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 email@example.com with any questions.
Chitons are locally common in New Zealand, and several studies have suggested that their valves are resistant to dissolution, so it seems contradictory that they are under-represented in the sediment and fossil records of New Zealand. Indeed, special resistance to dissolution seems counterintuitive since the valves are primarily made of aragonite. Here we examine the resistance of chiton skeletal material to dissolution in order to expand our understanding of how taphonomic forces affect chitons and to provide insight into the preservation potential of chiton valves. Live individuals of eight species of chitons were collected from Otago Peninsula, South Island, New Zealand. The valves were subjected to one of two pH treatments: ambient pH of 8.10 and reduced pH of 7.70. Notoplax violacea, Sypharochiton pelliserpentis, and S. sinclairi were the most resistant to dissolution while Acanthochitona zelandica, Chiton glaucus, Onithochiton neglectus, and Ischnochiton maorianus were more vulnerable to dissolution. Leptochiton inquinatus lost the most mass in both treatments, but did not show a significant difference between them. SEM images of the dorsal and ventral surfaces on each valve revealed low-pH damage to crystal structures in the articulamentum, while the tegmentum showed no significant damage. Chiton skeletal material in general does not appear to resist dissolution any better than other examined mollusks, but the resistant tegmentum confers considerable resilience to lowered pH. Chiton valves can last up to an estimated 45 years before becoming unrecognizable, which is much shorter than the normal temperate shallow-water exposure time of hundreds to thousands of years.
Cambrian deposits bearing exceptional preservation of soft-bodied fossils offer unique insights into the diversity and paleobiology of the Cambrian biota. In recent years, a growing number of studies have provided evidence that highly labile tissues, such as nervous tissues, may be more common in fossils from Burgess Shale-type deposits than previously appreciated. These discoveries have provided novel insights into the anatomy and phylogeny of Cambrian animals and have also challenged the basic understanding of the limits on the resolution of anatomical information that may be captured by Burgess Shale-type preservation. Evidence used to infer primary blood chemistry was recently reported from the iconic Burgess Shale fossil Marrella splendens, the first such report from a Cambrian Lagerstätte. Diminutive crystals of a copper sulfide mineral found in association with Marrella were interpreted as evidence that the animal's blood contained the Cu-bearing protein hemocyanin. Copper minerals in shales in general, and in the Burgess Shale in particular, however, may have a complex geologic history. Using microscopy and SEM-EDS elemental mapping of material from the Walcott Quarry as well as from the Burgess Shale fossil quarry at Marble Canyon, we assess the timing of emplacement of Cu-sulfides in fossil-bearing mudrocks of the Burgess Shale. These data demonstrate that copper minerals are most conspicuously concentrated within veins that are oriented sub-perpendicular to bedding. These late-stage fluid flow paths allowed Cu-mineralizing fluids to infiltrate bedding plane-parallel cracks, which commonly run through Burgess Shale fossils and extend into the matrix around them. Based upon the relationships of mineral phases associated with fossils, it appears that Cu-mineralization occurred after metamorphic volatilization of a large proportion of the carbonaceous material that originally comprised the fossils. We find that copper minerals in the Burgess Shale are of demonstrably post-Cambrian origin, a secondary overprint that cannot be used to infer original blood chemistry or tissue composition of its fossils.
Although environmental variability generates differences in the preservation of shell assemblages, intrinsic variations in shell characteristics can confound the effects of environment on preservation. However, several studies proposed that the composition of shell supply only affects the intensity of alteration but not its preservation trend along the environmental gradient and that environmental variability represents a major driver of taphofacies preservation. Here, we examine whether taphonomic differences among four infaunal and aragonitic bivalve species differing in shell thickness affect the definition of beach taphofacies in tropical carbonate environments on San Salvador Island (Bahamas). We show that (1) taphofacies can be discriminated with respect to (a) wave and storm activity as a function of exposure to Trade Winds, and (b) sandy beaches versus beaches with a mixture of sands and beach rock (representing a source of exhumed and cemented shells), and (2) species-specific bivalve assemblages show similar gradients in preservation, documenting that differences in preservation between species have minor effects on taphonomic discrimination of beach environments. Environments with a mixture of sands and beach rock are characterized by higher frequency of external cementation and abrasion than sandy beaches. Shells from low-energy beaches are more fragmented and discolored than shells from high-energy beaches. Previous studies showed that shells from San Salvador sandy beaches are more time-averaged than shells from rocky beaches. Differences in preservation between these two environments indicate two pathways: (1) assemblages on sandy beaches are degraded at higher rate but are enriched by old exhumed and lithified shells, and (2) assemblages on rocky beaches are cemented at higher rate. Old and lithified shells on sandy beaches are probably derived from submerged or exposed beach rock patches, leading to the mixture of young, well-preserved shells with old, poorly preserved shells. Shells on sandy beaches thus experience a complex history of burial and exhumation before their final deposition. Therefore, shell assemblages in lower energy carbonate environments from San Salvador Island are highly time-averaged, with the presence of old, poorly preserved shells. The depositional environment is thus the dominant factor controlling the structure of San Salvador beach taphofacies.
Ammonites are iconic members of Jurassic and Cretaceous marine communities, but many questions remain about their ecology. Because it contains a diverse assemblage of well-preserved macro- and microfossils, the upper Maastrichtian Owl Creek Formation exposed in Tippah County, Mississippi, is an excellent site at which to compare carbonate isotopic patterns within and among taxa. A previous isotopic study of two ammonite groups common in the study section (baculitids in the genera Baculites and Eubaculites and scaphitids in the genus Discoscaphites) indicated that Owl Creek baculitids and scaphitids lived near the seafloor. However, their wide range of measured isotopic values, coupled with oxygen isotopic values suggesting that the ammonite shells sometimes were secreted at temperatures cooler than those experienced by other benthic mollusks suggested the possibility that these taxa may have migrated. Here, we test the migration hypothesis by comparing ontogenetic isotopic trends in the shells of co-occurring ammonites, bivalves, and gastropods. We found the range in δ18O values to be generally similar among baculitids, scaphitids, nuculids (Bivalvia), and various gastropods and that there are no consistent δ18O trends through ontogeny in baculitids or scaphitids (or other mollusks). Thus, there is no need to invoke migration for either ammonite group. Additionally, the variability in δ18O values within individual shells is low in scaphitids and baculitids, intermediate among gastropods, and high in nuculids, indicating that benthic temperature variability occurred on timescales that were long relative to the rate of shell formation for ammonites but short relative to that of the lifespan of the gastropods and nuculids. This interpretation implies rapid growth among baculitids and scaphitids, a conclusion consistent with lower δ13C values in scaphitids and baculitids compared to other mollusks.