Euthycarcinoidea are an enigmatic group of extinct possibly amphibious

arthropod Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chitin, o ...

s that ranged from

Cambrian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ago ( ...

Triassic The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest period ...

times. Fossils are known from Europe, North America, Argentina, Australia and Antarctica.

Description

The euthycarcinoid body was divided into a cephalon (head), preabdomen, and postabdomen. The cephalon consisted of two segments and included

mandibles In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower teeth in place. The mandible sits beneath the maxilla. It is the only movable bone ...

, antennae and presumed eyes. The preabdomen consisted of five to fourteen

tergites A ''tergum'' (Latin for "the back"; plural ''terga'', associated adjective tergal) is the dorsal ('upper') portion of an arthropod segment other than the head. The anterior edge is called the 'base' and posterior edge is called the 'apex' or 'ma ...

, each having up to three

somites The somites (outdated term: primitive segments) are a set of bilaterally paired blocks of paraxial mesoderm that form in the embryonic stage of somitogenesis, along the head-to-tail axis in segmented animals. In vertebrates, somites subdivide in ...

. Each somite had in turn a pair of

uniramous The arthropod leg is a form of jointed appendage of arthropods, usually used for walking. Many of the terms used for arthropod leg segments (called podomeres) are of Latin origin, and may be confused with terms for bones: ''coxa'' (meaning hip, plu ...

, segmented legs. The postabdomen was limbless and consisted of up to six segments and a terminal tail spine.

Affinities

Due to its particular combination of characteristics, the position of the Euthycarcinoidea within the Arthropoda has been ambiguous; previous authors have allied euthycarcinoids with crustaceans (interpreted as

copepod Copepods (; meaning "oar-feet") are a group of small crustaceans found in nearly every freshwater and saltwater habitat. Some species are planktonic (inhabiting sea waters), some are benthic (living on the ocean floor), a number of species have p ...

branchiopod Branchiopoda is a class of crustaceans. It comprises fairy shrimp, clam shrimp, Diplostraca (or Cladocera), Notostraca and the Devonian ''Lepidocaris''. They are mostly small, freshwater animals that feed on plankton and detritus. Description ...

s, or an independent group), with trilobites, or the merostomatans (

horseshoe crabs Horseshoe crabs are marine and brackish water arthropods of the family Limulidae and the only living members of the order Xiphosura. Despite their name, they are not true crabs or crustaceans: they are chelicerates, most closely related to arac ...

and sea scorpions, now an obsolete group). However, due to the general features and the discovery of fossils from this group in Cambrian rocks, a 2010 study suggested that they may have given rise to the mandibulates, the group that includes the myriapods (

centipede Centipedes (from New Latin , "hundred", and Latin , "foot") are predatory arthropods belonging to the class Chilopoda (Ancient Greek , ''kheilos'', lip, and New Latin suffix , "foot", describing the forcipules) of the subphylum Myriapoda, an a ...

s, millipedes and the like),

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 ...

s, and hexapods (insects, etc.). However, a 2020 study identified several characters, including

compound eye A compound eye is a visual organ found in arthropods such as insects and crustaceans. It may consist of thousands of ommatidia, which are tiny independent photoreception units that consist of a cornea, lens, and photoreceptor cells which disti ...

s and various details of the preoral chamber, that suggested instead a position as the closest relatives of living myriapods. This would help to close the gap between the earliest body fossils of

crown-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. ...

myriapods in the Silurian and molecular clock data suggesting a divergence from their closest relatives during the Ediacaran or

. This had already been suggested by the cladogram of a previous study. Mosineia macnaughtoni

Environment and life habits

Euthycarcinoid fossils have been found in marine,

brackish Brackish water, sometimes termed brack water, is water occurring in a natural environment that has more salinity than freshwater, but not as much as seawater. It may result from mixing seawater (salt water) and fresh water together, as in estuar ...

and freshwater deposits. Taxa from the Cambrian are from marine or intertidal sediments, while all specimens from the Ordovician to the Triassic are freshwater or brackish. Fossil impressions of euthycarcinoid postabdomens in association with '' Protichnites'' trackways in Cambrian

intertidal The intertidal zone, also known as the foreshore, is the area above water level at low tide and underwater at high tide (in other words, the area within the tidal range). This area can include several types of habitats with various species of ...

