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Two ichnofabrics characterized by abundant vertical and helical burrows (ichnogenus Gyrolithes) are described from the Pliocene siliciclastic facies of the southwestern sector of the Guadalquivir Basin (Lepe, Huelva, SW Spain). These ichnofabrics, associated with shallow and marginal marine environments, characterize two consecutive and concordant stratigraphic units: (1) the lower one is dominated by G. nodosus (together with other pellet-lined ichnotaxa), occurs in fine- to medium-grained, massive sands and silty sands, and is characterized by moderate to high bioturbation; (2) the upper ichnofabric is dominated by G. variabilis (and other unlined ichnofossils), occurs in sandy silts, and is characterized by low to high bioturbation. The transition of these two ichnofabrics clearly reflects the ability of an infaunal community to assimilate environmental changes over time. Additionally, new observations at the type locality of G. nodosus, the description of a new locality for G. variabilis and review of existing literature on this ichnogenus have provided the bases for emending the diagnoses of both ichnospecies, to propose a neotype for G. nodosus and to suggest a new type locality for G. variabitis. According to the main architectural features of Gyrolithes specimens studied herein and by comparison with modern analogues, ‘thalassinidean’ shrimps are proposed as their most likely tracemakers. Although it is known that these kinds of crustaceans exhibit a great variability in regards to their burrowing behaviors, further study is needed in order to more fully understand the purpose of these helical bioturbation structures.
Limestone-marl alternations (LMA) are rhythmical successions of carbonate-rich sedimentary rocks. They are often assumed to record cyclic sedimentation linked to Milankovitch cycles. In spite of the importance of LMA for a range of questions in geosciences, it is not unequivocally understood how they originate. The two models explaining their origin both assume carbonate redistribution, either by late diagenetic pressure dissolution amplifying primary depositional differences, or through early diagenetic aragonite dissolution and reprecipitation as calcite, creating LMA even in the absence of primary differences. The latter model is known as differential diagenesis. As both models can imply different interpretations of paleoenvironmental conditions, the identification of the generating process is essential. This study addresses the question how to distinguish the generating process through statistical comparison of taphonomic characteristics of marls and limestones in thin sections by: (1) measuring the relative abundance of originally aragonitic and calcitic components in the fossil assemblages, and (2) by analysis of their orientations. Based on four sets of thin sections from different paleoenvironments from the Upper Ordovician to the Permian, the model of late diagenetic pressure-induced carbonate redistribution is ruled out. The results point towards early diagenetic aragonite dissolution and reprecipitation as calcite as the main diagenetic process generating LMA. Furthermore, the influence of primary sedimentary differences is demonstrated. This approach offers a tool to gauge conditions during sedimentation and a way to assess the systematically poorer preservation of aragonitic components in marine deposystems (aragonite bias) quantitatively.
Considerable refinement of the surficial geology and biostratigraphy Bermuda has resulted in the proper ordering of the phylogenetic sequence of Poecilozonites, and thus offers an opportunity to examine evolutionary pathways within this taxon. Paedomorphism, the retention of juvenile ontogenetic characteristics into adulthood, is a recurrent morphological manifestation in fossil land snail shells of the subgenus P. (Poecilozonites) on this isolated oceanic island. The paleontology of this endemic taxon has been examined over the past century and was a key example of “punctuated equilibria” (PE) in the late 1960s and early 1970s. In a previous study, we documented the biostratigraphy and geochronology from the known fossil record of P. (Poecilozonites) representing atleast the past 500 kyr. Here we focus specifically on paedomorphic forms that appeared in shells at the onset of the last interglaciation, marine isotopes stages (MIS) 6/5e and again at the beginning of the Holocene (MIS 2/1). There are many possible mechanisms to explain the occurrence of paedomorphism including PE, but of importance to this discussion is that neither the size nor the fossil record of Poecilozonites show three independent lineages as proposed by Gould (1969). Gould's several named paedomorphic forms supposedly branching from P. bermudensis over the past 300 kyr, occur only during the last interglacial (sensu lato) MIS 5, and the Holocene (MIS 1). Both punctuation and stasis characterize the morphological changes of this taxon over the past 140 ka, but these changes are reversible, and no speciation is evident.
Taphonomic processes may filter in a biased manner the tiny fraction of leaves preserved as fossils. A common perception is that large leaves are underrepresented; this is based both on intuition (large leaves are more likely to break apart) and some observations of extant vegetation. Characterizing leaf area correctly is critical for reconstructing climate and for studying evolutionary and biogeographic patterns. In extant dicotyledonous angiosperms, leaf area generally scales with the inverse of second-order vein density. This scaling offers the potential to test if fossil leaf fragments were derived from leaves that were larger than complete (or nearly complete) fossil leaves of the same species. Here we test vein scaling on 573 complete leaves from the latest Cretaceous Hell Creek Formation and earliest Paleocene Fort Union Formation in the Williston Basin of western North and South Dakota. We find a strong scaling similar to extant vegetation, with a somewhat shallower slope (1.67 vs. 2.04) and lower r2 (0.64 vs. 0.80). We apply these two scalings to 41 species-site pairs from the Williston Basin that are each represented by complete (n = 355) and fragmented (n = 387) leaves. With both scalings, the reconstructed leaf areas of fragments are on average 10% larger (±36% 1σ) than their complete companions. This small but noisy signal means that the underrepresentation of large leaves, as captured by our study design, is probably not critical for most fossil applications. Comparing directly the reconstructed areas of complete and fragmented leaves appears reasonable, thus expanding the usefulness of fossil leaf fragments.
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