Temporal range: Cambrian–Middle Triassic
Gall & Grauvogel, 1964
The euthycarcinoid body was divided into 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, segmented legs. The postabdomen was limbless and consisted of up to six segments and a terminal tailspine.
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). However, the general features and the discovery of fossils from this group in Cambrian rocks suggest that its may have given rise to the mandibulates, the group that includes the myriapods (centipedes, millipedes and the like), crustaceans, and hexapods (insects, etc.).
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.
- Apankura machu (Cambrian), from marine deposits in Argentina
- Heterocrania rhyniensis (Lower Devonian), from freshwater deposits of the United Kingdom
- Kalbarria brimmellae (Ordovician or Late Silurian), from freshwater deposits of Australia
- 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
- Mosinieia 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
- Schramixerxes gerem (Late Pennsylvanian: Stephanian stage), from freshwater deposits in France
- 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
- Synaustrus brookvalensis (Triassic) from freshwater deposits of Australia
- Ortega-Hernandez et al., 2010, page 195.
- Racheboeuf, Patrick R.; Vannier, Jean; Schram, Frederick R.; Chabard, Dominique; Sotty, Daniel (2008). "The euthycarcinoid arthropods from Montceau-les-Mines, France: functional morphology and affinities". Earth and Environmental Science Transactions of the Royal Society of Edinburgh 99 (1): 11–25. doi:10.1017/S1755691008006130.
- Ortega-Hernandez et al., 2010, page 196.
- Ortega-Hernandez et al., 2010, page 196 & 197.
- Collette & Hagadorn, 2010.
- Collette et al., 2012.
- MacNaughton et al., 2002. page 394.
- Collette et al., 2012, pages 452 & 453.
- Collette & Hagadorn, 2010.
- Anderson, L. I.; Trewin, N. 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, J. H., Gass, K. C. & Hagadorn, J. 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.
- Collette, J. H. & Hagadorn, J. 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.
- MacNaughton, R. B., Cole, J. M., Dalrymple, R. W., Braddy, S. J., Briggs, D. E. G., & Lukie, T. 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.
- McNamara, K. J. & Trewin, N. H. (1993). "A euthycarcinoid arthropod fauna from the Silurian of Western Australia". Palaeontology 36: 319–335.
- Ortega-Hernandez, J., Tremewan, J., & Braddy, S. J. (2010). "Euthycarcinoids". Geology Today 26 (5): 195–198. doi:10.1111/j.1365-2451.2010.00770.x.
- Vaccari, N. E., Edgecome, G. D. & Escudero, C.; Edgecombe; Escudero (2004). "Cambrian origins and affinities of an enigmatic fossil group of arthropods". Nature 430 (6999): 554–557. Bibcode:2004Natur.430..554V. doi:10.1038/nature02705. PMID 15282604.
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