Abstract

Hypermecaspidinae (Trilobita; Olenidae) constitutes a monophyletic clade but its internal relationships are unknown and its phylogenetic position with respect to the rest of Olenida has been recently questioned. In order to propose a phylogenetic hypothesis and to resolve the status of the group, a cladistic analysis of the hypermecaspidinids was performed. One hundred characters (28 quantitative and 72 qualitative) of holaspid morphology were coded for 30 taxa (18 Hypermecaspidinae). Two groups of analysis were performed considering the percentage of missing data. For each group, discrete partition was treated as totally unordered or with some characters that describe intensity, position or shape of certain structures, as ordered. The analyses were performed with TNT using maximum parsimony and implied weighting as optimality criteria. Hypermecaspididae constitutes a monophyletic family within Olenida, sister of pterocephaliids. In the light of the present analysis, an emended diagnosis of the family is provided. The results suggest that “Tropidopyge” broeggeri should be reassigned to Hypermecaspis. Hypermecaspidids would have originated in the late Furongian and rapidly diversified across different parts of the world in the Early Ordovician. With the exception of H. armata and H. chamarrensis, the species of Hypermecaspis exhibit a proportionately large pygidium with a wide doublure and a distinct postaxial field. An evolutionary trend to increase the size of these characters is observed in the genus. The cranidium shows a tendency to enlarge the glabella and the palpebral lobes, together with a narrowing of the interocular genae.