Mainbrookia, 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 : 11-12

Subfamily Deiphoninae Raymond, 1913

Genera included. Deiphon Barrande, 1850 ; Hemisphaerocoryphe Reed, 1896 ;? Hinggania Zhao, Zhang, Cheng, and Shu, 1997; Mainbrookia n. gen.; Onycopyge Woodward, 1880 ; Sphaerocoryphe Angelin, 1854 .

Diagnosis. Glabella strongly inflated; librigena small, with strongly curved lateral margin; 13 thoracopygidial segments, though this is altered in late, derived forms; basal taxa with thorax of 10 segments and pygidium with three axial rings and two or three pairs of pygidial border spines; derived taxa with thorax of 9 segments and additional pygidial segment anterior to that with the major spine pair.

Discussion. The classification and phylogenetic structure of Deiphoninae has been uncertain. There are two obvious, mostly Silurian, clades, Deiphon and Onycopyge . Most Middle and Upper Ordovician species have been classified in Sphaerocoryphe . There has been disagreement whether the Darriwilian–Sandbian Hemisphaerocoryphe should be recognized as distinct from Sphaerocoryphe . The discovery of a well preserved Laurentian Darriwilian species, along with greater appreciation of the work of Krueger (1994, 1996) clarifies the basic structure of the subfamily, though questions remain regarding the basal node and the structure of the paraphyletic Sphaerocoryphe and specific relationships of its species to those of Deiphon and Onycopyge .

The monotypic Hinggania, proposed by Zhao et al. (1997) for a Darriwilian species from the Hinggan Region, Heilongjiang, China, could be either a deiphonine or a “cyrtometopine.” The poor preservation of the illustrated sclerites make detailed interpretation difficult, and it is not considered further.

The basal node of the family lies somewhere within the as yet uncertain phylogenetic structure of species of the paraphyletic group “Cyrtometopinae Öpik, 1937.” In fact, Pärnaste (2004a) classified Sphaerocoryphe , Hemisphaerocoryphe , and Cyrtometopella (here regarded as a junior subjective synonym of Hemisphaerocoryphe ) as Cyrtometopinae. “Bubble-headedness,” the marked subspherical inflation of the glabella, along with reduction in the size of L1, has been considered characteristic of Deiphoninae , but it occurs also in species with greater number of thoracic and pygidial segments presently classified as Cyrtometopinae, such as Cyrtometopus aries ( Eichwald, 1843) (see Klikushin et al. [2009, figs 513–515]; see further discussion under Hemisphaerocoryphe below). Hence, it is conceivable that as the “cyrtometopines” become better known and better understood, additional, more basal, species might be classified as Deiphoninae .

We elect, in the present state of knowledge, to draw the basal node to include species that have 13 post-cephalic body segments. This number may be reduced in highly derived later forms via merging and tagmatization of the pygidium, and it is increased in Onycopyge , which has a high number of pygidial axial rings versus pleural segments, similar to the situation seen in encrinurids. A 13 segment thoracopygidium, however, is stable across most of the Ordovician history of the group.

In contrast to prior discussions of the utility or potential monophyly of Hemisphaerocoryphe , which were written largely without knowledge of the thoracic segment count or pygidium, and in light of the important work of Krueger (1994, 1996), it now seems apparent that this genus is a monophyletic Darriwilian-Sandbian Baltic group characterized by 10 thoracic segments and especially by derived pygidia with three axial rings but only two pairs of spines, with the first pair greatly enlarged and the second pair tiny. Hemisphaerocoryphe is discussed at greater length below.

A second fairly obvious clade is comprised of the genera Sphaerocoryphe , Deiphon , and Onycopyge . These taxa share a nine segment thorax which seems to have been derived via an evolutionary event (presumably paedomorphic) in which the M9 meraspid condition was frozen as the holaspid stage, with the tenth thoracic segment remaining fused to the pygidium. All species of Sphaerocoryphe , Deiphon , and Onycoypyge, for which information is available have this condition. Sphaerocoryphe exserta Webby, 1974 , was thought to belong to Hemisphaerocoryphe by Přibyl and Vaněk in Přibyl et al. (1985, p. 155) and Zhou et al. (1998, p. 725–726) because of its cranidial morphology. It is best regarded on the basis of the anterior pygidial segment as a species of Sphaerocoryphe .

The new Darriwilian deiphonine species from the Table Cove Formation does not have this derived pygidial morphology. It has a pygidium of three segments, with three pairs of spines, and there is no additional pygidial segment anterior to the pair with large major spines. Hence it lies outside the phylogenetic structure of Sphaerocoryphe + Deiphon + Onycopyge . It does, however, have a feature that uniquely groups it with this clade and suggests it is part of its sister taxon. This is the retention in mature adults of a prominent cranidial spine on the fixigenal lateral border (Pl. 5, figs 7, 10). This feature is definitively absent from Hemisphaerocoryphe (see illustrations of most species in Krueger [1994, 1996]), but present in all species of Sphaerocoryphe + Deiphon + Onycopyge . There are only two other named Darriwilian Laurentian deiphonine species. Sphaerocoryphe akimbo Tripp, 1967 , also shows a three segment pygidium and a cranidial fixigenal lateral border spine ( Tripp, 1967, pl. 4, figs 1–9). Sphaerocoryphe saba Tripp, 1962 , lacks an associated pygidium, but definitely has the fixigenal lateral border spine ( Tripp, 1962, pl. 3, fig. 11). A second feature that as far as is known is uniquely shared among taxa of this group is the presence of a pair of ventrally directed pygidial marginal spines. In derived taxa such as Silurian species of Deiphon , these features are distinct, subcylindrical spines (e.g., Chatterton and Perry, 1984, pl. 26, figs 17, 26, 30) set at the posterolateral corner of the margin, beneath the major border spines. Homologous structures are present in Mainbrookia becki , but they are blunt, subtriangular, ventrally directed portions of the margin, apparently reflecting the lateral extremes of the transverse dorsoventral arch described by the margin (e.g., Pl. 5, fig. 13, Pl. 6, figs 8, 9). It appears that these ventrally extended subtriangular protrusions developed into subcylindrical spines in more derived, younger members of the group. While this feature is only observable in the Mainbrookia + Sphaerocoryphe + Deiphon + Onycopyge clade, caution must be exercised, as no pygidia of species of Hemisphaerocoryphe have been illustrated with the ventral margin exposed, and the status of the margin in this genus is currently unknown. We group the Laurentian Darriwilian species as Mainbrookia n. gen. and suggest, on the basis of the shared fixigenal spine, and potentially the pygidial ventral marginal spines, that it is sister to Sphaerocoryphe + Deiphon + Onycopyge , and that Hemisphaerocoryphe is sister to Mainbrookia + Sphaerocoryphe + Deiphon + Onycopyge , based on sharing a 13 segment thoracopygidium. As Hemisphaerocoryphe has had a history of taxonomic flux, we discuss it below at some length in order to establish its monophyly and scope.