Robin et al.—Fossil crabs from the Kerguelen and gills preservation 1 Quaternary sediments
Basaltic uplands Volcano-plutonic complex Trachyte
Basanite/limburgite Ice cap
Cap Milon outcrop 2 3 N
467
Prince of Wales Foreland
Figure 1. Fossil site location. (1) Simplified geological map of the Kerguelen Islands (modified after Nougier, 1970 and Lorand et al., 2004); (2) Cap Milon tufas yielding the crab-bearing nodules (110m above sea level; from Nougier, 1970); (3) satellite south-eastern view of Prince of Wales Foreland showing the Cap Milon and the Miocene outcrops (black arrow). Stars represent the two other fossiliferous deposits in Kerguelen (black=Mont Rond site; white=Cat Ears locality). Scale bars=50km (1), 500m (3).
preservation with cuticles in situ, fragile endoskeleton parts (internal pleurites), and even gills preserved in 3D. Here, we study these fossil crabs through two complementary aspects: systematics and preservation with petrographic and geochem- ical analyses.
Geological setting
The Miocene layers of Cap Milon crop out on the southern slope of the Prince of Wales Foreland, at the southeastern extremity of the Courbet Peninsula (Kerguelen Islands). This fossil locality is of limited extent (some tens of m2) and of very difficult access, requiring coming through the western side of the foreland (Pointe Suzanne), which is much less steep than the sharp southern cliffs (Lauriat-Rage et al., 2002). The sedimentary layers are exposed below some substantial outpourings of basanite and limburgite, which cover the whole south of the foreland (Fig. 1.1). The outcrop, ~150m above sea level, con- sists of basaltic conglomerates ~50m thick, the aspect of which indicates a marine beach deposit (e.g., slope of the waves breaking). Within these conglomerates, an 8m thick sub- horizontal deposit of very fine-grained tufa that includes sphe- rical fossiliferous sandstone nodules (Fig. 1.2, 1.3). These nodules contain remains of a monospecific assemblage of crabs, but also bivalve and gastropod mollusks, as well as bones and teeth of vertebrates (actinopterygians, chondrichtyans, and sirenians) (Duchêne, 1989; Lemaire, personal observation). These fossils characterize a proximal shore facies that was further exhumed, along with the whole foreland, due to the activity of a fault located north of the Passe Royale (entrance of the Morbihan Gulf; Nougier, 1970; Duchêne, 1989). The age of the layers could not be dated more precisely than Miocene.
Materials and methods
Studied specimens.—We examined 122 specimens of fossil crabs preserved in nodules, and deposited at the MNHN (MNHN.F.A59107–A53219, A59618–A59626). All of them were successively collected in 1974 and 1987 by B. Richer de Forges and L. Lemaire, respectively, at the Cap Milon site, during winters in Kerguelen. Among these fossils, 35 display a measurable carapace, one of which (MNHN.F.A59117) dis- plays fouling bryozoans. Three crabs display preserved gills; two are open endoskeletons (MNHN.F.A59110, MNHN.F. A59115) and one is an external mold of a partially broken car- apace (MNHN.F.A59116). The geochemistry of the fossils and their surrounding matrix was characterized on specimen MNHN.F.A59114 in order to document the taphonomic con- ditions. Modern gills were sampled for comparative observa- tions on a con-familial specimen of Cancer pagurus Linnaeus, 1758 from Finistère (France), which were equivalent in size to the studied fossil crabs (MNHN-IU-2014-12647).
SEM, EDS, and DRX analyses.—For fossil material, the gills and the fouling bryozoans were observed in situ. The modern gills were sampled after immersion of the crab in successive ethanol baths for 20 minutes each to reach 100% ethanol equi- librium (dilutions at 80%, 90%, and 100%). The gills were dried at the critical point (EmiTechK850, carbon dioxide solvent) to allow their 3D preservation for SEM and then mounted on stubs and sputter-coated in carbon. Observations were made using a Tescan SEM (VEGA II LSU) linked to an X-ray detector SD3 (Bruker) (Direction des Collections, MNHN, Paris). The chemical composition of the fossil crab bodies and the non- fossiliferous matrix, as well as that of the modern gills, were semi-quantitatively determined by energy dispersive X-ray
Courbet Peninsula
Morbihan Gulf
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