195 FOSSILS © Blackwell Publishing Ltd, The Geologists’ Association & The Geological Society of London, Geology Today, Vol. 26, No. 5, September–October 2010 Fossils explained 59 Euthycarcinoids Javier Ortega- Hernández 1 , David A. Legg 2 , Jonathan Tremewan 3 & Simon J. Braddy 3 1 Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK (jo314@esc.cam. ac.uk) 2 Department of Earth Science and Engineering, Royal School of Mines, Imperial College London, London SW7 2AZ, UK (d.legg10@imperial.ac.uk) 3 Department of Earth Sciences, University of Bristol, Queen’s Road, Bristol BS8 1RJ, UK (s.j.braddy@bristol.ac.uk) The euthycarcinoids are a mysterious group of extinct, possibly amphibious, arthropods. Like many groups of arthropods their affinities remain enigmatic. They comprise 15 species known mainly from the Late Carboniferous Coal Measures of Europe and North America, although they have been found in older rocks in Argentina and Australia. The oldest known euthycarcinoid is Apankura machu, from the Late Cambrian of Argentina, and the youngest is Euthycarcinus kessleri, from the Middle Triassic of France. Anatomy Two distinct groups of euthycarcinoids can be recog- nized (Table 1), distinguished by the number of trunk segments and leg count: the Sottyxerxiformes and the Euthycarciniformes. The sottyxerxiforms have an elongate body and resemble living millipedes. This group, typified by Sot- tyxerxes pieckoae (Fig. 1d), from the Late Carbonifer- ous of France, is distinguished by its ‘preabdomen’ consisting of 14 dorsal body plates (known as tergites) and up to 28 body segments (somites), each of which bears a pair of walking legs. The ‘postabdomen’ is short, consisting of five segments and a small tail spine. The euthycarciniforms are larger than sottyx- erxiforms; the species Schramixerxes gerem is typical (Fig. 1b). This species has five tergites in its ‘pre- abdomen’, five somites in its post-abdomen, and an elongate tail spine. Euthycarcinoids have a distinctive pattern of head segmentation. The main part of the head is known as the cephalon (a term used also for trilobites) con- sists of two segments or tergites: one at the front, the ‘procephalon’, and one at the rear, termed the ‘gnathocephalon’. The procephalon bears a pair of multi-segmented antennae and mysterious ‘sphaer- oidal processes’ that may have functioned as eyes, while the gnathocephalon contains the mouthparts and related structures (Fig. 1). These mouthparts vary, either consisting of millipede-like mandibles, as in Heterocrania rhyniensis (Fig. 1c); a single plate, as in Schramixerxes gerem (Fig. 1b); or a pair of plates, as in Sottyxerxes multiplex (Table 1). In some species there is a small plate at the back of the cephalon, the ‘monosomite’, although this is rarely visible. In euthycarcinoids, the dorsal body plates or tergites are notable for their lack of differentiation along the body. In some species they are divided into three lobes, an axial lobe and two lobes either side. In many modern arthropod groups each dorsal tergite also corresponds to a single plate on the lower, or ventral side (a sternite). However, this is rarely the case in euthycarcinoids, which typically have two or three sternites per dorsal tergite. In all cases, the limbs are cylindrical and undif- ferentiated. These consist of numerous (12–14) leg segments (podomeres). Some species have hair-like setae on their legs, which may have performed a filter-feeding function. The setae give the limbs of Ko- ttixerxes globosus a feather-like appearance (Fig. 1a) Fig. 1. Euthycarcinoid anatomy. Euthycarciniformes: a. dorsal view of Kottyxerxes gloriosus; b. ventral view of Schramixerxes gerem; c. ventral view of Heterocrania rhyniensis cephalon. Sottyxerxiformes: d. Sottyxerxes pieckoae (redrawn from Schram and Rolfe, 1982; legs omitted and postabdomen not preserved).