Protopliomerella seegeri, Adrain, 2011

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 : 23-24

Protopliomerella seegeri n. sp.

( Plates 31–33)

? 1951 Protopliomerops contracta Ross , p. 136, pl. 33, fig. 15 (only; pl. 33, figs 16, 17, 22 = Protopliomerella contracta ; pl. 33, figs 18, 19, 23–32 = P. kerouaci ).

1973 Genus and species undetermined A; Demeter, p. 64, pl. 5, fig. 9 (only; pl. 5, figs 2–4, 7, 8 = Cybelopsis ? spp.; pl. 5, fig. 13 = gen. and sp. indet.; pl. 5, fig. 15 = Protopliomerella kerouaci ).

2009 Protopliomerella contracta ; Adrain et al., p. 561.

Material. Holotype, pygidium, SUI 126265, and assigned specimens SUI 126247–126254, 126257–126264, 126266–126268, 126270, from Section G 181.8 m, Fillmore Formation (lower Floian; Tulean; low Psalikilus hestoni Zone ), southern Confusion Range, Ibex area, Millard County, western Utah, USA; SUI 126255, 126256, 126269, 126271, from Section HC5 217.0 T m, Garden City Formation (lower Floian; Tulean; low Psalikilus hestoni Zone ), Bear River Range, Franklin County, southeastern Idaho, USA.

Etymology. After Pete Seeger.

Diagnosis. Glabella short and wide; hypostome with dense granules on entire anterior lobe of middle body; six-segmented pygidium with dense sculpture of small tubercles on axis and inner pleurae; pygidial spines very finely tapered.

Description. Protopliomerella seegeri is very morphologically similar to P. bowlesi , and is described with a differential discussion. Ratios are given for comparison to other species. Cranidia of Protopliomerella seegeri differ from those of P. bowlesi in being shorter and narrower overall; with shorter, more anteriorly positioned palpebro-ocular ridges; a slightly more arcuate palpebro-ocular furrow; a shorter, wider, less anteriorly tapered glabella; much wider LF with faint S4; longer posterior fixigenae; and having coarser granules lining the cranidial furrows. The sagittal length of the cranidium is 50.2% (42.5–56.1%) width across the genal angles, and the width of the glabella across L1 is 92.1% (79.5–101.9%) its sagittal length.

The hypostome of P. seegeri is relatively longer, narrower, and less posteriorly tapered than that of P. bowlesi , with a narrower hypostomal suture and anterior wings, more strongly inflated middle body with a more granulose anterior lobe, a wider lateral border, longer posterior border, and longer border spines. The width across the shoulders is 68.4% (66.5, 70.4%) the sagittal length.

The librigenae of P. seegeri are unknown for comparison, and the rostral plate is unknown for either species.

Thoracic segments of P. seegeri and P. bowlesi are very similar, but those of the former have shorter articulating furrows; slightly more inflated axial rings and posterior pleural bands; and tiny tubercles on the axial rings. The width of the axial ring is 41.7% (36.2–48.9%) the width of the posterior pleural band.

The pygidium of P. seegeri is slightly longer and narrower than that of P. bowlesi , with width across anterior pleural band 110.6% (104.7–117.4%) sagittal length; the pleurae are narrower; the axial rings are shorter; the elongate median areas of the inter-ring furrows are wider and a little shorter; the axial and overall pygidial sculpture is more coarsely granulose; the terminal piece is wider and longer; the pleural spines are slightly narrower at the tips (posterior view); and the doublure (anterior view) is shorter, more medially pointed, and more strongly anterolaterally tapered.

Ontogeny. Cranidial ontogenetic changes include effacement of the dense, coarsely granulose sculpture of the smallest specimens (e.g., Pl. 31, fig. 17) to mainly small granules lining furrows; development of fixigenal pitting; widening and shortening of the glabella; slight elongation of the eye ridges; widening and lengthening of all cranidial furrows; shortening and widening of LO; and reduction of the genal spines to small nubs.

Size ranges of hypostomes and thoracic segments are inappropriate for observation of ontogenetic changes. Librigenae are unknown.

Pygidial specimens are fairly close in size, and none are very large, but it is possible to observe some narrowing and elongation of the pygidium overall; lengthening of the inter-ring furrows, especially the median section of the first few furrows; slight widening of the axial furrows; deepening of the interpleural furrows; and development of the terminal piece impression.

Discussion. The left half of one anterior border of one cranidium (Pl. 31, fig. 6) is bent posterolaterally and partially obscures the frontal lobe of the glabella on that side. It is possible that this is taphonomic, but it may also represent a teratology or healed injury to the border or LF. The partial articulated specimen (Pl. 31, fig. 1), shows that the anteriormost thoracic segment has a single-tipped pleural spine. The spines of the disarticulated thoracic segments are not well preserved, but two (Pl. 32, figs 20, 26) appear to possess notched spines. One small pygidium (Pl. 33, fig. 24) shows the line of suture for the anteriormost segment as partially open ventrally, but undetectable dorsally. This may be an indication of the mechanism by which the pygidium develops six segments: the suture never fully separates to shed the anteriormost segment into the thorax.

Adrain et al. (2009, p. 561) reported the Protopliomerella contracta Zone from HC5 217.0T, but further investigation has revealed that P. seegeri occurs at this horizon, and that it represents the low Psalikilus hestoni Zone.

Compared to the older species P. stegneri , P. seegeri shares a mostly effaced cranidium and a similar wide glabella, although it is relatively longer and more anteriorly tapered; the anterior border is narrower, and the palpebroocular ridges are shorter and more anteriorly located. The hypostome has shorter and much narrower anterior wings with smaller wing process pits, a more densely and extensively sculptured middle body, and longer and wider posterior and lateral borders with slightly larger spines. The pygidia are very distinct, as that of P. seegeri is longer and narrower, with six segments, longer and narrower pleurae, a tuberculate axis, a smaller terminal piece, and a much shorter (particularly laterally) doublure.

Protopliomerella seegeri differs from the stratigraphically closest species, P. kerouaci , in possessing a wider, somewhat shorter glabella with a much shorter (oblique) and less well impressed S4, palpebro-ocular ridges separated from the glabella and anterior border by a narrow section of interocular fixigena, and longer and less pitted fixigenae. The hypostome is longer and somewhat narrower, with a more tuberculate anterior lobe of the middle body. The pygidia are similar in proportion, but that of P. seegeri has six segments, a much smaller terminal piece without a complex indentation, is more posteriorly tapered, and has a tuberculate axis.

Protopliomerella seegeri is compared to P. contracta in the discussion section of that species.