Reassessment of the Order Trinucleida (Trilobita)

Abstract

Trinucleoids have been considered as a superfamily of the Order Asaphida, based on: a pre-occipital glabellar tubercle, a cephalic median suture in most ancestral forms, and the presence of broadly similar globular protaspid larvae in the life cycle. Recent discoveries cast doubts on the monophyly of the two last characters. This study provides a phylogenetic analysis whose main purpose is to test the affiliation of trinucleoids. The topology of the strict consensus tree obtained indicates that trinucleoids share a more recent common ancestor with members of the Order Ptychopariida than they do with any asaphid. The stratigraphically higher and most derived trinucleoids are well defined and easy to recognize. Their basal representatives can also be identified with confidence, in spite of substantial variation in the diagnostic characters of the group. These early trinucleoids are similar enough to the typical basal ‘ptychopariid’ bauplan to suggest ancestry within that diffuse and poorly defined trilobite order. Indeed, Ptychopariida is presently regarded as being paraphyletic. We consider that raising trinucleids to ordinal status, by proposing the Order Trinucleida, will make it easier to decipher higher level relationships among trilobites. This action, sustained by improved morphological and ontogenetic information of Cambro-Ordovician trilobites, reveals convergences of the three characters that have been used to support the inclusion of trinucleids within Asaphida. Based on this new information, we propose six distinct morphotypes of commutavi protaspis and, as a probable sensorial organ, that the cephalic axial tubercle migration might be caused by an increase in the height of the glabella and thus may not bear an important systematic signal. Moreover, the tree topology provided herein offers a preliminary overview of evolution within the order. Alsataspididae and Raphiophoridae are paraphyletic, suggesting that more investigation is needed to define them clearly. Dionidae appears to be more closely related to Raphiophoridae than to Trinucleidae, suggesting that the character state of ‘a perforated fringe’ evolved more than once.