Tesselacauda depressa 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 : 209-213

Tesselacauda depressa Ross, 1951

( Plates 1–8 )

1951 Tesselacauda depressa Ross , p. 130, pl. 31, figs 27–31, pl. 34, figs 1–4, 18. 1955 Tesselacauda depressa Ross ; Hupé, fig. 231.11. 1962 Tesselacauda depressa Ross ; Maksimova, p. 143.

non 1973 Tesselacauda depressa Ross ; Terrell (partim), p. 48, pl. 1, figs 5, 6, 10, 14, 15 (only; pl. 1, figs 5, 6, 10, 15 = T. kriegerae n. sp.; pl. 1, fig. 14 = Pilekiinae gen. nov. 3 sp. nov. B of Adrain et al. [2014, p. 179, fig. 12EE]).

? 1973 Tesselacauda depressa Ross ; Demeter, p. 89, pl. 2, figs 6, 9, 13. 1985 Tessalacauda [sic] depressa Ross ; Jell, p. 79. 1989 Tesselacauda depressa Ross ; Dean, p. 17.

? 1997 Tesselacauda depressa Ross ; Lee and Chatterton, p. 685, figs 3.1–3.6, 4.1–4.13, 5.1, 8.2, 8.5, 8.8, 8.12, 8.16. 1997 Tesselacauda depressa Ross ; Ross et al., p. 45. 1999 Tesselacauda depressa Ross ; Edgecombe et al., p. 1171. 2003 Tesselacauda depressa Ross ; Jell and Adrain, p. 452. 2012 Tesselacauda depressa ; Dai and Zhang, p. 650. 2014 Tesselacauda depressa Ross ; Adrain et al., p. 178, fig. 10Q, U.

Material. Assigned specimens SUI 147509–147511, 147514–147520, from Section HC5 106.7 m, east side of Hillyard Canyon, SUI 147513 from Section HC6 88.3, west side of Hillyard Canyon, and SUI 147507, 147508, and 147512, from Section FB7 102.1 m, Franklin Basin, all Garden City Formation (upper Tremadocian; Stairsian; Bearriverops loganensis Zone ), Bear River Range, Franklin County, southeastern Idaho. Assigned specimens SUI 134129, 134130, 147511–147523, 147524–147546, 147547–147551, from Section MME 75.5 m, Fillmore Formation (upper Tremadocian; Stairsian; Bearriverops loganensis Zone ) and SUI 147552, 147554, 147555, 147557, 147560, 147561, from Section MME 84.0 m (upper Tremadocian; Stairsian; Bearriverops deltaensis Zone ), both Middle Mountain, and SUI 147556, 147558, 147562, and 147563, from Section AAA 79.5 m (upper Tremadocian; Stairsian; Bearriverops deltaensis Zone ), northern House Range, all Ibex area, Millard County, western Utah. Assigned specimens SUI 147553, 147559, from Section HC6 107.5 m, Garden City Formation (upper Tremadocian; Stairsian; Bearriverops deltaensis Zone ), west side of Hillyard Canyon, Bear River Range, Franklin County, Idaho, USA.

Diagnosis. Glabella with moderately dense sculpture of fine tubercles; anterior border medially transverse and only slightly forwardly arched, longer sagittally in dorsal view than in other species; posterior projections less downturned than in other species, cephalon less vaulted; exsagittal length of fixigena behind palpebral lobe relatively short; librigenal lateral border with prominent band largely free of tuberculate sculpture; pygidial axial rings with moderately dense tuberculate sculpture, terminal piece wider than long.

