Cheiruridae, Hawle and Corda, 1847

Adrain, Jonathan M. & Karim, Talia S., 2019, Revision of the Early Ordovician (late Tremadocian; Stairsian) cheirurid trilobite Tesselacauda Ross, with species from the Great Basin, western USA, Zootaxa 4661 (2), pp. 201-255 : 202-203

Cheiruridae vs Pliomeridae

Due to uncertain stratigraphic occurrences it is difficult to determine the age of the earliest definite cheiruroidean trilobites. The genera Eocheirurus Rozova, 1960 , and Emsurina Rozova, 1960 , were proposed as cheirurids. Each contains obviously cheiruroidean species from the Tolstochi’kha Formation of Salair Ridge in the Altai-Sayan fold belt of southwestern Siberia. Rozova (1960) also proposed the genus Emsurella and considered it a cheirurid, but subsequent authors ( Westrop, 1986, p. 68; Jell and Adrain, 2003, p. 371) have suggested it belongs to Cheilocephalidae Shaw, 1956 . It is known from two species each represented only by cranidia. These are apparently proparian and with cheiruroidean dimensions, but seem to lack glabellar furrows. The similarly aged Siberian species Parapliomera njuensis ( Maksimova, 1955) , however, has an assigned cranidium ( Maksimova, 1962, pl. 17, fig. 13; see also Timohin, 1989, pl. 9, figs 8, 10, 14) that also has effaced furrows and the pygidia of this species certainly resemble those of pliomerids. Lane (1971) assigned all three genera to Pilekiinae , though he queried the affinity of Emsurella . Rozova (1960) considered all of the species she described to be late Cambrian in age. This was accepted by Přibyl and Vaněk (in Přibyl et al., 1985), who considered Eocheirurus to be a pilekiine cheirurid, Emsurina a sphaerexochine cheirurid, and Emsurella a pliomerid. Lane (1971, p. 73) suggested the age of these genera might be Tremadocian and Edgecombe (1992, p. 168–169) considered a Late Cambrian age suspect. It is now clear from conodont work that the Tolstochi’kha Formation spans the Cambrian–Ordovician boundary ( Sennikov et al., 2015, p. 606), with its upper part including assemblages assignable to the early Tremadocian Iapetognathus fluctivagus Zone and Cordylodus angulatus Zone (see also Petrunina et al., 2001). Rozova (1960) gave the provenance of all of the relevant species as the upper horizon of the formation. Hence the age remains equivocal and could be Late Cambrian, but is possible that they are all from the Ordovician (early Tremadocian).

“ Anacheirurus ? sp.” of Apollonov and Čugaeva (in Apollonov et al., 1984, pl. 22, figs 5–7) was based on specimens derived from a horizon at 214 m in the Batyrbai Section, Malyi Karatau Range, southern Kazakhstan. This horizon lies within the Cordylodus prion assemblage of Apollonov et al. (1981, fig. 1), which correlates with the latest Cambrian Cordylodus lindströmi Zone. Hence , this species, known from only three incomplete cranidia, but clearly cheiruroidean, may represent the oldest known, apparently latest Cambrian, cheiruroidean taxon. Other cheirurid genera with putative early Tremadocian species include Chashania Lu and Sun in Zhou et al. (1977) , Parapilekia Kobayashi, 1934 , Pseudopliomera Lu and Qian in Yin and Li, 1978 , Seisonia Kobayashi, 1934 , and Sinoparapilekia Peng, 1990 . It seems that cheiruroideans appeared in Siberia, North and South China, and other tropical terranes, around the time of the Cambrian–Ordovician transition.

No cheiruroideans are known from Laurentia prior to the beginning of the Stairsian Stage in the upper Tremadocian. Rossaspis pliomeris Demeter, 1973 , is the oldest known Laurentian species, occurring only a few metres above the Skullrockian–Stairsian boundary. Rossaspis Harrington, 1957 , will be revised with several new species in a forthcoming work. It appears to be a pliomerid. The northern Laurentian Stairsian contains both early pliomerids and considerable numbers of pilekiine cheirurids, including species belonging to Pilekia Barton, 1915 , and several new genera, all currently under study.

Among all of these early cheiruroideans there has been considerable confusion over which taxa belong to Cheiruridae versus Pliomeridae , and what potential synapomorphies characterize each clade. Tesselacauda is emblematic of this, as it has been assigned to Pliomeridae by many authors (e.g., Ross, 1951 [Ross made no formal family assignments in his monograph, but he considered { Ross, 1951, fig. 2} Tesselacauda to be part of an evolutionary trend with the pliomerids Rossaspis , Hintzeia Harrington, 1957 , and Pseudocybele Ross, 1951 ]; Kobayashi, 1955; Maksimova, 1962; Demeter, 1973; Jell and Adrain, 2003) and to Cheiruridae by others (e.g., Černyševa, 1960 [as “ Pilekiidae ”]; Hupé, 1955; Lane, 1971; Dean, 1989; Lee and Chatterton, 1997; Edgecombe et al., 1999; Adrain, 2013; Adrain et al., 2014). Other early cheiruroidean genera have similar taxonomic histories.

Whittington (1961, p. 912–913) gave a diagnosis for Pliomeridae and contrasted pliomerids with pilekiine cheirurids. He considered that pilekiines differed from pliomerids in that they have “the glabella parallel-sided or narrowing forward, eye lobe situated far forward, eye ridge running inward to the axial furrow, and pleural furrow situated medially.” In the current state of knowledge none of these features seem at all diagnostic, as each is found in both early cheirurids and pliomerids. Other early diagnoses of Pliomeridae (e.g., Holliday, 1942, p. 473) were similarly composed of general features shared with cheirurids, and there have been no synthetic modern treatments of the group. In order to justify a familial assignment of Tesselacauda , and in an attempt to better characterize the early morphologies of pliomerids and cheirurids, we discuss below what seem to be the most salient differences between them.

In dorsal cephalic features, there are few differences between the groups. Floian pliomerids develop an adaxially curtailed fusion of the palpebral lobe and eye ridge which was termed a palpebro-ocular ridge by McAdams and Adrain (2009). In the genera Hintzeia, Panisaspis McAdams and Adrain, 2011b , Pseudocybele , and others, this feature, when not set in a crowded anterior position, is always separated from the axial furrow posteriorly by a pitted fixigenal field. However it is a feature that seems to have been developed at some point within the group, as it is absent from those Stairsian species we assign to Pliomeridae . The only other strong contrast between the groups is that some cheirurids develop median occipital spines or glabellar spines, and these features are completely unknown in pliomerids. Cheirurids have a tendency to possess more prominent tuberculate sculpture, but there is considerable overlap in this feature. There are few other dorsal cephalic differences.