Hemisphaerocoryphe Reed, 1896

Adrain, Jonathan M. & Pérez-Peris, Francesc, 2021, Middle Ordovician (Darriwilian) cheirurid trilobites from the Table Cove Formation, western Newfoundland, Canada, Zootaxa 5041 (1), pp. 1-73 : 12-14

Hemisphaerocoryphe Reed, 1896

= Cyrtometopella Nikolaisen, 1961

Type species. By subsequent designation of Barton (1915, p. 126); Sphaerexochus pseudohemicranium Nieszkowski, 1859 , from the Kahula Formation (Sandbian; upper Haljala Stage), Harju County, Estonia (Baltica).

Other species. Sphaerocoryphe granulata Angelin, 1854 , Kullsberg Limestone (Sandbian), Dalarna, Sweden (Baltica); Cheirurus (Sphaerocoryphe) hübneri Schmidt, 1881 , Haljala Stage (Sandbian), Lääne-Viru County, Estonia (Baltica) (= Hemisphaerocoryphe inflatus Nikolaisen, 1961 , Furuberget Formation (Sandbian), Innlandet county, Norway (Baltica); fide Krueger [1994, p. 469]); Cheirurus (Cyrtometopus) pseudohemicranium var. dolichocephala Schmidt, 1881 , Pihla Formation (Sandbian; Kukruse Stage), Harju County, Estonia (Baltica); Trilobites sphaerica Esmark, 1833, Vollen Formation (Sandbian), Oslo, Norway (Baltica); Cyrtometopella tumula Nikolaisen, 1961 , Arnestad Formation (Sandbian), Asker, Viken county, Norway (Baltica).

Species provisionally assigned. Cyrtometopella askerensis Nikolaisen, 1961 , Vollen Formation (Sandbian), Asker, Viken county, Norway (Baltica); Hemisphaerocoryphe sulcata Thorslund, 1940 , Dalby Limestone (Sandbian), Jämtland, Sweden (Baltica).

Diagnosis. Cranidial fixigenal border spine absent; thorax of 10 segments; pygidium with three axial rings, spine pairs developed on first two segments; first pygidial spine pair very large; second pair tiny.

Discussion. Hemisphaerocoryphe has variably been considered a junior subjective synonym (or probable synonym) of Sphaerocoryphe Angelin, 1854 ( Lane, 1971, p. 58; Holloway and Campbell, 1974, p. 412), a subgenus of Sphaerocoryphe (Přibyl and Vaněk in Přibyl et al., 1985, pp. 47, 154; Zhou et al., 1998, p. 725), or as a separate genus ( Krueger, 1994, p. 470; Chen and Zhou, 2002, p. 221; Pärnaste, 2004a, p. 32; Hansen, 2005, p. 205; Zhou et al., 2014, p. 110). Much of the early confusion was due to poor knowledge of the potential constituent species. As recently as Tripp et al. (1997, p. 772) and Zhou et al. (1998), arguments for synonymy exclusively cited cranidial features. Zhou et al. (1998, pp. 725–726) claimed that “the only known pygidium of Hemisphaerocoryphe was described as Sphaerocoryphe exserta Webby, 1974 ....” In fact, Krueger (1994, 1996) had published excellent photographs of either assigned pygidia or complete articulated specimens for many of the Baltic species normally assigned to Hemisphaerocoryphe . What these revealed is that, where known, Hemisphaerocoryphe has ten thoracic segments and a pygidium with three segments. The first pair of pygidial spines is the largest, the second pair small, and the third pair is lost, though the axial ring is retained.

In light of this, Hemisphaerocoryphe appears to be a monophyletic group of presumptively basal deiphonines with ten thoracic segments and a three segment pygidium, known exclusively from Baltica. Two species that seem to belong but which are known only from cranidia are provisionally assigned.

Ellipsocoryphe Lu, 1975 (type species E. elliptica Lu, 1975 , from the Meitan Formation (Darriwilian), Sichuan, China [South China]) has been considered a junior subjective synonym of Hemisphaerocoryphe (Přibyl and Vaněk in Přibyl et al., 1985; Zhou et al., 1998; Pärnaste, 2004a; Zhou and Zhou, 2008; Zhou et al., 2014), though Tripp et al. (1997) thought it should be recognized. The type species was based on a single partial internal mold of a cranidium ( Lu, 1975, pl. 43, figs 12–14). It is difficult to compare material from elsewhere to this incomplete specimen, but Zhou et al. (1998, p. 726) assigned a partial dorsal exoskeleton from the Dawangou Formation (Darriwilian), Xinjiang (Tarim). They noted morphological differences with the type specimen, but considered that they might be ontogenetic as the Tarim specimen was much larger. Given that there are morphological differences and that the specimens are from different paleocontinents, it seems unlikely that they will prove conspecific. Chen and Zhou (2002, p. 222) assigned material from the Guniutan Formation (Darriwilian), southern Shaanxi (South China) to the species. Importantly, this included a nearly complete specimen which revealed an eleven segment thorax in combination with a three segment pygidium. This is similar to the morphology seen in the species Cyrtometopus ? aries , discussed below, and suggests that Ellipsocoryphe also represents a similar “cyrtometopine” lying outside the phylogenetic structure of Deiphoninae . We hence exclude it from Hemisphaerocoryphe .

