Pliocene stratigraphic paleobiology in Tuscany and the fossil record of marine megafauna

Abstract

Tuscany has a rich Pliocene record of marine megafauna (MM), including mysticetes, odontocetes, sirenians and seals among the mammals, and six orders of sharks among the elasmobranchs. This is reviewed with respect to paleogeography and sequence-stratigraphy in six different basins. Conditions at the ancient seafloor are explored by means of sedimentary facies analysis, taphonomy and multivariate techniques applied to a large quantitative dataset of benthic molluscs. MM is rare or absent in most basins during the Zanclean, except in one basin, and most abundant in Piacenzian deposits in all six basins. MM occurs preferentially in fine-grained, shelfal highstand-deposits of small-scale depositional sequences, or at condensed horizons of the maximum flooding interval. It is rare in shallow marine paleonvironments and nearly absent in bathyal paleosettings. Paleogeographic and paleoecological evidence and a comparison with modern patterns of marine upwelling suggest that a wedge of nutrient-rich waters sustained in the offshore during the Pliocene a high biomass of primary producers and a community of apex consumers and mesopredators, similarly to the modern northwestern Mediterranean Sea, with a species-richness higher than the modern and a more complex trophic structure. The highest MM diversity coincides with the mid-Piacenzian warm period, suggesting that facies control does not obscure a link between climate and diversity. We underline however that not all marine environments were suitable for marine mammal preservation. Buoyant carcasses were preferentially dismembered and destroyed in high-energy shallow waters, with the possible exception of delta front deposits, where sudden sediment input occasionally buried pristine carcasses. We hypothesise that carcasses sunken on the seafloor below the shelf break underwent destruction through the activity of a whale-fall biota of modern type, specialised in the consumption of decomposing tissues, both soft and mineralised. A taphonomic window was left between storm wave base and the shelf break. Here water pressure is high enough to prevent the formation of decomposing gases and the resurfacing of carcasses, while the lack of a specialised whale-fall biota slows down bone degradation with respect to deeper settings. Sedimentation rate was high enough to cover skeletal material before its complete destruction. An estimate of paleobathymetries based on multivariate techniques suggests that the preferential depth for the inclusion of MM in the fossil record was 30-300 m. The results are compared with major Mesozoic and Cenozoic MM records worldwide. Available evidence suggests that the late Neogene radiation of large whales, true ecosystem engineers, and their size increase, triggered the radiation of a bone-eating fauna that hampered, and hampers, MM preservation in the deep sea. Stratigraphic paleobiology and an ecosystem-level approach deliver useful insights to the nature of the fossil record.

First record of Monodontidae (Cetacea, Odontoceti) in the Mediterranean Basin from the Pliocene sands of Arcille (Grosseto, Tuscany, Italy)

“…These characteristics include: the depression of the premaxillae in front of the premaxillary sac fossae; the constriction of the medial exposure of the maxilla between the premaxillae just posterior to the anterior end of the external bony nares; the lateral expansion of the left premaxilla anterior to the left premaxillary foramen; the very small size of the external bony nares; the triangular nasals (which are even more slender and posteriorly elongated than in D. leucas). Considering the subtropical climate conditions of the Mediterranean Sea during the early Pliocene (Bianucci et al, 2009;Prista et al, 2015;Dominici et al, 2018), the finding of MSNUP I17602 supports the hypothesis of an ancestral adaptation of Monodontidae to warm climate conditions followed by a dispersion to high-latitude environments which ultimately led to the origin of the modern monodontid genera.…”

Stratigraphic paleobiology of an evolutionary radiation: taphonomy and facies distribution of cetaceans in the last 23 million years

“…The fact that this passage coincides with the onset of the evolutionary radiation of very large baleen whales and with their almost doubling in maximum size (Marx et al, 2016) -which should instead contribute a far better record -must have also triggered the radiation of bone eaters, with the counterintuitive effect of making extremely unlikely for cetacean bones to become preserved. Modern worldwide data confirm that even the largest whale skeletons are rapidly destroyed at deep-water settings (see references in Dominici et al, 2018).…”

“…In the Pliocene of Tuscany articulated specimens and rather complete skeletons are associated with offshore mudstones deposited at an estimated depth of 30-300 m, suggesting that very shallow and very deep (bathyal) depths are generally unfavourable to the preservation of bones (Dominici et al, 2018). An analogous environmental control on the degree of articulation and completeness of fossil cetacean skeletons is also observed in the Pliocene of Emilia and Piedmont, positively correlated with offshore mudstones and biocalcarenite transgressive shell-beds, whereas no skeletons are associated with shoreface sandstones, and rare occurrences in epibathyal mudstones (Freschi & Cau, 2016).…”

“…offer a particularly rich cetacean fossil record and an area where available studies allow to explore this key time of cetacean evolution at a stratigraphic resolution finer than the stage. An increase in cetaceans diversity is recorded around 3.2 -3.0 Ma, in coincidence of the mid-Piacenzian climatic optimum, and a drastic decline at the Piacenzian-Gelasian boundary (Dominici et al, 2018;Freschi et al, in press). Starting from this evidence, the present study aims at exploring if measured taxonomic diversity can be taken at face value, or if it is an artifact of the fossil record, by adding data from the literature of Neogene cetaceans of other geographic areas.…”

The “Brunella” Project: preparation and study of a mysticete from the Early Pliocene of Tuscany

“…Preliminary investigations revealed that: it belonged to the family Balaenopteridae; it does not belong to any of the living taxa; based on the length of its dentary, it was estimated that the total body length of "Brunella" should have been between c. 6 and c. 7.5 m and its weight was c. 5000 kg; the age of this whale was preliminarily assessed based on foraminifers and mollusks and was constrained between 3.7 and 4.5 Ma. "Brunella" was framed in publications on the taphonomy and stratigraphic palaeobiology of baleen whales from Italy (Dominici et al, 2009;2018). Given its degree of completeness and preservation, it is expected that "Brunella" will shed further light on the evolution of balaenopterid mysticetes from the Early Pliocene of the Mediterranean basin.…”