# A New Species of the Durophagous Mosasaurid Carinodens from the Late Maastrichtian Phosphates of Morocco and Implications for Maastrichtian Mosasaurid Diversity

Nicholas R Longrich, Xabier Pereda-Suberbiola, Nour-Eddine Jalil, Nathalie Bardet

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Nicholas R Longrich, Xabier Pereda-Suberbiola, Nour-Eddine Jalil, Nathalie Bardet. A New Species of the Durophagous Mosasaurid Carinodens from the Late Maastrichtian Phosphates of Morocco and Implications for Maastrichtian Mosasaurid Diversity. Diversity, 2024, 17 (1), pp.25. 10.3390/d17010025. [https://hal.science/hal-05293321v1](https://hal.science/hal-05293321v1)

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Article

# A New Species of the Durophagous Mosasaurid *Carinodens* from the Late Maastrichtian Phosphates of Morocco and Implications for Maastrichtian Mosasaurid Diversity †

Nicholas R. Longrich <fnref n="1" />,* , Xabier Pereda-Suberbiola <fnref n="2" />, Nour-Eddine Jalil <fnref n="3" />,<fnref n="4" /> and Nathalie Bardet <fnref n="3" />

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Academic Editor: Luc Legal

Received: 18 October 2024

Revised: 20 December 2024

Accepted: 24 December 2024

Published: 29 December 2024

**Citation:** Longrich, N.R.; Pereda-Suberbiola, X.; Jalil, N.-E.; Bardet, N. A New Species of the Durophagous Mosasaurid *Carinodens* from the Late Maastrichtian Phosphates of Morocco and Implications for Maastrichtian Mosasaurid Diversity. *Diversity* **2025**, *17*, 25. https://doi.org/10.3390/d17010025

**Copyright:** © 2024 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license
(https://creativecommons.org/licenses/by/4.0/).

**Abstract:** Late Cretaceous marine ecosystems saw a major adaptive radiation of mosasaurids, which evolved highly disparate jaw and tooth morphologies to feed on different prey. A striking pattern seen in mosasaurids was a high diversity of durophagous forms. Durophagy likely evolved several times independently in the mosasaurine genera *Globidens*, *Prognathodon*, and *Carinodens*. *Carinodens* is unusual in having low, rectangular, laterally compressed teeth. The genus is known from around the world, with the species *Carinodens minalmamar* and *C. belgicus* previously reported from the Maastrichtian Phosphates of Morocco. Here, we report a new species of *Carinodens*, *C. acrodon*, from the same Maastrichtian Phosphates. It is characterized by teeth with tall crowns, triangular apices, and broad bases. Many of the diagnostic features of this species appear to be plesiomorphies, suggesting the persistence of a primitive species of *Carinodens* into the latest Maastrichtian alongside the more derived *C. minalmamar* and *C. belgicus*. The new species contributes to an emerging pattern of mosasaurid hyperdiversity in the late Maastrichtian of Morocco. A revised faunal list, including stratigraphic range extensions of *Khinjaria* and *Stelladens* into the latest Maastrichtian of upper Couche III of the Phosphates of Morocco, suggests at least 16 species of mosasauroid coexisted here.

**Keywords:** Late Cretaceous; Maastrichtian; Mosasauridae; *Carinodens*

## 1. Introduction

The Late Cretaceous saw a major radiation of mosasaurids, a family of highly specialized marine squamates [1–3]. Following the appearance of small and primitive aigialosaurids in the Cenomanian, the advanced, large-bodied mosasaurids radiated in the Late Cretaceous [1–3]. By the end of the Cretaceous, mosasaurids had achieved high species diversity and disparity. They evolved disparity in body size, as well as jaw and tooth morphologies, associated with different diets and feeding strategies [3–5], as well as a worldwide distribution [4,6–14].

Early, basal mosasaurids had small, conical, and recurved teeth, an adaptation for preying on relatively small prey such as fish and soft-bodied cephalopods [15,16]. By the end of the Cretaceous, mosasaurids had evolved highly diverse tooth morphologies [4].

<footnote n="1">¹ Department of Life Sciences, University of Bath, Claverton Down, Bath BA2 7AY, UK</footnote>

<footnote n="2">² Departamento de Geologia, Facultad de Ciencia y Tecnología, Universidad del País Vasco/Euskal Herriko Unibertsitatea, Apartado 644, 48080 Bilbao, Spain; xabier.pereda@ehu.eus</footnote>

<footnote n="3">³ CR2P Centre de Recherche en Paléontologie—Paris, CNRS/MNHN/SU, Muséum National d'Histoire Naturelle, CP 38, 57 rue Cuvier, 75005 Paris, France; nour-eddine.jalil@mnhn.fr (N.-E.J.); nathalie.bardet@mnhn.fr (N.B.)</footnote>

<footnote n="4">⁴ Muséum d'Histoire Naturelle de Marrakech, Dpt de Géologie, FSSM-UCAM, Marrakech 40000, Morocco</footnote>

<footnote>* Correspondence: nrl22@bath.ac.uk</footnote>

<footnote>† urn:lsid:zoobank.org:pub:43FACB2A-84C8-47F1-9E58-B527231D6488.</footnote>
These included massive, conical teeth for seizing and tearing apart prey [3], blunt teeth for crushing bone [17], knifelike [18] and bladelike teeth [19] for stabbing and cutting large prey, sawlike teeth for cutting [20], and low and bulbous teeth for crushing hard-shelled invertebrates [21–23].

