# Reconstructing the trophic structure of Maastrichtian elasmobranch communities in Morocco using calcium isotopes

Alexandre Assemat, Sylvain Adnet, Jeremy E. Martin

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Alexandre Assemat, Sylvain Adnet, Jeremy E. Martin. Reconstructing the trophic structure of Maastrichtian elasmobranch communities in Morocco using calcium isotopes. Gondwana Research, 2025, 146, pp.228-234. 10.1016/j.gr.2025.05.021. hal-05166160

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GR Letter

# Reconstructing the trophic structure of Maastrichtian elasmobranch communities in Morocco using calcium isotopes

Alexandre Assemat<sup>a, *</sup>, Sylvain Adnet<sup>a</sup>, Jeremy E. Martin<sup>b</sup>

<figure><img src="image_3.png" /></figure>

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## ARTICLE INFO

Handling Editor: Andrea Festa

Keywords:

Squalicorax pristodontus

Calcium isotopes

Trophic ecology

late Cretaceous

## ABSTRACT

The diversification of elasmobranchs through time is accompanied by an incredible diversity of feeding ecologies reflecting every trophic position and placing elasmobranchs as cornerstone organisms of the marine food webs. Quantifying their trophic ecology in the fossil record brings precious insights about their successes and the evolution of trophic structure through deep time. Here we quantitatively describe the trophic organization of a rich Maastrichtian elasmobranch assemblage from Morocco presenting taxa from all the trophic levels using the calcium isotopes of their teeth. We highlight overall similarities with modern trophic food webs presenting batomorphs at the lower trophic levels including the first estimate of diet in the extinct suborder of sclerorhynchid and large selachimorphs as apex predators. Moreover, δ<sup>44/42</sup>Ca values in *Squalicorax pristodontus* reveal peculiar feeding behavior underlining that apex elasmobranchs were already divided into two categories including large piscivorous predators and tetrapod feeders.

## 1. Introduction

Chondrichthyes display one of the most successful radiation through the last 420 million years that has led to the colonization of all marine ecosystems as well as some freshwater ones (Compagno 1990; Kriwet et al. 2008). This is highlighted by the high diversity displayed by modern species as well as in their fossil record. The highly mineralized enameloid of their teeth allows the preservation of an abundant fossil record for elasmobranchs presenting a huge diversity of shape that has been the cornerstone of paleo-dietary reconstructions for this group (Frazzetta 1988, Kent 1994, Hubbell 1996). This diversity is associated with a wide range of feeding behaviors that place elasmobranchs at every level of the trophic food webs from planktonophagous species to apex predators (Cappetta 1986, 2012). Among the latter, occur species with a preference for tetrapod consumption (marine turtles and mammals), such as *Galeocerdo cuvier* (Tiger shark) or some populations of *Carcharodon carcharias* (Great white shark) (Simpfendorfer et al. 2001, Dicken et al. 2017, Assemat et al. 2022). In the fossil record, estimating tetrapod predation by elasmobranchs is limited to the presence of direct evidence such as teeth embedded in bones and bite marks, or to approximate suppositions based on dental morphology.

The phosphoric deposits from the Ganntour Basin in Morocco are known to display a remarkable and highly diverse fossil association of elasmobranch, osteichthyan fish and marine reptile teeth through the Maastrichtian and the Paleogene (Arambourg, 1952; Cappetta et al., 2014a; Bardet et al., 2017). These outstanding assemblages, composed of thousands of teeth, offer the opportunity to explore the paleoecology of elasmobranchs at several scales from species to community level. New tools based on stable isotope analyses (SIA) have opened new pathways to explore the paleoecology and paleodiet in these groups (Martin et al. 2015, 2017, Kast et al., 2016, 2022, Leichliter et al. 2021, McCormack et al. 2022, 2023). Recently, the exploration of calcium stable isotopes among extant and extinct marine vertebrates and especially sharks has highlighted that δ<sup>44/42</sup>Ca values in their tooth enameloid are directly linked to their trophic levels and allow new perspectives for paleo dietary reconstructions (Martin et al. 2015, Martin et al. 2017, Assemat et al. 2022). This tool permits to discern the different types of trophic position from primary consumers to apex predators in elasmobranchs as well as the possibility to discern peculiar feeding behaviors such as tetrapod consumption in South African white sharks (Assemat et al. 2022).

