Pseudoolenoides derbyi Shaw, 1974

Adrain, Jonathan M. & Karim, Talia S., 2012, 3467, Zootaxa 3467, pp. 1-97 : 24-25

Pseudoolenoides derbyi Shaw, 1974

Plate 12, figs 1–14, 16–19, Plates 13–15

1974 Pseudoolenoides derbyi Shaw , p. 26, pl. 1, figs 23, 25–28.

1996 Pseudoolenoides derbyi Shaw ; Fortey and Droser, p. 87.

Material. Holotype, cranidium, OU 7952 (Pl. 12, figs 1, 4, 7), and assigned specimens OU 7948–7950 , 13961– 13963 , 38881 , 38884–38886 , 38888–38890 , 38894 , 38897 , 38899 , 38902 , 38904 , 38905 , 38912 , 38914 , 38917– 38921 , from Section WSC 46.3–51.5 m, Oil Creek Formation (Darriwilian), West Spring Creek , Murray County, Oklahoma, USA. Assigned specimen OU 7951 , from Section N 1 161.5 m, Oil Creek Formation (Darriwilian), Carter County, Oklahoma, USA.

Diagnosis. Glabella strongly and fairly coarsely tuberculate; line of tubercles on slightly swollen anterior edge of L1; eye ridge very prominent; raised line arc on librigenal field and frontal area surmounted by line of partially merged tubercles; pygidium with four unreleased segments, but pleural spines relatively small; pygidium with strong dorsal convexity, especially of pleurae; posteromedian spine short.

Discussion. Shaw's (1974, p. 26) list of autapomorphic features of Pseudoolenoides derbyi is accurate, and is supplemented in the diagnosis above. Pseudoolenoides derbyi is a distinctive species with its combination of a strongly tuberculate glabella with mostly nontuberculate fixigenae and librigenae and a raised librigenal field/ frontal area arc that features aligned, partially merged tubercles. Parsimony analysis retrieves it as sister to a clade including P. acicaudus , P. oilcreekensis , P. fossilmountainensis , and P. carterensis ( Fig. 3). The tuberculate glabella of P. derbyi is a plesiomorphic feature and is transformed in the more derived group to a sculpture composed entirely of raised lines. Pseudoolenoides derbyi also retains a forward eye position whereas the derived group are characterized by a posteriorly placed eye (though this is reversed in P. carterensis ).

Among known species, P. derbyi is most similar to P. acicaudus , which is retrieved as the basal species of its derived sister group. Pygidia of the species are closely comparable, except that those of P. acicaudus feature an extra unreleased segment, have slightly longer pleural spines, have a completely smooth axis versus a sculpture of subdued scattered small tubercles, and have an apparently longer posteromedian spine, though this structure is broken at the base in most specimens of P. derbyi (but see the internal mold of Pl. 15, fig. 14, which seems to give an indication of a short, narrow spine). Librigenae differ in that those of P. derbyi have an acute versus obtuse angle between the rear margin of the posterior border and the inner margin of the genal spine, have a distinctly broader field with a prominent tuberculate raised arc versus very subdued and non-tuberculate sculpture, have a posterior border that widens versus narrows adaxially, and which appears to lack the posterior point at the contact with the facial suture seen in P. acicaudus . Cranidia are similar in proportions, but differ in the tuberculate sculpture and anterior eye position of P. derbyi , as well as its somewhat deeper anterior border furrow.

Almost all of the material of this species is from a narrow stratigraphic interval at the West Spring Creek section in Murray County, where its highest occurrence is some 56 metres below the occurrence of P. carterensis . The exception is a single cranidium (Pl. 12, figs 11, 12) from a section in Carter County, which Shaw (1974, p. 26) listed as occurring in the same collection as P. carterensis . The occurrence seems anomalous, given the wide stratigraphic separation of the species at West Spring Creek, but was taken by Shaw (1974, p. 26) to indicate a much longer range for P. derbyi .