MIDDLE AND UPPER JURASSIC BRACHYURAN CRABS – PHYLOGENETIC AND PALAEOENVIRONMENTAL SIGNIFICANCE OF THEIR EARLY EVOLUTIONARY STAGE

Our knowledge on early, Mesozoic crabs is still rudimentary, mainly due to their poor preservational potential of crustacean decapods. Fossil evidence suggests that the family Prosopidae is ancestral to all other brachyurans including Dromiacea and Eubrachyura, with very close phylogeny relation to Homolodromiidae, Dromiidae, Homolidae, Latreillidae, Dynomenidae, Xantidae, Cyclodorippoidea and Calappoidea. Prosopidae is an extinct family, consisting mostly of Mesozoic species, almost exclusively known by carapace. This family appeared in the late Early Jurassic (Late Pliensbachian) but disappeared at the end of the Danian. The Early Jurassic (Pliensbachian) oldest true 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 - Upper Pliensbachian). The mode of life of Middle Jurassic prosopids, and therefore, brachyuran evolution started on moderately deep, soft bottom environment. This oldest prosopid species, lived on a silty sea floor, as did the first known crab (late Lower Pliensbachian) and could be similar to that of their presumed ancestors (Pemphicidae) that were probably also shallow water organisms. The Middle Jurassic prosopids lived both in shallow warm waters within organic buildups and silty sea floor of black shale sedimentation of Bajocian/Bathonian age (new example from central Poland). Prosopids had their climax during the Late Jurassic time with a wide distribution in cyanobacterial-sponge (Oxfordian) buildups and/or coral reef (Kimmeridgian-Tithonian) environments of Europe (Pithonoton, Coelopus, Longodromites, Prosopon, Foersteria, Nodoprosopon, Lecythocaris, Glaessneropsis).

Research has been financially supported by the AGH (MK - grant No. 11.11.140.888)