Protocalymene Ross, 1967

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 : 16-17

Protocalymene Ross, 1967

Type species. Protocalymene mcallisteri Ross, 1967 , from the Antelope Valley Formation (Dapingian) of California, USA .

Other species. Protocalymene sp. of Ross (1970, p. 92, pl. 18, figs 2–5), Antelope Valley Formation, Ike’s Canyon, Monitor Range, Nye County, Nevada; Protocalymene sp. of Loch and Ethington (2017, p. 310, fig. 13.6), uppermost Ninemile Formation, Whiterock Canyon Narrows, Monitor Range, Nye County, Nevada.

Diagnosis. Frontal and preglabellar area long and uninflated; anterior border furrow shallow; anterior bor- der not strongly dorsally inflated; L1 prominent, displaced laterally so glabella bell-shaped in outline; L2 well inflated; glabella with lateral indentation in front of L2 where faint oblique eye ridge runs into axial furrow from anterior part of palpebral lobe; median occipital node prominent; cranidial sculpture of medium sized tubercles; posterior projections prominent, bearing robust and long, posteriorly-directed genal spines; hypostome with ventrally smooth middle body and large, triangular posterior spines; librigena with nearly straight anterior and posterior sections of the facial suture, visual surface set atop independently inflated band of socle; pygidium with four axial rings discernible in most specimens, often with larger pair of tubercles on each ring; pygidium relatively narrow, each posterior pleural band with prominent tubercle at fulcrum.

Discussion. Whittington (1971, p. 456) excluded Protocalymene from his discussion of calymenid taxonomy “because of the difficulty in determining the systematic position of the small specimens on which it is based.” Similarly, Fortey and Droser (1999, p. 197) noted that Ross’s (1967) illustrated material was all “rather small”. This is true, but in light of our new collections it appears that this is because the species reached a diminutive maximum size. Our new silicified collections contain thousands of trilobite sclerites and P. mcallisteri is one of the most common species. The largest cranidia we have recovered are only about 3.3 mm in sagittal length (e.g., Pl. 8, figs 1, 3). There is no reason to suspect the silicified sample is inherently biased toward lack of preservation of larger specimens. Specimens belonging to a species of the pliomerid Ectenonotus Raymond, 1920 , for example, are common. Many cranidia of this species have been recovered that are as large or larger than the single calcareous cranidium assigned by Fortey and Droser (1996, fig. 16.3, 16.6, 16.7) to their new E. progenitor . These specimens are more than double the size of the largest P. mcallisteri cranidia. Given that the species are of broadly similar general morphology, it is likely also that hydrodynamic sorting can be ruled out as an explanation for the small maximum size of P. mcallisteri material. Very large illaenid specimens are also common in the collections. Protocalymene mcallisteri appears simply to have been a small trilobite. In addition to the species noted above, Ross (1972, p. 14) listed “ Protocalymene sp.” from the Antelope Valley Formation above the massive bioherm at Meiklejohn Peak, Nye County, Nevada. This occurrence has not been illustrated.

Almost all previous commentators have treated Protocalymene as Calymenidae , and often as Calymeninae (=Flexicalymeninae; see above) ( Fortey, 1975, p. 346; Hammann, 1983, p. 46; Fortey, 1990, p. 568; Fortey and Droser, 1999, p. 197; Jell and Adrain, 2003, pp. 431, 468; Loch and Ethington, 2017, p. 310), and as detailed above there has been repeated confusion of Ross’s taxon with the material described by Whittington (1965) in various open nomenclature assignments and formalized here as Atlanticalymene bardensis . The latter, as argued above, is an unambiguous calymenine calymenid. The affinity of Protocalymene is much less clear.A few authors have regarded Protocalymene as of less certain affinity. Kobayashi and Hamada (1977, pp. 124–125), for example, questionably assigned it to a calymenid Subfamily Ptychometopinae Balašova in Černyševa, 1960 , along with Ptychometopus Schmidt, 1894 , and Endocrania Kobayashi, 1956 . Ptychometopus is variably considered either a pharostomatid (e.g., Jell and Adrain, 2003, pp. 436, 476) or a calymenid (e.g., Fortey and Cocks, 2003, p. 267). Endocrania is a junior subjective synonym of the leiostegiid Leiostegium Raymond, 1913 ( Zhou and Fortey, 1986, p. 172).

Although it has been regarded as a calymenine, few aspects of the morphology of P. mcallisteri resemble members of this subfamily. In particular, the species has a “normal” preglabellar area and anterior border. In most calymenids, the frontal regions are inflated and the preglabellar field is lost, with a more or less inflated anterior border abutting the front of the glabella. This morphology is reminiscent of that of some species of pharostomatids, such as Pharostoma narinosum Siveter, 1977 (fig. 3A–H). Protocalymene mcallisteri does not appear to be a pharostomatid, however, as species of that group have a posterior facial suture which cuts the anterior border adaxial to the genal spine, with the genal spine positioned on the librigena. In all other calymenoideans the genal spine is on the cranidium, and the posterior facial suture cuts the lateral border, not the posterior border. This is emphatically the case in P. mcallisteri , which has huge genal spines developed on the cranidium. These spines themselves are unusual features in a calymenoidean. Some bathycheilids have large and long genal spines (e.g., Bathycheilus gallicus Dean, 1965 ) but, again, these are librigenal, not cranidial features.

Protocalymene mcallisteri does not resemble species of Reedocalymeninae or Colpocoryphinae , all of which have prominent adaptations for coaptation which P. mcallisteri entirely lacks.

Despite its unusual morphology, P. mcallisteri has unmistakably calymenoidean features. The rostral plate (Pl. 13, figs 15, 16, 18, 19) is typically calymenoidean, with a border sector and a doublural sector. The hypostome (Pl. 13, figs 1–3) is of typical calymenoidean aspect, with a pair of large posterior spines. The librigena is slightly odd looking due to its nearly straight facial sutures, but it is once again of standard calymenoidean form. In the absence of any obvious close comparisons, but equally obvious calymenoidean affinity, for the time being we elect not to make a family assignment of the genus.