Registered users receive a variety of benefits including the ability to customize email alerts, create favorite journals list, and save searches.
Please note that a BioOne web account does not automatically grant access to full-text content. An institutional or society member subscription is required to view non-Open Access content.
Contact firstname.lastname@example.org with any questions.
Actualistic studies in freshwater environments are scarce, limiting the interpretation of paleoenvironmental information obtained from the fossil record. The objectives of this study are to assess the taphonomic factors that affect the formation of mollusk assemblages in freshwater environments of the pampean region. Twenty sites were analyzed at regional (La Brava and Nahuel Rucá) and environmental (lentic and lotic) scales. Shells were primarily affected by loss of proteinaceous parts, fragmentation, and fine-scale surface alteration. Taphonomic differences were observed in dead shell condition related to environmental conditions and faunistic composition (thin-/thick-shelled species proportion) in spite of the dominance of one species, Heleobia parchappii. The differences were related to extrinsic (environmental) factors probably due to differential influence of organic decay, microbioerosion, and/or dissolution. However, intrinsic factors, especially those related to differential preservation and input rates of thin-/thick-shelled species, may be also biasing the compositional fidelity of death assemblages. Although the degree of taphonomic alteration across locales and environments was evident, all mollusk assemblages still preserved their biological signature from the precursor communities.
Beginning with the discovery of the middle Cambrian Burgess Shale, the rare examples of exceptional preservation of soft tissues in fine-grained strata have been celebrated as windows into the diversity and complexity of ancient marine animal life. Even so, with the loss of original organic compounds, the physiological attributes of the fossils can be inferred only on the basis of modern descendants or sister groups among extant representatives that can serve as analogs. To test whether soft-bodied fossils from the Burgess Shale and similar Cambrian Lagerstätten could preserve chemical evidence of their original biology, a number of specimens from different groups were analyzed with synchrotron-based X-ray fluorescence imaging to determine the spatial distribution of a range of elements, especially those in exceedingly trace amounts that cannot be detected by more conventional analytical methods. The relative distribution of common elements like Fe, K, Ti, and Ca is related to the composition of the host shale, abundance of pyrite, and presence or absence of preserved exoskeletal calcite. However, the black stain commonly preserved with the arthropod Marrella splendens is preferentially enriched in Cu due to localized residue in the carbon and minute crystals of chalcopyrite. This is interpreted as indicative of the original chemical composition of a primary biotic fluid, likely blood. Since modern-day arthropods generally utilize Cu-containing hemocyanin for oxygen transport, it is hypothesized that hemocyanin was the biogenic source of the observed Cu. This is the first chemical evidence for blood composition in the invertebrate fossil record. These observations reinforce the consensus molecular clock age for the hemocyanin gene family, which traces its origins back to the ‘Cambrian explosion’, thus providing evidence for the early origin of this oxygen carrier among some arthropod groups.
A variety of competitive advantages accompany colonization of hard substrates in aquatic settings, and as a result, many organisms adopt this mode of life. Similarly, adverse effects may occur to organisms that have themselves been encrusted. Many marine basibiont faunas (organisms that provide a substrate for populations of attached epibiontic organisms), including bivalve mollusks, are known to employ a range of chemical and physical deterrents to minimize the incidence of epibiotic attachment, with its attendant detrimental impacts. Freshwater basibiont and epibiont associations, however, are poorly documented. We collected an assemblage of dead, empty freshwater mussel (unionid bivalve) shells from the Saint John River, New Brunswick, Canada. Some unionid specimens exhibited colonization by attached Trichoptera (caddisfly) cases, in either their final instar larval or pupal developmental stages. The trichopteran assemblage, identified from case morphology, included abundant Neophylax sp. together with common Goera sp. and rarer Helicopsyche borealis. For all genera, site-selective attachment occurred upon the shell surfaces. Crystalline aragonite and organic layers exposed within taphonomically damaged regions of molluscan shell were not colonized. Attachment was instead confined to areas of intact periostracum posterior to the umbonal region of the shell, the periostracum perhaps being selected for its rough surface texture in comparison to aragonitic layers. Trichoptera cases dislodged during handling of specimens revealed no damage to the underlying shell. Observations undertaken on the bivalve shells using scanning electron microscopy suggest that physical barriers to attachment are absent within these faunas, and evidently, any chemical barriers, if present, are largely ineffective.
Ichnological analysis in cores has grown substantially in the past two decades because of its importance in fields such as paleoecology, biostratigraphy, or reservoir characterization. Yet core analysis entails some added difficulties in comparison to ichnological outcrop study, especially in modern marine sediments. Quantitative pixel analysis conducted on high-resolution images is used to facilitate the study of bioturbation in marine cores from site U1385 of IODP Expedition 339 in the Gulf of Cádiz. Lateral and vertical variation of pixel values obtained from the infilling trace fossils and from the host sediment are revealed to be highly useful to: (1) differentiate between trace fossils and biodeformational structures, including the host sediment; (2) discriminate between ichnotaxa (i.e., Palaeophycus vs. Planolites); (3) differentiate between passively and actively infilled structures (e.g., Thalassinoides vs. Planolites), and (4) characterize the horizon of bioturbation and thus evaluate the depth of penetration of particular tracemakers.
We describe and interpret a tracksite in the Lower Jurassic Aztec Sandstone in Valley of Fire State Park, southern Nevada. The site contains approximately one hundred tracks of the ichnogenus Brasilichnium, arranged in twelve, subparallel trackways, all on the same foreset bedding plane. The Brasilichnium trackmaker was most probably a fossorial, tritylodontid therapsid. Sedimentologicial analyses indicate that the trackway surface is a wind-ripple horizon with a primary dip of about 25°, and that the animals climbed straight up the slip face of the dune. A combination of features leads us to conclude that the footprints were impressed into a crust of moist, cohesive sand, leaving two modes of preserved tracks: (1) shallow, well-defined tracks without associated sand crescents, and (2) deeper, less well defined tracks with associated sand crescents. We interpret this assemblage of tracks to record gregarious behavior in a mixed-age group of tritylodontid therapsids. In the correlative Navajo Sandstone, other researchers have documented the presence of complex networks of burrows concentrated in elevated mounds, reminiscent of colonies of North American prairie dogs. The Brasilichnium trackmaker is a good candidate to have excavated the burrows. Although we cannot directly associate the Brasilichnium trackmaker with the burrow complexes, we hypothesize that these gregarious, fossorial animals lived in prairie-dog–town–like colonies. This study supports the aridity food-distribution hypothesis, which posits that the patchy distribution of food resources in arid environments creates selective pressure for colonial behavior.