Lotagnostus morrisoni, 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 : 50-52

Lotagnostus morrisoni n. sp.

( Plate 13 )

Diagnosis. Non-scrobiculate species with parts of glabella and pygidial axis that lie slightly below dorsally convex proximal areas of acrolobes; anterior end of glabella and posterior end of pygidial posteroaxis particularly lowlying relative to preglabellar field and postaxial pleurae, respectively. Approximately inner third of acrolobe slopes gently toward axial furrow while distal two-thirds descend steeply abaxially to border furrow. Cephalic acrolobe narrows (sag.) only slightly in front of short (sag.), bluntly rounded anteroglabella. A pair of small, elliptical nodes at posterior ends of lateral lobes of M3 discernible on exfoliated cephala. Small, circular glabellar node sits in line with posterior end of very broad (exsag.) F2. Pygidial F1 discontinuous, terminating adaxially at longitudinal furrow that bounds confluent central lobes of M1 and M2. Relatively short (sag.) posteroaxis weakly trisected with intranotular axis bounded only by change in slope along most of its length, but by weakly impressed notular furrows along posterior-most one fourth to one fifth. Pygidial acrolobe constant in width from midlength of posteroaxis to midline behind axis.

Etymology. Named after Scott Morrison for invaluable assistance in conducting a thorough review of the relevant literature.

Material and occurrence. Holotype CM 41389 is a pygidium from collection 5/22/08D: assigned specimens include 1 cephalon CM 41388 from collection 5/22/08D; 1 cephalon CM 41387 from collection 5/22/08C; and 1 cephalon USNM 775747 and 4 pygidia USNM 775748–775751 from collection D3381-CO. Lotagnostus rushtoni Fauna : collections 5/22/08C (1-2), 5/22/08D (1-2) and D3381-CO (10-15) from Windfall Formation at Ninemile Canyon , Nevada.

Description. Cephalon semiovate with maximum cephalic width equal to cephalic length (97–105%); strongly convex (sag.) posterior to glabellar node, gently convex from node to anterior margin; moderately to strongly convex (tr.). Glabella moderately long (sag.), accounts for 77% (71–84%) of cephalic length, and a third of cephalic width (tr.) at M3; slightly convex (tr.), barely rises above adjacent pleural fields in anterior and lateral views. Axial furrows broad, moderately impressed; strongly convergent (30º angle to midline) from posterior margin to anterior end of basal lobes, continuing nearly straight until slightly angular turn at anterolateral corners of glabella, converge strongly (60-65º angle to midline) to intersection with median preglabellar furrow.Basal lobes long, 33% (27–35%) of glabellar length; basal furrow moderately impressed, but slightly fainter than axial furrows. Border rim-like, of even width. Border furrow moderately impressed, uniform in depth. Acrolobe semi-circular, maximum width opposite of F2 furrow; moderately convex in lateral and anterior views. Median preglabellar furrow moderately impressed. F1 as slight widening of axial furrows at anterior end of basal lobes; M2 equal in width (tr.) to M1, with small, circular glabellar node centered on anterior half; F2 moderately impressed abaxially, wide (exsag.) and short (tr.), extending only halfway to midline. M3 weakly trisected by very faint longitudinal furrows or change in convexity into wider (tr), anteriorly tapering medial lobe between very slightly inflated, elliptical lateral lobes; smaller, elliptical lobes discernible at posterior ends of lateral lobes on exfoliated specimens. F3 transglabellar, moderately impressed, weakly anteriorly concave. Anteroglabella subpentagonal in outline with rounded anterolateral corners and bluntly pointed front; accounts for 33% (31–36%) of glabellar length. Genae widest (tr.) at F2, narrowing very slightly anteriorly to where preglabellar field accounts for 20% of cephalic length at midline; genae moderately to strongly convex with adaxial third inclined toward axial furrow and distal two thirds descending steeply to border furrow. Adaxial slope of proximal third results in parts of the glabella lying below highest areas of adjacent genal field, imparting a depressed or “sunken” appearance to the anterior third to half of the glabella.

