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Fossilized shell assemblages are often the result of postmortem transportation. It is therefore crucial to identify biases introduced due to differences in the hydrodynamic properties of shells to ensure the validity of ecological interpretations. In a flow tank study with the bivalve Donax scortum Linnaeus, 1758, we found that shell size, shape, and ornamentation played an important role in dictating the hydrodynamic properties of shells. This study demonstrates that threshold current velocity for the entrainment of a convex-up shell is generally determined by its size, a result corroborating previous findings. We found that a smaller shell is mobilized with a lower velocity compared to a larger one. Additionally, we found ornamentation of a shell also played a significant role in transportation. Unlike previous studies, we have demonstrated that altered smooth shells require a higher velocity for transportation compared to fresh shells with pronounced ornamentation. The movement trajectory of a shell depends on its asymmetry. The right and left valves of a single individual are deflected in distinctly different directions. This study shows that the extent of such deflection is dependent on the size of the valve and the velocity of the flow. Using a simulation based on our experimental data, we have also demonstrated the effect that transportation bias can have in modifying a shell assemblage. The results of this study underscore the concept that the final distribution of shells following a transportation event may yield a bivalve population significantly different from the original one.
The middle–upper Miocene (Serravallian-Tortonian) Gatun Formation of Panama is one of the most diverse marine fossil assemblages in the Caribbean, and has been widely used as a baseline in studies investigating the effects of the late Neogene closure of the Central American Seaway. Despite being the focus of exhaustive paleoecological sampling studies, previous studies have not revealed the fine-scale spatial and temporal variation in the composition of the Gatun fauna. High-resoluton faunal inventories were conducted from over 600 stratigraphic horizons in the Gatun Formation, capturing more than 200 species, which represent mostly widespread or common, and typically macroscopic taxa. Cluster and detrended correspondence analyses (DCA) reveal a stratigraphic pattern of variation in the faunal composition of individual horizons as well as differences between sampling sites. Stratigraphic curves of DCA axis scores indicate relative shifts in water depth and sedimentation through the stratigraphy and among localities. Overall the Gatun reveals both coarse- and fine-scale temporal variations in paleobathymetry, reflecting long-term shifts in basin subsidence and accommodation space that is potentially overprinted by fifth- or sixth-order glacio-eustatic cycles. The formation accumulated during an overall phase of increasing accommodation space, with major intervals of transgression (lower Gatun), regression (middle Gatun), and further transgression to mid-shelf depths (upper Gatun). Shallowing through the middle part of the Gatun is largely the result of an increase in supply of pyroclastic and volcaniclastic sediments from the adjacent volcanic arc. These findings have implications for understanding the tectonic evolution of the Canal Basin.
Salmonid vertebrae in mixed faunal remains from North Iberian archaeological sites of the upper Paleolithic were analyzed to determine specimen age, migratory status, and the seasonality of catch, based on annual growth marks. Fish size was back calculated from vertebra size using published equations. Although sample size is very small, significant changes in the average size and migratory status of fished specimens, and in the seasonality of fishing were detected. Salmonids (Atlantic salmon Salmo salar and brown trout Salmo trutta) were present as a resource in human diet in different climatic conditions. However, preferential winter harvest was prevalent starting after the Last Glacial Maximum, although this may be an artifact of the data related to sea-level rise. The main potential impact of prehistoric fishing habits on salmonid populations was the removal of the larger breeders from the rivers, thus indirectly promoting the reproduction of smaller fish (i.e., selection for small size, although most likely unintended). The methodology described in this study, if applied to larger collections and/or samples, can provide information on how salmonids reacted to past changes in harvest or climate, and could help to predict the consequences of current environmental and climatic changes.
Various intrinsic as well as extrinsic factors significantly control the preservation of insects. Few studies have presented a quantitative comparison of biostratinomic patterns in different groups, especially at the familial level. Here, we systematically investigated the preservation of 277 specimens of Palaeontinidae and 113 of Tettigarctidae, two hemipterous families from the Jurassic Daohugou beds lacustrine Konservat-Lagerstätte. We carried out quantitative analyses of their size and taphonomic characters, including body orientation, articulation, and preservational quality, and also performed a preliminary experiment to understand the floating and decay process of cicadas. Our statistical analyses reveal significant differences in both body orientation and preservational quality between the two families. Palaeontinidae experienced longer flotation time (mostly over one month) before settling through the water column due to their high SM index (wing surface/body mass ratio) and unfolded wings, increasing the opportunity to decompose on the water surface and resulting in the dorsoventral preservation position with lower preservational quality. In contrast, Tettigarctidae have a comparatively low SM index and overlapping wings, so that their drifting period on the water surface might have been short (mostly within 2 weeks), leading to the lateral preservation position with higher preservational quality. Our results show that the wing-folding behavior and relative size of wings (SM index) influence the buoyancy times and significantly control the preservation patterns of both groups.
Trace fossils of basal, apterygote (wingless) insects from the Pennsylvanian-aged Rhode Island Formation of southeastern New England include the body imprint Tonganoxichnus buildexensis and the trackways Mitchellichnus cf. ferrydenensis, Siskemia elegans, Stiallia pilosa, and Stiaria intermedia. Trackways with double and triple medial impressions (S. elegans and M. ferrydenensis) are rare among trace fossils attributed to such insects. To determine how these unusual trackway morphologies were produced, and why they are so rare, experiments were conducted over a range of media ( = sedimentary substrates) with modern archaeognathans (jumping bristletails) and thysanurans (silverfish). Our experimental results suggest that archaeognathans produced the fossil body imprints, as well as trackways exhibiting opposite symmetry, but cannot rule out thysanurans for trackways exhibiting alternate symmetry. The results also show that a variety of appendages leave medial impressions. The rarity of fossil trackways with more than one medial impression results from the low mass of the animal and the delicate nature of the appendages making such structures, coupled with behavior (jumping and landing) and media properties (saturation and cohesiveness). The production of different numbers of medial impressions by a single archaeognathan species suggests that fossils should show similar variety, albeit in rare cases, and that the named ichnotaxa are likely derived from a single biological taxon. Finally, the experimental work provides new insights into the production and preservation in Stiallia and Tonganoxichnus.
Fossilized trackways have rarely been analyzed quantitatively to examine major trends and patterns in evolution despite their potential utility, especially in understanding locomotory evolution. In the present study, trackways of Triassic archosauriforms were analyzed. The analyses showed foot and stride lengths of archosauriforms increased from the Early to Middle Triassic, especially those of dinosauromorphs, which tripled. Dinosauromorphs were much smaller in foot length and stride length compared to other archosauriforms during the Early Triassic. They reached similar stride length compared with other archosauriforms during the Middle Triassic and similar foot length in the Late Triassic. Stride/foot ratio is significantly higher in dinosauromorphs compared to other archosauriforms throughout the Triassic. This relatively long stride length of dinosauromorphs is attributed to either faster speed or higher relative hip height that was probably caused by their digitigrade foot posture. Analyses of trackway data sets, especially in combination with precise trackmaker assignment and age determination, would bring us more thorough knowledge about locomotory evolution of tetrapods that complements body fossil evidence.