Tesselacauda morrisoni, Adrain & Karim, 2019

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 : 213-214

Tesselacauda morrisoni n. sp.

( Plates 9–11 , Plate 12 , figs 1–12, 17)

2014 Tesselacauda sp. nov. 1; Adrain et al., p. 174, fig. 8A, E.

Material. Holotype, cranidium, SUI 134075 (Pl., 9, figs 3, 5, 6, 9, 12), and assigned specimens SUI 147564 , 147565–147586 , from Section MME 49.8 m, Fillmore Formation ( upper Tremadocian ; Stairsian ; Rossaspis leboni Zone ), southern Confusion Range , Ibex area , Millard County, western Utah.

Etymology. After Scott Morrison.

Diagnosis. Dorsal surface of cranidium almost entirely lacking tuberculate sculpture in larger specimens, with only a scattering of very fine tubercles developed on the anterior border and anterior aspect of the glabella in some specimens; posterior projections narrow, exsagittally long behind palpebral lobe, and strongly downturned; pygidium with tuberculate sculpture restricted to distal pleura on most large specimens; pygidial axial terminal piece large, significantly longer than wide.

Description. Tesselacauda morrisoni is similar to T. depressa and a detailed comparison of their differences is provided rather than a separate description. Cranidia of T. morrisoni possess narrower, longer (exsag.) posterior projections (especially abaxially) that are more strongly ventrally flexed; fixigenal field possesses a less obviously pitted sculpture; the palpebral lobes are thinner and the external margins less sinuous, with the strong (almost 90) bend present posteriorly on T. depressa being much more obtuse on T. morrisoni , so that the posterior extensions of the palpebral lobes are much smaller and more posterolaterally directed; the lobes are also slightly more flattened and narrower (tr.) in anterior view, with the furrows bounding anteromedial margins more clearly defined especially adjacent to the fossulae, which are more prominent; palpebral furrow intersects axial furrow about two thirds of the way along L3 instead of closer to the anterolateral corner of L3; the glabella is slightly narrower, more tapered and rounded anteriorly, with stronger dorsal inflation; larger specimens lacking tubercles on the glabella, except for a few scattered on the anterior portion of the frontal lobe (e.g., Pl. 9, fig. 9); the axial furrows are more outwardly bowed; the anterior border is shorter (sag., exsag.); LO is slightly longer (sag.) and effaced.

The hypostome and rostral plate of T. morrisoni have not been identified so a comparison cannot be made.

Librigena of T. morrisoni are much more densely tuberculate with tubercles extending across entire border to furrow rather than restricted to a dense strip along the external margin of the border; the border furrow is generally more even in width along the entire course rather than being expanded medially; row of fine tubercles along margins of border furrow is much less prominently developed.

Three isolated thoracic segments have been identified for T. morrisoni (see Pl. 12). Thoracic segments strongly dorsally arched, with distal tips flexed gently outward from main arc; articulating half-ring with sagittal length 83.4% (82.6–84.1%) that of axial ring, width 80.0% (79.6–80.5%) that of axial ring, anterior margin anteriorly arched, with posterior margin more transverse to gently anteriorly arched medially and lined with row of small tubercles; articulating furrow deep; axial ring with distal portion rounded and slightly bulbous, in posterior view distal portion of ring extended into short lateral projections (e.g., Pl. 12, fig. 9); axial furrows deep with small articulating surface present anteriorly abaxial to articulating half-ring; pleurae with short (exsag.) articulating flange developed along anterior margin, distal portion of flange longer (exsag.); pleurae clearly divided into subequal anterior and posterior bands by deep pleural furrow; both bands with proximal terminations rounded and anterior band extended further adaxially than posterior band; anterior band pinched out abaxially, with posterior band extending past termination of anterior band into short anterolaterally directed blunt spine bearing scattering of granules (e.g., Pl. 12, fig. 1), pleurae otherwise generally smooth.

Pygidia of T. morrisoni appear taller in posterior view, with ventral margin of tips of pleurae forming a gentle concave arc, whereas on T. depressa they form a more transverse margin; in ventral view, the pleural spines are longer and less merged, forming more isolated tab-like spines; axis is more tapered, with a longer (sag.) terminal piece with length (sag.) much greater than width (tr.); the anterior band of the second pleural segment is generally smaller than that of T. depressa ; fewer tubercles are present and are generally limited to the portion of pygidium distal to the fulcrum, with the axis almost totally lacking tubercles, except on the terminal piece; pygidia of T. morrisoni also lack the rows of fine tubercles along margins of furrows, except for along the posterior margin of the articulating half-ring.

Ontogeny. Smaller cranidia are densely tuberculate with a small genal spine. Throughout ontogeny the tubercles become nearly effaced, except on the anterior border and frontal lobe of the glabella; glabella becomes broader; distinct genal spine on smaller specimens (e.g., Pl. 10, figs 7, 8, 10, 12) becomes reduced to a small nubbin.

