Lotagnostus clarki, Taylor & Loch & Repetski, 2024

Taylor, John F., Loch, James D. & Repetski, John E., 2024, Taxonomy and stratigraphic distribution of Lotagnostus (Agnostida: Agnostidae) and associated trilobites and conodonts in the Upper Cambrian (Furongian) of Laurentia, Zootaxa 5422 (1), pp. 1-66 : 43-46

Lotagnostus clarki n. sp.

( Plate 1.6–1.10, 1.17–1.22 , Plate 10 , Plate 11 )

Diagnosis. Genae of large cephala covered by weakly to moderately impressed scrobiculae; pygidia and small cephala non-scrobiculate; M2, at maximum width (tr.) behind glabellar node, is significantly wider than M1 and slightly wider than M3; longitudinal furrows that converge and shallow anteriorly trisect M3, defining elliptical lateral lobes in larger specimens; trisection of pygidial posteroaxis absent or very faint, with sides of intranotular axis defined only by minor change in slope rather than furrows or notulae; pleural field widest (tr.) near midlength of anteroaxis, narrowing (sag.) behind axis; pygidial border furrow in larger specimens widens (tr.) posteriorly from anterolateral corners, then narrows behind posteroaxis to midline.

Etymology. Named after Dr. Joseph C. Clark, our colleague and mentor (Indiana University of Pennsylvania).

Material and occurrence. Holotype CM 41373 is a cephalon from collection 5/22/08B: assigned specimens include 9 cephala CM 41310–41314 , 41374–41377 and 10 pygidia CM 41315–41319 , 41378–41382 from collection 5/22/08B; and 5 cephala USNM 775729–775733 and 5 pygidia USNM 775734–775738 from collection D3362- CO. Lotagnostus nolani Fauna : collection 5/22/08B (495-383) and 3362-CO (38-46) from the Windfall Formation at Ninemile Canyon , Nevada.

Description. Cephalon semiovate in outline with maximum width approximately equal (89–105%) to length (sag.); moderately convex (tr.) and slightly convex (sag.). Glabella long, 75% (72–79%) of cephalic length (sag.), glabellar width at F3 30% (27–31%) of maximum cephalic width (tr.); slightly convex (sag. and tr.) standing in low relief above genae. Axial furrows moderately impressed in smallest specimens, faintly impressed in larger specimens; straight and strongly convergent (20º to midline) along basal lobes, slightly convergent to gently angular anterolateral corners of glabella; widest (tr.) from anterior tip of basal lobes to midlength of M3; meet medially at obtuse angle (approximately 120 º). Basal lobes subtriangular in outline, moderately long, 28% (24–36%) of glabellar length; end anteriorly at or just in front of F1; end posteriorly in line with small terminal glabellar node; moderately impressed basal furrows straight anterior to posterolateral corners of M1, turning sharply inward posteriorly to follow bluntly pointed back of M1 to glabellar culmination. Occipital band short (sag.). F1 absent, or a very faintly impressed, anteromedially directed furrow extending less than a third of the way to the midline. Slight abaxial shift of axial furrow at F1 expands the posterior half of M2, making it as wide (tr.) or slightly wider than M3; constriction of anterior half of M2 produces narrowest (tr.) part of glabella and widest segment of axial furrow; small, nearly circular glabellar node centered on anterior half of M2. M3 trisected in larger specimens by longitudinal furrows that converge and shallow anteriorly, defining slightly elongate (exsag.), subelliptical lateral lobes. F2 faintly to moderately impressed, anteromedially directed, terminating at posterior ends of longitudinal furrows of M3. F3 transglabellar, moderately impressed, straight to gently concave anteriorly in smaller specimens, tripartite in larger specimens with transverse central segment and lateral segments in front of lateral lobes of M2 directed anterolaterally abaxially to intersection with axial furrow. Anteroglabella accounts for 30% (27-33%) of glabellar length (sag.); subpentagonal in outline with gently angular corners and front. Genae moderately convex and moderately declined (tr.); preglabellar field constitutes 20% (13-23% of cephalic length (sag.), slightly less than width (tr.) of genae laterally; genae smooth in small specimens, weakly scrobiculate in specimens longer (sag.) than 4mm. Median preglabellar furrow moderately impressed, of constant depth and width (tr.), intersects axial and border furrows. Border convex, narrow, slightly wider at midline, slightly tapered posteriorly from level of M3. Border furrow well-impressed and narrower than border in small specimens; as wide or slightly wider in large specimens; narrowing posteriorly along posterior third of cephalon; deflected slightly posteriorly at intersection with median preglabellar furrow.

