Calymeninae, Adrain & Karim, 2020

Adrain, Jonathan M., Karim, Talia S. & McAdams, Neo E. B., 2020, Atlanticalymene, a new genus of Middle Ordovician (Darriwilian) calymenine trilobites, and revision of the calymenoidean genus Protocalymene Ross, Zootaxa 4859 (1), pp. 1-55 : 13-14

Calymeninae n. gen. n. sp.

Fig. 4

1999 Protocalymene new species A; Fortey and Droser, p. 197, fig. 8.11–8.14.

2013 “unnamed species described as ‘ Protocalymene n. sp. A’ by Fortey & Droser (1999) ”; Adrain, p. 310.

Material. Figured specimens USNM 495698, 495699, and 511808 from the Antelope Valley Formation (Dapingian), “olenid bed” at Little Rawhide Mountain, Nye County, central Nevada, USA. Note that USNM 511808 was listed by Fortey and Droser (1999, p. 197) as “ USNM 495700”. However they had earlier in their paper assigned the latter number to a librigena of a species of Dimeropygiella ( Fortey and Droser, 1999, p. 192, explanation of fig. 5.5); the calymenid specimen reillustrated herein has subsequently been assigned the number USNM 511808.

Discussion. Fortey and Droser (1999) illustrated specimens from a deep water olenid-dominated assemblage in the Dapingian part of the Antelope Valley Formation. They assigned the material to Protocalymene . In its cranidial dimensions the species appears to be a calymenine. Its most unusual feature is the presence of a very long preglabellar field (e.g., Fig. 4.4 ) with fine tuberculate sculpture similar to that on the rest of the cranidium. Some younger calymenines have a long area between the glabella and anterior border, but it is typically a deep well that resembles an expanded anterior border furrow (e.g., Calymene (s.l.) frontosa Lindström, 1885 —see Clarkson and Howells [1981, pl. 79, figs 6, 7, 11]; species of Arcticalymene Adrain and Edgecombe, 1997 ). None show what appears to be a standard, sculptured preglabellar field as most have the front of the glabella separated from the anterior border only by a relatively deep and short anterior border furrow. The morphology in Fortey and Droser’s species could represent a transitional form. The anterior parts of the fixigenae are inflated and typically calymenine. They are separated from the sculptured preglabellar area by abrupt changes in slope running forward from the axial furrows. The anterior border appears to be calymenine in aspect, though it is not well known. Alternatively, the inferred environment of deposition is deep water and dysaerobic. The cranidia are very broad and have much more prominent caecal sculpture (including, especially, the bicomposite eye ridge) underlying the fine tubercles than is typical for a calymenine. It is conceivable that the unusual preglabellar morphology is part of a suite of adaptations to an oxygenstressed environment.

While this Dapingian species broadly resembles A. bardensis in cranidial dimensions, especially the wide interocular fixigenae, it lacks the glabellar morphology considered apomorphic for Atlanticalymene . It has well de- veloped L2 and L3 and a deep S2. The glabella is relatively short compared to that of A. bardensis , but it is nearly parallel-sided versus the bell shape typical of calymenines. We agree with Fortey and Droser (1999) that the species, while very distinctive, is not well enough known to formally name. It is important as the oldest known Laurentian calymenine and the only known Dapingian species. It appears to represent a genus distinct from Atlanticalymene , formal naming of which must await the recovery of more material or related species from elsewhere.

Figures

FIGURE 4. Calymeninae n. gen. n. sp., from the Antelope Valley Formation (Dapingian), “olenid bed” at Little Rawhide Mountain, Nye County, central Nevada (“Protocalymene new species A” of Fortey and Droser, 1999, p. 197, fig. 8.11–8.14). 1, 3, 5, 7. Cranidium, USNM 495699 (original of Fortey and Droser, 1999, fig. 8.13, 8.14), dorsal, oblique, and anterodorsal views of latex from external mold, and dorsal view of testate counterpart, x10. 2, 4. Cranidium, USNM 495698 (original of Fortey and Droser, 1999, fig. 8.11, 8.12), dorsal and oblique views, x7.5. 6. Left librigena, USNM 511808, external view, x10 (not illustrated by Fortey and Droser, 1999, but listed by them as USNM 495700, see text for explanation).