Bienvillia eurekensis, 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 : 25-28

Bienvillia eurekensis n. sp.

( Plate 3 )

Diagnosis. A species of Bienvillia with a subrectangular glabella with evenly rounded anterolateral corners, a long preglabellar field that is equal in length (sag.) to that of the occipital ring, and S1 lateral glabellar furrows that do not connect across the axis.

Etymology. Named after Eureka County, Nevada.

Material and occurrence. Holotype CM 41320 is a cranidium from collection 5/22/08B; assigned specimens include 4 cranidia CM 41321-41324 , 1 pygidium CM41326 , and 1 librigena CM 41325 from collection 5/22/08B; and 3 cranidia USNM 775713–775715 , 1 pygidium USNM 775717 , and 1 librigena USNM 775716 from USGS collection D3362-CO. Lotagnostus nolani Fauna : collections 5/22/08B (139-1-9) and D3362-CO (12-1-4) from the Windfall Formation at Ninemile Canyon , Nevada.

Description. Cranidium subtrapezoidal in outline; of moderate size compared to associated polymeroids, up to 6mm in length; moderate in convexity (sag., tr.). Prominent subrectangular glabella with evenly rounded anterolateral corners, glabellar length exclusive of occipital ring 60% of total cranidial length (59-65%); glabellar length (sag.) 83% (79-89%) of maximum glabellar width (tr.); slightly keeled in some specimens, moderately convex (tr.) to stand in moderate relief above adjacent fixigenae, slightly convex (sag.) to descend gently into preglabellar furrow. Axial furrows moderately impressed, subparallel along glabella, curved adaxially from occipital furrow to posterior margin forming rounded posterolateral extremity to occipital ring. Preglabellar furrow moderately impressed, slightly shallowing and widening across midline. Four pairs of lateral glabellar furrows: S1 well impressed, slightly shallowed abaxially, curved, slightly greater than 33% maximum glabellar width, terminate adaxially in short (tr.), transverse segment; S2 well impressed, slightly shallowed abaxially, curved, approximately 33% maximum glabellar width; S3 absent or very faintly impressed, slightly curved, short (tr.), 10-15% of maximum glabellar width, positioned in front of proximal half of S2; S4 absent or very faintly impressed, nearly straight, short, 15- 20% of maximum glabellar width, directed antero-laterally at approximately 20 degrees from transverse line from intersection with axial furrow opposite proximal end of eye ridge. Occipital furrow tripartite with central, anteriorly convex, slightly shallower and wider (sag.) segment constituting 40% of width (tr.); lateral segments well impressed, directed slightly anteriorly abaxially, nearly reaching the axial furrow. Occipital ring moderate in length (sag.), 18% (16-22%) of cranidial length; low, slightly elongate occipital node of moderate size centered near midlength (sag.). Anterior margin an even curve viewed dorsally, low anterior arch in anterior view. Anterior border a narrow (sag.), upturned rim, less than 5% of cranidial length, very gently tapering abaxially; anterior border furrow faintly to moderately impressed, shallowing across midline. Preglabellar field moderate in length (sag.), 18% (16-19%) of cranidial length; moderately convex dorsally. Preocular area narrow (tr.), slightly convex, moderately declined toward anterolateral corners. Faint radiating genal caecae cover preglabellar field and preocular area. Low eye ridge intersects axial furrow opposite S4 glabellar furrow; merges distally with short, slightly upturned palpebral lobe; length (exsag.) of palpebral lobes 33% of glabellar length exclusive of L0, centered slightly anterior to distal end of S2, end posteriorly near midlength of L2, at approximate mid-length of glabella. Palpebral areas narrow, 25-33% of glabellar width at palpebral lobe midlength, short (exsag.), 25% of cranidial length; slightly declined (tr., exsag.). Posterior area triangular in outline, long (exsag.), approximately 33% cranidial length; slightly convex, moderately declined; bear straight, moderately impressed posterior border furrow. Anterior branch of facial sutures gently curved and slightly convergent from palpebral lobes to anterior border furrow, then sharply turned adaxially. Posterior branch of facial suture divergent from posterior end of palpebral lobes at approximately 45 degrees.

Librigenae crescentic in outline with long, slender genal spine approximately the length (exsag.) of the genal field; prominent, adaxially tapering anterior extension of border slightly longer than the length (exsag.) of the eye; lateral margin curved anterior to posterior end of palpebral lobe, continues straight posteriorly. Genal field narrow (tr.), slightly convex dorsally, slightly declined. Lateral border narrow (tr.) increasing slightly in width toward genal angle; posterior border narrows abaxially; lateral and posterior border furrows broad, faintly impressed, slightly deeper where they intersect at approximately 60º angle.