Description. Measurements for the cranidium were made on the largest and best preserved specimens of Plates 1 , 3 and were doubled from the mid-line where preservation was incomplete. Cranidium very broad and generally flattened in appearance with gentle dorsal inflation, sagittal length 45.4% (39.8–48.2%) maximum width (excluding genal spine); length 110.3% (104.1–121.6%) width across ε, 75.2% (72.3–78.6%) across γ; anterior border moder- ately short (sag.), sagittal length 5.7% (4.8–6.8%) cranidial length, generally describing broad anteriorly directed arc, with medial portion slightly more transverse, in anterior view border very gently dorsally arched medially, independently inflated sitting below anterior margin of glabella (Pl. 3, fig. 9), sculpture of densely spaced granules present on anterior rim (in dorsal view) and anterior face so that in anterior view border appears covered in granules (e.g., Pl. 3, fig. 6); anterior border furrow deeply incised, gently bowed anteriorly as anterior border; glabella with length (sag.) 96.8% (86.6–104.3%) maximum width (tr.), with lateral margins slightly bowed outward, maximum width achieved between anterior portion of L1 and L2, gently tapered (tr.) anteriorly, anterolateral corners more evenly rounded than posterolateral corners, which are more squared off, anterior margin broadly rounded, posterior margin with medial portion gently anteriorly bowed, weak dorsal inflation, anterior portion sloped downward from horizontal, sculpture of fine tubercles covers glabella, tubercles generally evenly spaced, but more concentrated medially along sagittal axis on some specimens; S1–3 deeply incised, narrow, extending about a third across glabella; S1–2 subparallel, directed gently posteromedially, with proximal tip of S1 extending a little more inward and gently posteriorly curved (Pl. 3, Fig. 13 ); S3 directed more strongly posteromedially than S1–2; SO similar in depth and length (sag., exsag.) to other glabellar furrows; LO moderately long, with sagittal length 14.0% (12.3–15.6%) cranidial length, tapered abaxially with distal ends gently swollen and rounded, posterior margin transverse, inflation similar to that of glabella, sculpture of small scattered tubercles concentrated on median portion of LO with distal ends more smooth; L1 wedge shaped, posterior margin swollen, but otherwise lacking independent inflation; fixige- nal field with maximum length (exsag.) 109.6% (101.7–114.5%) width; proximal posterior corner of field is extend- ed around L1 forming small rounded extension of field that is situated obliquely opposite the anterolateral corner of LO, field ends just behind S3 anteriorly, posterior margin of field gently anterolaterally directed, inter-ocular portion of field small, subtriangular; post-ocular field relatively short (exsag.); field with sculpture of deep pits overlain by scattered small tubercles, except for narrow smooth bands opposite posterior border and glabella; posterior projections gently flexed anteroventrally distal to fulcrum; fulcrum set relatively close to glabella (e.g., Pl. 3, figs 4, 21); posterior border furrow narrowest adaxially where fixigenal field is pinched toward LO, lengthens abaxially so that is longest opposite medial portion of field, pinches down again towards genal angle; lateral border furrow similar to narrower portion of posterior border furrow, ends abruptly before reaching intersection with facial suture (Pl. 3, fig. 8); posterior border shortest (exsag.) opposite LO, gradually lengthens abaxially, reaches maximum length at point where border abruptly changes course from transverse to anterolaterally directed; genal spine forming small nubbin (Pl. 1, figs 7, 8; Pl. 3, figs 12, 14); lateral border widest at genal angle, pinches out anteriorly; palpebral lobe small, set anteriorly opposite anterior portion of L3, bound anteriorly by very shallow furrow expressed more as change in slope, external margin strongly sinuous with nearly 90 bend before posterior termination, posterior portion of lobe extending into short laterally directed “tail” abaxially from this bend; palpebral furrow deep, narrow, sinuous, running from intersection with axial furrow just opposite anterolateral corner of L3 posteriorly toward facial suture, but ending abruptly just before reaching suture; axial furrows similar in depth to glabellar furrows; fossula forming deep pit just anterior to intersection of S3 with axial furrow, clearly visible ventrally as laterally directed projection (e.g. Pl. 3, fig. 17); all dorsal furrows are fringed by row of fine granules or microtubercles (especially visible on specimens from MME); doublure beneath LO short, only extending about half way across LO, with small articulating flange at posterolateral corner; short strip of doublure present beneath transverse portion of posterior border, with narrow articulating groove present along posterior margin; small triangular piece of doublure situated obliquely beneath genal angle.

Rostral plate identified ventrally on one partially articulated specimen (Pl. 1, fig. 2), with plate flipped so that the dorsal (interior) surface is visible; forming moderately wide (tr.) strip, with length (sag., exsag.) much shorter than width; narrow ridge developed along hypostomal suture; sculpture of coarse granules present adjacent to hypostomal suture, becoming progressively effaced across plate.

