Euthycarcinoidea are an enigmatic group of extinct possibly amphibious

arthropod Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and Arth ...

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 ag ...

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 Eurypterids, often informally called sea scorpions, are a group of extinct arthropods that form the order Eurypterida. The earliest known eurypterids date to the Darriwilian stage of the Ordovician period 467.3 million years ago. The group is ...

. 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. 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. Some species are planktonic (inhabiting sea waters), some are benthic (living on the ocean floor), a number of species have p ...

s, branchiopods, or an independent group), with

trilobite Trilobites (; meaning "three lobes") are extinct marine arthropods that form the class Trilobita. Trilobites form one of the earliest-known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the ...

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. The fossil record of myriapods reaches back into the late Silurian, a ...

s (

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 ...

millipede Millipedes 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 derived from this feature. Each double-legged segment is a re ...

s 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 gro ...

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 dis ...

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 myriapods in the

Silurian The Silurian ( ) is a geologic period and system spanning 24.6 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 shortest period of the Paleozoi ...

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

Ediacaran The Ediacaran Period ( ) is a geological period that spans 96 million years from the end of the Cryogenian Period 635 million years ago (Mya), to the beginning of the Cambrian Period 538.8 Mya. It marks the end of the Proterozoic Eon, and t ...

. 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 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 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 ...

/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 ...

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, 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 Pennsylvanian may refer to: * A person or thing from Pennsylvania * Pennsylvanian (geology) The Pennsylvanian ( , also known as Upper Carboniferous or Late Carboniferous) is, in the International Commission on Stratigraphy, ICS geologic timesca ...

: Westphalian), from freshwater deposits in Germany ** ''Euthycarcinus martensi'' (

Permian The Permian ( ) is a geologic period and 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 251.9 Mya. It is the last period of the Pale ...

), 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