Some of the most unusual tooth morphologies are seen in the durophagous mosasaurid genus *Carinodens*, a highly specialized mosasaurine appearing worldwide in the Maastrichtian [24–29]. *Carinodens* is characterized by relatively small size, long and slender jaws, and a tooth morphology that is unique among mosasaurids, or other vertebrates. The teeth are relatively low in profile, being longer anteroposteriorly than high, mediolaterally compressed, with a prominently projecting anterior carina and finely anastomosed enamel.

Three species of *Carinodens* have been described [24–28]. *Carinodens belgicus* is known from the Maastrichtian of Belgium and the Netherlands [25], as well as Crimea and Russia [24], Maryland [29], Angola [30], Brazil [31], and possibly Morocco [27,32] (but see Section 4.1). More recently, two additional species of *Carinodens* have been described: *C. minalmamar* from the upper Maastrichtian Phosphates of Morocco [27] and Denmark [33], and *C. palistinicus* from the Maastrichtian of Jordan [28], published in a non-reviewed book.

Here, we report a new, morphologically primitive species of *Carinodens* from the upper Maastrichtian Phosphates of Morocco, which coexisted with the more derived *C. minalmamar* and perhaps *C. belgicus* (see Section 4.1). It is also the first *Carinodens* known with well-preserved upper and lower jaws, helping to expand our knowledge of this enigmatic animal.

Institutional abbreviations. MHNM, Muséum d'Histoire Naturelle de Marrakech, Université Cadi Ayyad, Marrakech, Morocco.

## 2. Geographical and Geological Settings

The Phosphates of Morocco were laid down in a warm, subtropical sea that extended across much of North Africa during the Maastrichtian. Upwelling, driven by trade winds driving the surface waters to the west [2], brought nutrient-rich bottom waters to the surface, feeding phytoplankton. Plankton in turn supported a rich marine food chain of bony fish, sharks, and marine reptiles, including mosasaurid squamates, elasmosaurid plesiosaurs, and chelonioid turtles [34].

Mosasaurids are far and away the most diverse and abundant component of the reptile assemblage. The Maastrichtian Phosphates of Morocco preserve the most diverse known mosasaurid fauna, and perhaps the most diverse known Mesozoic marine reptile fauna. Mosasaurids are represented by four major groups. The Mosasaurinae are the most diverse and abundant [3, 4, 19, 34–37], but Halisaurinae [38,39] and Plioplatecarpinae [18,40] are common, and Tylosaurinae [41] occur rarely. Other marine reptiles include pachyvaranid squamates [42], elasmosaurid plesiosaurs [43,44], chelonioid turtles [45,46], and rare crocodylomorphs [47]. A diverse pterosaur fauna is also known [48,49]; dinosaurs are rare but diverse [50–53]. Bony fish [54] and selachians [54,55] are highly diverse and abundant.

The Phosphates crop out in several basins of central Morocco, the main ones being the Oulad Abdoun and Ganntour basins [54] (Figure 1). In the Oulad Abdoun Basin the phosphatic deposits extend from the Maastrichtian into the Early Eocene. They are divided into a series of beds or couches (in French): the upper Couches, 0 and I, are Eocene; Couche II is Paleocene, Couche III is Late Maastrichtian [55,56] (Figure 2).
<figure><img src="image_4.png" /><figcaption>**Figure 1.** The Maastrichtian Phosphates of Morocco. (A,B) Maps showing the location of the Oulad Abdoun Basin in central Morocco and of the Sidi Chennane mine that produced the holotype of *Carinodens acrodon* in the southern part of the Oulad Abdoun Basin.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Figure 2.** The Maastrichtian Phosphates of Morocco. Synthetical stratigraphic column of the Late Cretaceous–Palaeogene series of the Oulad Abdoun Basin, showing the stratigraphical occurrence of *Carinodens acrodon* in the Upper Couche III. After [56].</figcaption></figure>

Couche III is divided into Lower and Upper Couche III (Figure 2). Upper Couche III is dated to the latest Maastrichtian, based on the selachian fauna [55], and has been
constrained to the final ~1 Ma based on carbon and oxygen stable isotope chemostratigraphy [56]. Lower Couche III appears to contain distinct species of mosasaurids, but the selachians are similar to those of Upper Couche, III, suggesting it may be slightly older, perhaps late but not latest Maastrichtian in age.

**3. Results**

**3.1. Systematic Paleontology**

Squamata Oppel, 1811 [57].

Mosasauridae Gervais, 1852 [58].

Mosasaurinae Gervais, 1852 [58].

*Carinodens* Thurmond, 1969 [59].

*Carinodens acrodon* n. sp.

