Pseudoolenoides Hintze, 1953

Adrain, Jonathan M. & Karim, Talia S., 2012, 3467, Zootaxa 3467, pp. 1-97 : 11-13

Pseudoolenoides Hintze, 1953

Type species. Pseudoolenoides dilectus Hintze, 1953 , Kanosh Formation , western Utah, USA .

Other species. Pseudoolenoides acicaudus Hintze, 1953 , Kanosh Formation, western Utah; P. aspinosus Fortey and Droser, 1996 , Juab Formation, western Utah; P. carterensis Shaw, 1974 , Oil Creek Formation, Oklahoma; P. derbyi Shaw, 1974 , Oil Creek Formation, Oklahoma; P. fossilmountainensis n. sp., Kanosh Formation, western Utah; P. pogonipensis n. sp., Kanosh Formation, western Utah; Goniotelus ? ludificatus Hintze, 1953 , Kanosh Formation, western Utah; P. oilcreekensis n. sp, Oil Creek Formation, Oklahoma; Pseudoolenoides cf. dilectus Hintze, 1953 , Kanosh Formation, western Utah.

Hintze (1953, p. 226) pointed out that Symphysurus ? goldfussi Walcott, 1884, from the "Pogonip group" near Eureka, Nevada, is certainly a species of Pseudoolenoides , but the unique holotype cranidium is both too poorly preserved for meaningful comparison and also lost. Hence the species should be restricted to its missing holotype.

Diagnosis. Librigenae with entire lateral border and abaxial edge of genal spine flattened into very sharp ventrolateral edge; posterior border flattened and with faint accessory furrow near posterior edge; posterior margin of posterior border posteriorly arcuate to lobate.

Discussion. No diagnosis has previously been provided for Pseudoolenoides . As often happens once notions of phylogenetic relationship are made explicit, there is an obvious, well supported group which forms the derived core of the taxon. This is the group which Hintze (1953) and Shaw (1974) recognized as Pseudoolenoides . However, following Fortey and Droser (1996) and with the addition of P. pogonipensis herein, there is also a series of successive, less derived, sister species on the stem leading to the derived group. At issue is how to treat these less derived species and where to draw the basal node of the genus.

The question is placed more sharply in relief for Pseudoolenoides by the extreme morphological novelty of the derived group. The multiple unreversed changes at this node (encompassing P. dilectus , P. derbyi , P. acicaudus , P. oilcreekensis , P. fossilmountainensis , and P. carterensis ) affect all sclerites, but seem largely to be associated with the initiation of paedomorphosis. Information from articulated specimens is not as extensive as one would like within Bathyuridae , but there are strong indications that the basic body plan involves 14 well expressed thoracopygidial segments plus a terminal piece which does not usually display additional segmentation. Where known within the Acidiphorus group, the plan is 10 thoracic segments and a pygidium with four well expressed rings plus a terminal piece [e.g., Aponileus aasei Adrain and McAdams, 2012 , pl. 2; Platyantyx arcuata ( Billings, 1865) ( Whittington, 1953, pl. 68, fig. 28); Acidiphorus kindlei ( Whittington, 1963, pl. 14, fig. 4)]. Pygidia of early (lower Tulean) species generally have four axial rings, so this arrangement may be basal for the family. Within the taxa with more or less fan-shaped pygidia, it is common to reduce the number of thoracic segments to nine (e.g., Uromystrum validum ( Billings, 1865) ( Whittington, 1953, pl. 67, fig. 10); Punka nitida ( Billings, 1865) ( Whittington, 1963, pl. 11, fig. 11)). In these species there appear to be five pygidial axial rings, though it can be difficult to count due to effacement and the generally small photographs available in the literature.

