Lotagnostus rushtoni, 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 : 47-50

Lotagnostus rushtoni n. sp.

( Plate 12 )

Diagnosis. Non-scrobiculate Lotagnostus with narrow, strongly curved anterior margin; glabella long and ogival; lateral lobes of M3 defined only by inflation with no bounding longitudinal furrows; slightly elongate glabellar node sits high on elliptical elevated platform on anterior half of M2; large, inflated basal lobes with convex lateral margins extend well behind glabellar culmination; pygidium with transaxial F1; axial furrows narrow and moderately impressed; broad (tr.) posteroaxis with intranotular area undefined or extremely faint; acrolobe unconstricted; post-axial pleural field narrow (sag.); border furrow wide, deep, constant in width, with sharp boundaries with pleural field and border.

Etymology. Named after A.W.A. Rushton for his meticulous work on Cambrian taxa, among them Lotagnostus .

Material and occurrence. Holotype CM 41385 is a pygidium from collection 5/22/08D: assigned specimens include 2 cephala CM 41383–41384 and 1 pygidium CM41386 from collection 5/22/08D; 2 pygidia USNM 775743 , 775746 from collection D3381-CO; and 4 cephala USNM 775739–775742 and 2 pygidia USNM 775744–775745 from collection D3383-CO. Lotagnostus rushtoni Fauna : collection 5/22/08C (32-40), 5/22/08D (36-34), D3381- CO (0-3) and D3383-CO (9-18) from the Windfall Formation at Ninemile Canyon , Nevada.

Description. Cephalon semiovate in outline with maximum cephalic width equal to cephalic length (97– 105%); slightly convex (sag.) to moderately convex (tr.). Glabella long (sag.), 76% (71–81%) of cephalic length; narrow, accounting for approximately a third of cephalic width at M3; slightly convex (tr.), rising only slightly above genae in anterior and lateral views. Axial furrows moderately impressed; strongly convergent (30º angle to midline) from posterior margin to anterior end of basal lobes, parallel to very slightly convergent to poorly defined anterolateral corners of glabella, strongly convergent (45º) to front of glabella. Basal lobes long, 32% (26–35%) of glabellar length; basal furrow moderately impressed, but slightly narrower than axial furrows. Very short (sag.), gently curved, posteriorly convex occipital band. F1 a very shallow (tr.) indentation in side of glabella near anterior tip of basal lobe. Glabellar culmination very bluntly pointed with very faint terminal node. M2 equal in width (tr.) or slightly narrower than M3, with rounded anterolateral corners; prominent, slightly elongate glabellar node centered on inflated, elliptical area on posterior half of M2; F2 moderately impressed abaxially, faintly impressed medially in small specimens, incomplete across axis in larger specimens, directly slightly anteromedially. M3 divided transversely into a shorter (sag.), anteriorly tapering, medial lobe flanked by slightly inflated, elongate, subelliptical lateral lobes; boundaries between medial and lateral lobes defined by change in slope rather than furrows. Transglabellar F3 moderately impressed, slightly to moderately concave anteriorly in larger specimens. Ogival anteroglabella constitutes 33% (31–36%) of glabellar length (sag.). Width of genal field, equal to glabellar width (tr.) at F2, steadily decreases anteriorly to where preglabellar field constitutes only 16% (12–18%) of cephalic length at midline; slope angle of genal field increases abaxially from near-horizontal adjacent to axial furrow to vertical at border furrow, causing genal field to overhang border at posterolateral corners. Genal and preglabellar fields nonscrobiculate. Border narrow, convex, narrowing slightly posteriorly. Border furrow well impressed, uniform in depth and width, slightly narrower than border.

