Sunday, 13 October 2019

Lessiniabatis aenigmatica: A new species of Stingray from the Eocene of northern Italy.

Stingrays, Myliobatiformes, first appeared during the Early Cretaceous, and underwent a major evolutionary radiation following the End Cretaceous Extinction. The group are still highly successful, with eleven extant families and more than 360 species. However, despite this long evolutionary history, the fossil record of Stingrays is rather poor, with only their teeth being common. This is because Stingrays are Cartilaginous Fish, Chonrichthys, and lack a mineralised skeleton, so that body fossils are only preserved under exceptional circumstances, so that little is known about the total morphology of many fossil Stingray groups.

In a paper published in the journal Scientific Reports on 1 October 2019, Giuseppe Marramà of the Department of Palaeontology at the University of Vienna, Giorgio Carnevale of the Dipartimento di Scienze della Terra at the Università degli Studi di Torino, Luca Giusberti of the Dipartimento di Geoscienze, at the Università degli Studi di Padova, Gavin Naylor, of the Florida Museum of Natural History at the University of Florida, and Jürgen Kriwet, also of the Department of Palaeontology at the University of Vienna, describe a new species of Stingray from the early Eocene Monte Bolca Lagerstätte of northern Italy.

The Monte Bolca Lagerstätte comprises a suite of limestones laid down in the Tethys Ocean between about 50 and 48 million years ago, at the time of the Eocene Thermal Maximum, and subsequently uplifted during the closure of the Tethys Ocean and subsequent formation of the Alps. The limestones extend laterally for about 19 km, with several layers producing marine fossils with exceptional preservation. The Monte Bolca is one of the oldest recognised fossil producing sites in Europe, having been worked since at least the sixteenth century.

The new species is named Lessiniabatis aenigmatica, where 'Lessiniabatis' means 'Ray from Lessinia' after the region where the Monte Bolca Lagerstätte outcrops, and 'aenigmatica' means 'enigmatic', due to the puzzling nature of the fossil. The species is described from three specimens between 375.1 and 568.2 mm in width. Lessiniabatis aenigmatica is unique in having a thoracolumbar synarcual (the extended flattened cartilage that surrounds the spine and gives rays their distinct shape) that extends backwards past the pelvic girdle, forming an extended pectoral disc, combined with a very short tail that does not protrude beyond this disk.

 Lessiniabatis aenigmatica from the Eocene of Bolca Lagerstätte. (a), (b) the paratype MSNFI IGF 103555 in part and counterpart; (c) the paratype MFSN GP.864. Scale bars equal 100 mm. Marramà et al. (2019).

Most new species are described from a holotype, a specimen kept in a museum or university collection (in some living species from groups considered to be conservation priorities this is replaced with genetic material or even photographs). The specimen designated as the holotype of Lessiniabatis aenigmatica by Marramà et al., MNHN F.Bol.566 (previously labelled as 10997–11001) is in the collections of the Museum National d’Histoire Naturelle of Paris, where it is identified as the specimen designated as the holotype of Urolophus crassicaudatus by Henri Marie Ducrotay de Blainville in 1818. However the specimen is nothing like Blainville's description, and it is assumed that the specimens have been mislabled at some point, and the holotype of Urolophus crassicaudatus is lost.

Lessiniabatis aenigmatica from the Eocene of Bolca Lagerstätte. Details of the pelvic girdle and tail region in MNHN F.Bol.566 (a) and MFSN GP.864 (b). Abbreviations: mpt, metapterygium; pg, pelvic girdle; syn2, thoracolumbar synarcual; tv, tail vertebrae. Scale bars equal 50 mm. Marramà et al. (2019).

Lessiniabatis aenigmatica exhibits a unique bodyplan, unlike any other known fossil or living Stingray, while showing enough typical features to be confidently placed in the group (the absence of a rostral cartilage, its dentition, etc.). It lacks a sting on its tail, but this is thought to be a secondary loss and is seen in other Stingray species, such as the Manta Ray and some Eagle Rays. More surprising is the effective loss of the tail as a locomotive organ, as this is the main means of forward propulsion in modern Rays. Marramà et al. suggest that Lessiniabatis aenigmatica must have been more reliant on undulating movement than modern Stingrays, and that its evolution represents a unique evolutionary experiment in the period between the End Cretaceous Extinction and Eocene Thermal Maximum.

Silhouettes of selected living and fossil taxa as representatives for the modern Stingray families and holomorphic fossil taxa. (a) Hexatrygon bickelli (Hexatrygonidae, Sixgill Stingrays); (b) Dasyatis marmorata (Dasyatidae, Whiptail Stingrays); (c) Potamotrygon tigrina (Potamotrygonidae, South American Freshwater Stingrays); (d) Urobatis halleri (Urobatidae, Round Stingrays); (e) Plesiobatis daviesi (Plesiobatidae, Deepwater Stingrays); (f) Urolophus kapalensis (Urolophidae, Stingarees);  (g) Lessiniabatis aenigmatica (this study, Eocene, Monte Bolca, Italy); (h) Asterotrygon maloneyi (Eocene, Green River Formation, Wyoming); (i) Heliobatis radians (Eocene, Green River Formation, Wyoming); (j) Gymnura altavela (Gymnuridae, Butterfly Rays); (k) Promyliobatis gazolai (Eocene, Monte Bolca, Italy); (l) Myliobatis hamlyni (Myliobatidae, Eagle Rays); (m) Aetobatus laticeps (Aetobatidae, Longheaded Eagle Rays); (n) Rhinoptera bonasus (Rhinopteridae, Cownose Rays); (o) Mobula mobular (Mobulidae, Manta Rays). Figures not to scale. Marramà et al. (2019).

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