Here we attempt to reconstruct the trophic ecology of a wide panel of

<footnote><sup>a</sup> ISEM, Université Montpellier, CNRS, EPHE, IRD, Montpellier, France</footnote>

<footnote><sup>b</sup> Univ Lyon, Univ Lyon 1, ENSL, CNRS, LGL-TPE, Villeurbanne, France</footnote>

<footnote>* Corresponding author at: Université Montpellier, Campus Triolet, Institut des sciences de l'évolution de Montpellier (ISEM), CC 064, Place E. Bataillon, 34095 Montpellier cedex 05, France.</footnote>

<footnote>E-mail addresses: alexandre.assemat@umontpellier.fr, alexandre.assemat@gmail.com (A. Assemat).</footnote>

<footnote>https://doi.org/10.1016/j.gr.2025.05.021</footnote>

<footnote>Received 5 March 2025; Received in revised form 23 May 2025; Accepted 23 May 2025</footnote>

<footnote>Available online 14 June 2025</footnote>

<footnote>1342-937X/© 2025 The Author(s). Published by Elsevier B.V. on behalf of International Association for Gondwana Research. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).</footnote>
extinct elasmobranchs from the Maastrichtian of Morocco using the calcium isotopic record of their teeth with a particular emphasis on alleged tetrapod consumption by *Squalicorax pristodontus*.

## 2. Material and methods

### 2.1. Geological settings

The studied material comes from the Maastrichtian phosphatic levels of the Cretaceous/Paleogene section of the Benguerir locality that belongs to the largest phosphoric deposits from Ganntour Basin in Morocco (Arambourg, 1952; Cappetta et al., 2014a; Bardet et al., 2017). These series span the whole Maastrichtian and expand to the Ypresian. The local biostratigraphy is essentially based on the abundant record of elasmobranch teeth that allow the identification of Maastrichtian, Danian-Thanetian and Ypresian biozones (Arambourg, 1952; Cappetta, 1987a, 1987b; Noubhani and Cappetta, 1997), which largely correspond to the sequence stratigraphic units.

The Maastrichtian in this section is composed of about 20 m of phosphates displaying alternatively soft phosphate levels, marly horizons and hard phosphatic limestones in which isolated teeth of marine vertebrates are extremely numerous (e.g. Arambourg, 1952; Bardet et al., 2015; Cappetta et al., 2014a). Maastrichtian marine vertebrates from Benguerir are distributed through five main fossiliferous levels (L6 to L2) spanning all the Maastrichtian (Kocsis et al. 2014) until the K/Pg event (Aubineau et al., 2024).

### 2.2. Inferring elasmobranch ecology using calcium isotopes

As a general rule, the observed decline in $\delta^{44/42}$Ca values across the marine food web is thought to result from the progressive depletion of heavy calcium isotopes ($^{44}$Ca) in mineralized tissues from lower to higher trophic levels (Skulan & DePaolo, 1999; Martin et al., 2015). This trend is attributed to the preferential assimilation of lighter calcium isotopes during physiological processes such as bone mineralization and urinary excretion, leading to isotopic fractionation from the dietary source. At the base of the food web, invertebrates with soft bodies, whether or not they possess hard shells, tend to show elevated $\delta^{44/42}$Ca values, reflecting calcium uptake and/or biomineralization in equilibrium with seawater composition. As a result, primary consumers feeding on these prey items present slightly lower $\delta^{44/42}$Ca values.

