Euthycarcinoidea are a group of extinct, possibly amphibious arthropods that ranged from the Cambrian to the Triassic.A new Devonian euthycarcinoid reveals the use of different respiratory strategies during the marine-to-terrestrial transition in the myriapod lineage Fossils are known from Europe, North America, Argentina, Australia, and Antarctica. While previously considered enigmatic, they are now widely considered to be members of Mandibulata, and possibly the closest known relatives of Myriapoda (which contains millipedes and centipedes, among others).
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. Each somite had in turn a pair of uniramous (unbranched), 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 copepods, branchiopods, or an independent group), with trilobites, 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 myriapods (centipedes, millipedes and the like), crustaceans, and hexapods (insects, etc.).
However, a 2020 study identified several characters, including compound eyes 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 and molecular clock data suggesting a divergence from their closest relatives during the Ediacaran or Cambrian. This had already been suggested by the cladogram of a previous study. Cladogram of Mandibulata after Laville et al. (2025), showing proposed position of Euthycarcinida as sister to Myriapoda:
Environment and life habits Euthycarcinoid fossils have been found in marine, brackish 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 biofilms and microbial mats that covered much of the vast tidal flats 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.
+FamilyGenusSpeciesAgeType of depositsLocation of depositsEuthycarcinidaeEuthycarcinusE. ibbenburensisPennsylvanian: WestphalianFreshwaterGermanyE. kessleriTriassicFreshwaterFranceE. martensiPermianFreshwaterGermanySynaustrusS. brookvalensisTriassicFreshwaterAustraliaKottixerxidaeHeterocraniaH. rhyniensisLower DevonianFreshwaterUnited KingdomKalbarriaK. brimmellaeOrdovician or Late Silurian (age is controversial)Freshwater (habitat is controversial)AustraliaKottixerxesK. anglicusPennsylvanian: WestphalianBrackish to freshwaterUnited KingdomK. gloriosusPennsylvanian: WestphalianBrackish to freshwaterUnited StatesSchramixerxesS. geremLate Pennsylvanian: StephanianFreshwaterFranceSmithixerxesS. juliarumPennsylvanian: WestphalianBrackish to freshwaterUnited StatesS. pustulosusPennsylvanian: WestphalianBrackish to freshwaterUnited KingdomMictomeridaeMictomerusM. melochevillensisCambrianIntertidalCanadaincertae sedisAntarcticarcinusA. pagodaPennsylvanian: Gzhelian to Permian: AsselianFreshwaterPagoda Formation, AntarcticaApankuraA. machuCambrianMarineArgentinaEricixerxesE. potiiUpper DevonianBrackishBelgiumMosineiaM. macnaughtoniCambrianIntertidalUnited StatesPieckoxerxesP. pieckoaePennsylvanian: WestphalianBrackish to freshwaterUnited StatesSottyxerxesS. multiplexLate Pennsylvanian: StephanianFreshwaterFrance
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