Taxonomy and morphology of the skate genus Atlantoraja (Rajiformes: Arhynchobatidae)

Abstract

The skate genus Atlantoraja is composed of three species (A. castelnaui, A. cyclophora, and A. platana) which differ from the other Riorajini species, Rioraja agassizii, in regards to their clasper features, squamation and presence of a caudal fin. Despite of being distributed along Southwestern Atlantic and commonly captured by fisheries in Brazil, Uruguay and Argentina, detailed accounts on external and internal morphology are scarce and the taxonomic status of Atlantoraja species was not revised so far. The aim of this study, therefore, is to review the taxonomy of the genus Atlantoraja, updating information on type specimens and clarifying misidentifications among species, and to describe in detail anatomical structures such as neurocranium, visceral arches, pelvic girdle, dermal denticles and teeth. Lectotypes and paralectotypes are designated for Atlantoraja castelnaui and A. cyclophora. Notes on intraspecific variation within species are also provided. Atlantoraja cyclophora and A. platana are more similar to each other than to A. castelnaui in regards to the squamation (body dorsal surface smooth vs. densely covered by prickles) and color pattern of body dorsal surface, position of orbital foramina, overall shape of neucrocranium and body measurements. Lastly, we discuss the morphological differences among species of Atlantoraja and recommend the inclusion of characters presented here in future cladistic analyses.

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

“…Similarities between A. cyclophora and A. platana were described by previous authors (Barbosa & Gomes, 1998;Soares, Moreira, Silva, et al, 2021) and a closer relationship between both species was already recovered by Naylor et al (2012), Coelho et al (2020) and Crobe et al (2021). Atlantoraja castelnaui differs from its congeners by having a rougher dorsal surface with prickles on the interorbital region and thorns around the eye margins, nuchal and scapular regions (characters 13, 15 and 16), preorbital process poorly developed (45, 1) and anterior fontanelle with a straight posterior margin (50, 1).…”

“…52. Relative position of the foramen for the passage of the trochlear nerve (IV): (0) near to the optic foramen, (1) distant from the optic foramen [compiled from Soares, Moreira, Silva, et al, 2021] (CI = 20; RI = 33).…”

“…53. Relative position of the anterior cerebral vein: (0) dorsal or at the same level to the optic nerve foramen, (1) at the same level, (2) ventral to the optic nerve foramen [compiled from Soares, Moreira, Silva, et al, 2021] (CI = 22; RI = 36).…”

“…According to McEachran and Miyake (1990) and McEachran and Dunn (1998), the monophyly of the tribe is supported by four synapomorphies: the presence of a flange extending from the preorbital process to the supraorbital crest (preorbital process continuous with the supraorbital crest), dorsal surface of the disc free of dermal denticles (contrasting to the description of A. castelnaui provided by Soares, Moreira, Silva, et al, 2021), alar thorns strongly reclinated laterally and scapulocoracoid with slender anterior bridge.…”

“…Moreover, some characters analysed in that study are not independent, or their state characters are not mutually exclusive, requiring a reexamination and reinterpretation of these. Detailed descriptions on anatomical structures and ontogenetic changes in species of Rioraja and Atlantoraja were recently provided (Moreira et al, 2017; Soares et al, 2020; Soares, Moreira, & Gomes, 2021; Soares, Moreira, Silva, et al, 2021) but phylogenetic relationships within the tribe remain not fully understood since no cladistic analysis including morphological characters has ever been performed.…”