Acanthoparyphinae Whittington and Evitt, 1954

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 : 18-21

Subfamily Acanthoparyphinae Whittington and Evitt, 1954

Genera included. Acanthoparypha Whittington and Evitt, 1954 ; Cydonocephalus Whittington, 1963 ; Forteyops Přibyl and Vaněk in Přibyl et al., 1985; Hammannopyge Přibyl and Vaněk in Přibyl et al., 1985; Holia Bradley, 1930 (= Ainoa Männil, 1958 ); Hyrokybe Lane, 1972 (= Shiqiania Zhang, 1974 ); Kawina Barton, 1915 (= Kolymella Čugaeva, 1973 ); Nieszkowskia Schmidt, 1881 ; Pandaspinapyga Esker and Levin, 1964 ; Parayoungia Chatterton and Perry, 1984 (= Ichiyamella Kobayashi and Hamada, 1986 ); Pompeckia Warburg, 1925 ; Xystocrania Whittington, 1965 (= Xialiangshania Zhen in Li et al., 1975); Youngia Lindström, 1885 .

Discussion. Whittington and Evitt (1954, p. 70) proposed Acanthoparyphinae and included three genera: Acanthoparypha , Nieszkowskia , and Holia . They did not formally diagnose their new subfamily, but discussed features shared between the three genera. Many of these are general (ovate glabella with maximum width across the basal lobes, hypostome with prominent lateral shoulder and relatively wide lateral notch, etc.). They also noted that the pleural furrows of species all three genera are expressed as a transverse row of pits. This is true, but it is also potentially a symplesiomorphy, as a similar feature is seen in members of several other cheirurid subfamilies (e.g., Eccoptochile clavigera ( Beyrich, 1845) , see Přibyl et al. [1985, pl. 4, fig. 4], and Eccoptochile perlata Hawle and Corda, 1847 , see Šnajdr [1984, pl. 15, fig. 6)] Eccoptochilinae; Valongia wattisoni ( Curtis, 1961) , see Curtis [1961, pl. 4], “Cyrtometopinae”; Reraspis plautini ( Schmidt, 1881) , see Öpik [1937, pl. 11, fig. 2], “Cyrtometopinae,” etc.). One feature that unambiguously differentiates the three genera from the others listed by Whittington and Evitt (1954, p. 70), though, is the presence of only two pairs of pygidial spines, which is seen elsewhere in Cheiruridae only in Heliomerinae , and this has been the enduring basis for recognition of the subfamily and its included genera. Lane (1971, p. 81) characterized the subfamily as “the sudden appearance in the latest Llandeilo to earliest Caradoc of a group of well differentiated genera...” and considered that this sudden appearance made both within-group phylogeny and overall group affinity impossible to determine. Contrary to this, we argue, following Adrain and Karim (2019, p. 208), that the group was established by the Floian in Laurentia and has a rich intervening Floian, Dapingian, and Darriwilian history. The earlier taxa have in the past been misclassified as Sphaerexochinae , mainly because they have three instead of two pairs of pygidial spines.

The concept of Sphaerexochinae followed by most workers dates to Whittington (1965, p. 411), who considered that it represented “a group of trilobites particularly characteristic of Middle Ordovician rocks in North America, only Sphaerexochus being more widely known at this time and ranging into Silurian rocks.” He had earlier (Whittington, 1963) assigned Cydonocephalus and Kawina to the subfamily, and in 1965 assigned Xystocrania and Heliomera Raymond, 1905 . He further considered that Kawina sexapugia Ross, 1951 (now the type species of Forteyops Přibyl and Vaněk in Přibyl et al., 1985) might represent an Early Ordovician member of the group. Lane (1971, p. 53) added Pompeckia Warburg, 1925 . With the exception of the question of assignment of Heliomera (and Heliomeroides Evitt, 1951 ), which we discuss below under Sphaerexochinae , this collection of genera has subsequently become the consensus scope of the family. Congreve and Lieberman (2011), for example, conducted a cladistic analysis on the assumption that the subfamily so conceived is monophyletic, though they excluded Xystocrania and Pompeckia from their matrix on grounds of limited material and unclear affinity.