Pygidium known only from fragmentary material; transverse, more than three times as wide (tr.) as long. Moderately tapered pygidial axis with articulating half-ring, two axial rings, terminal axial piece; articulating and first inter-ring furrow moderately impressed, second inter-ring furrow very faintly impressed separating poorly defined second ring and terminal axial piece; terminal axial piece with two faint lateral knobs; axis extends to pygidial margin. Pleural fields with moderately impressed anterior pleural furrow, faintly impressed second pleural furrow, both directed slightly posteriorly. Very faint interpleural furrows roughly perpendicular to midline.

Discussion. The association of cranidia and librigenae for Bienvillia eurekensis is straightforward given the smooth surfaces on the fixigenae and the genal field of the librigena. The associated pygidium is the only other polymeroid pygidium available from collection 5/22/08B. Several other species assigned to Bienvillia possess pygidial axes with two rings and a terminal axial piece, consistent with the association of sclerites from the Windfall.

Taylor (1976, p. 689, pl. 2, figs 10–11) illustrated Bienvillia sp. from the Hales Limestone from the nearby Eureka mining district, Eureka County, Nevada. Bienvillia eurekensis differs from Taylor’s illustrated specimens in exhibiting a glabella with anterolateral corners that are more rounded, larger palpebral lobes which extend posterior to S2, and an S1 that is discontinuous across the axis.

Bienvillia eurekensis n. sp. differs from the type species, B. corax ( Billings, 1865; see Ludvigsen et al., 1989, p. 15, pl. 4, figs 18–22), in having a subrectangular glabella rather than one that is ovate and which bears S1 and S2 furrows that are incomplete across the axis rather than transglabellar. Furthermore, the preglabellar field in B. eurekensis is long, subequal in length to the occipital ring; the preglabellar field of B. corax is shorter than the occipital ring.

Tortello (2014, p. 303, fig. 6.3, 6.4) reillustrated Bienvillia ? australis ( Rusconi, 1951a, p. 3, fig. 3a–3c) from the Furongian of Argentina. The cranidium of Bienvillia eurekensis differs from that of B.? australis in having a subrectangular glabella, rather than one that is markedly tapered, a longer preglabellar field, and a S1 furrow that is not continuous across the axis. The pygidia of these species are more distinct: the axis of B.? australis has three axial rings, rather than two, and does not extend to the posterior margin.

The remaining species previously assigned to Bienvillia are all Tremadocian or younger in age ( Rasetti, 1954; Harrington & Leanza, 1957; Robison & Pantoja-Alor, 1968; Fortey, 1974; Ludvigsen & Tuffnell, 1983). All but one of these younger species display significantly shorter (sag.) preglabellar fields than B. eurekensis . The exception, B. terranovica Rasetti (1954) , differs from B. eurekensis in having only faintly impressed glabellar furrows, a much shorter (sag.) occipital ring that tapers rapidly abaxially, weakly developed eye ridges, and broader (tr.) librigenae that lack a distinct border furrow.

Figures

PLATE 3. Bienvillia eurekensis n. sp. Each white scale bar represents 2mm. 1, 6. Cranidium, holotype, CM 41320, from 5/22/08B, dorsal and anterior views. 2, 5. Cranidium, CM 41321, from 5/22/08B, dorsal and left lateral views. 3, 4. Cranidium, USMN 775713, from D3362-CO, dorsal and anterior views. 7. Cranidium, CM 41322, with Mendoparabolina nyensis, from 5/22/08B, dorsal view. 8, 11, 13. Cranidium, CM 41323, from 5/22/08B, dorsal, left lateral, and anterior views. 9. Cranidium, CM 41324, from 5/22/08B, dorsal view. 10, 14. Cranidium, USMN 775714, from D3362-CO, dorsal and right lateral views. 12. Cranidium, USMN 775715, from D3362-CO, dorsal view. 15, 19. Right librigena, CM 41325, from 5/22/08B, external and dorsal views. 16, 17, 20. Pygidium CM 41326, from 5/22/08B, dorsal, posterior oblique and posterior views. 18. Left librigena, USMN 775716, from D3362-CO, dorsal view. 21. Pygidium, USMN 775717, from D3362-CO, dorsal view.