Among elasmobranchs, filter-feeding species exhibit the highest $\delta^{44/42}$Ca values (0.1 to 0.3 ‰), while apex predators, those that consume mineralized tissues from prey like other elasmobranchs or bony fishes, display the lowest values (0.7 to 1 ‰) (Skulan & DePaolo, 1999; Martin et al., 2015; Akhtar et al., 2020). However, due to the opportunistic feeding habits of many sharks, $\delta^{44/42}$Ca values can vary widely within species, making it challenging to clearly delineate trophic groups. Since Martin et al. (2015), the expanding dataset on elasmobranch calcium isotopes (e.g., Akhtar et al., 2020; Assemat et al., 2022) has allowed to refine the categorization of elasmobranchs into the four prey-based groups (plankton feeders, primary, secondary, and tertiary consumers) using modern elasmobranch bioapatite.

Furthermore, Assemat et al. (2022) observed that in modern large sharks, such as the sixgill shark (Hexanchus griseus) and certain South African populations of the great white shark (Carcharodon carcharias), an increase in $\delta^{44/42}$Ca values can be attributed to the consumption or scavenging of tetrapods. This behavior involves ingesting soft tissues while regurgitating heavily mineralized components like bones, which limits the intake of lighter Ca isotopes into the body.

Such a comprehensive framework allowing to explore inter and intraspecific ecology of elasmobranchs using calcium isotopes now offers a foundation for investigating the ecology of extinct taxa.

### 2.3. Sample collection

Specimens were sampled from the collections of the University of Montpellier acquired by Henri Cappetta and Sylvain Adnet under the "Cadre Phosphapal" of the cooperation between OCP, Ministère de l'Energie, des Mines, de l'Eau et de l'Environnement du Royaume du Maroc, Universite Cadi Ayyad, Marrakech, Morocco and MNHN, France. The original material consists in enameloid fragments collected from isolated tooth crown belonging to *Squalicorax pristodontus* (Agassiz, 1835), *Squalicorax basanii* (Gemmellaro, 1920), *Cretolamna maroccana* (Arambourg, 1952), *Scapanorhynchus rapax* (Quaas, 1902), *Carcharias sp.*, *Plicatostyllium gharbii* (Noubhani and Cappetta 1997), *Ganopristis leptodon* (Arambourg, 1935), *Coupatezia arambourgi* (Noubhani and Cappetta 1997), and *Rhombodus binkhorsti* (Dames, 1881). Enameloid fragments weighting between 0.3 mg and 1 mg were separated from dentine under a binocular microscope in order to avoid bias linked to differential incorporation of calcium between enameloid and dentine (Akhtar et al., 2020; Martin et al., 2015). The lightest samples ~ 0.3 mg comes from the batoids that display really thin enameloid.

All the original sample data (this study) are presented in Supplementary Data (Table S1).

This new dataset is completed with compiled data of invertebrates and vertebrates from the literature including modern and fossil elasmobranchs, bony fishes, marine mammals and reptiles (Skulan et al. 1997, Skulan and DePaolo, 1999, Clementz et al. 2003, Blättler 2012, Martin et al. 2015, 2017, Akhtar et al. 2020, Assemat et al. 2022, Messa et al. 2022) presented in Supplementary Data (Table S1). Data from original and literature material are illustrated on Fig. 1. Attribution to Fig. 1 classes are available in the Supplementary Data (Table S1).

### 2.4. Chemical analysis

Each sample was dissolved in 1 mL of concentrated HNO₃ and heated until complete dissolution of the sample and then evaporated.

Calcium purification follows Tacail et al. (2014) and Martin et al. (2015, 2017). Three steps of elution were performed using nitric and hydrochloric acids. The first elution has been performed through a cationic resin AG50W-X12 which aims to remove matrix elements that could interfere with the calcium isotopic measurements. The second elution step has been performed using an anionic resin AG1 X8 in order to remove the iron resulting from fossilization and that could interfere with calcium measurements. Then the third elution was conducted using a Sr-specific resin (Sr-spec Eichrom®) that allows to separate strontium from calcium. Procedural blanks (of 0.1 µg) and standard samples with known concentrations of calcium were added to the analyses to assess the congruence of the results.

