Millardaspis knoxi, E.B & Adrain & Karim, 2018

E. B, Neo, Adrain, Jonathan M. & Karim, Talia S., 2018, The pliomerid trilobite Ibexaspis and related new genera, with species from the Early Ordovician (Floian; Tulean, Blackhillsian) of the Great Basin, western USA, Zootaxa 4525 (1), pp. 1-152 : 40-41

Millardaspis knoxi n. sp.

Plate 55

Material. Holotype, pygidium, SUI 129548 (Pl. 55, figs 12, 17, 18) and assigned specimens SUI 129544–129547 , 129549–129552 from Section H 163.3 m, Fillmore Formation (Tulean; Panisaspis sevierensis Zone), southern Confusion Range , Ibex area, Millard County, western Utah, USA.

Etymology. After Benjamin Knox.

Diagnosis. Librigena with short posterior branch of facial suture; pygidium with very coarse tuberculate sculpture and line of large tubercles running down the middle of each pygidial pleura and onto the spine; shallow and wide pygidial axial furrow; lacks terminal piece.

Discussion. Millardaspis knoxi is compared to Millardaspis milsteadi , noting all morphological differences, in the discussion of that species. Compared to the stratigraphically nearest species of the Ibexaspis group, Deltapliomera heimbergi , Millardaspis knoxi (cf. Pl. 24, fig. 1, Pl. 55, fig. 2) has a narrower, more strongly vaulted (sag., tr.) cranidium; a more anteriorly bowed anterior border; a shorter, slightly more inflated glabella with slightly longer and shallower lateral furrows; a longer, more laterally tapered LO; more laterally directed palpebral lobes; shorter and narrower posterior fixigenae; and more densely spaced sculpture.

Librigenae (cf. Pl. 24, figs 16, 17, Pl. 55, fig. 3) of Millardaspis knoxi possesses a shorter librigenal field with more tubercles, and a lateral border with a relatively longer anterior projection and coarser sculpture near the border furrow.

Thoracic segments (cf. Pl. 25, fig. 27, Pl. 55, fig. 11) mainly differ in that the specimen assigned to Millardaspis knoxi has a more inflated axial ring and pleurae than that of D. heimbergi ; the axial ring is longer; the axial furrows are shallower; there is a transverse line of granules on the posterior margin of the articulating half ring; and the sculpture is also more densely spaced, particularly on the pleural spine. The longer pleural spines of Millardaspis knoxi may be attributable to a different position in the thorax, or may be another difference, but more thoracic segments from both species are needed for comparison.

The pygidia (cf. Pl. 26, fig. 2, Pl. 27, fig. 13, pygidia of Pl. 55) are slightly narrower and much more strongly vaulted (sag., tr.) in both the axis and the pleurae; the pleurae are more strongly backswept, much more strongly downturned, and less splayed; the axial rings are more strongly independently inflated, as are the pleural ribs; the axial furrows are wider, especially across the first and second pleurae; the sculpture is composed of larger, more closely spaced tubercles; and the terminal piece is apparently never expressed, as opposed to its presence in about half of the D. heimbergi pygidia.

Millardaspis knoxi uniquely (among all members of the Ibexaspis group) possesses a line of large tubercles running down the middle of each pygidial pleura and onto the spine.