/supratidal deposits also suggest that euthycarcinoids may have been the first arthropods to walk on land. It has been suggested that the

biofilm A biofilm comprises any syntrophic consortium of microorganisms in which cells stick to each other and often also to a surface. These adherent cells become embedded within a slimy extracellular matrix that is composed of extracellular p ...

s and

microbial mats A microbial mat is a multi-layered sheet of microorganisms, mainly bacteria and archaea, or bacteria alone. Microbial mats grow at interfaces between different types of material, mostly on submerged or moist surfaces, but a few survive in desert ...

that covered much of the vast

tidal flat Mudflats or mud flats, also known as tidal flats or, in Ireland, slob or slobs, are coastal wetlands that form in intertidal areas where sediments have been deposited by tides or rivers. A global analysis published in 2019 suggested that tidal f ...

s during the Cambrian Period in North America may have provided the nourishment that lured these arthropods onto the land. Fossil evidence also suggests the possibility that some euthycarcinoids came onto the land to lay and fertilize their eggs via

amplexus Amplexus (Latin "embrace") is a type of mating behavior exhibited by some externally fertilizing species (chiefly amphibians and horseshoe crabs) in which a male grasps a female with his front legs as part of the mating process, and at the same ...

, as do the modern horseshoe crabs.

Classification

The known species of euthycarcinoids and their distribution were reviewed by Racheboeuf ''et al.'' in 2008. Additional species were described by Collette and Hagadorn in 2010. * '' Antarcticarcinus'' ** ''Antarcticarcinus pagoda'' Pagoda Formation Antarctica, Upper Carboniferous to Lower Permian. * '' Apankura machu'' (Cambrian), from marine deposits in Argentina * '' Euthycarcinus'' ** ''Euthycarcinus ibbenburensis'' ( Pennsylvanian: Westphalian), from freshwater deposits in Germany ** ''Euthycarcinus martensi'' ( Permian), from freshwater deposits in Germany ** ''Euthycarcinus kessleri'' (Triassic), from freshwater deposits in France * '' Mosineia macnaughtoni'' (Cambrian), from intertidal deposits in the United States * '' Mictomerus melochevillensis'' (Cambrian), from intertidal deposits in Canada * '' Pieckoxerxes pieckoae'' (Pennsylvanian: Westphalian), from brackish to freshwater deposits of the United States * '' Synaustrus brookvalensis'' (Triassic) from freshwater deposits of Australia Family Kottixexidae Starobogatov, 1988 * '' Heterocrania rhyniensis'' (Lower Devonian), from freshwater deposits of the United Kingdom * '' Kalbarria brimmellae'' (Ordovician or Late Silurian), from freshwater deposits of Australia (Age and habitat are controversial) *'' Kottixerxes'' ** ''Kottixerxes anglicus'' (Pennsylvanian: Westphalian), from brackish to freshwater deposits of the United Kingdom ** ''Kottyxerxes gloriosus'' (Pennsylvanian: Westphalian), from brackish to freshwater deposits of the United States * '' Schramixerxes gerem'' (Late Pennsylvanian: Stephanian stage), from freshwater deposits in France * '' Smithixerxes'' ** ''Smithixerxes juliarum'' (Pennsylvanian: Westphalian), from brackish to freshwater deposits of the United States ** ''Smithixerxes pustulosus'' (Pennsylvanian: Westphalian), from brackish to freshwater deposits of the United Kingdom * '' Sottyxerxes multiplex'' (Late Pennsylvanian: Stephanian stage), from freshwater deposits in France

References

Bibliography

* * * * * * * * {{Taxonbar, from=Q5414519 Arthropod enigmatic taxa Paleozoic arthropods Triassic arthropods Fossil taxa described in 1964 Cambrian first appearances Middle Triassic extinctions Arthropod subclasses