Robin et al.—Fossil crabs from the Kerguelen and gills preservation


personal communication, 2017; Fig. 3.3, 3.4), but this cannot be confirmed. The location of the colonies indicates a post-mortem fouling of the carapace, which had to be dislocated to allow their internal colonization by bryozoans (Jakobsen and Feldmann, 2004). The large size of the colonies implies a quite long exposure of the broken carapace lying on the seabed before its burial, and thus a certain resistance of the carapace dorsal side (Amui-Vedel et al., 2007; Robin et al., 2015).


Preserved gills.—Different views of the preserved gills exist in the studied material (Fig. 4.1–4.6). The outer view of the broken carapace reveals the dorsal side of the gill lamellae (Fig. 4.2, 4.5). If it is possible to spot the limits of these different gills in dorsal view (Fig. 4.5), it is impossible to identify them. On the other hand, on the endoskeletons, the nature of the different gills structures can be suggested. Extant Cancer (Cancridae) displays nine gills at each side, plus three mastigobranchiae (Pearson, 1908; Calman, 1909). These nine elements are located on thoracic somite 2 (g1, g2), somite 3 (g3, g4, g5), somite 4 (g6, g7), somite 5 (g8), and somite 6 (g9). On its left side, specimen MNHN.F.A59110 exhibits nine series of branchial lamellae in transversal view. These series may be identified as the paired lamellae (anterior and posterior), which compose g2–g7, with g8 and g9 missing and the anterior lamellae of g2 likely not visible. The g1 (a podobranch) is clearly recog- nizable. It lies with its outer face in contact with the posterior side of the branchial groove, and its inner face closely applied to the basal portions of gills 2–6. Its apex points backward and outward (Pearson, 1908). The gill segment, which lies outer of g5, could correspond to the g3, which is known on extant Cancer to be short and wedged in between g1 and g5 (Fig. 4.4.). On its right side, specimen MNHN.F.A59115 displays four


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lamellae series that could correspond to g4 and g5 (Fig. 4.5). The afferent (dorsal) and efferent (ventral) vessels, which enable the dioxygen exchanges in extant Cancer pagurus (Fig. 4.7–4.9), are not even partially preserved on the fossil material. The similarity between the fossil and extant lamellae shapes is difficult to appreciate due to the stiffening of fossil structures through diagenesis. However, we note that the fossil lamellae exhibit a general cup-shape very similar to that observed on the extant representatives (Fig. 4.8).


Matrix and gills mineralization.—The combination of XRD analyses on crushed matrix (Fig. 5) and local X-EDS analyses on grains and crab fragments allowed analysis of the mineral composition of the nodule. The nodule matrix displays for its most internal part a sandstone microfacies with volcanic remains (e.g., neat basaltic lithoclasts) and minerals stemming from the alteration of theses basalts, such as granulated feldspars (anorthoclase and anorthite), clinopyroxenes (likely augite and diopside), quartz, angular and opaque iron oxides identified as titanomagnetite and ilmenite, as well as a few micas (likely phlogopite) (Fig. 5.2, 5.3). Carbonate-fluoroapatite (CARFAP) constitutes an important cryptocrystalline phase (Fig. 5.1, 5.3–5.6) corresponding to: (1) the continuous layer that borders the edges of most of the clasts (Fig. 5.3–5.6), (2) dark angular to round phosphoclasts (Fig. 5.3), and (3) micritic bioturbation remains (fecal pellets and/or burrowing traces; Fig. 5.4). This 10 µm phosphate edging indicates diagenetic submarine phosphatization. More generally, this cryptocrystalline clay- phosphate-ferrous cement seems to encompass most of the grains. The crabs display a global CARFAP composition of the exoskeleton, endoskeleton, and of the gills. This CARFAP is partially mixed with clayey elements (Fig. 5.7). But a striking


Figure 3. Encrusting cheilostome bryozoans (MNHN.F.A59882) on MNHN.F.A59117. (1) Internal mold of the carapace; (2) SEM view of the colonies; (3) SEM close-up on the basal imprints of the zooecia; (4) SEM view of the zooecia imprints evoking Calloporidae. Scale bars=1cm(1), 4mm (2), 1mm (3), 100 µm(4). Photographs, N. Robin, S. Pont.


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