Ca isotopes from the purified fraction of calcium were measured using a Thermo Neptune Plus MC-ICP-MS equipped with an Aridus for sample introduction (see Tacail et al., 2014 for details). All samples were diluted in 0.05 M HNO₃ before uptake to a concentration of 1.5 ppm. Measurements were conducted at medium or high resolution, with each analysis consisting of 40 measurements of 4.2 s integrations on $^{42}$Ca+, $^{43}$Ca + and $^{44}$Ca + ions in static mode. 43.5 m/z signals, corresponding to $^{87}$Sr$^{2+}$, were measured for correction of Sr double charge interferences on Ca isotopes ($^{88}$Sr$^{2+}$ on $^{44}$Ca+, $^{86}$Sr$^{2+}$ on $^{43}$Ca+, and $^{84}$Sr$^{2+}$ on $^{42}$Ca$^+$). Each analysis was preceded by a washout pumping in 0.5 M HNO₃. A second washout pumping was carried out in 0.05 M HNO₃ and measured as blank, this background being subtracted online before calculation of isotope ratios. Delta values were obtained using the standard bracketing method with the Lyon ICP Ca standard, referred to as ICP Ca Lyon (Tacail et al., 2014) and issued from a Specpure calcium plasma standard solution (Alfa Aesar). SRM 1486 was used as a secondary standard. For each analytical session of samples, SRM 1486 was measured in turn. Such a sequence was repeated at least twice to check so that a given sample was replicated.

Moreover, Martin et al. (2017) have shown that $\delta^{44/42}$Ca
<figure><img src="image_4.png" /><figcaption>**Fig. 1.** Comparison of trophic structuration of marine communities between present day and the Maastrichtian assemblages from the Benguerir (Ganntour Basin) locality in Morocco using $\delta^{44/42}$Ca. $\delta^{44/42}$Ca values are given relatively to ICP$_{Lyon}$ and converted to SRM915b standard (see Tacail et al. 2014). Boxplot display the median values of the groups. Banners represent the trophic level (Blue: primary consumers; Yellow: Secondary consumers; Red: Apex predators). Maastrichtian faunas are presented regarding to a seawater offset of 0.07‰. Original elasmobranch data from Benguerir are completed with data of modern vertebrates and in-vertebrates as well as Maastrichtian vertebrates from the literature (Skulan et al. 1997, Skulan and DePaolo, 1999, Clementz et al. 2003, Blättler et al., 2012, Martin et al. 2015, 2017, Akhtar et al. 2020, Assemat et al. 2022, Messa et al. 2022). Data and associated references are presented in Supplementary Data (Table S1). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

composition of Maastrichtian seawater at Benguerir, Morocco, differs from modern seawater $\delta^{44/42}$Ca values by 0.05 to 0.07 ‰ ICP-Ca Lyon resulting in differences smaller than analytical error and allowing to compare easily modern and Cretaceous marine vertebrate faunas. Calcium isotopes ratios were measured on individual specimens and standards and replicated at least twice to avoid measurement bias. Here we express the $\delta^{44/42}$Ca values relative to the ICP-Ca Lyon standard previously used for modern and fossils marine vertebrates (Martin et al., 2015; Martin et al., 2017, Assemat et al. 2022) and converted to SRM915a.

## 2.5. Statistical analysis

Kruskal-Wallis tests were performed to test the significance of the difference between $\delta^{44/42}$Ca(ICP-Ca$_{Lyon}$) and the analyzed species. Differences of $\delta^{44/42}$Ca between species were tested by paired samples using a Wilcoxon signed-rank test between each group.