Hypostome with sagittal length 85.1% (76.4–90.7%) maximum width (tr. across anterior wings); with lateral margins rounded and gently expanded laterally across shoulders, posteriorly lateral margin gently constricted (tr.); anterior margin gently anteriorly arched; anterior border short (sag., exsag.), merging smoothly with anterior wings, differentiated from middle body by shallow, but distinct anterior border furrow; anterior wings prominent, subtriangular, extending laterally just beyond lateral margins of hypostome, with elongate (tr.) pit developed near anterior margin of wing, pit expressed dorsally as large strongly dorsally deflected wing process (e.g., Pl. 2, fig. 15); lateral notch prominent and moderately deep in lateral profile (e.g., Pl. 2, fig. 15), smooth; posterior wings small, strongly deflected dorsally; middle body long, subovoid, with anterior lobe much longer (sag.) than posterior lobe; anterior lobe oval shaped with posterior margin more pointed medially and anterior margin more evenly rounded, inflation moderate; posterior lobe of middle body short (sag.), U-shaped, inflation distinct, but slightly less than that of anterior lobe; anterior and posterior lobes separated by very shallow middle furrow, that is most distinct laterally and nearly effaced medially; maculae small, oval shaped with long axis directed anterolaterally, smooth; lateral border furrow deep, narrow, clearly setting off anterior portion of middle body from lateral border; border furrow disrupted at anterior margin of posterior lobe of middle body; furrow deep posterolaterally, clearly setting off posterior lobe of middle body from border; lateral and posterior borders moderately broad, expanded laterally at shoulder and posterolateral corner of hypostome, posterior border shortest (sag.) medially, lengthening (exsag.) abaxially, somewhat flattened in lateral and posterior views; border deflected dorsally in lateral profile at posterior shoulder; in posterior view border nearly transverse; two sets of lateral spines developed; smaller spine pair present at posterior edge of shoulder, directed laterally to slightly posterolaterally; larger spine pair present at posterolateral corner of hypostome, directed more strongly posterolaterally; posterior margin of hypostome broadly W-shaped with slight median embayment (more well developed on some specimens than others; cf. Pl. 2, fig. 3 and Pl. 5, fig. 1); anterior lobe with most densely spaced and coarsest sculpture, granules larger anteriorly, progressively finer posteriorly; posterior lobe with finer granular sculpture that is nearly effaced in some specimens; lateral and posterior borders with granular sculpture that is less densely spaced than that on anterior lobe; ventrally, doublure broad and smooth beneath lateral and posterior borders.

Librigena elongate, slender; eye small, bulbous, set far forward on librigena, differentiated from field by greater inflation and very shallow furrow developed around base of eye, furrow slightly deeper along posterior margin; field expressed as long thin triangular extension posteriorly from base of eye, posterior extension of field generally longer than anterior portion with anterior portion minute in some specimens, field is somewhat variable in length with some specimens possessing a rather longer pinched out tail posteriorly, while others terminate more rapidly (cf. Pl. 4, figs 9 and 10); deep furrow separates field from border, sinuous, on most specimens furrow is slightly wider opposite posterior margin of eye and field, furrow significantly tapered toward posterior facial suture, furrow terminated abruptly before reaching anterior facial suture, row of very fine granules lines margins of furrow; border elongate, medial portion broadest, shorter anteriorly and posteriorly, in ventrolateral view anterior portion of border flexed upwards before anterior projection strongly flexed downward; inner portion of border generally smooth, lacking tubercles, outer margin and external surface with band of densely spaced fine tubercles, sculpture stops abruptly and does not continue ventrally onto doublure (Pl. 4, fig. 11); anterior projection subtriangular, much shorter than posterior projection, strongly flexed downward in ventrolateral view; posterior projection extended into long sweeping projection, in external view tip of posterior projection almost level with top of eye and portion of doublure visible distally, held roughly in line with main portion of border in ventrolateral view; anterior facial suture outwardly convex opposite eye, distinct change in course opposite border furrow, nearly straight across anterior projection; posterior facial suture long, sinuous with slight outwardly convex bump across field just behind eye, suture describing gently concave arc across posterior projection, with arc more strongly curved distally along tip of posterior projection; doublure smooth, extending inward to dorsal expression of border furrow anteriorly and posteriorly, but medial portion only extending about halfway across border to furrow.

