Coarse-grained sediments deposited in high-energy environments are usually considered unfavorable to the preservation of fossil tracks. Here we report dinosaur footprints showing good physical preservation, despite being found in coarse-grained sandstones of alluvial origin from the Upper Triassic of Ardèche, southeastern France. The ichnoassemblage, dominated by Grallator isp., raises questions about the processes leading to the formation and preservation of tracks in coarse-grained sediments. The track-bearing surface is a medium- to coarse-grained quartz arenite that is microconglomeratic locally. The tracking surface grain size ranges from 0.2 to 2 mm and numerous pebbles are present. It is overlain by a succession of thin, intercalated layers of claystones and siltstones, themselves covered by a mix of siltstones and coarse-grained sandstones. We interpret this succession as a progressive decrease in energy due to channel migration culminating in channel abandonment, and the establishment of a lower energy setting where the tracking surface formed. Sedimentological and taphonomic observations indicate that the trackmakers walked on fine-grained layers (clay, silt) in which true tracks formed. The passage of the animals along the tracking surface deformed the older coarse-grained sand layers and resulted in the formation of the transmitted undertracks. The fine-grained layer helped record the pedal anatomy of the trackmakers and contributed to protecting the transmitted undertracks from destruction. Overall, we suggest that the fossil footprints were preserved by abiotic processes only, the main factor being the lithological contrast between successive sedimentary layers. The exceptional preservation of those relatively high quality undertracks in coarse-grained deposits contrasts sharply with the prevailing models of true track formation involving fine-grained sediments and microbial mats present in low-energy environments. This mode of undertrack formation may have been relatively frequent elsewhere but potentially overlooked in previous studies.

A large brachyuran, the blue land crab (Cardisoma guanhumiLatreille, 1825) is a keystone species and a prolific burrower in a variety of low-latitude supratidal habitats; however, few studies have addressed its ichnological aspects. Here we present a detailed morphological assessment of C. guanhumi burrows and make comparisons to Macanopsis plataniformis, a trace fossil thought to be constructed by a similar brachyuran. Using nine burrow casts from a mangrove wetland on San Salvador Island, Bahamas, 29 dimensionless morphometric parameters were compared. Results of Bray-Curtis tests show high overall similarity index (0.85) between casts, with complexity, tortuosity, and opening inclination angle having moderate to high similarity (> 0.8). End chamber angle of the blue land crab has highest similarity both within the modern morphometric parameter dataset (> 0.9) and to that of M. plataniformis (0.88). The large terminal chamber is particularly important because of its high preservation potential, as a repository for organic remains, and as a reliable water table (∼ sea-level) indicator. Cluster analysis suggests size-independent Macanopsis-resembling burrow morphology among different extant taxa can be discriminated via quantitative neoichnological methods. Our findings demonstrate how a quantitative assessment of modern burrows and their likely counterparts from the sedimentary record can help constrain tracemaker identity and aid paleoenvironmental reconstruction of low-energy microtidal settings.

The fluvial facies of the Catskill Formation record important ecological events that occurred during Late Devonian time. A well-exposed section between the towns of Blossburg and Covington, in north-central Pennsylvania, contains abundant macrofossils and sedimentary features, making it well-suited for linking Upper Devonian fossil occurrences with depositional environments and sedimentary processes. Strata consist of two distinct fluvial facies: floodplain lithofacies consist of mudrocks, with evidence of pedogenic overprinting and sharp-based sandstones interpreted as crevasse splays; channel-bar lithofacies consist of single- and multi-storied cross-stratified lenticular sandstone bodies interpreted as fluvial channel-bar complexes. Macrofossils occur in 22 discrete horizons spanning > 240 m of stratigraphic succession that include Archanodon bivalve shell impressions, two genera of “placoderms” (Bothriolepis, Phyllolepis), an unidentified acanthodian, and several taxa of sarcopterygian fishes, including lungfish (Dipnoi indet.), Holoptychius, Langlieria, and Sauripterus. Most vertebrate macrofossils are preserved as disarticulated, abraded plates, scales, and bone fragments in sandstone channel-bar deposits. Articulated, unabraded remains are preserved in proximal floodplain deposits. Miospores recovered from Catskill Formation fossil sites in the Blossburg-Covington section belong to the COR subzone of the VCo (Diducites versabilis-Grandispora cornuta) palynological zone, indicating deposition ca. 362 to 361.8 Ma during the late Famennian stage of the Late Devonian. Catskill Formation fluvial strata exposed tens of kilometers to the south and west yield latest Famennian palynomorphs. These broadly contemporaneous continental depositional environments supported Late Devonian vertebrate evolution, including the fin-to-limb transition in tetrapodomorphs, and the possible euryhalinity of vertebrates occupying marine-to-nonmarine transitional habitats.