Sphaerexochus has a suite of prominent synapomorphies, such as pervasive fine granular sculpture on almost all dorsal exoskeletal surfaces, including the librigenal field, a strong coaptative furrow on the posterior part of the librigenal lateral border, so that the border overhangs the margin of the doublure in internal view (e.g., Chatterton and Ludvigsen, 1976, pl. 13, figs 35, 36), a hypostome with a bilobate posterior border, very robust thoracic segments with faint pleural furrows, the distal spines of which are strongly downturned and form a secondary, quasifaceted articulation with adjacent segments, and highly tagmatized three segment pygidia with usually blocky, subquadrate spines (the spines are lobate and the terminuses rounded in younger Silurian species) and with the third axially ring partially or fully merged with a triangular terminal piece. In particular, all species known from articulated specimens (e.g., “ S. mirus ,” [ Thomas, 1981, pl. 16, fig. 1]; S. latifrons Angelin, 1854 [ Ramsköld, 1983, pl. 27, fig. 12a]; S. laciniatus Lindström, 1885 [ Ramsköld, 1983, pl. 28, fig. 11a]) have ten thoracic segments, a count seen elsewhere within Cheiruridae only in the deiphonine genus Hemisphaerocoryphe (see above) and the “cyrtometopine” Reraspis .

No members of the broader, mostly Laurentian, group that has been assigned to Sphaerexochinae share any of these states, with the exception of a pygidium with three segments. The only known segment count for a member of this group is 12, in Kawina arnoldi . Assuming this does not vary, then the three segment pygidia of this group are not serially homologous with those of Sphaerexochus , as they represent post-cephalic body segments 13–15 versus 11–13. It is therefore not surprising that they show very little detailed resemblance.

While taxa such as Kawina , Cydonocephalus , and Forteyops share few if any obvious characters with Sphaerexochus , they share many with traditional Acanthoparyphinae. Prominent among these is a unique type of sculpture, featuring large, smooth tubercles set against a background of much finer tubercles and with a central perforation. This type of sculpture is seen in multiple species of Acanthoparypha . Whittington and Evitt (1954) observed it on the spines of the librigenal border and used it as the basis of the name for the type species, A. perforata Whittington and Evitt, 1954 . The same sculpture can be seen in their higher magnification photograph of a pygidium of A. chiropyge Whittington and Evitt, 1954 (pl. 30, fig. 28) and on a higher magnification oblique photograph of A. evitti Chatterton and Ludvigsen, 1976 (pl. 10, fig. 41). We document the sculpture here on both the pygidium ( Fig. 3.6 ) and cranidium ( Fig. 4 ) of Pandaspinapyga salsa Esker, 1964 . Perforate spines are pervasive among Silurian species of Hyrokybe (see multiple examples in Chatterton and Perry [1984]).

Identical sculpture is a prominent feature of multiple older species with three segment pygidia, which we document in Figure 3 . It is present, for example, in the Floian Forteyops sexapugius ( Ross, 1951) ( Fig. 3.1, 3.3 ), the Floian “ Kawina ?” webbi Hintze, 1953 ( Fig. 3.4 ), and the Dapingian Kawina n. sp. ( Fig. 3.5, 3.7, 3.8 ). While we have pointed out above that replacement of thoracic and/or pygidial pleural furrows with transverse pit rows is a widely distributed feature, it is nevertheless common within traditional Acanthoparyphinae but completely absent from Sphaerexochus . We document it here in the pygidium of a new Floian species of Forteyops ( Fig. 3.2 ).

Hypostomes of the Early and Middle Ordovician three segment pygidia species are all but identical with those of species of Acanthoparypha . In Figure 5 we reillustrate the hypostomes of Cydonocephalus griphus Whittington, 1963 ( Fig. 5.1 ), and Kawina arnoldi Whittington, 1963 ( Fig. 5.2 ), and newly illustrate examples of Forteyops sexapugius ( Fig. 5.3 ), Kawina n. sp. ( Fig. 5.4 ), “ Kawina ?” webbi ( Fig. 5.5 ) and a second species of the new genus to which the latter will be assigned ( Fig. 5.6 ). These compare very closely with those of, for example, A. evitti ( Chatterton and Ludvigsen, 1976, pl. 10, figs 22, 25, 26, 28, 31), A. perforata ( Whittington and Evitt, 1954, pl. 17, figs 1, 4), and A. chiropyge ( Whittington and Evitt, 1954, pl. 30, figs 1–6, 8). The hypostome of C. griphus even has a bifurcate middle furrow, a feature common to most species of Acanthoparypha .