# 3. Results

## 3.1. Calcium isotope composition

All measured data are reported in Supplementary Data (Table S1). The fossil species analyzed in this paper display $\delta^{44/42}$Ca values ranging from -0.86 ‰ for Cretolamna maroccana to -0.26 ‰ for Rhombodus binkhorsti (Fig. 1).

The standard SRM 1486 was measured in triplicates during two sessions and present $\delta^{44/42}$Ca mean values of -0.92 ± 0.08 ‰ and -0.97 ± 0.08 ‰ (2SD). These values are congruent with the standard values

given by Tacail et al. (2014), Martin et al. (2015,2017) and Assemat et al. (2022).

Among the Maastrichtian taxa, large sharks such as Cretolamna maroccana and Scapanorhynchus rapax present the lowest $\delta^{44/42}$Ca values placing them among the tertiary consumers (Fig. 1, Supplementary Data Table S1). Cretolamna maroccana displays mean values of -0.70 ± 0.13 ‰ (n = 3) while the Scapanorhynchus rapax specimen analyzed displays $\delta^{44/42}$Ca value of -0.81 ‰ (n = 1). Carcharias displays a wider range of values with a mean $\delta^{44/42}$Ca value of -0.78 ± 0.28 ‰ (n = 2). Specimens of Squalicorax pristodontus present a mean value of -0.64 ± 0.13 ‰ (n = 6) associated with high variation ranging from -0.44 ‰ to -0.85 ‰ (Fig. 2, Supplementary Data Table S1). The Wilcoxon signed rank test shows Squalicorax pristodontus values to be significantly different from modern piscivorous taxa such as Isurus oxyrhynchus (p = 0.027) Isurus paucus (p = 0.026) or modern taxa with very occasional tetrapod consumption such as Carcharhinus leucas (p = 0.0043) but not significantly different from modern tetrapod feeders such as Galeocerdo cuvier (p = 0.23), Carcharodon carcharias (p = 0.64) and large Hexanchus griseus (p = 0.22) (Fig. 2).

Among the selachimorphs, small sharks are only represented by PlicatOScyllium garbi which displays $\delta^{44/42}$Ca values of -0.64 ‰, falling in the range of secondary consumers such as observed in modern reef sharks (Fig. 1, Supplementary Data Table S1).

Finally, the higher $\delta^{44/42}$Ca values reported in our dataset are displayed by species that belong to the batomorphs such as Rhombodus binkhorsti (-0.26 ‰), Coupatezia arambourgi (-0.34 ‰) and Ganopristis leptodon (-0.34 ‰) (Fig. 1, Supplementary Data Table S1).
<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Comparison of *Squalicorax pristodontus* δ<sup>44/42</sup>Ca values (corrected regarding modern seawater) with modern apex predators including two large piscivorous sharks (*Isurus paucus* and *Isurus oxyrinchus*), a piscivorous shark consuming occasionally some tetrapods (*Carcharhinus leucas*) and three large tetrapod feeders (*Hexanchus griseus*, *Carcharodon carcharias* and *Galeocerdo cuvier*). Boxplots display median value of each species. Differences of δ<sup>44/42</sup>Ca between *Squalicorax pristodontus* and each group have been tested using a Wilcoxon signed-rank test. \* represent the significant difference between the two tested groups.</figcaption></figure>

# 4. Discussion

## 4.1. Inferring trophic level in a Maastrichtian elasmobranch fauna

In the modern trophic food webs, the elasmobranchs with the lowest trophic positions are mostly represented by the filter-feeding taxa and by invertebrate feeders (Navia et al. 2017). Secondary consumers are often represented by piscivorous sharks. Finally, the apex predators among the elasmobranchs are characterized by taxa able to feed on several groups of marine vertebrates such as large bony fishes, other elasmobranchs and even mammals (Lowe et al. 1996, Ebert 2002, Navia et al. 2017, Ebert & Fowler 2021).

