Calcium Isotopic Evidence for Vulnerable Marine Ecosystem Structure Prior to the K/Pg Extinction

Abstract

The collapse of marine ecosystems during the end-Cretaceous mass extinction involved the base of the food chain [1] up to ubiquitous vertebrate apex predators [2-5]. Large marine reptiles became suddenly extinct at the Cretaceous-Paleogene (K/Pg) boundary, whereas other contemporaneous groups such as bothremydid turtles or dyrosaurid crocodylomorphs, although affected at the familial, genus, or species level, survived into post-crisis environments of the Paleocene [5-9] and could have found refuge in freshwater habitats [10-12]. A recent hypothesis proposes that the extinction of plesiosaurians and mosasaurids could have been caused by an important drop in sea level [13]. Mosasaurids are unusually diverse and locally abundant in the Maastrichtian phosphatic deposits of Morocco, and with large sharks and one species of elasmosaurid plesiosaurian recognized so far, contribute to an overabundance of apex predators [3, 7, 14, 15]. For this reason, high local diversity of marine reptiles exhibiting different body masses and a wealth of tooth morphologies hints at complex trophic interactions within this latest Cretaceous marine ecosystem. Using calcium isotopes, we investigated the trophic structure of this extinct assemblage. Our results are consistent with a calcium isotope pattern observed in modern marine ecosystems and show that plesiosaurians and mosasaurids indiscriminately fall in the tertiary piscivore group. This suggests that marine reptile apex predators relied onto a single dietary calcium source, compatible with the vulnerable wasp-waist food webs of the modern world [16]. This inferred peculiar ecosystem structure may help explain plesiosaurian and mosasaurid extinction following the end-Cretaceous biological crisis.

Calcium isotopic ecology of Turkana Basin hominins

“…Independent measurements of NIST SRM 1486 standards yielded a mean value of: −1.047 ± 0.13‰ 2 SD (n = 101). These values are the same as those measured in the same laboratory at LGLTPE 12,24,30,[32][33][34][52][53][54][55] and agree well with previously published values from other laboratories 31,[57][58][59] .…”

“…After purification, Ca samples were dissolved in ultrapure 0.05 M HNO 3 and Ca concentration was set at 2 ppm for all samples and standards. All Ca isotope compositions are expressed using the 'delta' notation defined as follows for the 44 Ca/ 42 Ca ratio: 12,24,30,32,33,[52][53][54][55] . All newly reported samples were measured as part of the same batches as the samples reported in Martin et al 29 , i.e., in June and August 2016 and in June and July 2017.…”

“…It is well known that diagenetic processes can alter the original isotopic composition of mineralized tissues. That Ca is a major constituent of bioapatite (at a concentration of about 35-40% in weight) represents a solid reason to investigate its isotopic composition in fossil bone and teeth as old as the Cretaceous 30,31,53 . Here, before chemical purification of Ca isotopes, all samples underwent the standard protocol that is applied for carbon isotopic analyses by removing potential secondary carbonates 51 .…”

A dyrosaurid from the Paleocene of Senegal

“…Jouve et al (2008) and Barbosa et al (2008) concurred with the idea that dyrosaurids may have been under competition with the diverse mosasaurs of the Maastrichtian. Whether the extinction of mosasaurs at the K/Pg boundary freed some ecological space for dyrosaurids during the Paleocene is possible but still untested (Martin et al 2017). It is equally possible that dyrosaurid diversity only started to increase later in the Paleocene but admittedly, the dating constraints on the Paleocene dyrosaurid record remains imprecise, despite the availability of some stratigraphic studies, notably from Mali (Hill et al 2008).…”

Calcium isotopic patterns in enamel reflect different nursing behaviors among South African early hominins

“…Calcium isotope abundance ratios ( 44 Ca/ 42 Ca and 43 Ca/ 42 Ca) were measured using a multicollector ICP-MS (Neptune Plus, Thermo Fisher Scientific) following previously described methods ( 9 , 16 , 18 , 19 , 33 ). After purification, Ca samples were dissolved in ultrapure 0.05 M HNO 3 , and Ca concentration was set at 1.5 ppm for all samples and standards.…”