Resolving taphonomic and preparation biases in silicified faunas through paired acid residues and X-ray microscopy.

Paired petrography and acid maceration has shown that preferential silicification of shelly faunas can bias recovery based on taxon and body size. Here, silicified fossils from the Upper Ordovician Edinburg Formation, Strasburg Junction, Virginia, USA, were analyzed using X-ray tomographic microscopy (µCT) in conjunction with recovered residues from acid maceration of the same materials to further examine sources of potential bias. Results reveal that very small (<~1 mm) fossils are poorly resolved in µCT when scanning at lower resolutions (~30 µm), underestimating abundance of taxa including ostracods and bryozoans. Acid maceration, meanwhile, fails to recover poorly silicified fossils prone to disarticulation and/or fragmentation during digestion. Tests for patterns of breakage, however, indicate no significant size or taxonomic bias during extraction. Comparisons of individual fossils from 3-D fossil renders and maceration residues reveal patterns of fragmentation that are taxon-specific and allow the differentiation of biostratinomic and preparational breakage. Multivariate ordinations and cluster analyses of µCT and residue data in general produce concordant results but indicate that the variation in taxonomic composition of our samples is compromised by the resolvability of small size classes in µCT imaging, limiting the utility of this method for addressing paleoecological questions in these specific samples. We suggest that comparability of results will depend strongly on the sample size, taphonomic history, textural, and compositional characteristics of the samples in question, as well as µCT scan parameters. Additionally, applying these methods to different deposits will test the general applicability of the conclusions drawn on the relative strengths and weaknesses of the methods.

Is the middle Cambrian Brooksella a hexactinellid sponge, trace fossil or pseudofossil?

First described as a medusoid jellyfish, the "star-shaped" Brooksella from the Conasauga shale Lagerstätten, Southeastern USA, was variously reconsidered as algae, feeding traces, gas bubbles, and most recently hexactinellid sponges. In this work, we present new morphological, chemical, and structural data to evaluate its hexactinellid affinities, as well as whether it could be a trace fossil or pseudofossil. External and cross-sectional surfaces, thin sections, X-ray computed tomography (CT) and micro-CT imaging, revealed no evidence that Brooksella is a hexactinellid sponge or a trace fossil. Although internally Brooksella contains abundant voids and variously orientated tubes consistent with multiple burrowing or bioeroding organisms, these structures have no relation to Brooksella's external lobe-like morphology. Furthermore, Brooksella has no pattern of growth comparable to the linear growth of early Paleozoic hexactinellids; rather, its growth is similar to syndepositional concretions. Lastly, Brooksella, except for its lobes and occasional central depression, is no different in microstructure to the silica concretions of the Conasauga Formation, strongly indicating it is a morphologically unusual endmember of the silica concretions of the formation. These findings highlight the need for thorough and accurate descriptions in Cambrian paleontology; wherein care must be taken to examine the full range of biotic and abiotic hypotheses for these compelling and unique fossils.

Actualistic approaches to interpreting the role of biological decomposition in microbial preservation.

Taphonomic processes, especially post-mortem biological decomposition, act as crucial controls on the microbial fossil record. Information loss during the fossilization process obscures interpretation of ancient microbial ecology and limits our view of preserved ecosystems. Conversely, taphonomic information can itself provide insight into fossilization pathways and processes. This information-gain approach requires specific attention to taphonomic patterns in ancient assemblages and robust modern analogue data to serve as points of reference. In this study, we combine experimental taphonomy with decomposition models in order to constrain taphonomic hypotheses regarding Proterozoic microfossil assemblages. Several filamentous and coccoidal prokaryotic and eukaryotic phototrophs were evaluated for taphonomic pattern over the course of a short (~100 days) decomposition experiment. In parallel, simple numerical models were constructed to explain potential taphonomic pathways. These analogue data were then compared to two Mesoproterozoic fossil assemblages, the ~1.5 Ga Kotuikan Formation, Siberia, and the ~1 Ga Angmaat Formation, Canada. Concordant with previous experiments and observations, our results suggest that sheath morphology is more persistent than cell/trichome morphology during early stages of decomposition. These experiments also suggest that taphonomic change in cell morphology may follow one of several trajectories, resulting in distinct taphonomic endpoints. Model output suggests two categories of underlying mechanism and resultant taphonomic trajectory: (1) uniform decomposition, resulting in a low overall taphonomic grade and poor preservation, and (2) faster decomposition of structurally compromised individuals, producing a final population with better overall preservation of very few individuals. In this experiment, cells of coccoidal organisms exhibit the first pattern and trichomes of filamentous organisms and some sheaths exhibit the second. Comparison with preserved microfossil assemblages suggests that differences in taphonomic pattern between parts of an assemblage could be useful in assessing taphonomic processes or degree of taphonomic loss in an entire assemblage.