Pygidium semi-elliptical in outline, length (sag.) 92% (87–100%) of maximum width (tr.); moderately convex (sag., tr.); in posterior view axis stands in low relief above moderately declined, slightly convex pleurae; in lateral profile axis slightly declined from axial node in even curve continued by post-axial pleural field. Axis long, 80% (69–85%) of pygidial length exclusive of articulating half-ring; slightly convex (sag., tr.) with exception of axial node; constricted at M2. Articulating half-ring short (sag.), crescentic. Articulating furrow straight, well impressed. Axial furrows moderately impressed, convergent posteriorly from anterior margin, convex laterally over M1, M2, and posteroaxis; slightly shallower around posterior lobe in larger specimens, shallowest medially. Anteroaxis strongly trisected by longitudinal furrows that extend from F2 to articulating furrow; confluent medial lobe of M1 and M2 much narrower (tr.) than lateral lobes of M2, slightly narrower than lateral lobes of M1; M1 lateral lobes slightly inflated, M2 lateral lobes very slightly inflated; asymmetrical axial node at posterior end of confluent medial lobe projects posteriorly to overhang F2. F1 moderately impressed, slightly curved, directed slightly anteromedially from axial furrow across 1/3 width of axis, terminating at intersection with longitudinal furrow. Transaxial F2 moderately impressed, tripartite with narrow (tr.) central segment along posterior margin of confluent medial lobe of anteroaxis, and longer, gently curved lateral segments directed anteromedially from axial furrow. Posteroaxis constitutes 59% (56–61%) of axis length (sag.), slightly convex, slightly declined (sag.); broadest (tr.) slightly anterior to lobe midlength; maximum width variable, averaging 95% (82–118%) of posteroaxis length; small terminal node present; very faint intranotular axis on some specimens, sides defined only by change in slope, except for faintly impressed, very short (exsag.) furrows outlining upturned posterior tip of intranotular axis on a few specimens. Pleural fields narrow (tr.), moderately and evenly convex, slightly declined laterally; typically smooth, rarely very faintly scrobiculate in large specimens; much narrower postaxially in large specimens. Border narrow (sag.), convex, widest medially, tapering steadily toward anterolateral corners. Pair of small posterolateral spines directed posterolaterally and positioned posterior to end of posteroaxis in small specimens; directed posteriorly and located slightly in front of posteroaxis in larger specimens. Border furrow on small specimens uniformly well impressed, nearly constant in width, narrower than border; width and variation in width increase with growth; wider than border in large specimens, widest adjacent to posteroaxis, tapering toward anterolateral corners and behind axis.

Discussion. Many of the specimens of Lotagnostus peladensis ( Rusconi, 1951a) illustrated by Tortello (2014) strongly resemble L. clarki , and some pygidia are nearly identical to those of the latter species. However, large, testate cephala of L. peladensis have much shallower furrows, and lack the scrobiculae that characterize L. clarki cephala of similar size. Exfoliated cephala of L. peladensis , which display more firmly impressed furrows, differ from those of L. clarki in having a longer (sag.) preglabellar field that is equal to or greater in length than the anteroglabella ( Figure 10.3 ). In L. clarki the length of the preglabellar field is shorter, averaging less than 90% of the anteroglabella length. Another species from Argentina, L. atenuatus ( Rusconi, 1955b) , has weakly scrobiculate genae similar to those of L. clarki , but its lateral lobes on M3 are less elongate and are bowed outward to make M3 significantly wider than M2. The pygidium of L. atenuatus is readily distinguished from that of L. clarki by a prominent intranotular axis bounded by well-impressed notulae.

