Crabs as overlooked trace fossil makers in freshwater settings

MELCHOR, R. N.; GENISE, J. F.; FARINA, J. L; SÁNCHEZ, M. V.; SARZETTI, L. C.; VISCONTI, G.

Lepe, España

Workshop; Workshop on Crustacean Bioturbation; 2010

The trace fossil record of true crabs (Decapoda: Brachyura) from nonmarine settings is limited to a few tentative assignments of fossil burrows from fluvial successions. This is in contrast with the recognition of a number of trace fossils assigned to freshwater burrowing crayfishes (Camborygma, Loloichnus, Lunulichnus, Dagnichnus and one species of Cellicalichnus), and with the several ichnotaxa assigned to marine or nearshore brachyuran decapod crustaceans, including Psilonichnus, Skolithos, Thalassinoides, Gyrolithes, Macanopsis, Spongeliomorpha and Foersterichnus (e.g. Frey et al., 1984). The rarity of trace fossils of freshwater crabs is also surprising if we consider that modern freshwater crabs account for about the 20% of the known extant crab species (Yeo et al., 2008) and that they play a significant role in the mixing of sediments. In this contribution we describe a new fossil burrow that is confidently attributed to freshwater crabs on the basis of neoichnological studies, which pose some questions about the scarce trace fossil record of freshwater crabs and give clues for the identification of their burrows in the fossil record. Fossil burrows (Fig. 1A) with predominantly vertical orientation, overall "L" shape (when fully developed), oval cross-section, absence of lining and burrow bifurcation, a distinct surface texture, massive burrow fill, and lack of burrow enlargements are the most common trace fossil in the fluvial succession of the Neogene Vinchina and Toro Negro formations (Sierra de 10s Colorados, northwestern La Rioja province, Argentina). These burrows are 50 - 80 mm in diameter and up to 350 mm long, and typically display an oval cross-section with a ratio between long and short burrow axis ranging from 1.0 to 1.5. The surface texture is composed of sets of three to five slightly curved and parallel ridges arranged oblique to burrow axis on burrow wall (Fig. 1A). Length of sets of ridges is comparable to burrow diameter. Width of individual sets of ridges is about 20 to 60% of large burrow diameter. In straight sections of burrows, sets of ridges converge with an acute angle along a line parallel to burrow axis describing a herringbone or chevron pattern. On the opposite side of the burrow filling the same pattern appears. Sets of ridges transverse to burrow axis are absent. Burrow terminations are blunt to rounded or slightly tapered, lacking enlargements. Burrow fill is structureless and typically composed of sandstone with abundant mudstone intraclasts. In these units, the burrows were found typically on the top of fluvial channels (channel bars) and in crevasse splay facies. The burrows commonly start in sandstone and ends in mudstone. In order to interpret these large burrows with a marked surface texture, neoichnologic observations were conducted on seasonal wetlands of the Rio Pilcomayo National Park (northeastern Argentina), which are considered as possible modern analogue of the sedimentary environment. At this park, we found that all large burrows with prominent marks in their walls (Fig. 1 B) were constructed by freshwater crabs, including Zilchiopsis collastinensis and Dilocarcinus pagei. These burrows can be distinguished by the overall architecture, the species and sex of its occupants, and the environmental setting where they were found. The study of these burrows and a comparison with the literature, allowed us to recognize neoichnologic signatures that can be used in the interpretation of the fossil example, which are: overall burrow architecture, the common oval burrow cross-section and the surface texture (see also Pemberton and Frey, 1985; Gingras et al., 2000; Gingras et al., 2002; Seike and Nara, 2007; Seilacher, 2007; Gingras et al., 2008). In particular, the surface texture in the modern crab burrows includes abundant comma shaped marks, sets of grooves oblique to burrow axis, and long segmented grooves. The predominantly oval cross-section, absence of lining, the strongly scratched burrow surface of the fossil burrows and its occurrence in fluvial environments suggest that they were produced by freshwater crabs. The key features of the surface texture are long sets of three-four ridges whose width is in average one third of the large diameter burrow. Other features that also point to a crab origin are a massive filling (suggesting open burrows), common burrow ends in a casing mudstone (to maintain water or moisture within the burrow), chevron pattern in sets of surface ridges (suggesting side burrowing), and possible cheliped marks. The relatively large cross-section and highly scratched burrow walls are features that can be also found in tetrapod burrows. The relationship between the width of sets of ridges and the diameter of the burrow can help to discriminate between a tetrapod or invertebrate producer. In the case of a high ratio (an average 0.32 in the case of the fossil burrows) the set of ridges can be more likely attributed to the independent and well separated appendages of an invertebrate (i.e. decapod legs and chelae), than to the digits of a single tetrapod foot. Considering the burrow diameter and the width of sets of ridges, such tetrapod would be disproportionately small in comparison with its large foots, which is unlikely. These findings may help in the recognition of additional trace fossils of freshwater crabs and suggest that its apparent scarcity may not be reflecting its actual abundance. The large ornamented fossil burrows are restricted to fluvial facies and are absent in the associated lacustrine deposits, thus they are a good indicator of fluvial or fluvial influenced facies, at least for the Vinchina basin.