Thorax composed of 12 segments (Pl. 4, fig. 1); articulating half-ring short (sag., exsag.), with anterior margin gently anteriorly arched, posterior margin nearly transverse, set off from axial ring by short, deep articulating furrow, with line of small tubercles across posterior margin of half-ring (Pl. 1, fig. 3; Pl. 8, fig. 6); axis broad (tr.), gently tapered posteriorly, with moderately weak dorsal inflation, sitting above pleurae in lateral profile; pleurae broad (tr.) anteriorly, progressively narrower posteriorly; axial ring very gently anteriorly bowed, more so on posterior segments, with distal tips rounded and slightly inflated, sculpture of scattered tubercles; axial furrow deep, broad; short (exsag.) articulating flange present along anterior margin of pleurae, set off from anterior pleural band by moderately shallow and short articulating furrow (Pl. 1, fig. 3); pleurae divided into clear anterior and posterior bands by deep pleural furrow, furrow shallower distal to fulcrum, then abruptly terminated before reaching end of pleural tip; anterior and posterior pleural bands of roughly equal length between axis and fulcrum, proximal termination of bands forming rounded tips with that of anterior band extending slightly further towards axis than posterior band; anterior segments with anterior and posterior bands less clearly differentiated toward termination of pleurae, which are lengthened into short, pointed, anteriorly flexed pleural spines (Pl. 4, figs 1, 2); on more posteriorly located segments, anterior pleural band occupies progressively less space distal to fulcrum becoming pinched out with the posterior band becoming more dominant (lengthened exsagittally) and clearly extended into a more tab-like and less pointed pleural tip; clear raised rim visible ventrally marking termination of segment, with pleural spine extended beyond; first six segments with pleural spines flexed anteriorly, posterior segments with spines directed progressively posteriorly so that they appear to wrap around thorax and pygidium.

Pygidial measurements were made on the largest and most complete specimens of Plates 2 , 5 , and 6. Pygidium with overall low dorsal inflation, broad with sagittal length (excluding articulating half-ring) 47.7% (43.3–52.6%) maximum width; fulcrum set relatively close to axis, with pleurae strongly downturned from fulcrum outward; articulating half-ring shorter (sag.) than first axial ring, anterior margin anteriorly bowed, posterior margin transverse, generally smooth except for row of scattered tubercles present along posterior margin, some tubercles very slightly elongated; axis broad, composed of four segments and terminal piece, length 104% (95–110.1%) maximum width across first segment, maximum width across first segment 41.5% (39.8–45.7%) pygidial width, axis remains relatively broad posteriorly, but sharply pinched (tr.) at terminal piece; first two axial segments of similar shape with the second being only slightly shorter (sag.), distal tips rounded or bulbous; third segment shorter (sag., exsag.) than first two with distal tips less bulbous; fourth segment with anterior margin broadly W-shaped so that medial portion is bowed slightly anteriorly and lateral portions posteriorly, terminating with distal tips swept anteriorly (e.g., Pl. 6, fig. 1), this curve is very flattened on some specimens so that the anterior margin appears nearly transverse; terminal piece triangular, with width (tr.) greater than length (sag.), clearly isolated by axial furrows, circumscribed by fourth posterior pleural band; inter-ring furrows with middle third shallowest, lateral thirds more deeply incised, articulat- ing furrow similarly incised; axial furrows similarly broad as inter-ring and pleural furrows, forming wider areas at intersection with ring and pleural furrows; very narrow articulating flange present along anterior pleural margin of first segment, set off by narrow articulating furrow; first pleural segment with anterior and posterior bands clearly defined by deep pleural furrow cutting obliquely across segment, furrow abruptly shallows before reaching pygidial margin, posterior band terminates in large tab that continues on past pygidial border as semi-fused pleural spine; second pleural segment similar to first in that segment is cut obliquely by clearly incised pleural furrow that divides segment into anterior and posterior bands, furrow is confluent with first interpleural furrow so that the anterior band is totally isolated; third and fourth pleural segments lacking anterior pleural band; fourth segment is subtriangular in dorsal view; all four pleurae extended into short semi-fused tab-like pleural spines; on one specimen from HC5 (Pl. 2, fig. 24) the tabs are more fused (cf. others specimens from both HC5 and MME where tabs are clearly separated (e.g., Pl. 6, fig. 8)); pygidial border expressed ventrally as pronounced short and narrow rim, extends anteriorly only to just cover posterior tip of terminal piece, in dorsal view the anterolateral tips of the rim are just visible as a small spike at the distal termination of the anterior band of the first pleural segment; doublure short, upturned so that it is best seen in anterior profile (e.g., Pl. 6, fig. 12), smooth; scattered small tubercles concentrated on medial portion of axis, few scattered tubercles on pleural region on most specimens, tubercles concentrated in a distinct rim around external margin of pygidium along with band of dense granules ringing posterior pygidial margin (e.g., Pl. 6, fig. 11), granular sculpture is continued ventrally to pygidial border (Pl. 2, figs 23, 24), pygidia from HC5 seem to possess fewer overall tubercles on dorsal surface compared to pygidia from MME; very fine row of tubercles also line the margins of most of the pygidial furrows.

