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

BIGNON, A.; VACCARI, N. E; WAISFELD, BEATRIZ G.; CHATTERTON, B.D.E.

Virtual

Encuentro; Zooming in on the GOBE 2020 Virtual Annual Meeting; 2020

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