474
Journal of Paleontology 92(3):466–477 The composition of the nodule matrix indicates that the
crabs likely evolved in an environment with a dark sandy beach, stemming from of the alteration of the Kerguelen prevalent volcanic basements (basalts). The early mineralization of soft tissues is a bacterially induced phenomenon that depends on local chemical conditions of the medium enclosing the body remains (Briggs and Kear, 1993). These conditions change according to the initial chemistry of the medium, the decay progression, the system permeability, and the release of micro- bial metabolism products. The precipitation of CaCO3 or Ca3(PO4)2 in marine environments has been compared to a ‘switch’ with the default set for the precipitation of calcium carbonate. Indeed, in a permeable aqueous system, the pre- cipitation of calcium phosphate is inhibited by high ambient concentration of microbially generated hydrogencarbonate ions (HCO3
- ). This explains why calcium carbonate normally pre-
cipitates preferentially during diagenesis (Allison, 1988; Briggs and Kear, 1993, 1994; Briggs and Wilby, 1996). When the system closes, the accumulation of the acidic bioproducts, stemming from microbial metabolism (CO2,H2S), triggers a pH decrease, which allows calcium phosphate precipitation. Thus, the phosphatization of the crab skeletons and soft tissues, associated with that of the matrix clasts, must have occurred early in diagenesis, likely on still unbound grains from an impervious sedimentary system that enclosed the crabs alive or
Figure 6. Hypothesized migration route of Romaleon based on previously described occurrences from Late Cretaceous to Holocene.
It is unclear if Metacarcinus traveled a similar route as Romaleon, from Chile along the Antarctic coast westward to New Zealand. An alternative migratory route for Metacarcinus may be from Japan southwards. In our opinion, such a route seems extremely unlikely for Romaleon to have followed. An alternative route from South America westward is far more assumable and would better suit the geographical dissemination of the genus through the Cenozoic. In any case, it has to be noticed that this genus is no longer present nowadays in Kerguelen, where a single species of decapod crustacean from a remote brachyuran group is known to live (Halicarcinus planatus, Majoidea). This species displays an uncommonly soft and uncalcified carapace whose protective role remains unob- vious (Richer de Forges, 1977; Duchêne, 1989). This pattern could be related to the calcareous poverty of theKerguelenwaters, even if this parameter could not entirely explain this carapace feature (Duchêne, 1989). In comparison, knowing the fragility of crab carcasses and regarding (1) the preservation state (in con- nection) of the studied specimens and (2) the long exposure that some specimens underwent to display the bryozoan foulings, it seems very unlikely that R. franciscae n. sp. exhibited such a softness of the dorsal carapace. These preservational aspects imply
that R. franciscae n. sp. displayed a likely higher carapace calci- fication than Halicarcinus planatus. Multiple factors could explain this variation in calcification of the crab carapaces, among which a substantial difference in Kerguelen water composition between nowadays and theMiocene.
soon after death. The formation of the clay-phosphate-ferrous film and its role in the preservation remain unclear, but it seems to have occurred secondarily in the diagenesis. The likely microbially induced formation of this covering could result of secondary changes in the permeability of the nodule system and/or of the activity of bacteria inducing silica precipitation (Birnbaum et al., 1989); but these issues would need to be further documented. The origin of dissolved phosphate in the system may both be intrinsic and extrinsic to crab composition. Indeed, Briggs and Kear (1993) showed in their experiments that phosphatization of decapod soft tissues occurred even where the sole source of phosphate is the crustacean itself. Since that, Bosselmann et al. (2007) quantitatively determined the proportion of phosphates in the exoskeleton of some decapod models, including Cancer pagurus, for the first time. They showed that PO4
3– represents up to 3.5% of the exoskeleton of
the species adult form, with a higher relative concentration in the carapace than in chelipeds (equivalent or slightly higher value than species studied by Briggs and Kear). Moreover, other structures have been documented as phosphates, such as mandibles, whose phosphatization is more widely applied among crustaceans than previously thought (see Bentov et al., 2016, but brachyurans are less involved than other decapods).A source could also be found in the phosphate granules produced in the digestive gland of many crabs (including C. pagurus), which plays a role in metal detoxication processes (Guary and Négrel, 1981; Luquet, 2012). In our case, the modern afferent/ efferent structures, which display a slightly lower phosphorous composition than the lamellae, are not preserved as fossils. Aware of the multiple gaps in our understanding of local phos- phatization in a fossil system, we wonder if this variation in composition could have affected the preservation. Phosphatization is known for offering a better morpho- logical fidelity than calcification in vertebrate soft tissues
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