ZooBank LSID: urn:lsid:zoobank.org:pub:43FACB2A-84C8-47F1-9E58-B527231D6488

*Etymology*. Greek *acro* = tall, and *odon* = tooth.

*Holotype*. MHNM.KHG.1510, complete left maxilla, partial right maxilla, almost complete right dentary, all bearing teeth, and three associated teeth (Figures 3–6).

<figure><img src="image_6.png" /><figcaption>**Figure 3.** *Carinodens acrodon* n. sp., MHNM.KHG.1510, holotype, left maxilla, Sidi Chennane, Oulad Abdoun Basin, Morocco, Upper Couche III, late Maastrichtian in: (A) lateral, (B) medial, (C) ventral, (D) dorsal views. Abbreviations: en, external naris; ju, jugal process; pm, premaxillary process.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>**Figure 4.** *Carinodens acrodon* n. sp., MHNM.KHG.1510, holotype, right maxilla, Sidi Chennane, Oulad Abdoun Basin, Morocco, Upper Couche III, late Maastrichtian in: (A) medial and (B) lateral views. Abbreviations: par, dental parapet; pm, premaxillary process.</figcaption></figure>

*Type Locality and Horizon.* Sidi Chennane phosphate mine, Khouribga Province, Morocco, Upper Couche III, latest Maastrichtian (Figures 1 and 2).

*Referred specimen.* MHNM.KHG.1563, isolated tooth crown from Sidi Chennane (Figure 7).
<figure><img src="image_8.png" /><figcaption>**Figure 5.** *Carinodens acrodon* n. sp., MHNM.KHG.1510, holotype, right dentary, Sidi Chennane, Oulad Abdoun Basin, Morocco, Upper Couche III, late Maastrichtian in: (A) lateral, (B) medial, and (C) dorsal views. Abbreviations: for, neurovascular foramen; mec, Meckelian canal; SDL, subdental lamina.</figcaption></figure>

*Diagnosis.* *Carinodens* species characterized by lateral teeth with high, triangular crowns, with a sharp apex, weak anterior expansion of the crown; straight, serrated posterior carinae "pinched" and rising above the main body of the crown, with the carinae barely rising above them rather than projecting to form prominent bladelike cutting edges; external nares extending from the 6th to 9th tooth position; medial maxillary and dentary parapet low, broadly exposing tooth roots; dentary with a ventrally expanded medial flange resulting in a very narrow, ventrally positioned Meckelian canal; dentary medial flange with a longitudinal shallow groove; dentary neurovascular foramina large, numerous, and closely packed almost constituting a continuous groove; dentary bearing ~15 teeth.
<figure><img src="image_9.png" /><figcaption>**Figure 6.** *Carinodens acrodon* n. sp., MHNM.KHG.1510, holotype, associated teeth, Sidi Chennane, Oulad Abdoun Basin, Morocco, Upper Couche III, late Maastrichtian. (A) Mid-posterior tooth (reversed); (B) mid-posterior tooth; (C) posterior tooth. In 1, lingual; 2, labial; 3, anterior; 4, posterior; 5, occlusal views.</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>**Figure 7.** *Carinodens acrodon*, n. sp., MHNM.KHG.1563, referred isolated middle tooth, Sidi Chennane, Oulad Abdoun Basin, Morocco, Upper Couche III, late Maastrichtian. (A) Anterior, (B) labial, (C) lingual, (D) posterior, and (E) apical views.</figcaption></figure>

## 3.2. *Description*

*General preservation* (Figures 3–6). MHNM.KHG.1510 comprises the nearly complete left maxilla (Figure 3) with a damaged posterodorsal margin and bearing seven in situ teeth, the anterior part of the right maxilla with two in situ teeth (Figure 4), an almost complete right dentary with three in situ teeth (Figure 5), missing only its anterior end, and three isolated teeth with roots (Figure 6). All elements are comparable in size, non-overlapping, and were found in close association, suggesting they belong to the same individual. The specimen has experienced some crushing following burial as well as additional damage after being uncovered in the phosphate mines. The dentary (Figure 5) is 200 mm long, suggesting a relatively small mosasaurid, with a skull about 450–500 mm long for a total length of about 4 m. An isolated, shed tooth crown (Figure 7) (MHNM.KHG.1563), referred to *C. acrodon*, comes from a significantly larger individual.

*Maxilla*. The maxilla (Figures 3 and 4) bears 13 teeth. In lateral view (Figure 3A), it is long and low. It measures 200 mm long, and 29 mm high at the front of the naris.

The maxilla is longer and lower than in *Xenodens* [20]. The maxilla of *Carinodens palistinicus* [28] is reconstructed as being relatively high, similar to *Xenodens*, although it remains embedded in the matrix and so its proportions are difficult to assess. A long, slender maxilla is typical of *Clidastes* and *Mosasaurus* spp. [60,61]; the maxilla is proportionately short and high in *Globidensini* [21] and *Prognathodontini* [3,62]. The ventral margin of the maxilla is slightly arched posteriorly, but almost straight along most of its length. The ventral margin of the maxilla in *Carinodens palistinicus* appears to be slightly arched as well [28]; it is straight in *Xenodens* [20].

