Mara Brady

Advisor: Ray Rogers
Senior Honors Thesis: 2005

An Experimental and Field-Based Approach to the Taphonomy of Microvertebrate Assemblages: A Case Study in the Judith River Formation of North-Central Montana

Mara Brady Microvertebrate assemblages, or “microsites,” are fossilized concentrations of small physico-chemically resistant vertebrate elements, including small bones and teeth from mammals, crocodiles, turtles, fish, and dinosaurs. These assemblages are generally preserved in two distinct depositional settings: 1) low-energy pond/lake environments and 2) high-energy channel facies. Many previous works have focused on the origins of channel-hosted concentrations. Several of these studies examined the behavior of bone in an active flow, and most concluded that bone material has great dispersal potential in water. Other studies focused on the characterization of fossil microvertebrate assemblages and generally discount the possibility of using these sites for paleobiological reconstruction, claiming that the bones in these assemblages have been transported great distances from their original habitats. However, previous studies generally fail to provide a mechanism of bone concentration in a fluvial environment. How does bone material concentrate in an active channel if its predicted behavior in water is dispersal?

This study takes a two-fold approach to understanding the origins of microvertebrate assemblages by 1) comparing sites found in both lacustrine and channel deposits and 2) conducting flume experiments in which an active flow is directed through a bed of sediment with a known concentration of dispersed bone material. Lacustrine and fluvial assemblages can be readily distinguished based on sedimentary context and the overall richness, or abundance, of bone material. Moreover, taphonomic features, such as the degree of surface alteration and trends in size sorting, differentiate the two site types. Bone material from the low-energy lacustrine assemblages (Type I) is dispersed as a background concentration (<1% bone by volume) throughout the fine-grained mudstone deposits, while bone material in channel-hosted assemblages (Type II) is concentrated in discrete lags (up to 3% bone by volume) localized at the base of fine-grained fluvial deposits. Microvertebrate elements from both types of sites exhibit a range of rounding, fragmentation, alteration, size, and shape. However, bone material from Type II assemblages shows evidence of greater surface alteration (discoloration, weathering) and is better sorted compared to the bone material from Type I assemblages.

Flume experiments were conducted to better constrain potential origins of microvertebrate assemblages. An active flow was directed through beds of fine-grained sand and dispersed bone material with three different initial background bone concentrations (0.1%, 1%, and 10% bone by volume). In all three experiments, bone material became less evenly dispersed through time as local accumulations developed. Fine bone material (0.5-1 mm) was preferentially winnowed to the distal portions of the flume, while the coarsest fraction of bone material (2-4 mm) was transported minimally and generally remained in the proximal portion of the flume as lag deposits.

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Chris Dwyer

Advisor: Ray Rogers
Senior Honors Thesis: 2005

A Comparative Investigation of Diagenesis in Fossil Teeth and Fish Scales from the Upper Cretaceous Two Medicine and Judith River Formations of Montana

Chris DwyerThe Campanian Two Medicine (TMF) and Judith River (JRF) Formations of Montana preserve coeval deposits of the semi-arid alluvial uplands (TMF) and the coastal plain lowlands (JRF). Previous work has documented the abundance and quality of vertebrate fossils in both formations. In this study I explore the diagenesis of dinosaur teeth, crocodile teeth, and gar scales using REE content and authigenic cements. The fossils under investigation were collected from several well-documented sites of fluvial and lacustrine origin.

Authigenic cements filling cracks and dentine tubules were examined in 23 theropod teeth, 19 hadrosaur teeth, 6 crocodile teeth, 4 ceratopsian, ankylosaur and myledaphus teeth, and 8 gar scales using polarized light microscopy and SEM-EDS. Fills include calcite, sulfates, sulfides, bornite, authigenic and detrital clays, iron oxides, and clastic particles. Patterns of authigenic cementation serve to describe the general process of permineralization, and also characterize pore water chemistry in the burial environment.

Rare earth element (REE) concentrations were determined for the same sample of teeth and scales using LA-ICP-MS. TMF teeth have higher REE concentrations than JRF teeth and are enriched in middle REE. JRF teeth are enriched in HREE. TMF teeth also tend to show more variability in REE concentrations. These REE patterns suggest that (1) TMF teeth likely spent the early stages of diagenesis in or upon alluvial soils, (2) TMF teeth experienced greater small-scale variability in the factors controlling REE uptake, and (3) JRF teeth likely spent early diagenesis in stagnant aqueous environments. These findings are consistent with independent sedimentologic and taphonomic data, and are also in line with previous studies of other workers that focused on REE patterns in TMF and JRF bone.

