Euthycarcinoids are an obscure group of arthropods that I confess I'd never heard of until I saw this article by Vaccari et al.. They are interesting because 1) their phylogenetic relationships have been ambiguous, which can be a sign that they may represent something transitional (or, as an alternative, something degenerate), 2) there is trace fossil evidence that suggests that this group may have been one of the early leaders in the transition to terrestrial life, and 3) the authors have just found the oldest fossils of members of this group, pushing back the record of their existence to the late Cambrian. Since I can't really say that I know much about these guys, I'll let the abstract speak for itself and show you a picture of the new euthycarcinoid species, Apankura machu.

Euthycarcinoids are one of the most enigmatic arthropod groups, having been assigned to nearly all major clades of Arthropoda. Recent work has endorsed closest relationships with crustaceans or a myriapod hexapod assemblage, a basal position in the Euarthropoda, or a placement in the Hexapoda or hexapod stem group. Euthycarcinoids are known from 13 species ranging in age from Late Ordovician or Early Silurian to Middle Triassic, all in freshwater or brackish water environments. Here we describe a euthycarcinoid from marine strata in Argentina dating from the latest Cambrian period, extending the group's record back as much as 50 million years. Despite its antiquity and marine occurrence, the Cambrian species demonstrates that morphological details were conserved in the transition to fresh water. Trackways in the same unit as the euthycarcinoid strengthen arguments that similar traces of subaerial origin from Cambro-Ordovician rocks were made by euthycarcinoids. Large mandibles in euthycarcinoids are confirmed by the Cambrian species. A morphology-based phylogeny resolves euthycarcinoids as stem-group Mandibulata, sister to the Myriapoda and Crustacea plus Hexapoda.

Holotype of Apankura machu. a, JUY-P24. Scale bar, 5 mm. b, Interpretative drawing of JUY-P24. act, anterior cephalic tergite; an, antenna; ap, apodemal rod; ?go, possible gonopore; gt, gut; li, limbs; ma, mandible; pct, posterior cephalic tergite; sp p, spheroidal process; T1-T5, pre-abdominal tergites 1-5; P1-P6, post-abdominal segments 1-6.

And here is the results of the phylogenetic analysis: it's right there all by itself between the chelicerates (spiders and scorpions) and the myriapods (centipedes and millipedes).

Vaccari NE, Edgecombe GD, Escudero C (2004) Cambrian origins and affinities of an enigmatic fossil group of arthropods. Nature 430: 554-557.