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.These proportions are representative of a living community (19) and not of an egg-laying habitat transiently occupied by females (20).
Because ammonites are one of the most diverse, abundant, and well-preserved clades in the history of life, they are a mainstay in macroevolutionary and biodiversity studies; however, their ecologies are poorly understood, and it is unknown whether taxa lived near the sea surface or seafloor.
This uncertainty undermines their use in paleoecological and paleoenvironmental reconstructions, which depend on knowledge of organisms’ depth preferences.
A variety of studies have attempted to determine the ecology of ammonites based on analogies with living relatives, shell morphology, facies distribution, faunal associations, and isotopic composition.
However, these studies have had limited success for both biological and geological reasons.
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).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 (δSr), which are used to estimate numerical age (6).Similarly, Tanabe (13) mapped the distribution of Turonian ammonites along an onshore–offshore transect, and noted that their postmortem distribution was broader than the settings they inhabited during life.Uncertainty in ammonites’ preferred habitat is especially concerning for temperature reconstructions based on their occurrence or isotopes because temperature varies both with depth and with distance to the shoreline.Further, many studies were undertaken in the WIS, where the water mass properties are poorly understood and controversial (16, 17).