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Abstract
Constraining the origin of animal groups is allowed, to some extent, by discoveries of Cambrian Lagerstätten that preserve both mineralizing and nonmineralizing organisms. A new species is reported here of the Cambrian arthropod Skania, which bears an exoskeleton that shares homologies with the Neoproterozoic (Ediacaran) organism Parvancorina and firmly establishes a Precambrian root for arthropods. A new monophyletic group, Parvancorinomorpha, is proposed as the first clade within the arthropod crown group demonstrably ranging across the Neoproterozoic–Paleozoic transition. The Parvancorinomorpha is interpreted to be the sister group of the Arachnomorpha. Incipient cephalization in Skania and related genera represents a step in the progression toward division of a cephalon from a large posterior trunk as shown in Cambrian arachnomorphs such as naraoiids and the addition of a pygidium and thoracic tergites as shown in the arachnomorph clade basal to trilobites. This evidence can serve as a new calibration point for estimating the divergence time for the last common ancestor of arthropods and priapulids based on molecular clock methods.
Acknowledgements
Many thanks to the following: the Liu brothers and relatives for assistance in collecting specimens; Derek E. G. Briggs, Nigel C. Hughes and James St John for reading earlier drafts. This work was supported by 2003 NSF East Asia Summer Research Fellowship (No. 0308714) to JPL, the US National Science Foundation (EAR 0073089, 0106883) to LEB and the National Natural Science Foundation of China (40162002, 40372023, 40232020) to YLZ.
Special thanks to the editor Ganeth Dyke and an anonymous reviewer for constructive comments.
Notes
† In April of 2005, Australia post issued a set of six stamps titled “Creatures of the Slime” featuring characteristic Ediacaran organisms, including Charniodiscus, Kimberella, Spriggina, Tribrachidium, Inaria and Dickinsonia. The stamp artist Peter Trusler carefully examined specimens and casts and illustrated a small three dimensional reconstruction of Parvancorina along with other Ediacaran creatures near the bottom left corner of the stamp sheetlet. Other previous interpretations of Parvancorina were also included in the stamp sheetlet but only visible under ultraviolet light.