Diversity, palaeoecology and palaeoenvironmental significance of the Eocene chondrichthyan assemblages of the Bolca Lagerstätte, Italy

Abstract

Over the last few years, the morphology, taxonomy and systematics of the cartilaginous fish taxa of the two main sites of the Bolca Lagerstätte, Italy, (Pesciara and Monte Postale sites) have been extensively discussed in a series of papers, resulting in a complete revision of this neglected component of the Eocene Tethyan ichthyofauna. Here, we provide a comprehensive overview of the diversity, palaeoecology and palaeoenvironmental significance of the two chondrichthyan assemblages of the Pesciara and Monte Postale sites. The assemblages include 14 shark species (Lamniformes and Carcharhiniformes) and batoids (Torpediniformes, Rhinopristiformes, Myliobatiformes, Platyrhinidae and Zanobatidae), as well as a single putative chimaeriform. The Pesciara and Monte Postale sites are characterized by eight chondrichthyan taxa each, but the taxonomic compositions are distinctly different reflecting the dissimilarities in the overall composition of both fish assemblages. Palaeoecological interpretations and habitat preferences of the two chondrichthyan assemblages are consistent with previously hypothesized palaeoenvironmental settings based on sedimentological, palaeontological and geochemical evidence. The chondrichthyan assemblages of the two sites appear to be constituted by ecologically vicariant taxa, with both characterized by a predominance of benthic species with durophagous/cancritrophic feeding modes. Taxonomic composition, habitat preferences and palaeobathymetric analyses support the hypothesis that both assemblages occupied tropical marine shallow waters (likely up to 50 m deep) of the inner portion of the Lessini Shelf. The taxonomic composition of both sites is considerably different from that of any other contemporaneous Tethyan and Boreal chondrichthyan assemblages.

Keywords: Monte Postale; Neoselachii; Pesciara; Tethys; Ypresian; palaeobathymetry.

Fig. 1

Chondrichthyan taxa from the Pesciara site: A, Galerorhinus cuvieri, MCSNV T.1124. B, Brachycarcharias lerichei, MCSNV IG.135779, isolated tooth. C, Eogaleus bolcensis, MCSNV T.415, isolated tooth. D, Tethytrygon muricatus, MNHN F. Bol564, holotype. E, Eorhinobatos primaevus, MGP‐PD 26278, holotype. F, Promyliobatis gazolai, MCSNV VII.B.90, holotype. G, Plesiozanobatus egertoni, MB.f 1608.1. H, Lessiniabatis aenigmatica, MNHN F. Bol.566, holotype. Scale bars: A, D–H = 100 mm; B–C = 2 mm.

Fig. 2

Chondrichthyan taxa from the Monte Postale site: A, Eogaleus bolcensis, MCSNV T.311, holotype. B, Titanonarke megapterygia, MCSNV IG.135576, holotype. C, Titanonarke molini, MCSNV IG.VR.67290. D, Pseudorhinobatos dezigni, NHMW 1853.XXVII.4, holotype. E, Eoplatyrhina bolcensis, MGP‐PD 26279C. F, Brachycarcharias lerichei, MCSNV IG.VR.69800, isolated tooth. G, Arechia crassicaudata, MCSNV IG.VR.27607, neotype. H, Ischyodus sp., dorsal‐fin spine, MCSNV IG.VR.61511. Scale bars: A–E, G = 100 mm; F = 2 mm; H = 20 mm.

Fig. 3

Bathymetric ranges estimated for the taxa of the Pesciara and Monte Postale palaeobiotopes based on the biology of their extant closest relatives. Dark blue indicates the most probably preferred range. Dashed lines indicate that the bathymetric range is greater. Question marks indicate that depth range for Lessiniabatis cannot be inferred, being difficult to accommodate it in any known stingray family.

Fig. 4

A, cluster analysis showing all chondrichthyan assemblages hierarchically grouped together as to reflect their palaeobiogeographical and palaeoenvironmental similarity; numbers on each node are bootstrap values (1000 replicates). B, non‐metric multidimensional scaling plot showing the same pattern. The Sørensen–Dice index was selected in both analyses because we use presence/absence data.

Fig. 5

Schematic presentation of the ecological role of chondrichthyan fishes in the palaeobiotopes of Pesciara and Monte Postale sites. Arrows indicate the direct trophic interactions. Images are courtesy of the Integration and Application Network, University of Maryland Center for Environmental Science (ian.umces.edu/symbols/), and Harvard Library, © [2020] President and Fellows of Harvard College, licenced under a Creative Commons Attribution 4.0 International License.