Ontogeny. Glabella slightly longer (sag.) than wide, becoming more subquadrate to slightly wider than long throughout ontogeny. Smaller cranidia are much more densely tuberculate, especially on the palpebral lobes and anterior border (Pl. 3, fig. 18). The fringe of small tubercles outlining the dorsal furrows is not obvious on smallest specimens, but develops and becomes more prominent throughout ontogeny. The hypostome becomes more elongate, with both sets of spine pairs becoming smaller, and the maculae becoming larger throughout ontogeny. Similar to the crandium, the fringe of fine tubercles outlining dorsal furrows on the pygidium is not as well developed on smaller specimens (cf. Pl. 2, fig. 29, Pl. 6, fig. 1) as it is on larger specimens. The dorsal furrows on the pygidium broaden, but their depth remains relatively constant.

Discussion. Tesselacauda depressa is compared with T. morrisoni and T. kriegerae in the differential description of those species. Terrell (1973, p. 89, pl. 2, figs 6, 9, 13) assigned three specimens to T. depressa . Unfortunately, he did not give the provenance for any of the specimens illustrated in his paper beyond listing the zone to which he assigned them. The illustrated specimens comprise a small cranidium, a pygidium, and a hypostome. The cranidium is juvenile, and is densely tuberculate. Similar small, highly tuberculate specimens are illustrated herein for T. depressa (e.g., Pl. 3, fig. 27), T. morrisoni (e.g., Pl. 10, figs 3, 7, 9), and T. kriegerae (e.g., Pl. 17, figs 3, 11). Given the small photographs, that the only illustrated cranidium is a juvenile, and the lack of horizon or section information, it is impossible to assign Terrell’s specimens with any confidence.

Species from the Bearriverops deltaensis Zone tend to be very similar to congeneric species from the underlying B. loganensis Zone. In most cases, however, they are clearly differentiated, as in the example of the zonal name bearers (see Adrain and Westrop [2007]). Whereas the sample of Tesselacauda sclerites from the B. deltaensis Zone ( Plates 7 , 8 ) is not as large as that from the B. loganensis Zone ( Plates 1–6 ), especially for cranidia, there are no apparent differences between them and T. depressa seems to be a rare example of a species ranging through both zones.

Figures

PLATE 1. Tesselacauda depressa Ross, 1951, from Section HC5 106.7 m, east side of Hillyard Canyon, and Section FB7 102.1 m, Franklin Basin, both Garden City Formation (upper Tremadocian; Stairsian; Bearriverops loganensis Zone), Bear River Range, Franklin County, southeastern Idaho, USA. 1, 2, 6, 9. Cephalon and thoracic segments, SUI 147507, dorsal, ventral, right lateral, and anterior views, x7.5 (FB7 102.1 m). 3–5, 10. Thoracic segments and pygidium, SUI 147508, dorsal, dorsal, ventral, posterior, and left lateral views, x7.5 (FB7 102.1 m). 7, 8, 11, 13. Cranidium, SUI 147509, dorsal, dorsal, right lateral, anterior, and ventral views, x10 (HC5 106.7 m). 12, 14, 15. Cranidium and thoracic segment, SUI 147510, dorsal, dorsal, right lateral, and anterior views, x10 (HC5 106.7 m).