In lateral view (Figures 3A and 4B), the anterior suture of the maxilla with the premaxilla rises up almost vertically, then extends posteriorly almost horizontally, forming a long, low, weakly convex flange along its anterodorsal margin. This differs from the premaxilla-maxilla suture of *Xenodens* [20], where the anterior margin is inclined at about 60°, followed by a long, straight contact. This contact is unknown in *C. palistinicus*.

The premaxilla contact extends backwards to end between tooth positions 5 and 6 (Figures 3A and 4B), although it is damaged posteriorly and could have extended further. The premaxilla-maxilla anterior suture ends between tooth positions 6 and 7 in
*Xenodens* [20], and remains unknown in *C. palistinicus* though it is reconstructed at the level of tooth position 3 [28]. The elongate premaxilla-maxilla contact is typical of mosasaurines such as *Mosasaurus* [60,61] and *Clidastes* [63]. The contact is shorter in globidensins, extending five tooth positions in *Globidens* [21]; it typically extends only three tooth positions in prognathodontins [3,62].

Behind this (Figure 3A), the naris is long and slightly concave, and the maxilla is low. The maxilla is higher here in *Xenodens* [20]. The external naris appears to extend up to the 9th tooth position.

The posterodorsal margin of the maxilla is damaged and obscured by matrix, making its shape difficult to discern, especially its contact with the prefrontal, but it appears to have been relatively low. The jugal process is long, low, and triangular.

The maxilla's lateral surface is slightly convex (Figure 3A), with a low, broad ridge that extends just dorsal to the ventral margin of the bone, from the third to the tenth tooth position. Atop this ridge are nine or ten large foramina, the anterior ones being ovoid and obliquely oriented. *Xenodens* [20] lacks this distinct ridge on the lateral surface of the maxilla, but has a row of large foramina, which are smaller and more numerous anteriorly.

In medial view (Figure 3B), the maxilla bears a prominent dental parapet bounding the alveoli medially. The parapet is low anteriorly but rapidly becomes deeper posteriorly, until it is half or more the height of the maxilla posteriorly; it then becomes shallower along the jugal process. The dental parapet hides the tooth roots anteriorly but broadly exposes them near the middle of the toothrow. A dental parapet that fails to conceal the tooth roots in medial view is also seen in *Xenodens* [20].

In ventral view (Figure 3C), the maxilla is straight from tooth positions 1–6, then strongly bowed outwards towards the back, suggesting a narrow snout anteriorly, and a much broader snout posteriorly. The maxilla in *Xenodens* [20] is almost straight for most of its length, then curves slightly outwards around the 9th and 10th tooth positions. The body of the maxilla is narrow anteriorly, but becomes much broader near the middle of the jaw, where the maxilla bears the largest teeth, as is typical of mosasaurids [14].

In dorsal view (Figure 3D), the anterodorsal margin of the maxilla is strongly convex dorsally and medially to where it would have contacted the premaxilla; the narial margin is angled obliquely outwards.

*Dentary* (Figure 5). The dentary has an estimated 15 teeth (see Dentition description). Mirroring the maxilla, it is long and slender (Figure 3A) and has straight margins, similar to that of *C. belgicus*, *C. minalmamar*, and possibly *C. palistinicus* [27,28].

The dentary's dorsal and ventral edges are relatively parallel, while in other *Carinodens* species the dorsal and ventral margins diverge so that the bone is tapered anteriorly [27,28] and deeper posteriorly. Posteriorly, the dentary ends with a short edentulous process posterior to the last tooth. Its posterior margin is posteroventrally obliquely oriented and bears a small groove on its upper third and, ventral to this groove, large horizontal striations, especially medially, as is usual in mosasaurids.

The lateral surface bears a unique row of large neurovascular foramina; the anterior part of the dentary is broken, and so it is impossible to know whether a second row of foramina was present ventral to it, as is usually the case in mosasaurids [14]. The neurovascular foramina are narrow, slitlike, and closely spaced, giving the appearance of a continuous narrow groove. In *C. belgicus* and *C. minalmamar*, the foramina are larger, more ovoid, fewer in number, and more widely spaced [27]. The condition is unknown in *C. palistinicus*.

In medial view (Figure 3B), the subdental parapet is deep anteriorly and becomes narrow towards the back of the jaw, as in other *Carinodens*; it is somewhat shallower than in *C. belgicus* [27]. This parapet, mirroring the condition in the maxilla, lies ventral relative to the labial dentary margin, such that the tooth roots are broadly exposed in medial view.
In *C. minalmamar* and especially *C. belgicus*, the medial parapet lies at about the same level as the lateral dentary margin [27].

Exposed tooth roots approach the primitive squamate subpleurodont condition [64] and in derived mosasaurids the subdental parapet is typically extended dorsally to conceal the tooth root and, along with the interdental ridges, create a theca or socket [14]. The exposed tooth roots represent a reversal to the primitive condition, perhaps associated with the highly modified teeth (see Section 4.2). A similar condition is seen in some *Globidens*, e.g., *G. simplex* [37].

