Middle and Late Jurassic roots of brachyuran crabs: Palaeoenvironmental distribution during their early evolution

Abstract

Knowledge of early, Mesozoic crabs is still rudimentary, mainly due to the poor preservational potential of crustacean decapods. Fossil evidence suggests that the family Prosopidae is ancestral to all other brachyurans, including Podotremata, with a very close phylogenetic relationship to Homolodromiidae, Dromiidae, Homolidae, Latreillidae, Dynomenidae, Xanthidae, Cyclodorippoidea and Calappoidea. Prosopidae is an extinct family, consisting mostly of Mesozoic species, almost exclusively known by their carapaces. This family appeared in the late Early Jurassic (Late Pliensbachian) and disappeared at the end of the Danian. The Early Jurassic (Pliensbachian) oldest crab species, Eocarcinus praecursor Withers, is transitional in many of its observable traits between the macruran Glypheoidea (Middle Triassic Pseudopemphix) and the early brachyuran prosopids, especially the earliest known species, Eoprosopon klugi Förster (Late Pliensbachian). Middle Jurassic prosopids lived in shallow-sea, soft bottom environments. The oldest known prosopid species lived on a silty sea floor, as did the first known crab (late Early Pliensbachian), and their presumed ancestors (Pemphicidae) were probably also shallow water organisms. Middle Jurassic prosopids lived both in shallow warm waters within organic buildups/shelly accumulations, and on silty sea-floors during the Bajocian/Bathonian, as illustrated by a new example from central Poland. Prosopids had an evolutionary climax during the Late Jurassic and were widely distributed in sponge-microbial (Oxfordian) buildups and coral reef (Kimmeridgian–Tithonian) environments of Europe ( Pithonoton, Coelopus, Longodromites, Prosopon, Nodoprosopon, Lecythocaris, Glaessneropsis). When biohermal and reef facies retreated at the end of the Jurassic, favourable conditions for prosopid crabs diminished and Cretaceous prosopids are rare and spatially dispersed. Their closely related descendants, the homolodromiids, preferentially inhabited soft muddy bottoms in deeper, colder waters, as is well documented by Cenozoic and Recent occurrences.