Discussion. As noted previously, T. morrisoni , the oldest species, is much more similar to T. kriegerae , the youngest, than either is to the intermediate species, T. depressa . Tesselacauda morrisoni and T. kriegerae both have anterior borders which are short sagittally and distinctly anteriorly bowed medially, versus the longer, medially transverse border of T. depressa . They share posterior projections which are exsagittally longer than in T. depressa as well as more steeply inclined. They differ in that the projections of T. morrisoni are longer and more inclined than those of T. kriegerae . Tesselacauda morrisoni also differs from T. kriegerae in its nearly completely effacement of dorsal cranidial tuberculation, versus variable development with fine, sparse tubercles retained on most specimens of T. kriegerae . Pygidia of T. morrisoni differ from those of T. kriegerae in: the absence of tubercles other than on the distal pleurae in most specimens, versus their common presence on the axis, and particularly in the possession of a much larger axial terminal piece which is much longer than wide.

Figures

PLATE 9. Tesselacauda morrisoni n. sp., from Section MME 49.8 m, Fillmore Formation (upper Tremadocian; Stairsian; Rossaspis leboni Zone), Middle Mountain, Ibex area, Millard County, western Utah, USA. 1, 2, 4, 7, 10. Cranidium, SUI 147564, dorsal, right lateral, ventral, anterior, and oblique views, x7.5. 3, 5, 6, 9, 12. Cranidium, holotype, SUI 134075, dorsal, left lateral, ventral, anterior, and oblique views, x10. 8, 11, 14. Cranidium, SUI 147565, left lateral, dorsal, and anterior views, x7.5. 13, 16, 18. Cranidium, SUI 147566, dorsal, anterior, and left lateral views, x6. 15, 19, 22. Cranidium, SUI 147567, dorsal, anterior, and left lateral views, x7.5. 17, 20, 21. Cranidium, SUI 147568, left lateral, anterior, and dorsal views, x7.5.

PLATE 10. Tesselacauda morrisoni n. sp., from Section MME 49.8 m, Fillmore Formation (upper Tremadocian; Stairsian; Rossaspis leboni Zone), Middle Mountain, Ibex area, Millard County, western Utah, USA. 1, 2, 4. Cranidium, SUI 147569, dorsal, right lateral, and anterior views, x10. 3, 5, 6. Cranidium, SUI 147570, dorsal, left lateral, and anterior views, x15. 7, 8, 11. Cranidium, SUI 147571, dorsal, right lateral, and anterior views, x15. 9, 13, 16. Cranidium, SUI 147572, dorsal, anterior, and left lateral views, x17. 10, 12, 14. Cranidium, SUI 147573, dorsal, left lateral, and anterior views, x15. 15. Left librigena, SUI 147574, external view, x12. 17, 20. Left librigena, SUI 147575, external and internal views, x12. 18, 21. Right librigena, SUI 147576, external and ventrolateral views, x12. 19, 22. Left librigena, SUI 147577, external and internal views, x12. 23. Left librigena, SUI 147578, external view, x12.

PLATE 11. Tesselacauda morrisoni n. sp., from Section MME 49.8 m, Fillmore Formation (upper Tremadocian; Stairsian; Rossaspis leboni Zone), Middle Mountain, Ibex area, Millard County, western Utah, USA. 1, 3, 4, 7, 10. Pygidium, SUI 147579, dorsal, right lateral, posterior, ventral, and anterior views, x12. 2, 5, 6, 8, 11. Pygidium, SUI 147580, dorsal, posterior, right lateral, ventral, and anterior views, x15. 9, 15, 18. Pygidium, SUI 147581, right lateral, dorsal, and posterior views, x12. 12, 13, 16. Pygidium, SUI 134076, left lateral, dorsal, and posterior views, x15. 14, 17, 21. Pygidium, SUI 147582 dorsal, posterior, and left lateral views, x12. 19, 20, 22. Pygidium, SUI 147583, posterior, dorsal, and left lateral views, x12.

PLATE 12. 1–12, 17. Tesselacauda morrisoni n. sp., from Section MME 49.8 m, Fillmore Formation (upper Tremadocian; Stairsian; Rossaspis leboni Zone), Middle Mountain, Ibex area, Millard County, western Utah, USA. 1, 4, 6, 11. Thoracic segment, SUI 147584, dorsal, anterior, posterior, and left lateral views, x10. 2, 3, 5, 7, 9. Thoracic segment, SUI 147585, right lateral, dorsal, ventral, anterior, and posterior views, x8. 8, 10, 12, 17. Thoracic segment, SUI 147586, dorsal, posterior, anterior, and left lateral views, x10. 13–16, 18–29. Tesselacauda n. sp. A, from Section MME 36.4 m, Fillmore Formation (upper Tremadocian; Stairsian; Rossaspis leboni Zone), Middle Mountain, Ibex area, Millard County, western Utah, USA. 13. Cranidial fragment (probably from same individual as fig. 14), SUI 147587, dorsal view, x7.5. 14. Cranidial fragment (probably from same individual as fig. 13), SUI 147588, dorsal view, x7.5. 15. Pygidium, SUI 147589, dorsal view, x15. 16, 20, 21. Pygidium, SUI 147590, dorsal, right lateral, and posterior views, x12. 18. Pygidium, SUI 147591, dorsal view, x12. 19, 23, 24. Pygidium, SUI 147592, dorsal, posterior, and right lateral views, x15. 22, 28, 29. Pygidium, SUI 147593, dorsal, posterior, and left lateral views, x12. 25–27. Pygidium, SUI 147594, posterior, right lateral, and dorsal views, x12.