|Protohimantura vorstmani (De Beaufort, 1926)|
Marramà, Klug, de Vos & Kriwet, 2018
The early Miocene stingray †Trygon vorstmani represented by a single specimen collected from the fish-bearing limestones of the Tonasa Formation of SW Sulawesi, Indonesia, is redescribed here in detail. This taxon exhibits a unique combination of features that clearly support the presence of a new genus, †Protohimantura gen. nov. and its assignment to the whiptail stingrays (Dasyatidae) of the subfamily Urogymninae. The morphological and phylogenetic affinities of †Protohimantura gen. nov. with the living whiprays suggest a close association of this taxon with tropical shallow-water habitats hypothesized for the SW Sulawesi palaeoenvironment during early Miocene. Moreover, this occurrence, which also represents the first holomorphic stingray specimen from the Neogene, provides new insights into the role of the Indo-Australian Archipelago for the evolutionary history of fishes associated with reefs in the context of the shift of the marine biodiversity hotspot across the globe during the last 50 million years.
Class Chondrichthyes Huxley, 1880
Superorder Batomorphii Cappetta, 1980
Order Myliobatiformes Compagno, 1973
Family Dasyatidae Jordan, 1888
Subfamily Urogymninae Gray, 1851 (Sensu; Last Et Al., 2016B)
Genus †Protohimantura Gen. Nov.
Type species: †Trygon vorstmani de Beaufort, 1926.
Etymology: From the Ancient Greek word prōto, meaning ‘first’, ‘foremost’, ‘earliest form of’, and Himantura, one of the living whipray genera, thus indicating a possible close relationship between both taxa.
Diagnosis: A whipray characterized by the following combination of characters and body proportions: eye small; interorbital width/eye diameter ratio of 3.5; nasal capsule width/neurocranial length ratio of 0.7; nasal capsule length/neurocranial length ratio of 0.2; anteroposterior fontanelle/neurocranial length ratio of 0.8; scapulocoracoid width/lateral face length ratio of 2.2; 55 propterygial radials; 17 mesopterygial radials; mid-dorsal surface of disc covered by heart-shaped denticles arranged in an antero-posteriorly directed patch having sharply defined outlines; teeth with semi-ovoid or subhexagonal crown with a second transverse keel; lingual and labial crown ornamentation absent.
Remarks: The species †Trygon vorstmani was created by de Beaufort (1926) who presented a short description (one page long) and figured this single specimen in part and counterpart, which was previously collected by Professor Brouwer at the beginning of the 20th century near Patoenoeang Asoe E in the Maros district of SW Sulawesi, Indonesia. The placement of this taxon in the family Dasyatidae [= Trygonidae of de Beaufort (1926)] was based on the presence of a propterygium that is bent inwards in front to the median line and pectorals of both sides meeting at the snout (de Beaufort, 1926). However, after this first brief report, no in-depth morphological analysis or identification of characters was provided to distinguish the specimen from other extant or extinct rays, with the exception of a preliminary study by Klug & Kriwet (2012) who recognized its close relationship with the genus Himantura. However, at present, Trygon is regarded as a junior synonym of Dasyatis Rafinesque, 1810, and the Sulawesi species shows several morphological features that distinguish it from Dasyatis, Himantura and all representatives of the family Dasyatidae (see Description and Discussion). On the contrary, the morphological characters observed in the examined specimen and discussed below corroborate the erection of a new genus to contain †Trygon vorstmani and its inclusion in the subfamily Urogymninae.
†Protohimantura vorstmani (De Beaufort, 1926)
†Trygon vorstmani de Beaufort, 1926: p. 119, pl. 1 (original occurrence of name, photograph and outline reconstruction); de Beaufort, 1931: p. 462.
†Himantura vorstmani (de Beaufort, 1926); Klug & Kriwet, 2012: p. 93.
Holotype: RGM 624420, single specimen in part and counterpart, lacking the posterior region of body.
Type locality and horizon: Patoenoeang Asoe E, Maros District, SW Sulawesi, Indonesia; Tonasa Formation, ?Burdigalian, early Miocene (see: Wilson, 2000; Wilson et al., 2000).
Although the early Miocene stingray from Sulawesi lacks portions of the posterior body, including the tail and the characteristic spines, several features are preserved and allow identification as a new representative of the family Dasyatidae, subfamily Urogymninae, and the creation of a new genus, †Protohimantura. A monophyletic family Dasyatidae is recovered based on the parsimony analyses. The phylogenetic analysis recovered a dichotomous nature of the relationships of the Myliobatiformes, which might reflect a phylogenetic signal in the nature of calcification of their pectoral radials, in their body shape and, consequently, in their swimming style. The analysis of the fossil record of the Urogymninae seems to suggest that the modern distribution of whiprays is the final result of their spatial dynamics across the Palaeogene and consistent, at least in part, with the eastward shift of the marine centre of palaeobiodiversity across the globe during the last 50 million years.
Giuseppe Marramà, Stefanie Klug, John de Vos and Jürgen Kriwet. 2018. Anatomy, Relationships and Palaeobiogeographic Implications of the First Neogene Holomorphic Stingray (Myliobatiformes: Dasyatidae) from the early Miocene of Sulawesi, Indonesia, SE Asia. Zoological Journal of the Linnean Society. zly020. DOI: 10.1093/zoolinnean/zly020