''Eocarcinus praecursor'' is a

Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The J ...

species of decapod

crustacean Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacean group ...

, sufficiently distinct from its relatives to be placed in its own

family Family (from la, familia) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its ...

(Eocarcinidae). Often considered the oldest true

crab Crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" (abdomen) ( el, βραχύς , translit=brachys = short, / = tail), usually hidden entirely under the thorax. They live in all the ...

, it was considered by a 2010 study to be an early member of the

Anomura Anomura (sometimes Anomala) is a group of Decapoda, decapod crustaceans, including hermit crabs and others. Although the names of many anomurans include the word ''crab'', all true crabs are in the sister group to the Anomura, the Brachyura (the ...

. However, a reanalysis in 2020 again found it to be the earliest known

stem-group In phylogenetics, the crown group or crown assemblage is a collection of species composed of the living representatives of the collection, the most recent common ancestor of the collection, and all descendants of the most recent common ancestor. ...

crab.

Distribution

It lived during the early

Pliensbachian The Pliensbachian is an age of the geologic timescale and stage in the stratigraphic column. It is part of the Early or Lower Jurassic Epoch or Series and spans the time between 190.8 ± 1.5 Ma and 182.7 ± 1.5 Ma (million years ago). The Plien ...

age (

Lower Jurassic The Early Jurassic Epoch (in chronostratigraphy corresponding to the Lower Jurassic Series) is the earliest of three epochs of the Jurassic Period. The Early Jurassic starts immediately after the Triassic-Jurassic extinction event, 201.3 Ma&nb ...

), and has been found in rocks at two sites in the

United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a country in Europe, off the north-western coast of the continental mainland. It comprises England, Scotland, Wales and North ...

– Mickelton Tunnel (near

Aston Magna Aston Magna is a hamlet located off Fosse Way in north Gloucestershire, England, between Moreton-in-Marsh and Shipston-on-Stour. It forms part of the civil parish of Blockley and the Church of England parish of Blockley with Aston Magna in the ...

Gloucestershire Gloucestershire ( abbreviated Glos) is a county in South West England. The county comprises part of the Cotswold Hills, part of the flat fertile valley of the River Severn and the entire Forest of Dean. The county town is the city of Gl ...

and

Runswick Bay Runswick Bay is a bay in the Scarborough Borough of North Yorkshire, England. It is also the name of a village located on the western edge of the bay (although the village is sometimes shortened to Runswick on UK road signs). It is north of ...

Yorkshire Yorkshire ( ; abbreviated Yorks), formally known as the County of York, is a Historic counties of England, historic county in northern England and by far the largest in the United Kingdom. Because of its large area in comparison with other Eng ...

Description

In many of its characters, it represents a transitional stage between the

Glypheoidea The Glypheoidea (containing the glypheoid lobsters), is a group of lobster-like decapod crustaceans which forms an important part of fossil faunas, such as the Solnhofen limestone. These fossils included taxa such as ''Glyphea'' (from which the ...

and the

Middle Jurassic The Middle Jurassic is the second epoch of the Jurassic Period. It lasted from about 174.1 to 163.5 million years ago. Fossils of land-dwelling animals, such as dinosaurs, from the Middle Jurassic are relatively rare, but geological formations co ...

crabs in the Prosopidae. Since its ancestors were long-tailed

decapods The Decapoda or decapods (literally "ten-footed") are an order of crustaceans within the class Malacostraca, including many familiar groups, such as crabs, lobsters, crayfish, shrimp and prawns. Most decapods are scavengers. The order is estim ...

, and its successors were short-tailed crabs, ''Eocarcinus'' has been described as "the lobster who decided to become a crab". Previously considered to be the oldest known true

, a 2010 revision concluded that ''Eocarcinus'' could not be accommodated among the Brachyura, and was instead transferred to the

. However, a 2020 renalysis found that it was again the earliest known stem-group crab, but that it had not undergone the process of

carcinisation Carcinisation (or carcinization) is an example of convergent evolution in which a crustacean evolves into a crab-like form from a non-crab-like form. The term was introduced into evolutionary biology by L. A. Borradaile, who describe ...

References

{{Taxonbar, from=Q774109 Anomura Jurassic crustaceans Monotypic arthropod genera Fossils of Great Britain