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| Dasyomyliobatis thomyorkei Marramà, Villalobos-Segura, Zorzin, Kriwet & Carnevale, 2023 DOI: 10.1111/pala.12669 Artworks by Fabrizio Lavezzi. |
Abstract
Studies of the origin of evolutionary novelties (novel traits, feeding modes, behaviours, ecological niches, etc.) have considered a number of taxa experimenting with new body plans, allowing them to occupy new habitats and exploit new trophic resources. In the marine realm, colonization of pelagic environments by marine fishes occurred recurrently through time. Stingrays (Myliobatiformes) are a diverse clade of batoid fishes commonly known to possess venomous tail stings. Current hypotheses suggest that stingrays experimented with a transition from a benthic to a pelagic/benthopelagic habitat coupled with a transition from a non-durophagous diet to extreme durophagy. However, there is no study detailing macroevolutionary patterns to understand how and when habitat shift and feeding specialization arose along their evolutionary history. A new exquisitely preserved fossil stingray from the Eocene Konservat-Lagerstätte of Bolca (Italy) exhibits a unique mosaic of plesiomorphic features of the rajobenthic ecomorph, and derived traits of aquilopelagic taxa, that helps to clarify the evolutionary origin of durophagy and pelagic lifestyle in stingrays. A scenario of early evolution of the aquilopelagic ecomorph is proposed based on new data, and the possible adaptive meaning of the observed evolutionary changes is discussed. The body plan of †Dasyomyliobatis thomyorkei gen. et sp. nov. is intermediate between the rajobenthic and more derived aquilopelagic stingrays, supporting its stem phylogenetic position and the hypothesis that the aquilopelagic body plan arose in association with the evolution of durophagy and pelagic lifestyle from a benthic, soft-prey feeder ancestor.
Keywords: durophagy, ecomorph, evolution, Myliobatiformes, pelagic lifestyle, stingray
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| †Dasyomyliobatis thomyorkei gen. et sp. nov. from the Eocene of Monte Bolca (Italy). MCSNV VR.21.107, holotype, dorsoventral view natural normal light. |
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| †Dasyomyliobatis thomyorkei gen. et sp. nov. (MCSNV VR.21.107, holotype) showing its peculiar combination of rajobenthic (A–D) and aquilopelagic (E–H) traits: A, small holaulacorhizous lateral teeth arranged in alternating rows; B, pectoral-fin radials with catenated calcification and no cross-bracing; C, free tail vertebrae without cartilaginous rod and caudal fin reduced to a ventral fold; D, soft, flexible pectoral disc with convex anterior and posterior margins; E, head protruding from pectoral disc; F, cephalic lobes; G, wing-like pectoral disc with positive FRD; H, enlarged hexagonal symphyseal/parasymphyseal polyaulacorhizous teeth in pavement-like arrangement. Scale bar represents 100 mm. |
SYSTEMATIC PALAEONTOLOGY
Class CHONDRICHTHYES Huxley, 1880
Superorder BATOIDEA Compagno, 1973
Order MYLIOBATIFORMES Compagno, 1973
Family †DASYOMYLIOBATIDAE nov.
Genus †DASYOMYLIOBATIS nov.
Type species: †Dasyomyliobatis thomyorkei sp. nov.
Derivation of names: Genus and family names refer to the peculiar mosaic of dasyatoid and myliobatoid traits.
†Dasyomyliobatis thomyorkei sp. nov.
Derivation of name: After the British composer and musician Thom Yorke.
Holotype: MCSNV VR.21.107/8 well preserved, complete, and articulated skeleton in part and counterpart, 99.9 cm disc width.
Type locality & horizon: Pesciara site, Bolca Konservat-Lagerstätte, Italy; early Eocene, late Ypresian, middle Cuisian, SBZ-11, †Alveolina dainelli Zone, c. 50 Ma (Papazzoni et al. 2014).
Diagnosis: Stingray unique in having a peculiar combination of dasyatoid and myliobatoid traits. Like dasyatoids, †Dasyomyliobatis has a soft and flexible pectoral disc with convex anterior and posterior fin margins supported by catenated radials with no cross-bracing; numerous (c. 40) labiolingually directed files of holaulacorhizous lateral teeth arranged in alternating rows; tail formed by free vertebrae not stiffened by a cartilaginous rod; caudal fin reduced to a ventral fold. Like myliobatoids, †Dasyomyliobatis shows a head protruding anterior to a wing-like pectoral disc; cephalic lobes contacting along their mesial edge forming a single, shovel-like structure; cephalic-lobe radials discontinuous with pectoral-fin radials; moderately enlarged hexagonal symphyseal/parasymphyseal polyaulacorhizous teeth in pavement-like arrangement, with bulbous/irregular interlocking mechanism, 3–4 wide-block and irregularly spaced root lobes. In addition, †Dasyomyliobatis has a pectoral-fin AR between 2.0 and 3.0, and positive FRD.
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| Hypothetical reconstruction of †Dasyomyliobatis thomyorkei gen. et sp. nov. in different views. Artwork by Fabrizio Lavezzi. |
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| Life reconstruction of †Dasyomyliobatis thomyorkei gen. et sp. nov. swimming in the marine tropical shallow waters of the western Tethys about 50 million years ago. Artwork by Fabrizio Lavezzi. |
CONCLUSIONS:
The unique body plan of †Dasyomyliobatis suggests that:
• †Dasyomyliobatis is a representative of a new stingray family with unique hybrid dentition and pectoral-fin morphology that allowed the shift from undulatory to oscillatory swimming, and to exploit a variety of prey (from soft-bodied to hard-shelled organisms).
• The evolutionary origin of durophagy and pelagic lifestyle was achieved through gradual transformation of morphological traits at least since the early Late Cretaceous.
• Cephalic lobes were already present in a stingray without crustal calcification, cross-bracing, or compagibus laminam corroborating the hypothesis that they originated before the shift toward exclusive oscillatory locomotion and occupation of pelagic environments.
• The phylogenetic analyses highlight that evolutionary modifications of the dentition related to a shift toward extreme durophagy seem to covariate with the evolution of traits of pectoral skeleton related to a shift toward a pelagic lifestyle, possibly reflecting high level of integration. One can speculate that although invasion of new habitats allowed initially stingrays to exploit wider ranges of food items, competition with other benthic batoids like skates, known to occupy today the same niche as benthic stingrays in deeper and cooler waters (Ebert & Compagno 2007) but also warm and shallow waters during the Late Cretaceous (Cappetta 1980), pushed pelagic/benthopelagic stingrays to specialize for a less exploited food resource, like hard-shelled invertebrates, although this hypothesis needs to be tested.
Giuseppe Marramà, Eduardo Villalobos-Segura, Roberto Zorzin, Jürgen Kriwet and Giorgio Carnevale. 2023. The Evolutionary Origin of the durophagous Pelagic Stingray Ecomorph. Palaeontology. DOI: 10.1111/pala.12669
