Atlanticalymene, 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 : 7-8

Atlanticalymene n. gen.

Type species. Atlanticalymene bardensis n. sp., from the Darriwilian Table Cove Formation of western Newfoundland, Canada .

Other species. Atlanticalymene n. sp. A (“? Ischyrophyma sp. ind.” of Whittington [1963, p. 50, pl. 6, figs. 13-15]), Shallow Bay Formation (lower Darriwilian), Lower Head, western Newfoundland; Sthenarocalymene sp. indet. of Zhou et al. (2014, p. 114, fig. 56), Yijianfang Formation (Dapingian), Yijianfang, Bachu, northwestern Tarim, Xinjiang, China.

Etymology. From the Atlantic Ocean, on the western edge of which the type locality is situated, and the genus name Calymene ; gender is feminine.

Diagnosis. Glabella long and parallel-sided anterior to L1; L1 well developed and nearly completely isolated; L2 and L3 scarcely developed, S2 faint and barely discernible, S3 not impressed; cranidial anterior border strongly bowed in anterior view; preglabellar area elongate (sag.; exsag.); eye ridge expressed dorsally; palpebral lobe small; sculpture of small to coarse fairly densely scattered tubercles over all skeletal parts except hypostome.

Discussion. As noted above, a consensus emerged in the recent literature ( Fortey, 1990; Fortey and Droser, 1999; Turvey, 2002) that Middle Ordovician Laurentian calymenoidean species should all be assigned to Protocalymene and that they are unambiguous calymenids. Revision on the basis of new photographs herein demonstrates that a calymenid affinity is true of only a handful of the occurrences (those assigned by Adrain and Fortey [1997] to Sthenarocalymene ), and not necessarily true of Protocalymene mcallisteri . The various reports break down into two morphological groups: taxa related to Darriwilian material described in open nomenclature by Whittington (1965) and taxa related to P. mcallisteri . The former collectively represent Atlanticalymene n.gen., whereas the latter, while certainly calymenoidean, are considered below to be of unclear family relationships.

The genus diagnosis is complicated by the fact that there is only one well known species, yet the genus is not monotypic. We have included morphological features of the cranidium that are shared between the known occurrences. Features that are known only from A. bardensis are listed in the species diagnosis of that taxon, with the understanding that both diagnoses should potentially be refined if other exoskeletal parts of other species are described.

Zhou et al. (2014) assigned a single partial cranidium to the genus Sthenarocalymene , following assignment of Whittington’s (1965) Table Cove Formation specimens by Adrain and Fortey (1997, p. 105). With description of A. bardensis on the basis of abundant silicified material, it no longer appears that assignment to Sthenarocalymene is justified. The type species of Sthenarocalymene , S. lirella Siveter, 1977 , is from the Vollen Formation (Sandbian), Oslo-Asker, Norway. The only other reliably assigned species is S. aldonensis ( Reed, 1935) , from the Superstes Formation (Sandbian) of the Laurentian-affinity Midland Valley Terrane, Scotland (see Tripp, 1976, p. 408, pl. 7, figs 8–12). Some workers (following Holloway [1980] and including, e.g., Edgecombe and Wright [2004], and Lin [2008]) have assigned a large number of Silurian and Devonian species, ranging from the Telychian to Emsian, considering Apocalymene Chatterton and Campbell, 1980 , a junior subjective synonym. The basis for this is little more than that these species, some 27 formally named, are taxa which occur in Silurian and Devonian rocks yet lack glabellar buttresses and share a broadly conserved glabellar outline. Others (e.g., Siveter and Chatterton [1996, p. 57]; Edgecombe [2007, p. 60]) have regarded the question as open. Given the large stratigraphic gap between the earliest “ Apocalymene ” (Telychian species such as the Australian Gravicalymene ? vaccina Holloway, 1994) and the species regarded here as Sthenarocalymene (Sandbian) , together with the absence of any detailed phylogenetic work on the Siluro-Devonian group, it seems premature at best to assume they together represent a clade and Sthenarocalymene is restricted herein to the two species listed above.

Sthenarocalymene lirella is known from an internal mold of an articulated specimen lacking librigenae, an external mold of a cranidium, one well preserved cuticular cranidium, and two internal molds of cranidia. Its similarity to A. bardensis is scant. The younger Baltic species has much larger L1, strongly inflated L2 and L3, strongly incised S2 and clearly visible S3, a bell-shaped versus subparallel sided glabella, and narrower interocular fixigenae. While the pygidium is known only from the internal mold example on the articulated specimen ( Siveter, 1977, fig. 12C), it is clearly narrower relative to its length, has axial furrows that are more strongly posteriorly convergent, and lacks the slightly bulbous paired swellings at the posterior of the axis characteristic of A. bardensis . Apart from both clearly being calymenines, there is little morphologically in common between the taxa and nothing that suggests A. bardensis should be assigned to Sthenarocalymene . Sthenarocalymene aldonensis is known only from poorly preserved molds. While difficult to interpret, the morphology of the available specimens seems to match closely that of the coeval S. lirella . One partial external mold of a librigena is known ( Tripp, 1976, pl. 7, fig 10), but it is too incomplete to meaningfully compare.

Zhou et al.’s (2014, p. 114, fig. 56) “ Sthenarocalymene sp.,” from the Dapingian of Tarim, consists only of a single incomplete cranidium. Its morphology clearly indicates relationship with A. bardensis , as Zhou et al.’s discussion (2014, p. 115) indicates. In particular, it shares the development of L1 as the only prominent, inflated, glabellar lobe, together with an elongate, parallel-sided glabella. The anterior border is broken off so cannot be compared. The interocular fixigenae of the Tarim specimen are much narrower than those of A. bardensis . The specimen is relatively small, and retains expressed juvenile paired fixigenal spines.

Zhou et al. (2016, p. 322, fig. 20J, K) described a single partial cranidium from the lower Darriwilian Zhuozishan Formation of Wuhai, Inner Mongolia, as “ Sthenarocalymene sp. indet.” The affinity of this specimen, which consists of an external mold of a glabella, part of an occipital ring, and a small portion of posterior fixigena and posterior border, is difficult to determine. It has strongly inflated L1 and L2, but L1 seems smaller than L2 and laterally contiguous with the glabella, which is unusual for a calymenine. The glabella is also strongly vaulted with the anterior part steeply forwardly sloping. This species does not resemble A. bardensis . Because there is so little morphological information there are many possibilities for its affinity. It could conceivably represent a species similar to “ Calymeninae n. gen. n. sp. ” (below), but alternatively it might not represent a calymenine at all.

Atlanticalymene is compared with the Dapingian “ Calymeninae n. gen. n. sp. ” below. Atlanticalymene bardensis has little in common with the three Gondwanan Armorican Floian species from the Montagne Noire assigned by Courtessole et al. (1983) to Platycalymene ( P. (P.) pradesensis from the La Maurerie Formation and P. (P.) minervensis and P. (P.) villerembertensis from the St.-Chinian Formation). All are known only from cranidia, which feature non-isolated L1 and strongly impressed S2 and S3. The interocular fixigenae of P. (P.) pradesensis and P. (P.) villerembertensis are very narrow and the eye is in a more anterior position than in A. bardensis .