992
Journal of Paleontology 91(5):987–993
Formation, the very strong morphological similarities between trace and inferred tracemaker are striking. We thus reinterpret these impressions as having been produced by the long lateral T1 processes of A. pagoda n. gen. n. sp., while the long, posterior longitudinal groove would have been produced by the legless postabdomen and telson of A. pagoda n. gen. n. sp. (compare Figs. 1, 3 with the reconstruction of A. pagoda n. gen. n. sp. in Fig. 1). Additionally, the Protichnites and Diplichnites trackways on the surface figured in Briggs et al. (2010, fig. 3a) may have been produced by Antarcticarcinus n. gen. walking on the sediment surface because these ichnogenera have been associated with eutycarcinoids (MacNaughton et al., 2002; Collette et al., 2012). All euthycarcinoid taxa except for the Middle Triassic
Synaustrus brookvalensis occur from approximately 30°N to 30°S. Both S. brookvalensis and A. pagoda n. gen. n. sp. occur at approximately 85°S, but S. brookvalensis occurred during greenhouse Middle Triassic time, while A. pagoda n. gen. n. sp. inhabited an ice-dominated lake. As inhabitants of a polar lake supplied by seasonally introduced glacial meltwater, A. pagoda n. gen. n. sp. must have been capable of withstanding extremely cold temperatures. Abundant evidence shows that continental glaciation in southern Gondwana was extensive during the latest Paleozoic (Crowell, 1978, 1999, p. 28–39; Coates, 1986; Collinson and Elliot, 1986; Miller, 1989). Based on the inferred temperature tolerances of coexisting conchostracans (Tasch, 1964), water temperatures in the Pagoda lake may have reached approximately 10°C for several weeks each year. Trace fossils, whose margins are commonly indistinct, were probably constructed in soft, unfrozen muds (Miller and Smail, 1996). Wood fragments present in the Pagoda Formation at Mt. Butters imply the proximity of terrestrial vegetation requiring an environment that received enough sunlight and warmth seasonally to promote growth (Isbell et al., 2001). The occur- rence of euthycarcinoids in low-diversity freshwater settings dominated by conchostracans is not unique to the Pagoda lake— Euthycarcinus martensi from Germany also co-occurs with conchostracans and few other animals (Schneider, 1983). Euthycarcinoids from the upper Carboniferous Mazon Creek (Schram and Rolfe, 1982) and Montceau-les-Mines (Rolfe et al., 1982; Schram and Rolfe, 1982; Schram and Rolfe, 1994) deposits co-occur with conchostracans, ostracodes, and numer- ous other taxa. Taken together, paleobiological and sedimento- logical evidence suggest periodic warming in high southern latitudes during the Permian (Isbell et al., 2001). The extreme rarity of non-mineralized arthropod body
fossils such as A. pagoda n. gen. n. sp. in these lacustrine deposits imply that: (1) difficult ecological conditions may have been limiting to many taxa, or (2) scavenging and/or microbial degradation proceeded rapidly due to limited resources. Remains of predaceous or scavenging animals are unknown from the Pagoda Formation, but trace-fossil evidence from the Mackellar Formation suggests that A. pagoda n. gen. n. sp. may not have been a rare animal because many cf. Orbiculichnus traces attributable to this taxon occur on a single small slab from Mt. Weeks (Briggs et al., 2010). The occurrence of Antarcticarcinus pagoda n. gen. n. sp. in
a polar lacustrine setting indicates that euthycarcinoids may have been more successful and geographically widespread than
previously thought. All known euthycarcinoids occur in shallow, emergent, nearshore, or lacustrine environments (Racheboeuf et al., 2008), and with the exceptions of A. pagoda n. gen. n. sp. and Arthrogyrinus platyurus (Wilson and Almond, 2001) from the Carboniferous of the UK, most do not have adaptations specific to an aquatic lifestyle. Together with their dispersed paleobiogeographic distribution, and very limited number of occurrences, this begs the question of whether euthycarcinoids were truly aquatic organisms, or whether perhaps they may have been amphibious. A pair of ventral exoskeletal sternal pores per preabdominal sternite is closely associated with internal tube-like structures in many euthy- carcinoid taxa (Schram, 1971; Edgecombe and Morgan, 1999; Anderson and Trewin, 2003, p. 482, text-fig. 12; Vaccari et al., 2004). Such an arrangement of external openings and internal tubes is strikingly similar to the respiratory system in millipedes where a pair of respiratory spiracles per segment opens intern- ally to a branched tracheal tree for distribution of gasses to tissues (Clarke, 1973, p. 101). Others have posited that the sternal pores could be coxal vesicles associated with eversible sacs of the type present in myriapods (Edgecombe and Morgan, 1999), where these structures are associated with moisture uptake (Clarke, 1973, p. 56). Either of these interpretations for euthycarcinoid sternal pores argues for an amphibious or subaerial lifestyle. Considering these interpretations, together with abundant trace-fossil evidence indicating subaerial euthy- carcinoid activity (McNamara and Trewin, 1993; MacNaughton et al., 2002; Vaccari et al., 2004; Collette and Hagadorn, 2010; Collette et al., 2012), the absence of euthycarcinoid remains in offshore deposits, it is possible that euthycarcinoids had devel- oped the ability to breathe air by Cambrian time, and that they adopted an amphibious lifestyle thereafter.
Acknowledgments
J.Collette thanks
L.Babcock for sending thematerial described in this paper for study andD. Briggs for providing the photograph of cf. Orbiculichnus. An earlier version of this manuscript was improved by the helpful suggestions from the following review- ers: G. Edgecombe, J. Hannibal, and J. Vannier. This work was supported by NSF grants OPP-9419962 and OPP-9615045 to J.I., and grants OPP-9417978 and OPP-9614709 toM.M.
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