Finally, the structure of the generally tuberculate pygidia is very similar between various members of the three segmented pygidia group and taxa like Pandaspinapyga and Acanthoparypha , comparing the portion of the pygidium posterior to the first segment in the three segment group with the entire two segment pygidia of younger species. Adrain and Karim (2019) advanced the argument that the two segment group (traditional Acanthoparyphinae) was derived from the three segment group simply by releasing the anterior pygidial segment during late ontogeny to become the posteriormost thoracic segment. Unfortunately, only three relevant species are known from articulated specimens. They are, however, consistent with this hypothesis. A well preserved specimen of Kawina arnoldi has 12 thoracic segments and three pygidial segments (Whittington, 1963, pl. 26, figs 7–9, 11, 12, 14). If the hypothesis is correct, then species of the two segment group would be expected to have 13 thoracic segments. In fact, as pointed out by Adrain and Karim (2019, pl. 208), this is exactly what the holotype articulated exoskeleton of Pandaspinapyga salsa displays. Previous illustrations of this specimen have been difficult to interpret (Esker, 1964, pl. 2, fig. 6) or small ( Shaw, 1974, pl. 8, fig. 3). We reillustrate it here ( Fig. 4 ) to remove any ambiguity. The space that the missing posterior pair of pygidial spines would occupy is marked by their faint impression on the underlying matrix. The left pleural spine of the first segment is the only part of the pygidium preserved as exoskeleton. In front of it is the axial ring and part of the left pleurae of the posteriormost thoracic segment, which is the 13th. This is in agreement with the segment count as reported by Shaw (1974, p. 33). The monotypic Hammannopyge (type species H. unica ( Thomson, 1857) , from the Balclatchie Formation [Sandbian], South Ayrshire, Scotland [Laurentian-affinity Midland Valley Terrane]) is a probable synonym of Pandaspinapyga . Hammannopyge unica also has two pairs of pygidial spines and a 13 segment pygidium ( Lane, 1971, pl. 15, fig. 2).

While Pandaspinapyga (and Hammannopyge ) have two pairs of pygidial spines, all species for which pygidia are known have three pygidial axial rings (e.g., Fig. 3.6 ). All other acanthoparyphines with two spine pairs have only two rings. We interpret this as a consequence of a peramorphic event leading to the release of a 13th thoracic segment during an extended developmental program. While development continued, an extra thoracic segment was released, and an extra (16th) post-cephalic body segment was generated posteriorly. This extra posterior segment was evidently lost in more derived taxa.

Hence, we regard all of these taxa as closely related, and expand the concept of Acanthoparyphinae to include the earlier genera previously assigned to Sphaerexochinae . The current status of Sphaerexochinae , and its possible basal morphology, are discussed under that subfamily below.

Adrain and Fortey (1997) proposed Mayopyge for a densely tuberculate cheirurid species from the Floian Tourmakeady Formation of western Ireland (Laurentian-affinity Northwestern Terrane), and discussed it along with Kawina as a taxon that would traditionally be assigned to Sphaerexochinae . Adrain in Jell and Adrain (2003, p. 403) considered Mayopyge a junior subjective synonym of Parasphaerexochus Čugaeva, 1973. Parasphaerexochus was based on two Floian species, P. confragosus and P. galeatus , from Magadan Oblast, far eastern Russia (Kolyma- Omolon Terrane). Both are known only from cranidia, but these closely resemble those of P. zapata ( Adrain and Fortey, 1997) . All are densely tuberculate, have strongly impressed S1, with S2 and S3 also robust, and in particular all three species have a prominent dorsal conical swelling at the rear of the glabella. In cephalic features (the hypostome and librigena are known for P. zapata ) the species resemble acanthoparyphines as understood herein. Thoracic and pygidial morphology, however, appears to rule out a relationship. Thoracic segments of P. zapata have a small adaxial region with anterior and posterior pleural bands expressed and an obliquely set pleural furrow. An obliquely set pleural furrow is a universal feature of Cheirurinae . However in that case the furrow runs posterolaterally. The furrow in P. zapata is set in the opposite direction, running anterolaterally from the axial furrow (seen most clearly on Adrain and Fortey [1997, pl. 10, figs 4a, 5b). Abaxial to the region with pleural bands and furrow, a tuberculate, semicylindrical spine is developed, which protrudes laterally and is turned posteriorly in its distal portion. The first of the three pygidial segments has similar morphology. The pygidia have three sets of pleural spines, but the spines are semicylindrical and not in contact with each other along their length. This is very different from the acanthoparyphine morphology in which the pleural furrow, if it is expressed, is transverse, the pleural bands are not independently inflated, the pleural spines run directly from the pleural region with no break in morphology, and the pygidial spines are dorsoventrally flattened and usually nearly in contact along most of their length. The morphology of P. zapata is unique among cheirurids and at this point the affinities of Parasphaerexochus are uncertain. Zhou and Zhou (2008, p. 12) considered Conicephalus Zhao in Zhao et al., 1997 (a homonym of the Middle Cambrian solenopleurid Conicephalus Yang in Yang et al., 1991) a junior subjective synonym of Parasphaerexochus, and we agree.

