Neoagnostus parki, 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 : 54-56

Neoagnostus parki n. sp.

( Plates 14 , 15 )

Diagnosis. Neoagnostus with a relatively long (sag.) preglabellar field. Median preglabellar furrow very weakly impressed to absent, shallowing anteriorly and not reaching the border furrow. F3 indistinct, expressed only as shallow notches in side of glabella anterior to moderately impressed, chevron-shaped, transglabellar F2. Narrow (tr.), angular glabellar culmination extending as blunt spine to posterior end of large basal glabellar lobes. Pygidium exhibits faintly impressed F1 furrows, moderately impressed F2 furrows and moderately impressed axial furrows that diverge posteriorly from F2, terminating a short distance posterior to pair of slightly inflated bosses.

Etymology. Named in honor of our undergraduate mentor and colleague at Indiana University of Pennsylvania, Frederick R. Park.

Material and occurrence. Holotype CM 41394 is a cephalon from collection 5/22/08B: assigned specimens include 4 cephala CM 41390–41393 and 5 pygidia CM 41352 , 41354 , 41355 from collection 5/22/08B and 1 cranidium USNM 775752 and 1 pygidium USNM 775753 from USGS collection D3362-CO. Lotagnostus nolani Fauna : collections 5/22/08B (62-48) and D3362-CO (4-6) from the Windfall Formation at Ninemile Canyon , Nevada.

Description. Cephalon subquadrate, cephalic length (sag.) equaling 91% (88–94%) of maximum width (tr.); width at posterior margin 80% (77–91%) of maximum width; moderately (sag.) and strongly (tr.) convex. Glabella long (sag.), 66% length of cephalon, narrow, width (tr.) 31% (27-35%) of cephalic width at cephalon midlength; slightly convex (sag., tr.) standing in low relief above genae. Axial furrows moderately impressed; strongly convergent (30º to midline) along basal lobes; wider (tr.), slightly convergent, and very slightly laterally concave from basal furrow to maximum width of M3; slightly convergent to anterolateral corner, then nearly transverse to midline in specimens with anteriorly truncate glabellas; continue from M3 to midline in even curve on specimens with narrowly rounded front. Confluent M1 and M2 form long composite lobe, 61% (53–69%) of glabellar length (sag.), hexagonal in outline; glabellar culmination angular, extending in most specimens as narrow point that reaches posterior end of basal lobes. Basal furrows moderately impressed, narrow, nearly straight, converging to almost meet, resulting in narrow (tr.) occipital band; basal lobes large but short (exsag.), 20% of glabellar length, triangular in outline. F1 as very shallow indentations in side of glabella at anterior tip of basal lobes or absent; F2 transglabellar, chevon-like, moderately impressed near axial furrow, narrower (sag.) and faintly impressed across middle half to two thirds of glabella. Weakly inflated glabellar node located at glabellar midlength, just posterior to medial peak of F2. Transglabellar F3 V-shaped, very faintly impressed, usually incomplete, fading medially, isolating most of M3 as long (exsag.), elliptical, slightly inflated lateral lobes. Anteroglabella short (sag.), approximately 16% (14-17%) of cephalic length; front narrowly rounded in some specimens, more truncate in others; descends evenly to axial furrow in lateral view. Acrolobe very slightly constricted in large specimens; quadrate in small specimens, semi-elliptical in larger cephala, widest (tr.) at midlength (sag.) of confluent M1 and M2, narrowing slightly anteriorly to end of glabella, curving inward to midline sharply in small specimens, in even curve in large cephala. Genae slightly convex, gently declined adaxially; moderately convex, steeply declined laterally; gently declined anteriorly from distal end of basal furrow to preglabellar furrow, moderately declined to anterior border furrow. Preglabellar field increases in length (sag.) with size; 20% of cephalic length in smallest specimen, 28% in largest cephalon; gently convex dorsally, moderately declined to border furrow. Border strongly convex, accounts for 7% of cephalic length (sag.) at midline; constant in width anteriorly, tapering rapidly posterior to F2. Border furrow well impressed; broad, widest at anterolateral corners, where it is twice width of border, narrows anteriorly and posteriorly.

Pygidium subquadrate in outline, length (sag.) 83% (74–91%) of maximum width at midlength opposite F2; nearly flat (sag.), very gently convex (tr.). Acrolobe quadrate in small specimens becoming subpentagonal and constricted with increased size. Articulating half ring with gently anteriorly convex margin, posterior margin deeply embayed laterally creating posteriorly projecting medial prong; articulating furrow well impressed, broad (sag.), bowed backward medially, deepest anterolaterally in embayments behind very narrow (exsag.) lateral thirds of half ring. Axial furrows moderately impressed, converge from anterior margin to F2 furrow with slight indentation at position of F1, diverge slightly and shallow, terminating approximately 45% of way to posterior margin. M1 undivided; M2 trisected by faintly impressed longitudinal furrows that outline elongate axial node that widens (tr.) slightly posteriorly, deflecting and slightly overhanging F2; crest of node declines anteriorly from posterior end, terminating at level of F1. F2 transglabellar, straight and directed posteromedially to longitudinal furrows, shallowing as curve around posterior end of axial node. Two slightly inflated lateral bosses and depressed central area at anterior end of posteroaxis; axial furrows posterior to paired bosses very faint to absent; small terminal node on midline at posterior margin of acrolobe confirms that posteroaxis extends to border furrow; very faint notular furrows present on a few exfoliated specimens. Pleural fields broad (tr.), nonscrobiculate, slightly convex, gently declined laterally and posteriorly. Border convex, bears one pair of long (exsag.), slender posterolateral spines directed posteriorly and slightly laterally from margin; border widest anterior to base of posterolateral spines, tapering anteriorly, even in width (sag., exsag.) posteriorly. Border furrow well impressed, broadest anterior to posterolateral spines, narrowing anteriorly, even in width (sag., exsag.) posteriorly.

