops quattuor , do in fact belong to the species, although a number of other sclerites assigned by Hintze (1953) and Demeter (1973) to P. quattuor do not. A full synonymy of Panisaspsis quattuor and discussion of both Hintze's and Demeter's sclerite associations was given by McAdams and Adrain (2011b).

Přibyl and Vaněk (in Přibyl et al., 1985) briefly compared Ibexaspis to Pilekia Barton, 1915 ; Parapilekia Kobayashi, 1934 ; and Protopliomerops Kobayashi, 1934 . The first two are cheirurid taxa with no close relationship to the pliomerid Ibexaspis . Protopliomerops is a "wastebasket" pliomerid taxon with a very poorly understood type species (see discussion in McAdams and Adrain [2011b, p. 7]). It currently contains a mix of cheirurid and pliomerid species, none of which are closely related to Ibexaspis . Přibyl and Vaněk (in Přibyl et al., 1985, p. 116) also discussed Ibexaspis as part of an "evolutionary lineage" from Eocheirurus Rozova, 1960 , to Parapilekia , to Ibexaspis . They also suggested (1985, p. 126) that Ibexaspis was the "ancestor" or sister taxon of Pseudosphaerexochus Schmidt, 1881 . Again, the other genera, as cheirurids, are not germane to the phylogenetic relationships of Ibexaspis . Ongoing work on a broader phylogeny of Pliomeridae (e.g., McAdams and Adrain, 2010b) indicates that the broad Ibexaspis group described herein is monophyletic and sister to Panisaspis.