Finally, the overall REE patterns in enamel and dentine are similar within a given element, but normalized total REE concentrations vary in these tissues from relatively low (enamel) to relatively high (dentine). This pattern is consistent with porosity and crystallite density contrasts in enamel and dentine, and illustrates the importance of these factors in diagenetic processes.

Brett Dennis-Duke

Advisor: Ray Rogers
Senior Honors Thesis: 2005

Revisiting the Magnetostratigraphy of the Upper Cretaceous Berivotra and Maevarano Formations, Northwestern Madagascar

Brett Dennis-Duke The Maevarano and Berivotra Formations of the Mahajunga Basin in northwestern Madagascar yield a diverse assemblage of paleobiologically and paleobiogeographically significant vertebrate fossils. In an effort to correlate these units to the geomagnetic polarity time scale, oriented rock samples were collected from 28 sites. Magnetic remanence was detected with a Superconducting Rock Magnetometer (SRM) at the University of Minnesota’s Institute for Rock Magnetism, employing both alternating field (AF) and thermal demagnetization techniques. Although signals were weak, analysis thus far corroborates work done by Michelle Casey in 2003. Both studies correlated the Berivotra Formation and the uppermost Maevarano Formation (more tentatively) to Chrons 29R and 30N (late Maastrichtian). Additionally, trends were observed that might offer insight into future study: 1) Fidelity of magnetic signal seems to be distinctly tied to Lithologic character, and 2) There may be a reversed polarity or excursion recorded in Berivotra sediments. Based on this study, it is recommended that further work be conducted on specific units that may have more interpretable signals (specifically the Berivotra Formation and the Miadana Member of the Maevarano Formation). It is also suggested that extensive sampling be done in the interval 15-20 meters below the Berivotra Formation – Betsiboka limestone contact to test the hypothesis that a reversal or excursion is present.

Josephine Williams

Advisor: Ray Rogers
Senior Honors Thesis: 2005

Authigenic cements and Rare Earth Element Concentration in Fossil Bones from the Upper Cretaceous Two Medicine Formation, Northwestern Montana

Josephine WilliamsThe Upper Cretaceous (Campanian) Two Medicine Formation of northwestern Montana preserves abundant vertebrate fossils. An understanding of the changes that take place within a given bone as it becomes a fossil is essential to an informed interpretation of that fossil material and its associated depositional and diagenetic system. I investigated authigenic cements and rare earth element (REE) concentrations in fossil bones from a variety of terrestrial depositional environments, including high-energy fluvial deposits (sandstones) and low-energy floodplain and coastal deposits (mudstones). I explored preservational patterns in a paleoenvironmental context by comparing trends in authigenic cementation and REE concentration across differing lithologies, between similar depositional environments, and between individual fossil bones within a site. Authigenic cements were investigated using polarized light microscopy and scanning electron microscopy (SEM). REE concentrations in fossil bone tissue were examined using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS).

The nature and relative abundance of authigenic cements vary among sites. This suggests that authigenic cements track environments, and that there exists a correlation between depositional environment and authigenic mineralization. REE concentrations tended to display distinctly different fractionation patterns by site, and thus tracked depositional settings. In general, fossil bones from sandstones show the most variation in both authigenic cements and REE concentration, and are typified by HREE enrichment and abundant calcite and iron oxide void fill. Fossil bones from a coastal mudstone locality exhibit the least variability in preservational state, and are characterized by presence of pyrite, replacement of bone tissue by calcite, and very low levels of REE fractionation. Fossil bones from upland low-energy sites display intermediate variability and are characterized by MREE enrichment, and the presence of chlorite and detrital clay. Additionally, certain authigenic minerals and REE fractionation patterns are indicative of specific Eh/pH conditions. Overall, the data collected during the course of this multifaceted study allow for characterization of local conditions during early diagenesis, and reveal novel insights into the regional microtaphonomy of the Two Medicine system.

Kirsten Fristad

Advisor: John Craddock
Senior Honors Thesis: 2005

The Valley of Ten Thousand Smokes in Katmai National Park, Alaska as a Mars Analogue

Kirsten FristadRecent NASA missions to Mars have returned increasingly detailed surface imagery, motivating the effort to find and study analogue features on Earth to understand processes on Mars. The 1912 eruption of Novarupta in Katmai National Park created a barren volcanic landscape, the Valley of Ten Thousand Smokes (VTTS), which remains devoid of vegetation today. The precisely dated VTTS, created by pyroclastic volcanism, contains many geomorphic features similar to Mars and is an excellent test case for studying life in an inhospitable environment. The landscape would be beneficial for planetary science field studies, remote sensing and rover testing.