Abstract
With approximately 1,500 extant species, freshwater crabs (Decapoda: Brachyura) are among the most diverse decapod crustaceans. Nevertheless, their fossil record is extremely limited: only Potamidae, Potamonautidae and Trichodactylidae are reported up to the Eocene of the Neotropics so far. This work documents unusually large decapod claws from the Upper Cretaceous (Campanian) continental deposits of Velaux and vicinity (southern France), in close association with large vertebrate remains. In addition to (1) the systematic assignment of these claws, the study addresses (2) the salinity trends in the deposit environment from its faunal assemblage and the elementary chemical patterns of fossils, and (3) the likely scenario for their auto/allochthony in the Velaux fluvial system. These claws belong to a new taxon, Dinocarcinus velauciensis n. gen. n. sp., referred to as Portunoidea sensu lato, a group of “true” crabs nowadays linked to marine systems. However, the faunal assemblage, the claw taphonomy and the carbonates Y/Ho signatures support their ancient freshwater/terrestrial ecology, making them the oldest reported continental brachyurans and extending the presence of crabs in freshwater environments by 40 Ma. Either as primary or as secondary freshwater crabs, the occurrence of these portunoids in Velaux is an evidence for the independent colonizations of continental environments by multiple brachyuran clades over time, as early as the Campanian.
Highlights
With approximately 1,500 extant species, freshwater crabs (Decapoda: Brachyura) are among the most diverse decapod crustaceans
The present paper reports the remains of brachyuran crabs from fluvial Late Cretaceous localities of southern France (Velaux-La Bastide Neuve and vicinity), fossilized in association with vertebrate remains
If direct comparisons with extant taxa is straightforward and often helpful for identifying Pliocene and Pleistocene brachyurans[33,34,35,36], this task becomes more complicated when working with material of Miocene age[37,38,39,40]
Summary
With approximately 1,500 extant species, freshwater crabs (Decapoda: Brachyura) are among the most diverse decapod crustaceans. Close associations of different and diverse fossil organisms may both (1) be the result of a long-distance transport of allochthonous remains or (2) testify of local biocoenoses for which members of quite restricted ecosystems have deposited altogether These claws are of exceptional large size compared to most Late Cretaceous marine crab claws; and interestingly do not conform to the morphology of any extant freshwater crab family. The study aims at (1) proposing a systematic assignment for these claws, (2) characterizing the actual salinity trend of their depositional environment, based on the channel fauna assemblage and elementary chemical patterns of fossils and (3) identifying the relevant taphonomic scenario for the presence of crab claws within a fluvial system As all these approaches support a freshwater or terrestrial signature for the paleoenvironment of these large-clawed brachyurans; we discuss the implications for presumed multiple invasions of freshwater habitats by crustacean decapods over time
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