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A first-time chemometric study of energy-dispersive X-ray (EDX) data of Charophyta gyrogonites is presented. Specimen provenance is a microbialitic carbonate lacustrine succession from the Triassic (Anisian, 243.8 ± 1.9 Ma) of the Cerro Puntudo Formation, San Juan, Argentina. Gyrogonites from three different strata of the succession are studied. Data obtained by EDX include major and minor components, which are analyzed by principal component analysis (PCA). The aim of this study is twofold: first to determine the preservation features of gyrogonites by way of a chemometric approach (i.e., EDX followed by PCA) and then to infer the likely, original chemical composition of the paleolake inhabited by charophytes. EDX spectra show the presence of O, Ca, and minor elements (e.g., Si and Mg), indicating a predominantly calcium carbonate (CaCO3) composition. Principal component analysis supports differences obtained between central and peripheral areas of the gyrogonites, indicating a higher CaCO3 content in their central part. On the other hand, in their outer part, the CaCO3 diminishes and the presence of Si compounds is recorded. No significant differences among gyrogonites from the three different strata are found, implying a similar preservation mode. This suggests a differential diagenetic pattern for the external cells of the gyrogonites than their centers. These results have implications regarding the chemical composition of the paleolake water (Si and Ca availability) and the provenance and catchment areas. Results are encouraging regarding the usefulness of a chemometric approach for studies of fossil remains in lacustrine environments when other techniques of chemical analysis are not available.
Crusts identified as a stromatolitic mat are described for the first time from a small, tectonically uplifted basin associated with a marine terrace along the central gulf coast of the Baja California peninsula at Punta Chivato. Microscopic analyses of crustal laminae exhibit calcified features directly comparable to microbial processes that precipitate carbonates in modern stromatolites. Floating calcified mats recently described from extremely hypersaline, closed lagoon systems in the northern Gulf of California (Isla Angel de la Guarda) provide the best analog to the Punta Chivato locality. Preservation of the Punta Chivato stromatolites is likely due to the persistence of arid conditions through multiple glacial-interglacial cycles. Comparison with the marine terrace sequence at Punta Chivato implies an age range of 334 ka to 712 ka for the deposit. This study lends support for a dedicated survey of closed lagoons and terraces where microbialites and their fossil stromatolite representatives may be found along the roughly 3000-km length of peninsular gulf shores and related islands.
Seagrass meadows are productive marine ecosystems that stabilize sediments and provide food and shelter for a diverse associated community. The recognition of these important habitats in the geological record is problematic, because marine angiosperms rarely fossilize. Thus, the presence of paleo-seagrass vegetation often has to be inferred from the occurrence of associated organisms with a higher potential for fossilization, such as mollusks. Because most mollusk taxa are not restricted to seagrass meadows, the species composition and feeding ecology of fossil mollusk faunas need to be considered when distinguishing paleo-seagrass meadows from other marine habitats. In this study the utility of faunal composition and feeding ecology of gastropods as an indicator of seagrass vegetation was tested using present-day ecosystems. Bulk-sediment samples containing gastropod death assemblages from shallow-water seagrass meadows and unvegetated sandflats from San Salvador Island, Bahamas were collected in July 2012. Vegetation varied across localities in terms of density and number of seagrass species. Twenty-four standardized (n = 200) samples of gastropods were compared in terms of species composition and relative abundance of feeding guilds. Multivariate analyses indicate that species composition is an effective tool for distinguishing between gastropod assemblages from vegetated versus unvegetated areas. To a lesser extent, species composition differs among vegetation zones on sandflats with seagrass cover. Feeding guild composition based on species richness also differs on seagrass-vegetated and unvegetated sandflats. The results suggest that gastropod assemblages are a useful proxy for seagrass meadows in the fossil record.
The Plio-Pleistocene cave site of Sterkfontein has yielded the remains of Cercopithecoides williamsi, a colobine monkey reconstructed as a terrestrial folivore given its dental morphology, heavily worn teeth and robustly built postcranial skeleton. To address the validity of these dietary and habitat inferences, patterns of dental microwear obtained from low-magnification stereomicroscopy for C. williamsi specimens from Sterkfontein Member 4 (n = 13) are compared to a broad comparative sample of extant primates, including Cebus apella (n = 10), Colobus polykomos, (n = 6), Papio anubis (n = 13), and Papio ursinus (n = 39) as well as extinct Parapapio broomi (n = 12) from Sterkfontein Member 4 and Theropithecus oswaldi danieli (n = 6) from Swartkrans Member 1. Cercopithecoides williamsi exhibits numerous large pits and puncture pits, and few fine scratches. When diet is considered, C. williamsi is closely associated with C. polykomos and C. apella, suggesting folivorous resources and some hard-objects were consumed, possibly from seeds or grit on fallen fruit, but more likely from grit adhering to the underground storage organs of C4 plants given its mixed C3/C4 isotopic signal. This fossil taxon groups with terrestrial primates more than arboreal ones when habitat is considered. Individuals attributed to C. williamsi largely cluster as a group and a comparison of multiple use-wear scars differentiates open or partially open habitats from forested ones, suggesting dental microwear provides a strong habitat signal.
The presence of six articulated exoskeletons of late holaspid specimens of the rare harpetid Eoharpes benignensis entombed under a pygidial shield of the large asaphid trilobite Nobiliasaphus repulsus from the Middle Ordovician Dobrotivá Formation of the Prague Basin, Czech Republic is interpreted as a unimodal monotaxic trilobite cluster. The sheltered preservation of the trilobites may be explained as; (1) hiding behavior associated with predation pressure; (2) storm disturbance; or (3) molting associated with feeding. It is herein suggested that these Middle Ordovician holaspid trilobites deliberately entered the restricted space under a large isolated asaphid trilobite pygidial shield to find a refuge and shared the space within restricted shelters with conspecifics. The completeness of all specimens of the rare taxon Eoharpes, combined with the presence of more than one individual in this restricted space, excludes the possibility of transportation by bottom currents. This exceptional find represents an example of “frozen behavior” and provides a new insight in the life strategy of Middle Ordovician benthic trilobites. Attack abatement, e.g., avoidance and dilution effects, is for the first time proposed as a possible explanation for this example of sheltered gregarious behavior in trilobites.