Protopliomerella Harrington, 1957

Adrain, Jonathan M., 2011, Revision of the Lower Ordovician (lower Floian; Tulean) pliomerid trilobite Protopliomerella, with new species from the Great Basin, western USA 3144, Zootaxa 3144, pp. 1-113 : 6-7

Protopliomerella Harrington, 1957

Type species. Protopliomerops contracta Ross, 1951 (lower Floian; Tulean; Protopliomerella contracta Zone ).

Other species. Protopliomerella stegneri n. sp. (lower Floian; Tulean; unzoned interval beneath Psalikilopsis cuspidicauda Zone ); P. bowlesi n. sp. (lower Floian; Tulean; low Psalikilopsis cuspidicauda Zone ); P. kerouaci n. sp. (lower Floian; Tulean; high Psalikilopsis cuspidicauda Zone and Psalikilus typicum Zone ); P. seegeri n. sp. (lower Floian; Tulean; low Psalikilus hestoni Zone ); P. okeeffeae n. sp. (lower Floian; Tulean; Heckethornia hyndeae Zone and Heckethornia bowiei Zone ); Protopliomerella n. sp. A (lower Floian; Tulean; Heckethornia hyndeae Zone ); Protopliomerella n. sp. B (lower Floian; Tulean; low Psalikilopsis cuspidicauda Zone ).

Diagnosis. Anterior border narrow and very gently arcuate; anteroventral rim of anterior border with shallow median rostral suture arc about 1/3 total width of border; anterior margin of glabella bluntly truncated at anterior border furrow; palpebro-ocular ridges short, located opposite LF, but separated anteriorly from axial furrows by narrow strip of interocular fixigena; glabellar and axial furrows lined with granules; pygidium of five to seven segments with small, but elongated triangular terminal piece; tips of pygidial pleural spines very closely spaced but distinctly separate.

Discussion. Harrington (1957), in erecting his monotypic genus, considered the anteriorly tapered glabella, the positions of S3 and S4, the location of the palpebro-ocular ridges, shape of the genal angle, relative width of the pygidial axis to the pleurae, and shape of the posterior margin of the hypostome to be diagnostic features of Protopliomerella . In light of the seven new species described herein, as well as better knowledge of Pseudocybele Ross, 1951 , and Lemureops McAdams and Adrain, 2009a , most of these characters are no longer diagnostic of either P. contracta or of the genus as a whole. Harrington based his work on the illustrated specimens of Ross (1951, pl. 33, figs 15–19, 22–32), but these do not all represent Protopliomerella contracta (see discussion under P. contracta below).

Lee and Chatterton (1997a) presented a new diagnosis for Protopliomerella , but it was largely composed of symplesiomorphic features, including the anteriorly tapered glabella with large palpebro-ocular ridges; the hypostome with an elliptical middle body, three pairs of lateral border spines and a median posterior border spine; and the five-segmented pygidium with short spines. These characters are common to many pliomerid taxa, including Lemureops , Pseudocybele , and Hintzeia celsaora ( Ross, 1951) .

Difficulty in diagnosing Protopliomerella in terms of synapomorphies may stem from the possibility that it is paraphyletic. A preliminary phylogenetic analysis ( McAdams and Adrain, 2009c) of Protopliomerella , Lemureops , and Pseudocybele resulted in resolution of Protopliomerella (including some but not all of the species described herein) as a grade-group at the base of a well-supported Lemureops + Pseudocybele clade. Adjustment of the taxonomy may be necessary once cladistic analysis is complete, but this must await revision and description of all relevant taxa, which is in progress. We present a diagnosis which effectively separates the Protopliomerella group from taxa not part of the three genera considered here, but recognize that it may largely consist of basal synapomorphies of the broader clade. Again, the structure of Protopliomerella and its diagnosis must be revisited in the context of a broader analysis beyond the scope of the present descriptive study.

In any case, species presently grouped in Protopliomerella are undoubtedly closely related to Pseudocybele and to Lemureops . They are more morphologically similar to species of Pseudocybele , but the taxa are easily distinguished, as Pseudocybele possesses a medially pointed and recurved anterior border, rounded anterior glabellar margin, LF impressions, rounded genal angles, a long median posterior hypostomal spine, and an elongate pygidium with a very large, complexly impressed terminal piece. Lemureops is more morphologically derived, and notably possesses a very long, triangular anterior border, wide, lacunate cranidial axial furrows, very long genal angles, a short hypostome, and a pygidium with a large terminal piece and a distinctly tapered triangular posterior margin.