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Cambrian origins and affinities of an enigmatic fossil group of arthropods


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 crustaceans1 or a myriapod–hexapod assemblage2, a basal position in the Euarthropoda3, or a placement in the Hexapoda4 or hexapod stem group5. Euthycarcinoids are known from 13 species ranging in age from Late Ordovician or Early Silurian to Middle Triassic, all in freshwater or brackish water environments6. 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 euthycarcinoids7,8. Large mandibles in euthycarcinoids6,9 are confirmed by the Cambrian species. A morphology-based phylogeny resolves euthycarcinoids as stem-group Mandibulata, sister to the Myriapoda and Crustacea plus Hexapoda.

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Figure 1: Holotype of Apankura machu.
Figure 2
Figure 3: Strict consensus of the nine shortest cladograms for euthycarcinoids and extant Euarthropoda favoured by successive weights and implied weights for concavity functions k = 1 to k = 6.
Figure 4: Trackway from the Casa Colorada Member, JUY-P 25.

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We thank C. Colarich, F. Rivero and E. Piovano for assistance in preparing Figs 1 and 2. Support was provided by CONICET and ANPCyT-FONCyT.

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Correspondence to N. E. Vaccari.

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Supplementary Information

This includes a list of the characters used in phylogenetic analysis, and supplementary references. (DOC 199 kb)

Supplementary Table

Taxa and character codings used in the phylogenetic analysis. (DOC 58 kb)

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Vaccari, N., Edgecombe, G. & Escudero, C. Cambrian origins and affinities of an enigmatic fossil group of arthropods. Nature 430, 554–557 (2004).

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