Dimeropygidae Hupé, 1953

Adrain, Jonathan M. & Westrop, Stephen R., 2006, New genus of dimeropygid trilobites from the earliest Ordovician of Laurentia, Acta Palaeontologica Polonica 51 (3), pp. 541-550 : 543

Family Dimeropygidae Hupé, 1953

Discussion.—Classification of the Dimeropygidae is in flux (therefore we omit higher categories, as the question of which are correct is currently open), but current generic content was given Jell and Adrain (2003: 469). Generic content of the related family Hystricuridae follows Jell and Adrain (2003: 472) with the addition of Millardicurus Adrain and Westrop, 2006 . Hystricuridae has often been considered a paraphyletic taxon which “gave rise” to (i.e., contains the sister taxa of) other elements of the order Proetida Fortey and Owens, 1975 . As argued by Adrain et al. (2003), the hystricurid group contains major monophyletic units, including the taxa Hystricurinae Hupé, 1953 (sensu Adrain et al. 2003) and Hintzecurinae Adrain, Lee, Westrop, Chatterton, and Landing, 2003. Just as the order Proetida is itself “cryptogenetic” ( Stubblefield 1959; Whittington 1981; Fortey 1997, 2001) in the context of higher trilobite phylogeny, the origins and sister group relationships of families of Proetida , including Aulacopleuridae Angelin, 1854 , Brachymetopidae Prantl and Přibyl, 1951 , Dimeropygidae Hupé, 1953 , Rorringtoniidae Owens in Owens and Hammann, 1990, Scharyiidae Osmólska, 1957 , and Telephinidae Marek, 1952 , are also essentially unknown.

Unravelling the phylogenetic structure of the group is beyond the scope of this paper, although ongoing descriptive work on the Ross/Hintze faunas will add a wealth of new data to help address the problems. Tulepyge does not compare closely with hystricurids (to which T. paucituberculata was first assigned), and in fact with knowledge of T. tulensis it is more similar in its vaulted, tuberculate exoskeleton and librigena with a very short genal spine to younger taxa such as Dimeropygiella Ross, 1951 , and Ischyrotoma Raymond, 1925 (see Adrain et al. 2001, for revisions of both), which have been considered members of Dimeropygidae . There are a great number of undescribed vaulted, tuberculate species of upper Skullrockian, Stairsian, and Tulean age in the Ibex sections. The closest comparison of Tulepyge is possibly with an essentially undescribed younger Skullrockian clade, sclerites of which have been reported in open nomenclature by Ross (1951: pl. 9: 25, 29, 30, pl. 14: 5, 8, 12). We have sampled multiple undescribed species of this group, all from the Red Canyon Member of the House Formation and coeval strata of the Garden City Formation. In general, cranidial dimensions and sculpture of these species are similar, especially to T. tulensis , and the structure of the librigena is almost identical. However, pygidia of the taxa are very different. Those of the upper Skullrockian forms ( Ross 1951: pl. 9: 25, 29, 30) have a corona of sharp pleural spines, beneath which the distal part of the pleura forms a nearly vertical wall not crossed by pleural or interpleural furrows. The tails also feature only two or three segments, versus five in Tulepyge . Confident hypotheses of relationship of Tulepyge must await much work in progress on younger taxa, but it is tentatively interpreted as a plesiomorphic member of a group also encompassing genera such as Dimeropygiella and Ischyrotoma , and this group is tentatively assigned to Dimeropygidae .