A nitrogen isotopic shift in fish otolith-bound organic matter during the Late Cretaceous.

The nitrogen isotopes of the organic matter preserved in fossil fish otoliths (ear stones) are a promising tool for reconstructing past environmental changes. We analyzed the 15N/14N ratio (δ15N) of fossil otolith-bound organic matter in Late Cretaceous fish otoliths (of Eutawichthys maastrichtiensis, Eutawichthys zideki and Pterothrissus sp.) from three deposits along the US east coast, with two of Campanian (83.6 to 77.9 Ma) and one Maastrichtian (72.1 to 66 Ma) age. δ15N and N content were insensitive to cleaning protocol and the preservation state of otolith morphological features, and N content differences among taxa were consistent across deposits, pointing to a fossil-native origin for the organic matter. All three species showed an increase in otolith-bound organic matter δ15N of ~4‰ from Campanian to Maastrichtian. As to its cause, the similar change in distinct genera argues against changing trophic level, and modern field data argue against the different locations of the sedimentary deposits. Rather, the lower δ15N in the Campanian is best interpreted as an environmental signal at the regional scale or greater, and it may be a consequence of the warmer global climate. A similar decrease has been observed in foraminifera-bound δ15N during warm periods of the Cenozoic, reflecting decreased water column denitrification and thus contraction of the ocean's oxygen deficient zones (ODZs) under warm conditions. The same δ15N-climate correlation in Cretaceous otoliths raises the prospect of an ODZ-to-climate relationship that has been consistent over the last ~80 My, applying before and after the end-Cretaceous mass extinction and spanning changes in continental configuration.

Pleistocene sea level changes and crocodile population histories on the Isthmus of Panama: A comment on Avila-Cervantes et al. (2020).

Avila-Cervantes et al. proposed that glacial-interglacial sea level changes played an important role in the evolutionary and demographic histories of the crocodile Crocodylus acutus on the Isthmus of Panama. However, the study used erroneous sea level proxy data that produced flawed paleogeographic reconstructions. We present new paleogeographic reconstructions and review the timing of, and proposed mechanisms behind, the demographic events estimated by Avila-Cervantes et al.. With the data currently available, we find little evidence to support the hypothesis that sea level changes drove population demographic events in crocodiles on the Isthmus. Alternative hypotheses, including changing climate and habitat suitability, are equally valid and should be considered along with well-supported sea level models.