The Meckelian groove is very deep and narrow as a result of a large flange extending ventrally from the subdental parapet and constricting it; this flange is more poorly developed in *C. minalmamar* and *C. belgicus* [27]. This flange bears a shallow horizontal groove along its length, unknown in other *Carinodens* species, probably for articulation with a tongue of the and/or to accommodate the dorsal part of the medial ala of the splenial. Anteriorly, the Meckelian groove lies very close to the ventral margin of the mandible. In *C. minalmamar* and *C. belgicus*, there is a thick ventral lip that bounds the Meckelian groove anteriorly.

In dorsal view (Figure 5C), the dentary is relatively narrow anteriorly and posteriorly, but is transversely expanded near its mid-length where the largest teeth are attached, with the jaw projecting outward here. The toothrow appears to be weakly bowed inwards, although crushing has slightly distorted the relationship of the anterior and posterior ends of the bone.

*Dentition* (Figures 3–7). There are 13 tooth positions in the maxilla (Figure 3B,C), with 7 teeth ankylosed and 6 empty alveoli. There are 11 tooth positions preserved in the dentary, with teeth ankylosed in tooth positions 2, 4, 6, 8, and 9, and 5 empty alveoli. The anterior part of the dentary being broken, the total number of teeth remains unknown. Assuming 13 maxillary teeth and 2 premaxillary teeth, it is estimated to be 15, similar to *Mosasaurus* [60] and *Eremiasaurus* [36]. The dental formula of *C. acrodon* (based on the dentary) would therefore differ from *C. belgicus* (17 teeth), *C. minalmamar* (18 teeth), and *C. palistinicus* (14 teeth) [27,28]. Three teeth are preserved separately from the jaws, having fallen out of their thecae. The teeth of the maxillae and dentary do not obviously differ, making it difficult to place them in the jaws; as such, they are all described together.

The teeth show extreme heterodonty (Figures 3–6), as is usual for *Carinodens* [27]. The dental series can be divided into two to three distinct parts, both in the maxilla and dentary: (1) small, conical teeth are positioned at the front of the jaws (positions 1 to 4, possibly 5, of the maxilla); (2) much larger, broader and lower teeth sit near the middle and back of the jaw, with largest teeth being in positions 7 to 10 (on the maxilla); (3) the last two teeth in both the maxilla and dentary are smaller; unfortunately, they are not preserved. Although highly heterodont, all teeth are characterized by a swollen base and thick enamel; many have large wear facets.

The anterior teeth are conical or teardrop-shaped, reminiscent of those of *Globidens phosphaticus* [23]. The crown H/L ratio is about 0.7 (length 2/3 of height). They are very weakly recurved, and only slightly swollen at the base compared to more posterior teeth. The tooth crowns become slightly stouter and straighter posteriorly. The enamel is ornamented near the apex, but smooth on the base of the crown. These teeth have a circular basal cross-section.

Posteriorly, the teeth become larger and stouter, with the mesiodistal length of the crown exceeding the crown height (H/L ratio lower than 1). Tooth crowns also become very compressed, being about twice as long as wide (W/L ratio ~ 2), and lenticular in shape with subequal convex labial and lingual surfaces. The crowns are relatively straight, with the apex set a little posterior to the center of the tooth.
Unlike other *Carinodens* species [27], teeth from the middle of the toothrow are slightly recurved, with a strongly convex anterior margin, and a posterior margin that is weakly convex towards the base, then slightly concave towards the tooth apex, such that the crown is slightly hooked posteriorly.

The apex lies near the center of the tooth or, towards the back of the toothrow, is displaced slightly posteriorly. In unworn teeth, the apex is sharp and tall, giving the teeth a teardrop shape.

In *C. minalmamar*, *C. palistinicus*, and to a lesser degree *C. belgicus* [27,28], the anterior margin of the crown is almost vertical, then angles abruptly backwards, forming a long horizontal plateau up to the apical nubbin, which is posteriorly located on the crown. The crowns are also strongly swollen basally, so that they are broader than the roots, as in other *Carinodens*, giving the teeth a sort of mushroomlike appearance [27].

Both the anterior and posterior margins of the crown bear carinae, as is typical of Mosasaurinae [3,35]. However, these margins are broadly rounded and “pinched” from the main shaft of the crown, reminiscent of what is observed in *Prognathodon currii* [17,23] with the carinae barely rising above them, rather than projecting to form prominent, bladelike cutting edges as seen in some mosasaurids. Both anterior and posterior carinae are finely serrated and weakly developed as is typical of *Carinodens* [32]. The posterior carina is narrow and straight or weakly convex basally, and concave apically, rather than being convex and inflated as in *C. belgicus* and *C. minalmamar*.

The enamel of the crown is highly ornamented, being covered by anastamosing and reticulating ridges that create a sort of network of striae on the enamel surface. This ornament extends almost two-thirds of the way down the crown, but is better-developed and thicker near the apex.

