Stable isotopes, niche partitioning and the paucity of elasmosaur remains in the Maastrichtian type area

Abstract

Remains of elasmosaurid plesiosaurs are exceedingly rare in the type-Maastrichtian strata (Late Cretaceous, southeast Netherlands and northeast Belgium), in stark contrast to relatively common skeletal remains of mosasaurs. Here, we present an analysis of δ13C stable isotope values for tooth enamel of two elasmosaur teeth from the type Maastrichtian. The δ13C signal is a proxy for foraging area, trophic level and diving behaviour, the net value of which in these rare elasmosaurs turns out to be not noticeably different from that for the much commoner mosasaurs in the type Maastrichtian. Therefore, the rarity of elasmosaurs in the area probably reflects a primary near-absence of such reptiles during the latest Cretaceous, rather than a taphonomic artefact

New plesiosaurid material from the Maastrichtian type area, the Netherlands

“…Taphonomy and faunal assemblages -Plesiosaur material from the type area of the Maastrichtian Stage is exceedingly rare and so far limited to the family of the Elasmosauridae. The absence of plesiosaurs is in striking contrast to the large numbers of mosasaur and other vertebrate fossils recovered from the same strata (Schulp et al, 2016). However, elsewhere, in particular at several localities in the Southern Hemisphere, elasmosaurs are much more abundant than mosasaurs.…”

“…Attempts have already been made to explain the nearabsence of Elasmosauridae in the type Maastrichtian. The δ 13 C ratio in marine vertebrates has been used as a proxy for trophic level, general habitat and diving behaviour, with the latter two aspects usually overshadowing the first (Schulp et al, 2016). In that paper, the authors noted that the δ 13 C ratio of their elasmosaur samples was similar to the range of δ 13 C values of the mosasaur samples they had studied, which is why they concluded that the near-absence of elasmosaurs in the Maastrichtian type area did not primarily reflect differences in trophic level or diving behaviour between the two groups.…”

“…In part, this may reflect habitat preferences (e.g. Schulp et al, 2016), but also when considered over a wider geographical area, the rather meagre record of latest Cretaceous elasmosaurs across the entire North Atlantic realm is striking.…”

Latest Maastrichtian middle- and high-latitude mosasaurs and fish isotopic composition: carbon source, thermoregulation strategy, and thermal latitudinal gradient

“…In our context, we would thus expect a given taxon from the highpaleolatitude site (Antarctica, 64°S) to have lower δ 13 C CO3 values than those from the medium-paleolatitude site (Patagonia, 45°S). Furthermore, anatomic and isotopic evidence suggests that some mosasaur taxa were divers and had deeper foraging grounds (Sheldon 1997;Schulp et al 2013Schulp et al , 2017Harrell et al 2016). A deeper and offshore food source would cause their bioapatite to have lower δ 13 C values than coexisting shallow-water, nearshore organisms.…”

“…Differences in the carbon fraction are thus expected between the two sites due to different paleolatitudes, and intertaxonomic differences relating to different habitats might also appear. In addition to diet, other factors may influence the carbon isotope composition of bioapatite: breath holding is thought to lower the δ 13 C values of pulmonate diving animals such as mosasaurs (Robbins et al 2008; Schulp et al 2013, 2017; van Baal et al 2013), while sharks are known to show particularly high δ 13 C values in their enameloid (Vennemann et al 2001; van Baal et al 2013; Kocsis et al 2014).…”