Reassessment of the Order Trinucleida and its phylogeny and systematics at familial level.

VACCARI, N. EMILIO; BIGNON, ARNAUD; CHATTERTON, BRIAN D. E.; WAISFELD, BEATRIZ G.

Congreso; Zooming in on the GOBE, Online Annual Meeting of the IGCP 653; 2020

IGCP 653

The Superfamily Trinucleoidea is an iconic trilobite group of the GOBE, but its evolutionary history has received little attention. In recent decades, this group has been included in the Order Asaphida. In a recent phylogenetic analysis, we have shown that this group is more closely related to Ptychopariida. As Ptychopariida is paraphyletic, we suggested that trinucleoids should be raised to ordinal status, the Trinucleida, with the aim to define a higher level taxon based only upon synapomorphies. A preliminary phylogenetic study raised several unexpected issues with regard to definition of some of the families within Trinucleida. Most important among these was that the Family Alsataspidae, as currently defined, is paraphyletic. Hence, we performed another phylogenetic analysis, focussed on this family. Two suborders appeared, Trinucleina encompassing the families Orometopidae, Raphiophoridae and Trinucleidae; and the other one being Dionidina, including the families Dionididae, Heterocaryonidae, Myindidae, and Alsataspididae. Interestingly, because of some similarities to trinucleids, we included several Harpetida in the analysis. As might be expected, most of the harpetids stay close to the outgroup taxa, but the basal family of harpetids, the Heterocaryonidae is included in the analysis as a basal member of the Dionidina. The family Liostracinidae is more closely related to Ptychopariida, so we have excluded them from the Trinucleida, making it easier to posit the characters of a hypothetical ancestor of the Order Trinucleida. These analyses offer new views of the evolutionary history of trinucleid trilobites. Indeed two lineages (suborders of Trinucleida) evolved rapidly during the Late Cambrian/Early Ordovician generating several families that are less diverse when compared to earlier views of Alsataspididae (that included all of these families, excepted Heterocaryonidae). Moreover, our analysis suggests that the bilaminar perforated fringe evolved separately (more than once) in distinct taxa (Trinucleidae, Dionididae and Myindidae). These taxa, despite having similar cephalic morphological innovations did not share equal evolutionary success. Indeed, only Trinucleidae and Raphiophoridae (the latter does not have a perforated fringe but does have a marginal suture) are characteristic of the GOBE (Whiterock fauna). Most derived Dionididae were not as successful (Ibex 2 fauna). However, the systematics of this family need to be examined in more depth and detail to understand better its evolutionary history. Myindidae bearing these features (perforated bilaminar fringe with marginal suture) are restricted to only two genera (Ibex 1 fauna).