Journal of Paleontology, 91(5), 2017, p. 987–993 Copyright © 2017, The Paleontological Society 0022-3360/17/0088-0906 doi: 10.1017/jpa.2017.28
A unique winged euthycarcinoid from the Permian of Antarctica
Joseph H. Collette,1 John L. Isbell,2 and Molly F. Miller3 1Department of Geosciences, Minot StateUniversity, 500 University AveWest,Minot,North Dakota, 58707,USA⟨
joseph.collette@
minotstateu.edu⟩ 2Department of Geosciences, University of Wisconsin—Milwaukee, P.O. Box 413, 2200 E. Kenwood Blvd, Milwaukee, Wisconsin, 53201, USA
⟨
jisbell@uwm.edu⟩ 3Department of Earth and Environmental Sciences, Vanderbilt University, 5726 Stevenson Center, 7th Floor, Nashville, Tennessee, 37240, USA ⟨
molly.miller@
vanderbilt.edu⟩
Abstract.—Euthycarcinoid arthropods (Cambrian–Triassic) were likely the first animals to transition from oceanic to freshwater and emergent environments. Although their basic bauplan is well known, they have a poor fossil record because their non-sclerotized exoskeleton was rarely preserved. Euthycarcinoids’ unusual morphology (varying numbers of body segments, seemingly dichotomous possession of either mandibles or a labrum, specialized or generalized limbs, and possession by some euthycarcinoid species of sternal pores—structures possibly analogous to coxal vesicles in myriapods) contribute to uncertainty regarding their relationship to other arthropod groups; while their poor fossil record masks the evolutionary transitions within and between the separate realms they inhabited (marine, freshwater, emergent). A new euthycarcinoid from a Permian polar proglacial lake is described herein that is morphologically unlike all other euthycarcinoids, and interpreted as being well adapted for a nekton-benthic lifestyle. Antarcticarcinus pagoda n. gen. n. sp. possesses a pair of large wing-like processes that project laterally from the preabdominal dorsal exoskeleton. A trace fossil from the overlying Mackellar Formation, cf. Orbiculichnus, which was previously interpreted as having been produced by insects taking off or landing on wet sediments, is reinter- preted herein as being produced by A. pagoda n. gen. n. sp. due to the high degree of morphological similarity between traces and body fossils. This occurrence indicates that euthycarcinoids were able to adapt to life in temperate freshwater environments, while possible subaerial adaptations hint at an ability to breathe air. Indeed, if euthy- carcinoids could breathe air, Cambrian terrestrial forays and rapid transition (by the Ordovician) into freshwater environments might be explained.
Introduction
Euthycarcinoids have suffered the indignity of many poorly known fossil groups—they have been shuffled from group to group as more taxa, and hence more characters, have been dis- covered. This is due to their extremely limited diversity (only 18 taxa distributed from Cambrian to Triassic deposits), low fos- silization potential within their preferred habitats, their unusual morphology, seemingly dichotomous possession of either mandibles or a labrum (Schram and Rolfe, 1982; Edgecombe and Morgan, 1999; Racheboeuf et al., 2008), specialized or generalized limbs (Schram and Rolfe, 1982; Vaccari et al., 2004; Collette and Hagadorn, 2010), and possession by some taxa of sternal pores—structures possibly analogous to coxal vesicles in myriapods (Edgecombe and Morgan, 1999). Euthy- carcinoids are of particular interest from an evolutionary per- spective because they are morphologically similar to many enigmatic stem-group arthropods such as Pisinnocaris sub- conigera from the lower Cambrian Chengjiang Biota and, par- ticularly, ?Pisinnocaris from the Hongjingshao Formation of Kunming, Yunnan Province, China (Hou and Bergström, 1998); because they transitioned from nearshore marine to freshwater
environments (Racheboeuf et al., 2008); and because they may have been the first animals to make forays onto dry land in the Cambrian (MacNaughton et al., 2002; Collette and Hagadorn, 2010; Collette et al., 2010; Hagadorn et al., 2011). Recent hypotheses regarding euthycarcinoid relationships range from a close affinity with the Uniramia (=Myriapoda+Hexapoda [Edgecombe and Morgan, 1999]), the Myriapoda (McNamara and Trewin, 1993), the Branchiopoda (Wilson and Almond, 2001), or with theHexapoda (Legg et al., 2013).While the genus- and species-level characters that define euthycarcinoid taxa vary substantially, grossly exaggerated or highly modified exoskeletal features have not thus far been reported, with the possible exception of Arthrogyrinus platyurus, a probable euthycarcinoid from the Permian that possesses a paddle-like, highly modified telson (Wilson and Almond, 2001). Antarcticarcinus pagoda n. gen. n. sp. is thus strikingly different from all other euthycarcinoid taxa in that it possesses a pair of large wing-like processes that project laterally from the preabdominal dorsal exoskeleton (see Fig. 1). Trace fossils from the overlying Mackellar Formation attributable to A. pagoda n. gen. n. sp. indicate that this taxon may have been nektonic, and that its stratigraphic range may include the entirety of the Pagoda
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