Lotagnostus asiaticus Troedsson (1937) is the species most similar to L. clarki among the species of Lotagnostus reported from Asian successions. Using a narrow concept for L. asiaticus that includes only weakly scrobiculate specimens similar to the holotype, a full exoskeleton re-illustrated by Peng et al. (2015, fig. 4), it is clear that several features that separate these species. Most of the paratypes illustrated by Troedsson as part of the type suite from Xinjiang, China were excluded from the analysis because they are strongly scrobiculate and display very deep furrows, as noted by Westrop & Landing (2016). Aside from the holotype, the specimens we used for comparison with L. clarki include (among others) those assigned to L. asiaticus from eastern China (western Zhejiang) by Lu & Lin (1980, 1989), and similar specimens from the Wa’ergang section in northwest Hunan ( Peng et al., 2015, figs 1, 2). The size series illustrated by Peng et al. (2015, fig. 7) was particularly informative regarding the range of variation within L. asiaticus as all specimens derived from a single collection level (GC26a) in the Siyanshan section of Lu & Lin (1980, 1989) in Zhejiang. This size series, along with the holotype and similar specimens from the other areas of China, confirms a similar pattern of ontogenetic change for L. asiaticus and L. clarki in which scrobiculae are absent from the genae and pleural fields of small specimens and become increasingly prominent with increasing size. However, pygidia of L. asiaticus of any size differ from those of L. clarki in having gently to moderately impressed notular furrows. Larger pygidia of L. asiaticus also display, albeit only faintly, the circular proximal area of the pleural field with radially arranged, short scrobiculae adjacent to the posteroaxis and M2 that occurs in several species of Lotagnostus , but not in L. clarki . At least three features distinguish L. asiaticus cephala from those of L. clarki . The former species has a proportionally longer anteroglabella that is more parallel-sided and broadly rounded in front than that of L. clarki , whose convergent sides produce a narrower, more angular anterior. Secondly, M 2 in L. asiaticus is approximately equal in width (tr.) to M1 and significantly narrower than M3. In contrast, M2 is wider than both M1 and M3 on the cephala of L. clarki . Finally, on most cephala of L. clarki , the anterior border widens (sag.) slightly at the midline along with a slight rearward deflection of border furrow where it intersects the median preglabellar furrow; no such widening or deflection is seen on the cephalon of L. asiaticus .

Figures

PLATE 1. Ontogenetic comparison of Lotagnostus nolani and L. clarki n. sp. All images at same scale and all from collection 5/22/08B. 1-5, 15. L. nolani cephala. 1. CM 41301. 2. CM 41302. 3. CM 41303. 4, 15. CM 41304. 5. CM 41305. 6-10, 21. L. clarki cephala. 6. CM 41310. 7. CM 41311. 8. CM 41312. 9, 21. CM 41313. 10. CM 41314. 11-14, 16. L. nolani pygidia. 11. CM 41306. 12. CM 41307. 13. CM 41308. 14,16. CM 41309. 17-20, 22. L. clarki pygidia. 17. CM 41315, 18. CM 41316. 19. CM 41317. 20. CM 41318. 22. CM 41319.

PLATE 10. Cephala of Lotagnostus clarki n. sp. Each white scale bar represents 2mm. 1-4. Holotype, CM 41373, from 5/22/08B, dorsal, anterior, right lateral, and anterior oblique views. 5. USMN 775729, from D3362-CO, dorsal view. 6. USMN 775730, from D3362-CO, dorsal view. 7. CM 41374, from 5/22/08B, dorsal view. 8. CM 41375, from 5/22/08B, dorsal view. 9, 10. USMN 775731, from D3362-CO, dorsal and anterior oblique views. 11-13. USMN 775732, from D3362-CO, dorsal, anterior, and right lateral views. 14, 15. Cephalon, CM 41376, from 5/22/08B, dorsal and anterior oblique views, x9.5. 16. Cephalon, USMN 775733, from D3362-CO, dorsal view, x7.3. 17-19. Cephalon, CM 41377, from 5/22/08B, dorsal, anterior, and left lateral views, x7.0.

FIGURE 10. Morphometric comparisons of some Lotagnostus species. (1) Dimensions measured on cephala. Abbreviations: AAL, Axial Anteroglabella Length; APL, Axial Posteroglabella Length; EBL, Exsaggital, Basal lobe Length; PFL, Preglabellar Field Length. (2) Scatter plot of EBL/APL ratio versus length of the cephalon for 3 species of Lotagnostus. L. “trisectus” refers to species identified by that name from Sweden in previous studies. (3) Scatter plot of PFL/AAL ratio versus length of the cephalon for 4 species of Lotagnostus. (4) Graphic comparison of posteroaxis length/length of axis as a function of pygidial length (excluding articulating half ring) for 3 species of Lotagnostus.

PLATE 11. Pygidia of Lotagnostus clarki n. sp. Each white scale bar represents 2mm. 1-4. USMN 775734, from D3362-CO, dorsal, posterior, right lateral, and posterior oblique views. 5, 7 USMN 775735, from D3362-CO, left lateral and dorsal views. 6. CM 41317, from 5/22/08B, dorsal view. 8-10. USMN 775736, from D3362-CO, dorsal, posterior, and left lateral views. 11. CM 41316, from 5/22/08B, dorsal view. 12. CM 41378, from 5/22/08B, dorsal view. 13-15. USMN 775737, from D3362-CO, dorsal, posterior, and right lateral views. 16. USMN 775738, from D3362-CO dorsal view. 17. CM 41379, from 5/22/08B, dorsal view. 18. CM 41380, from 5/22/08B, dorsal view. 19. CM 41381, from 5/22/08B, dorsal view. 20. CM 41382, from 5/22/08B, dorsal view.