Tesselacauda, Ross, 1951

Adrain, Jonathan M. & Karim, Talia S., 2019, Revision of the Early Ordovician (late Tremadocian; Stairsian) cheirurid trilobite Tesselacauda Ross, with species from the Great Basin, western USA, Zootaxa 4661 (2), pp. 201-255 : 217

Tesselacauda ? flabella Kobayashi, 1955

( Plate 14 , figs 17, 18, 20)

1955 Tesselacauda flabella Kobayashi , p. 417, pl. 2, figs 8a, b.

1989 Tesselacauda flabella Kobayashi ; Dean, p. 17, pl. 2, fig. 12 (non pl. 2, figs 2, 4, 7, 8, 10, 11).

Material. Assigned specimens SUI 147612 and 147613, from Section G 26.6 m, Fillmore Formation (upper Tremadocian; Stairsian; Bearriverops alsacharovi Zone ), southern Confusion Range, Ibex area, Millard County, western Utah.

Discussion. Kobayashi (1955, p. 417, pl. 2, figs 8a, b) established this species on the basis of a single illustrated pygidium from the McKay Group of southeastern British Columbia, Canada. Dean (1989, pl. 2, figs 2, 4, 7, 8, 10–12) reillustrated this holotype and assigned three pygidia (two fragmentary) and a fragmentary cranidium from the middle member of the Survey Peak Formation, Wilcox Pass, Jasper National Park, Alberta. While possibly representing a related species, Dean’s specimens are obviously much less effaced than Kobayashi’s holotype and are unlikely to be conspecific. In fact, the Survey Peak specimens are not effaced at all, and the effacement of both the posterior axial and ring furrows and most of the pleural and interpleural furrows is the most striking feature of the holotype. The affinity of the cranidial fragment associated by Dean is impossible to assess; it appears to represent a cheirurid, but consists only of part of a glabella and part of an occipital ring.

Two pygidia from the Bearriverops alsacharovi Zone appear much closer in morphology to Kobayashi’s (1955) holotype. A smaller specimen preserving the anterior part of the axis and left pleural region (Pl. 14, fig. 20) is nearly identical to the holotype. A specimen more than twice the size preserving most of the left pleural region (Pl. 14, figs 17, 18) seems conspecific, and, if so, shows that the pygidium became increasingly effaced with size. Unfortunately the species is extremely rare, as these are the only specimens found and no prospective cranidia were recovered. They are of some potential importance, however, in that they may anchor some of Kobayashi’s (1955) species to the trilobite zonal scheme presented by Adrain et al. (2014).

So little is known of the species that its affinities are impossible to determine with any confidence. The pygidia are broadly comparable to those of species of Tesselacauda , but the smaller silicified example (Pl. 14, fig. 20) appears to show the pleural region of the third segment with an expressed pleural furrow. This is absent from all large pygidia assigned to Tesselacauda herein. It is expressed on a single specimen, a very small pygidium assigned to T. kriegerae (Pl. 18, fig. 28). Much more information would be required to meaningfully evaluate the taxon, but it is documented here because of its possible biostratigraphic significance.

Figures

PLATE 14. 1–16, 19. Tesselacauda kriegerae n. sp., from Section G 26.6–27.0 m and Section C 111.6 m, both Fillmore Formation (upper Tremadocian; Stairsian; Bearriverops alsacharovi Zone), Ibex area, Millard County, western Utah, USA. 1, 2, 4. Cranidium, SUI 147602, dorsal, right lateral, and anterior views, x17 (G 27.0 m). 3. Right librigena, SUI 147603, external view, x15 (G 26.6 m). 5, 8, 10. Right librigena, SUI 147604, internal, external, and ventrolateral views, x12 (G 26.6 m). 6. Left librigena, SUI 147605, external view, x12 (G 26.6 m). 7. Right librigena, SUI 147606, external view, x15 (C 111.6 m). 9. Left librigena, SUI 147607, external view, x15 (G 26.6 m). 11. Left librigena, SUI 147608, external view, x15 (G 26.6 m). 12, 14, 16. Left librigena, SUI 147609, external, internal, and ventrolateral views, x12 (G 26.6 m). 13, 15. Right librigena, SUI 147610, external and ventrolateral views, x15 (G 26.6 m). 19. Left librigena, SUI 147611, external view, x12 (G 26.6 m). 17, 18, 20. Tesselacauda? flabella Kobayashi, 1955, from Section G 26.6 m, Fillmore Formation (upper Tremadocian; Stairsian; Bearriverops alsacharovi Zone), Ibex area, Millard County, western Utah, USA. 17, 18. Pygidium, SUI 147612, dorsal and left lateral views, x6. 20. Pygidium, SUI 147613, dorsal view, x12.