The derived group of Pseudoolenoides species appears to have evolved via progressive retention of posterior thoracic segments as part of the adult pygidium. That is, the pygidium in holaspids represents an arrested transitory pygidium including the fully tagmatized portion, which is homologous with the adult pygidium of other members of the Acidiphorus group, plus one or more unreleased thoracic segments fused to the anterior margin. The number of unreleased segments ranges from one ( P. dilectus and P. cf. dilectus ) to four ( P. fossilmountainensis , P. carterensis , P. derbyi , P. oilcreekensis ) to five ( P. acicaudus ). If the apparently universal body segment count of 14 is maintained, this hypothesis makes predictions about thoracic segment counts in these species: the number of unreleased thoracic segments incorporated into the adult pygidium should concomitantly reduce the number of thoracic segments.

While the evidence is sparse, such predictions are borne out. Pseudoolenoides dilectus retains one unreleased segment, predicting a nine segment thorax, and in fact this what it possesses (Pl. 11, fig. 1). Pseudoolenoides fossilmountainensis retains four unreleased segments in the adult pygidium, predicting a (highly unusual) six segment thorax. While no fully articulated individual is known, we have recovered a well preserved thoracopygidium (pl. 27, figs 18–26) which displays six thoracic segments. Of course, the thorax of this specimen could be incomplete, but the pleural spines of the first segment are reduced to small nubs, which is a common alteration of the anteriormost segment of laterally spinose forms to facilitate enrolment (cf. the first thoracic segment of P. dilectus, Pl. 11, fig. 1). Hence, as far as the available data indicate, the evolution of the derived group involved neotenic paedomorphosis.

Cephalic modifications are difficult to evaluate in terms of paedomorphosis, as no relevant ontogenies are known in any detail. Ontogenetic information is poor for bathyurids in general, and those developmental series that are at least partially known (e.g., Bathyurus ulu Ludvigsen, 1979 ; see Chatterton, 1980, pl. 6, figs 1–15) do not represent taxa of the Acidiphorus group. Cephalic modifications in derived Pseudoolenoides involve reduction in the relative area of the glabella, broadening of the fixigena, emphasis of the eye ridge, reduction in eye size, and migration of the eye to a more posterior position. A hypothesis of paedomorphosis predicts the presence of at least some of these features earlier in the ontogeny of less derived species, which can be tested via discovery of well preserved life histories. In the present state of knowledge, only the presence of a deeply impressed S1 is supported as paedomorphic based on available data, as S1 is transformed from well impressed in small individuals to weakly impressed or effaced in large specimens in several of the species of Bathyurus described by Ludvigsen (1979; e.g., B. platyparius , cf. pl. 2, figs 23, 20).

Following Hintze (1953) and Shaw (1974), the basal node of Pseudoolenoides would be taken as that subtending the derived and probably paedomorphic group, which was recovered on all 10,000 bootstrap pseudoreplicates ( Fig. 3). Fortey and Droser (1996) named an older species from the Juab Formation, P. aspinosus , and pointed out its affinities to the derived group. These authors were also first to point out that Goniotelus ? ludificatus Hintze, 1953 , also has clear affinity with the derived group, and they advocated an expanded concept of the genus. As is the case with Hintze (1953) and Shaw (1974), they did not diagnose this expanded genus. We here describe a third plesiomorphic species, P. pogonipensis , which also lies outside the derived group. The situation is very similar to that encountered in the bathyurid genus Psalikilopsis Ross, 1951 , discussed by Adrain et al. (2011). In both cases there is a highly autapomorphic derived group which includes the type species, along with a set of plesiomorphic species retrieved as successive sister species in a pectinate basal region of the cladogram. The options are to classify the basal species in a paraphyletic group of convenience (in the present case, presumably by assigning them to the currently broadly conceived Acidiphorus ), to erect new monotypic genera to receive them, or to draw the basal node of Pseudoolenoides to include them. As with Psalikilopsis , we consider the latter option the most informative and least disruptive course of action.

Electing to include the plesiomorphic species within Pseudoolenoides however, means that the striking features of the derived group cannot feature in the genus diagnosis. Nevertheless, the modifications to the cephalic margin and genal spines shared by all species, involving flattening, the development of sharp, blade-like ventrolateral margins, and posteriorly extended posterior borders with small accessory furrows between the posterior border furrows and the posterior margins, are all uniquely derived within the Acidiphorus group and serve to effectively diagnose the taxon.