PLATE 2. Tesselacauda depressa Ross, 1951, from Section HC5 106.7 m, east side of Hillyard Canyon, HC6 88.3 m, west side of Hillyard Canyon, and Section FB7 102.1 m, Franklin Basin, both Garden City Formation (upper Tremadocian; Stairsian; Bearriverops loganensis Zone), Bear River Range, Franklin County, southeastern Idaho, USA. 1, 2. Right librigena, SUI 147511, external and internal views, x10 (HC5 106.7 m). 3, 6, 9, 11. Hypostome, SUI 147512, ventral, right lateral, posterior, and anterior views, x8 (FB7 102.1 m). 4, 7, 10, 12. Hypostome, SUI 147513, ventral, left lateral, posterior, and anterior views, x10 (HC6 88.3 m). 5. Left librigena, SUI 147514, external view, x12 (HC5 106.7 m). 8. Right librigena, SUI 147515, external view, x15 (HC5 106.7 m). 13–16. Hypostome, SUI 147516, ventral, dorsal, right lateral, and posterior views, x8 (HC5 106.7 m). 17. Hypostome, SUI 147517, ventral view, x10 (HC5 106.7 m). 18, 21, 22, 24, 25. Pygidium, SUI 147518, dorsal, posterior, left lateral, ventral, and anterior views, x10 (HC5 106.7 m). 19, 20, 23, 27. Pygidium, SUI 147519, left lateral, dorsal, ventral, and posterior views, x10 (HC5 106.7 m). 26, 28, 29. Pygidium, SUI 147520, right lateral, posterior, and dorsal views, x14 (HC5 106.7 m).

PLATE 3. Tesselacauda depressa Ross, 1951, from Section MME 75.5 m, Fillmore Formation (upper Tremadocian; Stairsian; Bearriverops loganensis Zone), Middle Mountain, Ibex area, Millard County, western Utah, USA. 1, 4, 7. Cranidium, SUI 147521, dorsal, anterior, and left lateral views, x5. 2, 5, 10. Cranidium, SUI 147522, dorsal, left lateral, and anterior views, x6. 3, 6, 9. Cranidium, SUI 147523, dorsal, anterior, and left lateral views, x6. 8, 11, 17, 22. Cranidium, SUI 134129, left lateral, dorsal, ventral, and anterior views, x6. 12, 14, 15. Cranidium, SUI 147524, dorsal, left lateral, and anterior views, x6. 13, 16, 19. Cranidium, SUI 147525, dorsal, anterior, and left lateral views, x5. 18, 23, 30. Cranidium, SUI 147526, dorsal, anterior, and left lateral views, x9. 20, 21. Cranidium, SUI 147527, dorsal and anterior views, x6. 24, 26, 27. Cranidium, SUI 147528, anterior, left lateral, and dorsal views, x9. 25, 28, 29, 31. Cranidium, SUI 147529, anterior, left lateral, ventral, and dorsal views, x6. 32. Cranidium, SUI 147530, dorsal view, x6.

PLATE 4. Tesselacauda depressa Ross, 1951, from Section MME 75.5 m, Fillmore Formation (upper Tremadocian; Stairsian; Bearriverops loganensis Zone), Middle Mountain, Ibex area, Millard County, western Utah, USA. 1–3, 5, 7. Dorsal exoskeleton, SUI 147531, dorsal, ventral, left lateral, right lateral, and posterior views, x12. 4. Left librigena, SUI 147532, external view, x12. 6, 8. Right librigena, SUI 147533, ventrolateral and external views, x10. 9, 11, 13. Right librigena, SUI 147534, external, internal, and ventrolateral views, x12. 10. Right librigena, SUI 147535, external view, x10. 12, 14. Right librigena, SUI 147536, external and internal views, x12.