Discussion. Naimark (2016) retained 6 species within Neoagnostus based upon her emended generic diagnosis. All display F3 and median preglabellar furrows that are more continuous and firmly impressed than those of Neoagnostus parki n. sp. The hexagonal shape of the posterior half of the glabella, between the chevron-shaped F2 and pointed glabellar culmination, also sets N. parki apart from all other species of the genus. Although N. bilobus ( Shaw, 1951) possesses a nearly hexagonal confluent lobe similar to that of N. parki , its glabellar culmination is more broadly rounded. Comparison with other species must be done between sclerites of similar size, owing to significant ontogenetic variation arising from allometric growth. The cephalic acrolobe of N. parki is subrectangular in small specimens but increases in length (sag.), becoming semielliptical with gently curved and anteriorly convergent margins and a rounded front in larger specimens. Despite the elongation of the acrolobe, and an increase in relative length (sag.) of the frontal area, concomitant broadening of the border furrow and rounding of the anterolateral corners of the cephalon during growth modified the cephalic outline from subrectangular to semiovate.The pygidium of N. parki experienced similar elongation of the acrolobe, broadening of the border furrows, and transformation of marginal shape from subrectangular to semiovate during growth ( Plate 15 ).

Neoagnostus parki also differs from the type, N. aspidoides Kobayashi, 1955 (p. 473–474, pl. 7, figs 4–5, pl. 9, fig. 5; Shergold, 1977, pl. 16, fig. 16; Shergold et al., 1990, fig. 16, 1a) in having a less tapered glabella, longer preglabellar field, more rounded anterior margin, and narrower anteroglabella. Pygidial comparison is difficult due to the poor quality of the original illustration for the N. aspidoides pygidium. Additional features that distinguish N. bilobus ( Shaw, 1951, p. 112–113, pl. 24, figs 17–22, not pl. 22, fig. 10 =? Plicatolina kindlei ; Shergold, 1977, pl. 16, figs 7–8; Shergold, et al., 1990, fig. 16, 1b–1c) from N. parki include smaller basal glabellar lobes and a narrower and more strongly inflated pygidial acrolobe. The paired bosses on the front of the posteroaxis that inspired the name of N. bilobus are more inflated and surrounded by deep furrows. The pygidial axial node of N. bilobus is longer than that of N. parki , extending posteriorly past the mid-length of the paired bosses. Deeply incised furrows, a much broader (tr.) glabella, shorter frontal area, a nearly transverse F2, and very small basal lobes distinguish the cephalon of N. eckardti Jell, 1985 (p. 58–59, pl. 19, figs 1–5) from that of N. parki . While the pygidia of N. parki and N. eckardti are similar, the post-axial bosses on N. eckardti are more prominent and the axial furrows do not diverge posterior to the F2 furrow as they do in N. parki . Although comparison with N. longicollis Kobayashi , (1966, p. 283, fig. 7) is difficult due to the quality of the original illustrations, the holotype is clearly more quadrate than N. parki cephala of similar size and displays a more continuous median preglabellar furrow, a firmly impressed F3, and more transverse F2. The pygidium of N. longicollis exhibits a broader border and the axial furrows posterior to F2 are more strongly divergent. Although most of the illustrated sclerites of N. tarijensis ( Gilberto, 2010, p. 123, fig. 1.1–1.6) are deformed, they confirm that the cephalon has smaller basal glabellar lobes, a more continuous, V-shaped F3, and more transverse F2 than N. parki . Neoagnostus hangulensis (Lu et al., 1981, pl. 2, fig. 1) has a less constricted cephalic acrolobe, narrower border furrows, a continuous median preglabellar furrow that reaches the border furrow, and more inflated bosses at the front of the posteroaxis. The cephalon designated as holotype for Neoagnostus shiziluensis by Lu & Zhou, (1990, p. 14, pl. 1, fig.7) more closely resembles that of N. parki with respect to shape of the acrolobe, width of border furrow, and cephalic outline. It differs, however, in having a narrower glabella, smaller basal lobes, and a blunt rather than pointed glabellar culmination. Moreover, none of the three pygidia illustrated for that species compares favorably with that of N. parki . The semi-circular and inflated acrolobes of the two small pygidia (figs 9–10) are quite different from the weakly convex, subrectangular acrolobes of N. parki pygidia of comparable size. The larger, rectangular pygidium (fig. 11), which is wider than long and has a narrow border furrow of constant width, bears no resemblance to the large pygidia of N. parki . It is questionable as to whether that larger pygidium is conspecific with the cephalon and smaller pygidia, one of which came from the same horizon as the holotype. Although Naimark (2016, p. 62) suggested N. shiziluensis is a junior synonym of N. eckardti , we consider the differences in cephalic outline, glabellar proportions, frontal area length, and depth of furrows sufficient to retain separate species status for the former.

Figures

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.

PLATE 15. Pygidia of Neoagnostus parki n. sp. Each white scale bar represents 2mm. 1-3. CM 41354 from 5/22/08B, dorsal, posterior, and right lateral views. 4. CM 41395, from 5/22/08B, dorsal view. 5. CM 41352, from 5/22/08B, dorsal view. 6. USMN 775753, from D3362-CO, dorsal view. 7. CM 41397, from 5/22/08B, dorsal view. 8. CM 41399, from 5/22/08B, dorsal view. 9. CM 41398, from 5/22/08B, dorsal view. 10-12. CM 41396, from 5/22/08B, dorsal, right lateral, and posterior views. 13. CM 41355, from 5/22/08B, dorsal view. 14. CM 41400, from 5/22/08B, dorsal view.