Pygidium semiovate in outline, pygidial length exclusive of articulating half-ring 91% (85–94%) of maximum width (tr.) at F2; moderately convex (sag., tr.). Axis long, 82% (78–85%) of pygidial length; slightly convex (sag., tr.), standing in slight relief above adjacent pleural fields; post-axial pleural field continues gentle posterior slope of posteroaxis in lateral view. Anteroaxis accounts for less than half (42–46%) of length (sag.) of axis. Axial furrows moderately impressed; slightly convergent from anterior margin to F2, bowed outward along M1, less so along M2; bowed outward along sides of ogival posteroaxis. Articulating half ring short (sag.), crescentic; width (tr.) approximately 70% of M1; articulating furrow deep and broad (sag.), narrowing laterally in front of lateral lobes of M1. M1 wider (tr.) than M2 and posteroaxis, trisected by longitudinal furrows into slightly shorter (sag.) and narrower (tr.) medial lobe with gently abaxially convex sides, and inflated lateral lobes with broadly rounded anterolateral corners. F1 transaxial, bowed forward slightly, moderately impressed, shallowing over midline in some specimens. M2 trisected, longitudinal furrows slightly shallower than on M1; medial lobe narrower (tr.) than medial lobe of M1, longer (sag.) and narrower than lateral lobes of M2; large, strongly asymmetrical axial node on posterior half of M2 extends postero-dorsally to overhang F2. Transaxial F2 moderately impressed, transverse to gently anteriorly convex; wider (exsag.) and curved forward slightly at intersection with axial furrows on some specimens, producing rounded posterolateral corners on lateral lobes of M2; shallower and deflected sightly forward over midline at base of axial node. Posteroaxis slightly wider (tr.) than M2; maximum width slightly greater than length (sag.); some specimens display very faint intranotular axis circumscribed by slight change in convexity and flat, terminating in flat, upturned tip. Pleural fields narrow, widest (tr.) at level of slightly constricted M2, narrowing posteriorly to produce short (sag.) post-axial field; moderately convex (tr., sag.); moderately declined post-axially as seen in lateral view; slightly declined from axial furrows increasing to moderately sloping near border furrow. Non-scrobiculate. Border flatly convex, moderate in width, 8% (7–10%) of pygidial length exclusive of articulating half-ring; widest at midline, tapering toward anterolateral corners. Border furrow broad, well impressed with very steep boundaries with pleural field and border; narrows slightly anterior to short posterolateral spines that lie just anterior to end of axis. Shoulder furrow well impressed, narrow (exsag).

Discussion. Lotagnostus rushtoni is distinguished from L. morrisoni , with which it co-occurs in the Windfall Formation, by the gentler convexity and abaxial slope of the proximal genal and pleural fields, longer and more pointed anteroglabella, transaxial pygidial F1, relatively longer and narrower posteroaxis ( Figure 10.4 ), and absence of notular furrows. All Lotagnostus clarki cephala differ from those of L. rushtoni in exhibiting a longer preglabellar field, less pointed anteroglabella, and a smaller glabellar node not elevated by inflation of the surrounding area. Larger L. clarki cephala are weakly scrobiculate, whereas those of L. rushtoni are smooth. The transaxial F1 distinguishes all L. rushtoni pygidia from those of L. clarki and larger sclerites of the former display a deeper, more steep-sided border furrow and wider posterior border.

The transaxial pygidial F1 sets Lotagnostus rushtoni apart from all other non-scrobiculate species of the genus except L. hedini and L. aff. L. rushtoni . The latter species is discussed in more detail below. The cephalon of L. rushtoni differs from that of L. hedini in possessing longer and more inflated basal lobes that extend posteriorly beyond the glabellar culmination; the posterior ends of the basal lobes of L. hedini lie in line with, or slightly in front of the glabellar culmination. L. rushtoni also has a shorter (sag.) and more tapered glabella. The pygidium differs from that of L. hedini in having a proportionally shorter ( Figure 10.4 ) and more broadly rounded posteroaxis, an unconstricted acrolobe, and a deep, symmetrical, trough-like border furrow that is less variable in width, and is narrower than the border posterior to F2.

Figures

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 12. Lotagnostus rushtoni n. sp. Each white scale bar represents 2mm. 1-3. Cephalon, CM 41383, from 5/22/08D, dorsal, anterior, and left lateral views. 4-6. Cephalon, USMN 775739, from D3383-CO, dorsal, anterior, and right lateral views. 7. Cephalon, USMN 775740, from D3383-CO, dorsal view. 8. Cephalon, USMN 775741, from D3383-CO, dorsal view. 9. Cephalon, USMN 775742, from D3383-CO, dorsal view. 10. Cephalon, CM 41384, from 5/22/08D, dorsal view. 11- 14. Pygidium, holotype, CM 41385, from 5/22/08D, dorsal, posterior, right lateral, and posterior oblique views. 15. Pygidium, USMN 775743, from D3383-CO, dorsal view. 16. Pygidium, USMN 775744, from D3383-CO, dorsal view. 17-19. Pygidium, USMN 775745, from D3383-CO, dorsal, left lateral, and posterior views. 20. Pygidium, CM 41386, from 5/22/08D, dorsal view. 21. Pygidium, USMN 775746, from 3D3381-CO, dorsal view.