Ammonites are among the best-known fossils of the Phanerozoic, yet their habitat is poorly understood. Three common ammonite families (Baculitidae, Scaphitidae, and Sphenodiscidae) co-occur with wellpreserved planktonic and benthic organisms at the type locality of the upper Maastrichtian Owl Creek Formation, offering an excellent opportunity to constrain their depth habitats through isotopic comparisons among taxa. Based on sedimentary evidence and the micro-and macrofauna at this site, we infer that the 9-m-thick sequence was deposited at a paleodepth of 70-150 m. Taxa present throughout the sequence include a diverse assemblage of ammonites, bivalves, and gastropods, abundant benthic foraminifera, and rare planktonic foraminifera. No stratigraphic trends are observed in the isotopic data of any taxon, and thus all of the data from each taxon are considered as replicates. Oxygen isotope-based temperature estimates from the baculites and scaphites overlap with those of the benthos and are distinct from those of the plankton. In contrast, sphenodiscid temperature estimates span a range that includes estimates of the planktonic foraminifera and of the warmer half of the benthic values. These results suggest baculites and scaphites lived close to the seafloor, whereas sphenodiscids sometimes inhabited the upper water column and/or lived closer to shore. In fact, the rarity and poorer preservation of the sphenodiscids relative to the baculites and scaphites suggests that the sphenodiscid shells may have only reached the Owl Creek locality by drifting seaward after death.A mmonites have constituted a primary data source for the fields of evolution, paleoceanography, biostratigraphy, and paleoecology for more than a century; their ubiquity, diversity, occurrence in a wide variety of marine environments, and readily preservable shell account for their utility in both paleontological and geological studies. Ammonites have been used extensively in studies of heterochrony because their shells preserve distinct ontogenetic changes that can be tracked in evolving lineages (1, 2); they are valued in paleoceanographic research because, like most mollusks, they are inferred to have precipitated their aragonitic shells in isotopic equilibrium with the surrounding seawater (3, 4). Thus, shell chemistry may record temperature, via oxygen isotopes (δ 18 O) (5), and water mass properties, such as strontium isotopes ( 87 Sr/ 86 Sr), which are used to estimate numerical age (6). Ammonites are also a textbook example of an index fossil; besides being abundant and widespread, they evolved rapidly, making them the dominant Mesozoic tool for relative dating and correlation of shallow water strata. For example, the 35-My-long stratigraphic record of Upper Cretaceous deposits in the US Western Interior Seaway (WIS) has been partitioned into 66 ammonite zones (7). Finally, ammonites underwent a spectacular extinction at the close of the Mesozoic. Explanations for why the ammonites, which were flourishing immediately before the Cretaceous-P...
We describe an outcrop of the Cretaceous-Paleogene (K-Pg) boundary exposed due to construction near New Albany, Union County, Mississippi. It consists of the Owl Creek Formation and overlying Clayton Formation. The Owl Creek Formation is rich in the ammonites Discoscaphites iris and Eubaculites carinatus, which, along with biostratigraphically important dinoflagellate cysts and calcareous nannofossils, indicate deposition occurred within the last 1 million years, most likely last 500 kyrs, of the Cretaceous. The base of the overlying Clayton Formation marks the K-Pg boundary, and consists of a 15-30 cm thick muddy, poorly sorted quartz sand containing abundant spherules representing ejecta derived from the Chicxulub impact event. Impact spherules range in size from 0.5 mm to 1 mm in diameter and are hollow and well preserved, with details such as smaller vesicular spherules enclosed within. The spherules are altered to clay minerals such as smectite and are typical of those found at K-Pg boundary sites in the Gulf of Mexico and beyond. Spherules are scattered throughout the bed, and surface counts suggest an average of 4 spherules per cm 2 . Macrofossils within the spherule bed represent a rich fauna of ammonites, benthic molluscs (bivalves and gastropods), echinoids, as well as crabs and sharks. Macrofossil preservation ranges from whole to fragmentary, with most fossils preserved as internal moulds. The infill of the fossils is lithologically identical to the matrix of the spherule bed, including impact ejecta preserved within phragmocones and body chambers of ammonites, and differs from the underlying Owl Creek Formation. This suggests that the animals were either alive or loosely scattered on the sea floor at the time of deposition. Grain size changes indicate multiple events were responsible for deposition, and together with taphonomic evidence are consistent with dynamic high energy post-impact processes. Later sea level change during the Paleocene is responsible for a sharp contact at the top of the spherule bed. Geochemical evidence from the Owl Creek and Clayton Formations at this locality indicate numerous local paleoenvironmental changes affected the Mississippi Embayment at the time of the K-Pg boundary and mass extinction event.
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