Ultrastructure of Ediacaran cloudinids suggests diverse taphonomic histories and affinities with non-biomineralized annelids.

Cloudinids have long been considered the earliest biomineralizing metazoans, but their affinities have remained contentious and undetermined. Based on well-preserved ultrastructures of two taxa, we here propose new interpretations regarding both their extent of original biomineralization and their phylogenetic affinity. One of these taxa is a new cloudinid from Mongolia, Zuunia chimidtsereni gen. et sp. nov., which exhibits key characteristics of submicrometric kerogenous lamellae, plastic tube-wall deformation, and tube-wall delamination. Multiple carbonaceous lamellae are also discovered in Cloudina from Namibia and Paraguay, which we interpret to have originated from chitinous or collagenous fabrics. We deduce that these cloudinids were predominantly originally organic (chitinous or collagenous), and postmortem decay and taphonomic mineralization resulted in the formation of aragonite and/or calcite. Further, based on our ultrastructural characterization and other morphological similarities, we suggest that the cloudinids should most parsimoniously be assigned to annelids with originally organic tubes.

Discovery of bilaterian-type through-guts in cloudinomorphs from the terminal Ediacaran Period.

The fossil record of the terminal Ediacaran Period is typified by the iconic index fossil Cloudina and its relatives. These tube-dwellers are presumed to be primitive metazoans, but resolving their phylogenetic identity has remained a point of contention. The root of the problem is a lack of diagnostic features; that is, phylogenetic interpretations have largely centered on the only available source of information-their external tubes. Here, using tomographic analyses of fossils from the Wood Canyon Formation (Nevada, USA), we report evidence of recognizable soft tissues within their external tubes. Although alternative interpretations are plausible, these internal cylindrical structures may be most appropriately interpreted as digestive tracts, which would be, to date, the earliest-known occurrence of such features in the fossil record. If this interpretation is correct, their nature as one-way through-guts not only provides evidence for establishing these fossils as definitive bilaterians but also has implications for the long-debated phylogenetic position of the broader cloudinomorphs.

Melanosome diversity and convergence in the evolution of iridescent avian feathers-Implications for paleocolor reconstruction.

Some of the most varied colors in the natural world are created by iridescent nanostructures in bird feathers, formed by layers of melanin-containing melanosomes. The morphology of melanosomes in iridescent feathers is known to vary, but the extent of this diversity, and when it evolved, is unknown. We use scanning electron microscopy to quantify the diversity of melanosome morphology in iridescent feathers from 97 extant bird species, covering 11 orders. In addition, we assess melanosome morphology in two Eocene birds, which are the stem lineages of groups that respectively exhibit hollow and flat melanosomes today. We find that iridescent feathers contain the most varied melanosome morphologies of all types of bird coloration sampled to date. Using our extended dataset, we predict iridescence in an early Eocene trogon (cf. Primotrogon) but not in the early Eocene swift Scaniacypselus, and neither exhibit the derived melanosome morphologies seen in their modern relatives. Our findings confirm that iridescence is a labile trait that has evolved convergently in several lineages extending down to paravian theropods. The dataset provides a framework to detect iridescence with more confidence in fossil taxa based on melanosome morphology.