Despite the extensive literature on the diversity and paleoecology of elasmobranchs around the K/Pg boundary (Kriwet & Benton 2004, Belben et al. 2017, Feichtinger et al. 2023, Guinot & Condamine 2023, Vullo et al. 2024), only a few analyses have explored Cretaceous elasmobranch diets quantitatively, the latter focusing mainly on qualitative aspects of the tooth such as global morphology, outline, microwear or indirect evidences such as bite marks or coprolites (Schwimmer et al. 1997, Amalfitano et al. 2017, Hone et al. 2018, Bazzi et al. 2021, McLennan & Purnell 2021, Weber et al. 2021; Mazurek and Antczak, 2023; Paredes-Aliaga and Herraiz, 2024).

Martin et al. (2017) highlighted that Maastrichtian marine vertebrates follow a global pattern of δ<sup>44/42</sup>Ca variations similar to the one observed in modern faunas which is marked by the increasing fractionation of calcium isotopes relatively to seawater with position in the foodweb. However, elasmobranchs in their dataset are restricted to three apex elasmobranchs among the other vertebrates. This work brings new clues for the understanding of the trophic ecology of extinct

elasmobranchs before the Cretaceous-Paleogene event that drastically altered their evolutionary history.

Here, we bring data at a more resolved taxonomic and community scale including elasmobranch taxa from the bottom to the top of the Maastrichtian foodweb in the same locality allowing to describe and compare the overall structuration of the elasmobranch communities as well as more precise feeding behavior at the species level. We highlight the same global pattern among the elasmobranch communities from the Cretaceous of Morocco than in their modern counterparts (Fig. 1). Similarly to modern elasmobranchs, the highest δ<sup>44/42</sup>Ca values are found in the crushing/grinding batomorph such as fossil stingrays *Rhombodus binkhorsti* and *Coupatezia arambourgi* that were likely primary consumers feeding on benthic hard-shelled prey based on their dental morphology, similarly to their modern counterparts (myliobatids and dasyatids respectively (Jacobsen & Bennett 2013). The high δ<sup>44/42</sup>Ca values displayed by the sclerorhynchid *Ganopristis leptodon* also highlight a primary consumer feeding behavior as observed for modern batoids (Jacobsen & Bennett 2013, Martin et al. 2015, Last 2016). This represents the first attempt to estimate the diet of a representative of this extinct suborder of batoids, which exhibited long rostra with large denticles similar to sawfishes, also sharing the same trophic level (Nevatte et al., 2017). Among the selachimorphs, the highest values are presented by the fossil nurse shark *Plicatostyllium gharbii*, a small orec-tolobiform for which modern counterparts (Ginglymostomatidae) occupy a large spectrum of trophic position from primary to secondary consumers in the food webs by consuming prey ranging from in-vertebrates to other elasmobranchs (Cortés 1999, Castro 2000). As observed in modern marine ecosystems, strict piscivorous taxa such as the sand tiger shark *Carcharias* representatives appear to display lower
$\delta^{44/42}$Ca values that places them as secondary consumers (Assemat et al. 2022; Martin et al. 2015). However, if we refer to their modern representatives, often smaller in size, their ability to prey upon a wide range of fishes (from small bony ones to other elasmobranchs) give a higher range of $\delta^{44/42}$Ca values among this taxon. The lowest values are found in very large species such as *Scapanorhynchus rapax* and *Cretolamna maroccana* that display large teeth with cutting/piercing morphologies (Cappetta 1986) and that were expected to be apex piscivorous predators. These $\delta^{44/42}$Ca values fall in the same range than the ones observed in modern representatives of mackerel sharks *Isurus oxyrhinchus* and white sharks *Carcharodon carcharias* living in the Mediterranean Sea and preying mostly on tuna (Assemat et al. 2022). This highlights that these two large sharks would have occupied the rank of apex piscivorous feeders by the end of the Cretaceous. These results appear to be congruent with the $\delta^{15}$N values measured in some Santonian-Campanian representatives of the goblin shark *Scapanorhynchus* from the Gulf of Mexico by Comans et al. (2024) that place them as apex predators along with *Squalicorax*, *S. kaupi*. However, similarly to what is observed between modern *Carcharias* species and its Cretaceous representatives, *Scapanorhynchus rapax* appears to be a really large taxon compared to modern mitsukurinidae such as the smaller bathydemersal *Mitsukurina owsoni*, allowing to question the differences of ecology between modern and fossil mitsukurinidae. Akhtar et al. 2020 present data of several apex elasmobranchs from the Cretaceous of US such as *Squalicorax*, *Scapanorhynchus* or *Cretolamna*. However, even if their results appear to be congruent with the material from Morocco described herein, most of the material differs at the specific level excepting *Squalicorax pristodontus*. Moreover, the differences of ages and localities with the material from the present studies prevent from a direct comparison due to potential geographical and temporal variations in calcium baseline between the two localities.

