Euthycarcinoidea are an enigmatic group of extinct, possibly amphibious
arthropods that ranged from
Cambrian to
Triassic 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. 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.[1]
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[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 the
myriapods (
centipedes,
millipedes and the like),
crustaceans, and
hexapods (insects, etc.).[4]
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.[5] 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.[1] This had already been suggested by the cladogram of a previous study.[6]
Euthycarcinoid fossils have been found in
marine,
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 with Protichnites trackways in Cambrian
intertidal/supratidal deposits also suggest that euthycarcinoids may have been the first arthropods to walk on land.[9][10] 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.[11] 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.[12]
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.[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.