Phylogeny of the tribe Riorajini with comments on the morphological character evolution within Rajiformes (Chondrichthyes, Batoidea)

The tribe Riorajini is composed of four species, Rioraja agassizii, Atlantoraja castelnaui, A. cyclophora and A. platana, which are endemic to the inshore waters of the Southwestern Atlantic Ocean. Previous studies have hypothesized the monophyly of this group but interrelationships within its members remain not fully understood. In this study, we analysed quantitative as well as qualitative morphological characters related to external morphology, squamation, neurocranium, visceral arches, pectoral and pelvic girdles and claspers, totalizing 82 characters, representing a greater number than previously used in studies focused on rajiform systematics. 24 terminal taxa were included, and two different analyses were carried out testing Narcine brasiliensis and Pseudobatos horkelii as roots of the cladogram. The monophyly of Riorajini is supported by five quantitative and three discrete characters, including a caudal length smaller than disc length, sensory pores and mucous canals darkly pigmented on ventral surface of disc, and preorbital flange present. Clasper morphology was phylogenetically informative since it contributes to diagnose Rioraja in which an accessory terminal 2 cartilage is absent and to support a sister relationship between A. cyclophora + A. platana. Squamation, teeth and neurocranial morphology need to be thoroughly investigated in order to revise their terminology, propose and test hypotheses of homology and improve our knowledge on morphological variation regarding ontogeny and sexual dimorphism.