A new stingray from the miocene of río negro and a review of the fossil myliobatiformes of Argentina

Abstract

We analyze a dental plate belonging to Myliobatiformes (Chondrichthyes, Batoidea) from the Gran Bajo del Gualicho Formation (Saladar Member; Upper Early Miocene-Lower Middle Miocene) located in Gran Bajo del Gualicho locality, Río Negro Province, Argentina. The specimen analyzed (MAPBAR 3671.26) represents the first record of an associated, partially articulated dental plate of Myliobatiformes in Argentina, whose informative nature invites us to reconsider the fossil record of the group. MAPBAR 3671.26 consists of a partially complete lower dental plate, measuring 11 cm maximum labiolingual length, 8.2 cm maximum transverse width, and 2.4 cm maximum occlusal-basal height. This plate is characterized by the presence of: 1) teeth of regular basoapical thickness and hexagonal outline and with a medial row much wider than the lateral ones; 2) a labio-lingually extended medial depression resulting in a ‘lip’ shaped plate in labial/lingual views; 3) slightly curved and ‘M’ shaped medial teeth; 4) longitudinal medial groove and; 5) ridges or undulations on its occlusal surface; and 6) polyaulacorhizous root in the medial teeth. All these characteristics shows that MAPBAR 3671.26 is related with Myliobatidae. Furthermore, the specimen was subjected to a phylogenetic analysis, wherein it is incorporated into an existing data matrix that includes both current and fossil specimens, along with the inclusion of new characters and taxa (Myliobatis magister and Myliobatis crassus). These two shows a similar morphology and comes from coetaneous age than MAPBAR 3671.26. In the analysis, MAPBAR 3671.26 was obtained within Myliobatiformes, and specifically, in Myliobatidae. On the other hand, MAPBAR 3671.26 was recovered forming a trichotomy with Myliobatis magister and Myliobatis crassus. These taxa share unique characteristics (traits 2, 4); and thus, differ from the rest of the myliobatid genera. For this reason, we consider that the dental plates of MAPBAR 3671.26, Myliobatis crassus and Myliobatis magister should be grouped within a new generic entity. Additionally, a brief review of the records of Myliobatiformes in Argentina was carried out. Identifying the presence of: 1) Hypolophodon patagoniensis (Fm. Lefipán, Chubut; Fm. Jagüel, Río Negro. 2) Myliobatis sp. (Fm. Paraná, Entre Ríos; “Puelchense” locality, Punta Indio and Punta Médanos, Buenos Aires; and Fm. Pampeano, Buenos Aires). 3) Myliobatidae indet (Río Chico locality, Chubut; San Julián locality, Santa Cruz) and 4) MAPBAR 3671.26 (Fm. Gran Bajo del Gualicho, Río Negro).