Moreover, similarly to their interaction with mammals in modern ecosystems, data from Martin et al. (2017) show that Cretaceous elasmobranchs shared their apex niche in the trophic food webs with large tetrapods represented by marine reptiles that could act as well as prey or as predators of elasmobranchs.

## 4.2. Tetrapod consumption by *Squalicorax pristodontus*

Among the anacoracid sharks of the genus *Squalicorax*, the species *S. pristodontus* from the Maastrichtian is the largest one (estimated > 4 m TL, see Becker et al., 2008; Shimada et al., 2020) and is often compared to the modern tiger shark *Galeocerdo cuvier* in terms of ecology (Shimada and Cicimurri 2005) due to its estimated body size and the shape of its crenulated teeth. On the basis of several indirect marks on fossil remains of Cretaceous reptile bones such as teeth marks, this species has been interpreted as a scavenger as well as a possible active predator of mosasaurids (Dortangs et al. 2002, Everhart 2004, Rothschild et al. 2005). It has also been shown that within the fossil record, *Squalicorax pristodontus* teeth are often worn apically that correspond to enameloid chips that have been broken as a result of a bite on bony structures (Becker and Chamberlain 2012). These authors also highlight quite similar processes in modern opportunistic elasmobranch species such as the Tiger shark *Galeocerdo cuvier* or the great white shark *Carcharodon carcharias* that prey on a variety of tetrapods.

The analysis of $\delta^{44/42}$Ca values in the Maastrichtian Moroccan fauna brings new clues to describe the trophic level quantitatively and the diet that allows to distinguish categories among apex predators between high piscivorous sharks and the ones that predate tetrapods. This analysis of $\delta^{44/42}$Ca values allows to support previous hypotheses based on qualitative traits of *Squalicorax pristodontus*. Recently, it has been highlighted that among modern large sharks such as the sixgill shark (*Hexanchus griseus*) or some population of the great white shark (*Carcharodon carcharias*) from South Africa, preying or scavenging on mammals can explain an increase in $\delta^{44/42}$Ca values linked to the ingestion of soft tissues (Assemat et al. 2022), which in turn reduces the

inflow of heavy Ca in the body. The values reported here for *Squalicorax pristodontus* present similar ranges as observed for modern tetrapod feeders such as South African representatives of *Carcharodon carcharias*, or some of the *Galeocerdo cuvier* or *Hexanchus griseus* (Fig. 2). It supports the hypotheses of an opportunistic diet including the ingestion of soft tissues that could drive the $\delta^{44/42}$Ca values of such large estimated predator to rise and fall in the primary consumer level due to the lack of bone consumption. This also supports the possibility of scavenging or even hunting behaviors for *Squalicorax* on large marine reptiles during the end of the Cretaceous (Schwimmer et al. 1997; Everhart 2004; Neumann 2000; Shimada and Hooks, 2004; Becker & Chamberlain 2012) during which the bone-soft tissue ratio is much reduced. In the Maastrichtian levels of Benguerir the teeth attributed to *Squalicorax pristodontus* are known to increase in global size and number from the base of the section (level C6) to the upper part (level C2) (Cappetta et al. 2014a). In the same time, other coeval species of *Squalicorax* (Cappetta et al., 2014b), always smaller, appear or disappear and the populations of marine reptiles increases as well (Cappetta et al. 2014a) which could lead to a trophic balance between these two groups. However, physiological differences between fishes and tetrapods such as reptiles or mammals (e.g., involving tooth replacement, seawater consumption) prevent a direct comparison of $\delta^{44/42}$Ca values between these groups and trophic food-web inferences should be discussed considering these physiological differences (Martin et al., 2015, 2017, 2023, Assemat et al. 2022; Messa et al. 2022).