Tooth wear is located mainly on the tooth apex, taking the shape of flat, ovoid facets. Wear facets are oriented perpendicular to the long axis of the crown, and expose the dentine. Wear facets are generally small, but several are large (Figures 4, 5, and 7). In a few places, especially basally, there are large areas of worn enamel, often observed in *Carinodens* teeth (NB, pers. obs.) and possibly linked to repeated contact between adjacent teeth and/or upper and lower teeth during occlusion.

The implantation in *C. acrodon* shows several empty tooth sockets and shed teeth preserving their roots (Figures 3–6), an unusual character in squamates and observed in Maastrichtian *Globidens*, such as *G. phosphaticus* from Morocco and Angola [13,14] and *G. simplex* from Morocco [37] among mosasaurids. This illustrates dental gomphosis (unmineralized tooth attachment, as in archosaurs and mammals) and may represent an adaptative response of durophages to resist and absorb shocks during food crushing [22,37]. In *Carinodens*, the condition is however less extreme than in *Globidens* (see Discussion).

# 4. Discussion

## 4.1. Systematics of *Carinodens acrodon* and Diversity of *Carinodens*

*Carinodens acrodon* differs from *Carinodens minalmamar*, *C. belgicus*, and *C. palistinicus* in having an overall much more primitive tooth morphology, especially in middle and posterior teeth:

1. The apex of the tooth is taller and more strongly pointed, giving the teeth the shape of a posteriorly recurved cone;

2. The anterior margin is not strongly expanded anteriorly to create the distinct horizontal ‘plateau’ or ‘shoulder’ responsible for the characteristic rectangular shape seen in the other *Carinodens* species;

3. The posterior carina is relatively straight and narrow, rather than being convex and inflated (compare Figures 3–7 with Figures 8 and 9).
<figure><img src="image_11.png" /><figcaption>**Figure 8.** Dentaries of *Carinodens* spp. (*C. belgicus* and *C. minalmamar* (new assignment)) in medial view, showing diagnostic characters of *C. acrodon*: 1, narrow lip below the Meckelian groove; 2, narrow Meckelian groove constricted by flange; 3, broadly exposed tooth roots; 4, teeth tall and triangular. *Carinodens minalmamar* and *C. belgicus* from [27]. Scale = 5 cm.</figcaption></figure>

Overall, *C. acrodon* middle/posterior teeth have a roughly triangular shape, more closely resembling the primitive mosasaurid condition, whereas those of the other *Carinodens* species have a derived, rectangular one [27,28]. In this respect *C. acrodon* is primitive with respect to other species of *Carinodens*. This implication is that a morphologically primitive species of *Carinodens* persisted into the late Maastrichtian, alongside two more derived species.

Previously, three dentaries from the Phosphates of the Oulad Abdoun Basin and six isolated teeth from both the Oulad Abdoun and Ganntour basins were referred to *C. belgicus* [27,32]. The two associated dentaries OCP DEK/GE 454 differ from *C. belgicus*, however, in several features. The teeth have much finer ornament. The apex of the crown is extremely low, so that in some teeth, the anterior margin is almost horizontal, and the crown is more ovate and does not project posteriorly. In these features, these dentaries more closely resemble *C. minalmamar*, suggesting that the specimen may be a larger and more mature specimen of *C. minalmamar* (to which it is here reassigned).

Concerning some isolated teeth already published [32] and some newly found (Figure 9), they are however typical of *C. belgicus*, having more rectangular crowns and a strong posterior projection of the crown. More material needs to be found to differentiate *C. belgicus* and *C. minalmamar*; however, it seems likely that both were present in the assemblage. Accordingly, we identify three species of *Carinodens* in the late Maastrichtian Phosphates of Morocco: *C. minalmamar*, *C. acrodon*, and probably *C. belgicus*.
<figure><img src="image_12.png" /></figure>

**Figure 9.** *Carinodens* cf. *belgicus*, MHNM.KHG.1574, referred isolated tooth, Sidi Chennane, Oulad Abdoun Basin, Morocco, Upper Couche III, late Maastrichtian. (A) anterior, (B) lingual, (C) labial, (D) posterior, and (E) apical views.

*Carinodens* is a widespread genus in the Maastrichtian, and both *C. belgicus* and *C. minalmamar* have been reported from many different localities [27]. *C. acrodon* is so far known only from Morocco.

## 4.2. *Ecology of Carinodens*

*Carinodens* is thought to represent a durophage adapted to feeding on hard prey. However, the prey were likely smaller and thinner-shelled than those of *Globidens*, possibly including ammonites and other thin-shelled mollusks, arthropods and echinoderms [25,27,65–67]. The feeding strategy of *Carinodens* seems thus to have been distinct from that of other durophages.

Durophages typically exhibit characteristic low, blunt teeth with thick, rugose wrinkled enamel, adaptations to resist the stresses the teeth were subject to during feeding processes [25,68], and heavy tooth wear. Several groups of Mesozoic marine reptiles evolved durophagy, such as the placodont sauropterygians [69], several ichthyosaur lineages [70], and, among mosasaurid squamates, the mosasaurids *Globidens*, *Carinodens* [25] and *Prognathodon currii* [17]. Several sharks such as *Heterodon* and *Ptychodus* [71] and actinopterygian fishes such as pycnodontiforms are or were also durophagous. All these taxa are specialized for eating relatively hard prey items (cephalopods, bivalves, echinoderms, corals, sponges, etc.) and, to the extent *Carinodens* resembles them, it can be assumed to represent a durophage. Although the morphology of *Carinodens* is generally characteristic of durophages, significant differences exist between *Carinodens* and other durophagous taxa, including other durophagous mosasaurids.

