Euthycarcinoidea are an enigmatic group of extinct, possibly amphibious

arthropod Arthropods ( ) are invertebrates in the phylum Arthropoda. They possess an arthropod exoskeleton, exoskeleton with a cuticle made of chitin, often Mineralization (biology), mineralised with calcium carbonate, a body with differentiated (Metam ...

s that ranged from

Cambrian The Cambrian ( ) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordov ...

Triassic The Triassic ( ; sometimes symbolized 🝈) is a geologic period and system which spans 50.5 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.4 Mya. The Triassic is t ...

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, antennae and presumed eyes. The preabdomen consisted of five to fourteen tergites, 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somites The somites (outdated term: primitive segments) are a set of bilaterally paired blocks of paraxial mesoderm that form in the embryonic stage of somitogenesis, along the head-to-tail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, 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 (ecology), habitat. Some species are planktonic (living in the water column), some are benthos, benthic (living on the sedimen ...

s, branchiopods, or an independent group), with

trilobite Trilobites (; meaning "three-lobed entities") are extinction, extinct marine arthropods that form the class (biology), class Trilobita. One of the earliest groups of arthropods to appear in the fossil record, trilobites were among the most succ ...

s, or the merostomatans ( horseshoe crabs 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

myriapod Myriapods () are the members of subphylum Myriapoda, containing arthropods such as millipedes and centipedes. The group contains about 13,000 species, all of them terrestrial. Although molecular evidence and similar fossils suggests a diversifi ...

s (

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

millipede Millipedes (originating from the Latin , "thousand", and , "foot") are a group of arthropods that are characterised by having two pairs of jointed legs on most body segments; they are known scientifically as the class Diplopoda, the name derive ...

s and the like),

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

compound eye A compound eye is a Eye, visual organ found in arthropods such as insects and crustaceans. It may consist of thousands of ommatidium, ommatidia, which are tiny independent photoreception units that consist of a cornea, lens (anatomy), lens, and p ...

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 The Silurian ( ) is a geologic period and system spanning 23.5 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the third and shortest period of t ...

and molecular clock data suggesting a divergence from their closest relatives during the

Ediacaran The Ediacaran ( ) is a geological period of the Neoproterozoic geologic era, Era that spans 96 million years from the end of the Cryogenian Period at 635 Million years ago, Mya to the beginning of the Cambrian Period at 538.8 Mya. It is the last ...

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

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 estuari ...

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/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 is a Syntrophy, syntrophic Microbial consortium, community of microorganisms in which cell (biology), cells cell adhesion, stick to each other and often also to a surface. These adherent cells become embedded within a slimy ext ...

s and

microbial mats A microbial mat is a multi-layered sheet or biofilm of microbial colony (biology), colonies, composed of mainly bacteria and/or archaea. Microbial mats grow at interface (chemistry), interfaces between different types of material, mostly on submer ...

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 ...

s during the Cambrian Period in

North America North America is a continent in the Northern Hemisphere, Northern and Western Hemisphere, Western hemispheres. North America is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South Ameri ...

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, mating behavior exhibited by some External fertilization, externally fertilizing species (chiefly amphibians, Amphipoda, amphipods, and horseshoe crabs) in which a male grasps a female with his fro ...

, 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 The Permian ( ) is a geologic period and System (stratigraphy), stratigraphic system which spans 47 million years, from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.902 Mya. It is the s ...

), 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 * '' Ericixerxes potii'' (Upper Devonian) from brackish deposits in Belgium Family Kottixerxidae 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 Taxa described in 1964 Cambrian first appearances Middle Triassic extinctions Arthropod subclasses