Figures

FIGURE 3. Sclerites of species here assigned to Acanthoparyphinae, showing unique perforate tubercle sculpture found in multiple taxa and the occasional presence of transverse pit rows in place of pleural furrows. 1, 3. Forteyops sexapugius (Ross, 1951). 1. Pygidium, SUI 148403, dorsal view, x10, Garden City Formation (Floian; Blackhillsian; “Pseudocybele nasuta Zone”), Round Hill, near Mantua, Box Elder County, Utah, USA. 3. Left librigena, SUI 148404, external view, x20, Fillmore Formation (Floian; Blackhillsian; “Pseudocybele nasuta Zone”), Section H, 290.4 m, southern Confusion Range, Millard County, Utah, USA.. 2. Forteyops n. sp., pygidium, SUI 148405, dorsal view, x12, Wah Wah Formation (Floian; Blackhillsian; “Pseudocybele nasuta Zone”), Section J 46.8 m, southern Confusion Range, Millard County, Utah, USA. 4. Kawina? webbi Hintze, 1953, left librigena, SUI 148406 external view, x20, Wah Wah Formation (Floian; Blackhillsian; “Pseudocybele nasuta Zone”), Section J

FIGURE 4. The holotype incomplete dorsal exoskeleton of Pandaspinapyga salsa Esker, 1964 (provenance given in explanation of Fig. 3.6), demonstrating the presence of 13 thoracic segments in this two pygidial segment form of Acanthoparyphinae. OU 5200, dorsal view, x10. An example of an intact pygidium of this species is illustrated in Fig. 3.6.

FIGURE 5. Hypostomes of Early and Middle Ordovician species assigned herein to Acanthoparyphinae. 1. Cydonocephalus griphus Whittington, 1963, ventral view of hypostome in articulated cephalon, holotype specimen GSC 16272, x15, Shallow Bay Formation (lower Darriwilian), Lower Head, western Newfoundland, Canada. 2. Kawina arnoldi Whittington, 1963, ventral view of hypostome in articulated cephalon, holotype specimen GSC 162692, x7.5, Shallow Bay Formation (lower Darriwilian), Lower Head, western Newfoundland, Canada. 3. Forteyops sexapugius (Ross, 1951), hypostome, SUI 148410, ventral view, x10, Garden City Formation (Floian; Blackhillsian; “Pseudocybele nasuta Zone”), Round Hill, near Mantua, Box Elder County, Utah, USA. 4. Kawina n. sp., hypostome, SUI 148411, ventral view, x12, Kanosh Formation (Dapingian), Section K 1.5T m, southern Confusion Range, Millard County, Utah, USA. 5. Kawina? webbi Hintze, 1953, hypostome, SUI 147662, ventral view, Wah Wah Formation (Floian; Blackhillsian; “Pseudocybele nasuta Zone”), Section J 40.0 m, southern Confusion Range, Millard County, Utah, USA. 6. Acanthoparyphinae n. gen. n. sp., hypostome, SUI 148412, ventral view, x12, Yellow Hill Limestone (Floian; Blackhillsian; “Pseudocybele nasuta Zone”), Yellow Hill, near Pioche, Lincoln County, Nevada, USA.