First, the teeth of *Carinodens* are mediolaterally compressed, which is highly unusual compared to the broad teeth of other durophagous squamates such as the mosasaurid *Globidens* [21,23,72] and the semiaquatic Caiman Lizard *Dracaena* [73]. Placodonts exemplified this in evolving great enlargement and flattening of the marginal teeth coupled to a
reduction in tooth number, and the development of large and flat palatal teeth, resulting in an expanded, flat grinding surface [69,74,75]. Durophagous sharks such as the extinct *Ptychodus* [76], modern Port Jackson Shark (*Heterodontus*) and stingrays [77], also have teeth that are low and broad and typically packed together to create large, broad crushing surfaces. Durophagous actinopterygians, including teleosts such as the wolf eel (*Anarhichthys*), cichlids [78], and pycnodontiforms [79,80] also have low, wide crowns. Among mammals, sea otters and walruses have wide, flat crushing teeth [81]. The narrow crushing surfaces of *Carinodens* thus appear to be unique. Although some horn sharks [77] and the extinct hybodont *Lonchidion* [82] have relatively low, long, and narrow teeth, the crowns are packed together to create broad crushing plates; they do not create long, narrow crushing surfaces.

Second, the jaws of *Carinodens* are unusually long, straight, and slender for a durophage. Durophages generally have short, deep, and curved jaws, as seen, for example, in the mosasaurids *Globidens* [21,22,72] and *Prognathodon currii* [3,17], the caiman lizard *Dracaena* [73], and the wolf-eel *Anarichthys* [78]. Shortening the jaws improves their mechanical advantage. By reducing the length of the out-lever, the leverage of the jaws improves, much as the jaws of a pair of pliers are short to maximize leverage, increasing bite force. Furthermore, short, deep jaws are more resistant to loading in bending [83]. The relatively slender jaws of *Carinodens* would not have been able to exert or withstand high bite forces, which seems unusual in an animal adapted to crush hard-shelled prey.

Third, the less extreme dental gomphosis exhibited by *C. acrodon* compared to its more extreme development in *Globidens phosphaticus* and *G. simplex* [22,37] suggests a reduced bite force. Again, this indicates a unique approach to durophagy, especially compared to coeval durophagous mosasaurids living in the same environment.

This unusual combination of features seen in *Carinodens*—low but laterally compressed teeth, slender jaws, and specialized tooth attachment—suggests that if *Carinodens* was a durophage, it was unlikely to have fed in the same way as modern or extinct durophages. From all this can infer the following:

1. Paradoxically, *Carinodens* was a durophage with a weak bite force. This implies its prey had a hard but thin shell. This conclusion is broadly in agreement with previous workers. Dollo speculated that *Carinodens* fed on sea urchins [66]; Lingham-Soliar [25] suggested that it fed on cephalopods, crustaceans, and echinoids. Mechanical modeling also supports the hypothesis that *Carinodens* crushed thin-shelled prey like arthropods, small mollusks, and echinoderms [67].

2. *Carinodens* may have exploited a resource or niche which is no longer extant, given that no living animal has converged on its unusual tooth and jaw morphology. In other words, the morphology of *Carinodens* lacks modern analogs because its ecology lacks modern analogs. Ammonites have hard shells, but they are thin and would have been brittle, and would not require the high bite forces needed to crush clams. Ammonites also became extinct at the end of the Cretaceous, and so if *Carinodens* was highly specialized to eat ammonites, no marine organism in the past 66 million years would have converged on the anatomy of *Carinodens*.

### 4.3. Diversity of Latest Maastrichtian Mosasaurids

In recent years, a remarkable number of new mosasaur species have emerged from the latest Maastrichtian Phosphates of Morocco (Table 1). Although two of these taxa, *Khinjaria* [18] and *Stelladens* [5], were originally reported as coming from the lower part of the phosphatic deposits, in Lower Couche III (probably late but not latest Maastrichtian, see Geology), teeth referable to both genera have recently been discovered in Upper Couche III at Sidi Chennane (NL, pers. obs.), allowing these species to be added to the uppermost Maastrichtian faunal list. In total, we recognize 16 taxa. The extraordinary
diversity of mosasaurids in the Phosphates of Morocco makes this easily the most diverse known mosasaurid assemblage, and one of the most diverse Cretaceous marine reptile assemblages.

