Euthycarcinoidea are an enigmatic group of extinct, possibly amphibiousarthropods that ranged fromCambrian toTriassic times. Fossils are known from Europe, North America, Argentina, Australia, and Antarctica.
The euthycarcinoid body was divided into a cephalon (head), preabdomen, andpostabdomen. The cephalon consisted of twosegments and includedmandibles,antennae and presumed eyes. The preabdomen consisted of five to fourteentergites, each having up to threesomites. Each somite had in turn a pair ofuniramous, segmented legs. The postabdomen was limbless and consisted of up to six segments and a terminal tail spine.[1]
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 ascopepods,branchiopods, or an independent group), withtrilobites, or the merostomatans (horseshoe crabs andsea scorpions, now an obsolete group[2]).[3] 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 themyriapods (centipedes,millipedes and the like),crustaceans, andhexapods (insects, etc.).[4]
However, a 2020 study identified several characters, includingcompound eyes and various details of the preoral chamber, that suggested instead a position as the closest relatives of living myriapods.[5] This would help to close the gap between the earliest body fossils ofcrown-group myriapods in theSilurian and molecular clock data suggesting a divergence from their closest relatives during theEdiacaran orCambrian.[1] This had already been suggested by the cladogram of a previous study.[6]
The Cambrian euthycarcinoidMosineia macnaughtoni from the Elk Mound Group,Blackberry Hill, central Wisconsin. Cambrian euthycarcinoids such as this one may have been the first animals to walk and survive on land.[7]
Euthycarcinoid fossils have been found inmarine,brackish and freshwater deposits.[8] Taxa from the Cambrian are from marine or intertidal sediments, while all specimens from the Ordovician to the Triassic are freshwater or brackish.[3] Fossil impressions of euthycarcinoid postabdomens in association withProtichnites trackways in Cambrianintertidal/supratidal deposits also suggest that euthycarcinoids may have been the first arthropods to walk on land.[9][10] It has been suggested that thebiofilms andmicrobial mats that covered much of the vasttidal flats during the Cambrian Period inNorth America may have provided the nourishment that lured these arthropods onto the land.[11] Fossil evidence also suggests the possibility that some euthycarcinoids came onto the land to lay and fertilize their eggs viaamplexus, as do the modern horseshoe crabs.[12]
The known species of euthycarcinoids and their distribution were reviewed by Racheboeufet al. in 2008. Additional species were described by Collette and Hagadorn in 2010.[3][9]
Anderson, Lyall I.; Trewin, Nigel H. (1991). "An early Devonian arthropod fauna from the Windyfield cherts, Aberdeenshire, Scotland".Palaeontology.46 (3):467–509.doi:10.1111/1475-4983.00308.
Collette, Joseph H.; Gass, Kenneth C.; Hagadorn, James W. (2012). "Protichnites eremita unshelled? Experimental model-based neoichnology and new evidence for a euthycarcinoid affinity for this ichnospecies".Journal of Paleontology.86 (3):442–454.doi:10.1666/11-056.1.S2CID129234373.
Collette, Joseph H.; Hagadorn, James W. (2010). "Three-dimensionally preserved arthropods from Cambrian Lagerstatten of Quebec and Wisconsin".Journal of Paleontology.84 (4):646–667.doi:10.1666/09-075.1.S2CID130064618.
MacNaughton, Robert B.; Cole, Jennifer M.; Dalrymple, Robert W.; Braddy, Simon J.; Briggs, Derek E. G.; Lukie, Terrence D. (2002). "First steps on land: Arthropod trackways in Cambrian–Ordovician eolian sandstone, southeastern Ontario, Canada".Geology.30 (5):391–394.Bibcode:2002Geo....30..391M.doi:10.1130/0091-7613(2002)030<0391:FSOLAT>2.0.CO;2.S2CID130821454.
Ortega-Hernández, Javier; Legg, David A.; Tremewan, Jonathan; Braddy, Simon J. (2010). "Euthycarcinoids".Geology Today.26 (5):195–198.doi:10.1111/j.1365-2451.2010.00770.x.