Thus, more data would be necessary to test the presence of differences in *Squalicorax pristodontus* $\delta^{44/42}$Ca values between the different stratigraphic levels. As it stands, the overlapping of $\delta^{44/42}$Ca values of *Squalicorax pristodontus* specimens from different stratigraphic levels could indicate that the predation of this species upon marine reptiles could have occurred early in the Maastrichtian. Even if $\delta^{44/42}$Ca values from distant localities such as presented in Akhtar et al. (2020) cannot be directly compared to $\delta^{44/42}$Ca values in Morocco, we can observe that specimens of *Squalicorax pristodontus* (from the late Campanian) in their study appear to display higher $\delta^{44/42}$Ca (-0.71 ± 0.01) values than the other apex elasmobranchs comprising some other representatives of the genus *Squalicorax*. These higher values falling in the same range as $\delta^{44/42}$Ca values of *Squalicorax pristodontus* in Morocco, could support similar behavior among this taxon in different geographic places and geologic times.

Interestingly, Dortangs et al. (2002) described a specimen of mosa-saurid presenting scavenging marks and associated with teeth of *Squalicorax* and *Plicatostyllium* sharks. The authors propose that these two shark species would have been involved in scavenging the marine reptile carcass. Even if we only measured one specimen of *Plicatostyllium gharbii*, the relatively high $\delta^{44/42}$Ca value displayed by its teeth that falls into the same range as *Squalicorax pristodontus* could support the hypothesis of marine reptile scavenging in this species.

## 5. Conclusion

The present work explores $\delta^{44/42}$Ca values to describe the diet of extinct elasmobranch taxa (e.g. the Cretaceous sclerorhynchids, rhom-bodids) and the trophic structuration of a late Cretaceous community of sharks and rays and give insights into peculiar feeding behaviors such as scavenging (e.g. *Squalicorax pristodontus*). These results show the high similitude between a pre-K/Pg ecosystem and their modern counterparts with apex predators already divided into two categories including large piscivorous predators and tetrapod feeders. The usefulness of calcium isotopes to highlight tetrapod consumption in the fossil record opens the path for the exploration of the trophic interactions between Chon-drichthyes and tetrapods in deeper time and/or at a finer scale. Moreover, it opens the possibility to quantify the impact of biological crises on elasmobranch communities in the past for a better understanding of how they could be impacted by the modern crisis.
# 6. Fundings

The project was supported by the CNRS (INSU INTERRVIE).

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgements

We want to pay tribute to Dr. Henri Cappetta (1946-2024) for collecting and providing the samples of elasmobranchs from the Maastrichtian of Morocco. This work was performed under the convention "Cadre Phosphapal" of the cooperation between OCP, Ministère de l'Energie, des Mines, de l'Eau et de l'Environnement du Royaume de Maroc, Universite Cadi Ayyad, Marrakech, Morocco and MNHN, France.

We thank CNRS (INSU INTERRVIE) and LGL-TPE staff and facilities at ENS de Lyon, which permitted the geochemical analyses. This work has been performed with the support of LabEx CeMEB, an ANR "Investissements d'avenir" program (ANR-10-LABX-04-01), funded through the I-SITE Excellence Program of the University of Montpellier, under the Investissements France 2030.

This is contribution ISEM 2025-106

# Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.gr.2025.05.021.

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