Pygidium semiovate in outline, maximum width (tr.) slightly less (87–100%) than pygidial length exclusive of articulating half-ring; moderately convex (sag., tr.). Axis moderate in length, 70% (67–76%) of pygidial length; slightly convex (sag., tr.), margins lie slightly below height of elevated adjacent pleural fields; widest at M1, slightly constricted at M2, with broadly rounded posteroaxis nearly as wide (tr.) as M3. Axial furrows moderately impressed, slightly shallower along M1, subparallel along M2, abaxially convex along slightly inflated lateral lobes of M1 and posteroaxis. Anteroaxis short (sag.), less than half (42-46%) of length of axis, trisected by moderately impressed longitudinal furrows that create confluent, medial lobe of M1 and M2. F1 and F2 moderately impressed; F1discontinuous, straight, directed slightly anteromedially from axial furrow, terminating at intersection with longitudinal furrows; F2 transaxial, slightly concave forward along posterior margins of M2 lateral lobes, transverse and shallower along base of posteriorly projecting axial node positioned at posterior end of confluent medial lobe. Posteroaxis relatively short (sag.), length only 91% (81-95%) of maximum width; trisected with faint intranotular axis, more clearly defined in smaller specimens; notular furrows restricted to posterior, and in a few specimens anterior, ends of intranotular axis, with remainder delineated only by changes in convexity; faint terminal node present on some specimens. Pleural fields moderately to strongly convex (tr., sag.); moderately declined post-axially in lateral view; in posterior view genae rise from axial furrows before declining steeply to border furrow; proximal third anterior to F2 subhorizontal; width of pleural field constant from midline anteriorly to maximum width of posterior lobe, increasing along anteroaxis. Smooth, non-scrobiculate. Border convex; maximum width at midline 8% (7–10%) of pygidial length (sag.) exclusive of articulating half-ring; tapering anteriorly. One pair of small, posterolaterally directed posterolateral spines positioned slightly posterior to end of axis on holotype ( Pl. 13.11-14 ). Border furrow moderately impressed, asymmetrical with sharp boundary with pleural field and more gradual transition to border; as wide or wider than border anterior to posterolateral spines, narrowing behind axis to be narrower than border.

Discussion. Lotagnostus morrisoni n. sp. appears unique among non-scrobiculate species of Lotagnostus in the depressed appearance of the axial structures imparted by inflated genal and pleural fields that rise above the axial furrows. The lack of similarly depressed appearance of the axis in specimens of L. rushtoni in the same collections confirms that it is a primary character of the species rather than a taphonomic effect. Cephala from the Frederick Valley of Maryland identified as Lotagnostus sp. by Rasetti (1959) and reillustrated by Westrop & Landing (2016, fig. 12) display a similar position of the axis relative to the genae and pleural fields, but are easily distinguished by well-developed scrobiculae and strongly depressed lateral lobes of M2. Lotagnostus morrisoni also differs from the contemporaneous species L. rushtoni n. sp. in the presence of the slight nodes in the posterolateral corners of glabellar M3 and the relative proportions of the pygidium.

Figures

PLATE 13. Lotagnostus morrisoni n. sp. Each white scale bar represents 2mm. 1-4. Cephalon, USMN 775747, from D3381- CO, dorsal, anterior, right lateral, and anterior oblique views. 5, 10. Cephalon, CM 41387, from 5/22/08C, dorsal and anterior oblique views. 6-9. Cephalon, CM 41388, from 5/22/08D, dorsal, anterior, lateral, and anterior obliqueviews. 11-14. Pygidium, holotype, CM 41389, from 5/22/08D, dorsal view; posterior oblique, posterior, and right lateral views. 15-16. Pygidium, USNM 775748, from 3D3381-CO, dorsal and posterior oblique views. 17. Pygidium, USMN 775749, from 3D3381-CO, dorsal view. 18. Pygidium, USNM 775750, from 3D3381-CO, dorsal view. 19-20. Pygidium, USMN 775751, from 3D3381-CO, dorsal and posterior oblique views.

PLATE 14. Cephala of Neoagnostus parki n. sp. Each white scale bar represents 2mm. 1-3. Holotype, CM 41394, from 5/22/08B, dorsal, anterior, and left lateral views. 4. CM 41391, from 5/22/08B, dorsal view. 5-6. USMN 775752, from D3362- CO, dorsal and anterior views. 7. CM 41392, from 5/22/08B, dorsal view. 8-10. CM 41393, from 5/22/08B, dorsal, anterior, and right lateral views. 11-12. CM 41390, from 5/22/08B, dorsal and left lateral views.