Carcineretidae is a prehistoric family

Family (biology)

In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...

 of heterotremata

Heterotremata

Heterotremata is a clade of crabs, comprising those crabs in which the genital openings are on the sternum in females, but on the legs in males. It comprises 68 families in 28...

n crustacean

Crustacean

Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...

s. They are only known from Cretaceous

Cretaceous

The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...

 fossil

Fossil

Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...

s. These crab

Crab

True crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" , or where the reduced abdomen is entirely hidden under the thorax...

s are tentatively placed in the superfamily

Taxonomic rank

In biological classification, rank is the level in a taxonomic hierarchy. Examples of taxonomic ranks are species, genus, family, and class. Each rank subsumes under it a number of less general categories...

 Portunoidea

Portunoidea

Portunoidea is a superfamily of crabs that includes the family Portunidae, the swimming crabs. Which other crab families are also placed here is a matter of some contention, and may be revised following molecular phylogenetic analyses.-Description:...

 and resemble the swimming crabs (Portunidae) in having some paddle-shaped pereiopods. But it is not certain that this placement is correct, as the Carcineretidae also show some similarities to the Matutidae

Matutidae

Matutidae is a family of crabs, sometimes called moon crabs, adapted for swimming or digging. They differ from the swimming crabs of the family Portunidae in that all five pairs of legs are flattened, rather than just the last pair, as in Portunidae...

 of superfamily Leucosioidea

Leucosioidea

Leucosioidea is a superfamily of crabs containing the two families Leucosiidae and Iphiculidae....

 and the Goneplacidae

Goneplacidae

Goneplacidae is a family of crabs of the order Decapoda and the superfamily Goneplacoidea. It includes the following genera:*† Amydrocarcinus Schweitzer, Feldmann, Gonzáles-Barba & Vega, 2002*Bathyplax A. Milne-Edwards, 1880*Carcinoplax H...

 of superfamily Xanthoidea

Xanthoidea

Xanthoidea is a superfamily of crabs, comprising the three families Xanthidae, Panopeidae and Pseudorhombilidae. Formerly, a number of other families were included in Xanthoidea, but many of these have since been removed to other superfamilies. These include Carpilioidea, Eriphioidea, Hexapodoidea,...

.

Description

Their carapace

Carapace

A carapace is a dorsal section of the exoskeleton or shell in a number of animal groups, including arthropods such as crustaceans and arachnids, as well as vertebrates such as turtles and tortoises. In turtles and tortoises, the underside is called the plastron.-Crustaceans:In crustaceans, the...

 is usually wider than long and of square

Square (geometry)

In geometry, a square is a regular quadrilateral. This means that it has four equal sides and four equal angles...

 or ovate shape. Its epibranchial region is bulbous, and it bears at least one transverse

Transverse plane

The transverse plane is an imaginary plane that divides the body into superior and inferior parts. It is perpendicular to the coronal and sagittal planes....

 ridge in the protogastric, hepatic or branchial region. Unlike in the living Portunoidea

Portunoidea

Portunoidea is a superfamily of crabs that includes the family Portunidae, the swimming crabs. Which other crab families are also placed here is a matter of some contention, and may be revised following molecular phylogenetic analyses.-Description:...

, the front (the part of the carapace above the head) is narrow; it bears a few blunt spines in some species

Species

In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

. The hind part of the carapace is also narrowed, tapering from just behind the eyes. Around the eyes, also atypical for portunoideans, the carapace makes a wide curve and is beset with spines and notches.

The chelipeds are quite robust – more so than the second to fifth pereiopods – and in some the manus is keeled on the outside. Each "paddle" is formed by the flattened propodus and dactyls of the fourth and/or fifth pereiopods.

At least in males, the first and second sternite are fused, with no suture visible. The suture between sternites 2 and 3 is conspicuous and deep, the one between sternites 3 and 4 incomplete. The eighth sternite is not visible from below. The abdominal of males is narrow, with (probably) completely free somites, and the first to fourth abdominal somite entirely fill the space between the coxae

Arthropod leg

The arthropod leg is a form of jointed appendage of arthropods, usually used for walking. Many of the terms used for arthropod leg segments are of Latin origin, and may be confused with terms for bones: coxa , trochanter , femur, tibia, tarsus, ischium, metatarsus, carpus, dactylus ,...

 of the fifth pereiopods.

Genera

Four genera

Genus

In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 are recognised in the family Carcineretidae:

• Branchiocarcinus Vega, Feldmann & Sour-Tovar, 1995
• Cancrixantho Van Straelen, 1934
• Carcineretes Withers, 1922
• Mascaranada Vega & Feldmann, 1991