Psephosthenaspis Whittington, 1953

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 : 24-25

Psephosthenaspis Whittington, 1953

= Ludvigsenella Tremblay and Westrop, 1991

Type species. Bathyurellus strenuus Billings, 1865 , from a boulder at St. Antoine de Tilly , Quebec, Canada .

Other species. Ludvigsenella ellipsepyga Tremblay and Westrop, 1991 , Sunblood Formation (Dapingian), Northwest Territories, Canada; Psephosthenaspis glabrior Fortey and Droser, 1996 , Juab Formation (Dapingian), Utah, USA; P. microspinosa Fortey and Droser, 1996 , Juab Formation (Dapingian), Utah, USA; Acidiphorus ? pseudobathyurus Ross, 1967 , Garden City Formation (Dapingian), Utah, USA.

Diagnosis. See Fortey and Droser (1996, p. 82).

Discussion. Psephosthenaspis , prior to the work of Fortey and Droser (1996), was a taxon comparable to Aponileus : monotypic, little discussed, and with an incompletely known type species. Fortey and Droser (1996) recognized Acidiphorus ? pseudobathyurus Ross, 1967 , and Ludvigsenella ellipsepyga Tremblay and Westrop, 1991 , as members of the genus, and described two new species from the Dapingian of western Utah.

As noted above, Adrain (in Jell and Adrain, 2003) placed Aponileus in subjective synonymy of Psephosthenaspis . At the time Aponileus was known only from Hu's (1963) illustrations of its unique type species. There are certainly several points of similarity. Both genera feature species with an upturned anterior border, a forwardly expanding glabella which occupies most of the area of the cranidium anteriorly and often exhibits waisting, anastomosing raised line sculpture (often mixed to greater or lesser extent with tuberculate sculpture), very large palpebral lobes, librigenae with a blade-like genal spine, and pygidia of similar dimensions which often exhibit a very small posteromedian spine in large specimens.

These similarities, however, are strongest when comparing the youngest known species of Psephosthenaspis , P. glabrior , with younger species of Aponileus . Fortey and Droser (1996) demonstrated that three species of Psephosthenaspis occur in stratigraphic succession in the Juab Formation at Ibex. We will present a detailed phylogenetic analysis of the overall group in a later work, but we agree with Fortey and Droser that stratigraphic order correlates with phylogenetic order in these species and that Psephosthenaspis microspinosa Fortey and Droser , the oldest known species, is phylogenetically basal to the genus. There are many aspects of morphology, discussed by Fortey and Droser (1996) in which P. pseudobathyurus ( Ross, 1967) shows transition in states from the older P. microspinosa to the younger P. glabrior . This suggests that the morphology of P. glabrior is a poor comparison for Aponileus , as much of it is derived within Psephosthenaspis . The affinity of Psephosthenaspis is best considered with reference to the morphology of P. microspinosa .

Fortey and Droser (1996, p. 83) entertained the idea that Psephosthenaspis was related to species currently assigned to Jeffersonia Poulsen, 1927 , but instead favoured the hypothesis that Acidiphorus brevus ( Hintze, 1953) was closely related to, and possibly a member of, Psephosthenaspis . Acidiphorus brevus is presently not well known. We will revise it on the basis of rich new collections in a forthcoming work. We have also discovered several new species from the " Pseudocybele nasuta Zone " that are closely related to it. All will be described in detail later, but we briefly figure one of them here as Figure 6 for purposes of comparison. This species, from high in the "zone" (Assemblage 5 of Figure 1 ) is extremely morphologically similar to P. microspinosa , differing mainly in minor proportions. The undescribed species retains a slightly nasute border which is lost in Psephosthenaspis , but the border is very short as in P. microspinosa . The librigena of P. microspinosa shows a distinct dorsal ridge on the genal spine which foreshadows the blade-like structure developed in P. glabrior . That of " Acidiphorus " n. sp. does not have a distinct ridge, but is dorsally swollen in the same area. Pygidia differ only in the relatively better impressed border furrow in P. microspinosa .

Hence we agree with Fortey and Droser (1996) and consider that the sister taxon of the Dapingian Psephosthenaspis is among this upper Floian group which includes "A." brevus and "A." n. sp. ( Fig. 6 ). Detailed systematics of this group, including genus assignments, must await description of many undescribed species and comprehensive phylogenetic analysis of the Acidiphorus group. Recognition of the affinities of Psephosthenaspis nevertheless strongly indicates that it is a separate clade, not particularly closely related to Aponileus .

Figures

FIGURE 1. Schematic diagram of stratigraphic distribution of species of Aponileus and the outgroup species Strigigenalis plicolabeona (Young) in Blackhillsian (upper Floian) sections of the Fillmore Formation in western Utah. Measurements are in metres. Meterages are not to scale. Zonation is that established by Adrain et al. (2009), with new informal subdivisions of the "Pseudocybele nasuta Zone" discussed in text.

FIGURE 6. "Acidiphorus" n. sp., from Section J 40.0 m, Wah Wah Formation (upper Floian; Blackhillsian; "Pseudocybele nasuta Zone"), southern Confusion Range, Ibex area, Millard County, western Utah, USA. This species and other undescribed species from the "Pseudocybele nasuta Zone" are closely related to the younger Psephosthenaspis Whittington, 1953. It is illustrated to facilitate comparison and will be described in detail in a forthcoming work. See text for discussion. 1, 4. Cranidium, SUI 131853, dorsal and anterior views, x7.5. 2, 3, 6. Pygidium, SUI 131854, posterior, dorsal, and right lateral views, x6. 5. Right librigena, SUI 131855, external view, x5.