The Achelata is an infra-order of the

decapod The Decapoda or decapods, from Ancient Greek δεκάς (''dekás''), meaning "ten", and πούς (''poús''), meaning "foot", is a large order of crustaceans within the class Malacostraca, and includes crabs, lobsters, crayfish, shrimp, and p ...

crustacean Crustaceans (from Latin meaning: "those with shells" or "crusted ones") are invertebrate animals that constitute one group of arthropods that are traditionally a part of the subphylum Crustacea (), a large, diverse group of mainly aquatic arthrop ...

s, holding the spiny lobsters,

slipper lobster Slipper lobsters are a family (biology), family (Scyllaridae) of about 90 species of Achelata, achelate crustaceans, in the Decapoda clade Reptantia, found in all warm oceans and seas. They are not lobster, true lobsters, but are more closely rel ...

s and their fossil relatives.

Description

The name "Achelata" derives from the fact that all the members of this group lack the

chelae A chela ()also called a claw, nipper, or pinceris a pincer-shaped organ at the end of certain limbs of some arthropods. The name comes from Ancient Greek , through Neo-Latin '. The plural form is chelae. Legs bearing a chela are called chelipeds ...

(claws) that are found on almost all other decapods (from the

Ancient Greek Ancient Greek (, ; ) includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Greek ...

, = "not", , = "claw"). They are further united by the great enlargement of the second antennae, by the special "

phyllosoma The phyllosoma is the larval stage of spiny and slipper lobster (Palinuridae and Scyllaridae), and represents one of the most significant characteristics that unify them into the taxon Achelata. Its body is remarkably thin, flat, and transpare ...

" form of the

larva A larva (; : larvae ) is a distinct juvenile form many animals undergo before metamorphosis into their next life stage. Animals with indirect development such as insects, some arachnids, amphibians, or cnidarians typically have a larval phase ...

, and by a number of other characters. Larva de phyllosoma

Classification and fossil record

The

infraorder Order () is one of the eight major hierarchical taxonomic ranks in Linnaean taxonomy. It is classified between Family_(biology), family and Class_(biology), class. In biological classification, the order is a taxonomic rank used in the classific ...

Achelata belongs to the group

Reptantia Reptantia is a clade of decapod crustaceans named in 1880 which includes lobsters, crabs and many other well-known crustaceans. Classification In older classifications, Reptantia was one of the two suborders of Decapoda alongside Natantia, w ...

, which consists of the walking/crawling decapods (lobsters and

crabs Crabs are decapod crustaceans of the infraorder Brachyura (meaning "short tailed" in Greek), which typically have a very short projecting tail-like abdomen, usually hidden entirely under the thorax. Their exoskeleton is often thickened and ha ...

). The cladogram below shows Achelata's placement within the larger

order Order, ORDER or Orders may refer to: * A socio-political or established or existing order, e.g. World order, Ancien Regime, Pax Britannica * Categorization, the process in which ideas and objects are recognized, differentiated, and understood ...

Decapoda The Decapoda or decapods, from Ancient Greek δεκάς (''dekás''), meaning "ten", and πούς (''poús''), meaning "foot", is a large order of crustaceans within the class Malacostraca, and includes crabs, lobsters, crayfish, shrimp, a ...

, from analysis by Wolfe ''et al.'', 2019. Achelata contains the spiny lobsters (Palinuridae), the

s (Scyllaridae) and the furry lobsters (Synaxidae, now usually included in Palinuridae), as well as two extinct families,

Cancrinidae The Achelata is an infra-order of the Decapoda, decapod crustaceans, holding the spiny lobsters, slipper lobsters and their fossil relatives. Description The name "Achelata" derives from the fact that all the members of this group lack the chela ...

and

Tricarinidae ''Tricarina'' is an extinct genus of crustaceans in order Isopoda, known from a single incomplete fossil specimen from the Cretaceous of western Iran. It has a flattened body with three longitudinal ridges, which give it its name. Sources The si ...

. Both Palinuridae and Scyllaridae have a

fossil record A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

extending back to the

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...

. The two fossil families contain a single genus each; '' Tricarina'' is known from a single Cretaceous fossil, while '' Cancrinos'' is known from the

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 143.1 Mya. ...

and Cretaceous. One estimate of the divergence between Achelata and its closest relatives places it at about .

References

External links

* {{Taxonbar, from=Q1191353 Arthropod infraorders Extant Late Jurassic first appearances