Biotic Composition and Taphonomy of an Upper Cretaceous Konservat-Lagerstätte: The Ingersoll Shale, Eutaw Formation, Eastern Alabama

Abstract

The Ingersoll shale is a thin (<1m), laterally restricted clay lens within the Upper Cretaceous (Santonian) Eutaw Formation, Russell County, eastern Alabama. The clay lens represents abandoned tidal creek fill within an estuarine environment. Excavation of this marginal marine Konservat-Lagerstätte has produced an abundant, diverse, and extraordinarily well-preserved biotic assemblage, which is dominated by terrestrial plants but includes invertebrates and vertebrates. The macroflora consists of leaves from dicotyledon and monocotyledon angiosperms (over two dozen species), gymnosperms, and a variety of water plants (ferns and lycopsids). Many of these plants represent riparian vegetation that was introduced to the site via wind transport; they have minimal signs of damage from being transported by water. Although most plant tissues are carbonized and/or pyritized, some leaf cuticle is unmineralized, flexible, and easily removed from the matrix. Conifer stems are often preserved in three-dimensions via early authigenic pyritization. Some of these reveal in situ amber rods. The shale lens also contains a variety of plant reproductive organs (e.g., cones, and seedpods), megaspores from heterosporous isoetalean lycopsids and water ferns, a variety of fungal spores, pollen, acritarchs, and marine dinoflagellates. Many of these fossils were transported into the depositional site. Amber is very abundant in the lower portions of the lens and often contains inclusions, some of which are insect appendages, mites, and fungal hyphae. In the reducing environment represented by the Ingersoll shale, carbonates are completely absent, and phosphatic components such as vertebrate bones have not yet been found. However, well-preserved vertebrate integumentary structures, such as theropod feathers and fish scales, have been found. These structures represent the preservation of originally protein-based tissues (e.g., beta-keratin and collagen). Feathers are preserved by replacement by fossilized bacilliform bacteria and are especially significant. The Ingersoll-shale biota is more diverse than any previously documented Gulf coast Santonian deposit and provides a unique look into Late Cretaceous terrestrial plant and animal communities. In particular, the Ingersoll shale has yielded the largest collection of fossil feathers from the Mesozoic strata of North America. Furthermore, understanding the taphonomy of the Ingersoll shale biota provides insight into the conditions that govern the preservation of refractory soft tissues, thus enhancing future prospecting methods for other nearshore Konservat-Lagerstätten.