PLATE 5. Tesselacauda depressa Ross, 1951, from Section MME 75.5 m, Fillmore Formation (upper Tremadocian; Stairsian; Bearriverops loganensis Zone), Middle Mountain, Ibex area, Millard County, western Utah, USA. 1, 5, 9, 13. Hypostome, SUI 147537, ventral, right lateral, posterior, and anterior views, x10. 2, 6, 10, 14. Hypostome, SUI 147538, ventral, right lateral, posterior, and anterior views, x10. 3, 7, 11, 15, 19. Hypostome, SUI 147539, ventral, left lateral, posterior, anterior, and dorsal views, x10. 4, 8, 12, 16, 20. Hypostome, SUI 147540, ventral, right lateral, posterior, anterior, and dorsal views, x12. 17, 21, 23, 25. Hypostome, SUI 147541, ventral, right lateral, posterior, and anterior views, x12. 18, 22, 24, 26. Hypostome, SUI 147542, ventral, left lateral, posterior, and anterior views, x12. 27, 30, 32, 35. Hypostome, SUI 147543, ventral, left lateral, posterior, and anterior views, x15. 28, 33, 34, 36. Hypostome, SUI 147544, ventral, left lateral, posterior, and anterior views, x15. 29, 31, 37–39. Pygidium, SUI 147545, dorsal, right lateral, posterior, ventral, and anterior views, x15.

PLATE 6. Tesselacauda depressa Ross, 1951, from Section MME 75.5 m, Fillmore Formation (upper Tremadocian; Stairsian; Bearriverops loganensis Zone), Middle Mountain, Ibex area, Millard County, western Utah, USA. 1, 3, 4. Pygidium, SUI 147546, dorsal, left lateral, and posterior views, x10. 2, 5, 6, 8, 12. Pygidium, SUI 134130, dorsal, left lateral, posterior, ventral, and anterior views, x10. 7, 9, 11, 15, 16. Pygidium, SUI 147547, dorsal, right lateral, posterior, ventral, and anterior views, x10. 10, 18, 20. Pygidium, SUI 147548, right lateral, dorsal, and posterior views, x10. 13, 19, 23. Pygidium, SUI 147549, left lateral, dorsal, and posterior views, x10. 14, 21, 22. Pygidium, SUI 147550, right lateral, posterior, and dorsal views, x12. 17, 24, 25. Pygidium, SUI 147551, left lateral, posterior, and dorsal views, x12.

PLATE 7. Tesselacauda depressa Ross, 1951, from Section MME 84.0 m, Middle Mountain, AAA 79.5 m, northern House Range, both Fillmore Formation, Ibex area, Millard County, western Utah, USA, and HC6 107.5 m, Garden City Formation, west side of Hillyard Canyon, Bear River Range, Franklin County, Idaho, USA (all upper Tremadocian; Stairsian; Bearriverops deltaensis Zone). 1, 2, 3, 5. Cephalon and thoracic segments, SUI 147552, dorsal, ventral, anterior, and left lateral views, x9 (MME 84.0 m). 4, 7. Cranidium, SUI 147553, left lateral and dorsal views, x7.5 (HC6 107.5 m). 6, 9, 10. Cranidium, SUI 147554, dorsal, anterior, and right lateral views, x10 (MME 84.0 m). 8, 11, 12. Cranidium, SUI 147555, anterior, right lateral, and dorsal views, x12 (MME 84.0 m). 13, 15, 17. Right librigena, SUI 147556, internal, ventrolateral, and external views, x10 (AAA 79.5 m). 14, 18. Left librigena, SUI 147557, internal and external views, x12 (MME 84.0 m). 16. Left librigena, SUI 147558, external view, x12 (AAA 79.5 m).

PLATE 8. Tesselacauda depressa Ross, 1951, from Section MME 84.0 m, Middle Mountain, AAA 79.5 m, northern House Range, both Fillmore Formation, Ibex area, Millard County, western Utah, USA, and HC6 107.5 m, Garden City Formation, west side of Hillyard Canyon, Bear River Range, Franklin County, Idaho, USA (all upper Tremadocian; Stairsian; Bearriverops deltaensis Zone). 1–4, 8. Hypostome, SUI 147559, right lateral, posterior, ventral, anterior, and dorsal views, x12 (HC6 107.5 m). 5, 7, 11. Pygidium, SUI 147560, dorsal, posterior, and left lateral views, x12 (MME 84.0 m). 6, 9, 12, 14, 15. Pygidium and thoracic segment, SUI 147561, dorsal, posterior, ventral, right lateral, and anterior views, x12 (MME 84.0 m). 10, 13, 16, 17, 19. Pygidium, SUI 147562, dorsal, posterior, ventral, right lateral, and anterior views, x15 (AAA 79.5 m). 18, 20, 21. Pygidium, SUI 147563, dorsal, left lateral, and posterior views, x15 (AAA 79.5 m).

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.