Newfoundlandops, Adrain & Pérez-Peris, 2021

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 : 29-30

Subfamily Sphaerexochinae Öpik, 1937

Genera included. Newfoundlandops n. gen.?; Sphaerexochus Beyrich, 1845 .

Discussion. Transfer of most taxa traditionally assigned to Sphaerexochinae to Acanthoparyphinae and restriction of Sphaerexochinae to Sphaerexochus itself were discussed above. To reiterate, Sphaerexochus has a morphology of a ten segment thorax, highly tagmatized pygidium, and robust thoracic segments that is unique in Cheiruridae and its affinities within the family are difficult to determine. The oldest known species that has been assigned is the Floian S. trisulcatus Čugaeva, 1973 , from the Khiti Formation, Magadan Oblast, far eastern Russia (Kolyma- Omolon Terrane). This species is known only from three incomplete cranidia. While it is not inconceivable that they could represent a species of Sphaerexochus , they could also represent an acanthoparyphine. Without knowledge of the librigena and particularly the pygidium, it is difficult to accept an assignment to the genus, particularly as it would involve a substantial stratigraphic range extension. No other Floian species and no Dapingian species have been assigned. The oldest definite species are Darriwilian and are from the Laurentian-affinity Midland Valley Terrane, Scotland ( S. eurys Tripp, 1962 ; S. filius Tripp, 1967 ). These species already have the fully derived pygidial morphology typical of the genus and provide no obvious clue to its affinity.

Apart from Sphaerexochus itself and the taxa transferred above to Acanthoparyphinae, the other group that has often been classified as Sphaerexochinae is a small but highly distinctive clade including Heliomera and Heliomeroides . These taxa have had a history of taxonomic flux, with opinion varying on whether they are distinct genera (e.g., Evitt [1951], Shaw [1968], Chatterton and Ludvigsen [1976], Edgecombe et al. [1999], Zhou et al. [2016]), whether Heliomeroides should be considered a subgenus of Heliomera (e.g., Whittington [1965], Dean [1971], Morris and Fortey [1985], Přibyl and Vaněk in Přibyl et al. [1985]; Hammann [1992]), or whether Heliomeroides should be considered a junior subjective synonym of Heliomera (e.g., Tripp [1967], Lane [1971]). The genera were assigned to a new Subfamily Heliomerinae by Evitt (1951), and this was followed by Whittington (1963) before he later ( Whittington, 1965) transferred Heliomera to Sphaerexochinae . Since then, some workers have assigned them to Sphaerexochinae ( Lane, 1971; Přibyl and Vaněk in Přibyl et al., 1985; Edgecombe et al., 1999), some to Heliomerinae ( Shaw, 1968; Adrain, 1998), and some, following the arguments of Chatterton and Ludvigsen (1976, p. 65) to Acanthoparyphinae ( Ludvigsen, 1979; Chatterton, 1980; Ingham in Ingham et al., 1986; Hammann, 1992). Work is in progress on description of new genera and species of the group and consideration of its affinity. For the time being we treat its members as comprising a small Subfamily Heliomerinae . There are no compelling morphological reasons to suppose it is phylogenetically close to Sphaerexochus .

Sphaerexochus is a sprawling genus, with 50 formally named species currently accepted (along with four currently considered junior subjective synonyms) and at least 16 further reports of species in open nomenclature. This compares with Acanthoparyphinae (as understood herein) with a total of 112 formally named and accepted species, which are currently classified in 13 separate genera. Kobayashi and Hamada (1976) proposed a subgenus, Sphaerexochus (Onukia) , and Přibyl and Vaněk (in Přibyl et al., 1985) proposed two more, S. (Korolevium) and S. (Parvixochus). Onukia and Parvixochus are both monotypic, so do nothing to elucidate phylogenetic structure in the group. Korolevium grouped a small set of Ordovician species. This was in fact retrieved as a clade in the cladistic analysis of Congreve and Lieberman (2011, fig. 5), though that analysis included many species that are herein regarded as unrelated acanthoparyphines, and considered only 20 of the 50 named Sphaerexochus species. Sphaerexochus is a difficult taxon because it has limited morphological diversity. More detailed phylogenetic analysis is required in order to understand its structure and possible bases for subdivision.

The Table Cove Formation preserves a species that is the same age as the oldest known firmly assigned Sphaerexochus . In cephalic features, it strongly resembles Sphaerexochus , sharing in particular what seem to be synapomorphies of the librigena (see genus discussion below). In pygidial features, it is very different, apart from having three segments. The pygidial morphology suggests potential relationships that are very different from the traditional classification of Sphaerexochus . There are two possibilities: 1) the species represents the less derived sister species of Sphaerexochus ; or 2) the cephalic features of the species are convergent on those of Sphaerexochus and the species is unrelated. We tentatively advance the hypothesis that the first possibility is correct and assign the new species to a monotypic new genus classified with question as Sphaerexochinae .