Hansen (2005) assigned a Dapingian (Volkhov Stage) species, H. platinflata , from the St. Petersburg region, Russia, establishing it on the basis of two illustrated cranidia. He made comparisons mainly with Krattaspis Öpik, 1937 (which had been revised by Pärnaste [2003]) and suggested that the species - the oldest that had been assigned to Hemisphaerocoryphe - indicated a link between the genera. It is not obvious why H. platinflata should be assigned to Hemisphaerocoryphe , especially in the absence of information on the pygidium. It seems morphologically closest to species of Krattaspis , to which we tentatively assign it. Hansen (2005, p. 207) considered that “ Krattaspis may constitute the ancestor of Hemisphaerocoryphe .” Hemisphaerocoryphe (and hence Deiphoninae ) almost certainly roots within the paraphyletic “Cyrtometopinae,” but it is worth pointing out that the only species of Krattaspis for which the thoracic segment count is known, K. popovi Pärnaste, 2003 ( Pärnaste, 2003,text-fig. 8H) has 11 thoracic segments, whereas all species of Hemisphaerocoryphe for which the count is known have 10.

Hemisphaerocoryphe sinica Zhao, Zhang, Cheng, and Zhu, 1997 , from the “Lower Ordovician” Xiqiuhe Formation, Heilongjiang, China, is based on a single partial and distorted cranidium. Apart from it being “bubbleheaded” and there likely either a “cyrtometopine” or deiphonine, it is otherwise uninterpretable.

The unique holotype of H. sulcata Thorslund (1940 , pl. 10, figs 6, 7) is a partial cranidium consisting of only the glabella and anterior border and part of the left fixigena. Männil (1958, pp. 179, 207) thought that it was related to the species Cheirurus (Cyrtometopus) rosenthali Schmidt, 1881 , and Metopias aries Eichwald, 1843 , and suggested these species should be classified together in a new genus. Nikolaisen (1961, p. 288) assigned rosenthali to his new genus Cyrtometopella , but did not comment on aries or sulcata . Přibyl and Vaněk in Přibyl et al. (1985, p. 129) assigned all three to Cyrtometopella Nikolaisen, 1961 . Bruton et al. (1997, pp. 101, 102) followed Nikolaisen’s (1961) assignment of rosenthali to Cyrtometopella , and assigned aries to Reraspis Öpik, 1937 . Pärnaste (2004a) assigned rosenthali and sulcata , along with Cheirurus verrucosus Brögger, 1882 and Ellipsocoryphe elliptica Lu, 1975 with question to Hemisphaerocoryphe and aries with question to Cyrtometopus Angelin, 1854 . Pärnaste (2004b, p. 134) considered that Cyrtometopella askerensis Nikolaisen, 1961 was similar to rosenthali and that both also resembled sulcata . She argued that “the pygidia identified with askerensis resemble those of Hemisphaerocoryphe , possessing only one pair of pleural spines between the stout anterior pair.” However, no pygidia were associated with askerensis , which was established on the basis of two specimens, an incomplete cephalon and an incomplete cranidium. The pygidia illustrated by Niklolaisen (1961, pl. 1, figs 9, 10) were assigned to the type species, Cyrtometopella tumula . Pärnaste (2004b, p. 134) excluded this species from the above mentioned group because of the conical shape of the glabella and other features. Krueger (1994, p. 469), however, had already proposed the synonymy of Cyrtometopella with Hemisphaerocoryphe , based on the morphology of the pygidia of Cyrtometopella tumula . Krueger’s opinion is accepted herein, and the poorly known species are provisionally assigned.

A significant problem in assessing these species, and the probable reason for their taxonomic flux, is that they are mostly very poorly known, typically described on the basis of one or two incomplete cranidia. Krueger (1994, p. 470) appeared to supplement what is known of H. sulcata , assigning one more illustrated cranidium (of three listed in his materials section) from north German (Mecklenburg-Vorpommern) glacial erratics. This specimen, however, is from the Darriwilian Uhaku Stage, whereas H. sulcata is from the Sandbian Dalby Limestone. Due to the paucity of the material and the size of Thorslund’s photographs, it is next to impossible to meaningfully compare the specimens, but the age difference strongly suggests they are unlikely to be conspecific.

The species aries has cephalic morphology closely similar to species of Hemisphaerocoryphe . However, it has 11 thoracic segments and a pygidium with three pairs of robust spines ( Klikushin et al., 2009, figs 511–513), similar to Ellipsocoryphe elliptica . The relationships of species like this and other members of the paraphyletic “Cyrtometopinae,” both to each other and to the monophyletic groups Cheirurinae and Deiphoninae , need to be investigated with comprehensive revision and cladistic analysis, though this is beyond the scope of this work. Lane (2002, p. 155) assigned both aries , rosenthali , and verrucosus with question to Cyrtometopus . Krueger (1994, p. 469) also excluded rosenthali from Hemisphaerocoryphe , and we follow these authors by also excluding all three.