Euthycarcinoidea

Euthycarcinoidea are an enigmatic group of extinct, possibly amphibious arthropods that ranged from Cambrian to Triassic times, with the earliest known species being Apankura machu from the Cambrian and the youngest Euthycarcinus kessleri from the Middle Triassic. 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.

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 Myriapodia:

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.

• Family Euthycarcinidae Handlirsch, 1914 Genus Euthycarcinus Handlirsch, 1914 Species E. ibbenburensis Schultka, 1991 - Pennsylvanian: Westphalian, from freshwater deposits in Germany Species E. kessleri Handlirsch, 1914 - Triassic, from freshwater deposits in France Species E. martensi Schneider, 1983 - Permian, from freshwater deposits in Germany Genus Synaustrus Riek, 1964 Species S. brookvalensis Riek, 1964 - Triassic, from freshwater deposits of Australia

• Family Kottixerxidae Starobogatov, 1988 Genus Heterocrania Hirst & Maulik, 1926 Species H. rhyniensis Hirst & Maulik, 1926 - Lower Devonian, from freshwater deposits of the United Kingdom Genus Kalbarria McNamara & Trewin, 1993 Species K. brimmellae McNamara & Trewin, 1993 - Ordovician or Late Silurian, from freshwater deposits of Australia (age and habitat are controversial) Genus Kottixerxes Schram, 1971 Species K. anglicus Wilson & Almond, 2001 - Pennsylvanian: Westphalian, from brackish to freshwater deposits of the United Kingdom Species K. gloriosus Schram, 1971 - Pennsylvanian: Westphalian, from brackish to freshwater deposits of the United States Genus Schramixerxes Starobogatov, 1988 Species S. gerem (Schram & Rolfe, 1982) - Late Pennsylvanian: Stephanian stage, from freshwater deposits in France Genus Smithixerxes Schram & Rolfe, 1982 Species S. juliarum Schram & Rolfe, 1982 - Pennsylvanian: Westphalian, from brackish to freshwater deposits of the United States Species S. pustulosus Wilson & Almond, 2001 - Pennsylvanian: Westphalian, from brackish to freshwater deposits of the United Kingdom

• Family Mictomeridae Collette & Hagadorn, 2010 Genus Mictomerus Collette & Hagadorn, 2010 Species M. melochevillensis Collette & Hagadorn, 2010 - Cambrian, from intertidal deposits in Canada

• Family incertae sedis Genus Antarcticarcinus Collette, Isbell & Miller, 2017 Species A. pagoda Collette, Isbell & Miller, 2017 - Upper Carboniferous to Lower Permian, Pagoda Formation Antarctica. Genus Apankura Vaccari, Edgecombe & Escudero, 2004 Species A. machu Vaccari, Edgecombe & Escudero, 2004 - Cambrian, from marine deposits in Argentina Genus Ericixerxes Gueriau, Lamsdell, Wogelius, Manning, Egerton, Bergmann, Bertrand & Denayer, 2020 Species E. potii Gueriau, Lamsdell, Wogelius, Manning, Egerton, Bergmann, Bertrand & Denayer, 2020 - Upper Devonian, from brackish deposits in Belgium Genus Mosineia Collette & Hagadorn, 2010 Species M. macnaughtoni Collette & Hagadorn, 2010 - Cambrian, from intertidal deposits in the United States Genus Pieckoxerxes Starobogatov, 1988 Species P. pieckoae (Schram & Rolfe, 1982) - Pennsylvanian: Westphalian, from brackish to freshwater deposits of the United States Genus Sottyxerxes Schram & Rolfe, 1982 Species S. multiplex Schram & Rolfe, 1982 - Late Pennsylvanian: Stephanian stage, from freshwater deposits in France