<table><caption>**Table 1.** Revised faunal list of the mosasaurid species from the Upper Couche III (uppermost Maastrichtian) of the Phosphates of the Oulad Abdoun Basin, central Morocco.</caption><thead><tr><th colspan="2">Mosasaurinae</th></tr></thead><tbody><tr><td><i>Mosasaurus beaugei</i> [35]</td><td></td></tr><tr><td><i>Thalassotitan atrox</i> [3]</td><td></td></tr><tr><td><i>Prognathodon aff. currii</i> (this paper)</td><td></td></tr><tr><td><i>Eremiasaurus heterodon</i> [36]</td><td></td></tr><tr><td><i>Carinodens minalmamar</i> [27]</td><td></td></tr><tr><td><i>Carinodens (?) belgicus</i> (this paper)</td><td></td></tr><tr><td><b>Carinodens acrodon n. sp. (this paper)</b></td><td></td></tr><tr><td><i>Xenodens calminechari</i> [20]</td><td></td></tr><tr><td><i>Stelladens sp.</i> (this paper)</td><td></td></tr><tr><td><i>Globidens phosphaticus</i> [23]</td><td></td></tr><tr><td><i>Globidens simplex</i> [37]</td><td></td></tr><tr><td><b>Halisaurinae</b></td><td></td></tr><tr><td><i>Halisaurus arambourgi</i> [38]</td><td></td></tr><tr><td><i>Pluridens serpentis</i> [39]</td><td></td></tr><tr><td><b>Plioplatecarpinae</b></td><td></td></tr><tr><td><i>Gavialimimus almaghribensis</i> [40]</td><td></td></tr><tr><td><i>Khinjaria sp.</i> (this paper)</td><td></td></tr><tr><td><b>Tylosaurinae</b></td><td></td></tr><tr><td><i>Cf. Hainosaurus</i> [41]</td><td></td></tr></tbody></table>

The Phosphates appear to be unusual in the diversity and abundance of mosasaurids, and may represent a Maastrichtian biodiversity hotspot. However, mosasaurids were widespread in the late Maastrichtian, with faunas known from Belgium and the Netherlands [6,60,84], New Jersey [7,85], California [10,86], Patagonia [87,88], and Antarctica [89–91]. Many of these faunas contain species distinct either from those of the Phosphates, or each other. Global mosasaurid diversity was therefore considerably higher than the diversity of the Phosphates.

The high diversity of late Maastrichtian mosasaurids suggests (as previously argued) that marine ecosystems remained diverse until the end of the Cretaceous, indicating that gradual environmental changes preceding the K-Pg boundary did not cause a diversity decrease prior to the extinction. If anything, high diversity may have made the ecosystem more vulnerable, insofar as specialized species are less able to respond to sudden environmental changes and food availability in the wake of an extinction event.

More generally, the remarkable diversity of mosasaurids is striking compared to other marine reptiles such as ichthyosaurs and plesiosaurs, which exhibit a much narrower range of tooth morphologies; species richness may also have been higher for mosasaurids than for other marine reptiles. If so, why might that be?

Note that the high diversity of mosasaurids is primarily driven by one lineage, the Mosasaurini, characterized by high dental diversity and tooth complexity. Mosasaurine teeth show specializations such as elaborate ornament of the enamel, serrations, facets, prisms, accessory carinae, inflation of the crown, as well as a high degree of complexity. To a lesser degree, dental specializations are seen in tylosaurines and plioplatecarpines, which evolved bladelike teeth in *Hainosaurus* and *Khinjaria*. It is difficult to disentangle cause and effect; that is, does the ability to evolve high dental diversity drive the species diversity of mosasaurids, or does some other factor (e.g., diving adaptation) [92] facilitate their radiation into various niches, which then causes their jaw and teeth morphologies to
diverge? But one possibility is that some aspect of mosasaurid anatomy or biology meant they were simply better able to specialize and speciate than other lineages.

Some aspect of anatomy—perhaps their jaw and tooth morphology—allowed them to become highly specialized for very different niches. A similar dynamic has been proposed for modern snakes, which have extremely high diversity, evolve rapidly, and—critically—tend to be much more highly specialized in terms of feeding habits than other squamates [93]. The fact that plesiosaurs did not achieve similarly high diversity at the same time as mosasaurids suggests that some aspect of their biology prevented them from moving into new niches in the way mosasaurids did.

**5. Conclusions**

A new specimen of *Carinodens* from the latest Maastrichtian Phosphates of Morocco represents a new and primitive species of *Carinodens*, *C. acrodon* (Figure 10), characterized by the retention of primitive traits of the teeth and jaws. It is represented by maxillary and dentary material, as well as associated teeth. Middle and posterior teeth of the new species differ from other *Carinodens* species in having high, triangular crowns with a sharp apex. Tooth implantation also differs from coeval *Globidens* species. *Carinodens* was highly diverse and widespread in the Maastrichtian, suggesting a rapid radiation into a new niche as a small durophage, but the diet and feeding strategy of *Carinodens* remain unclear. Mosasaurid diversity in Morocco is exceptional and suggests that they continued to radiate until just prior to the K-Pg boundary, and that mosasaurids may have been more speciose and diverse in ecology than other Mesozoic marine clades.

<figure><img src="image_13.png" /><figcaption>**Figure 10.** *Carinodens acrodon* shown to scale. Scale = 1 m.</figcaption></figure>

**Author Contributions:** Conceptualization, N.R.L.; investigation, N.R.L., X.P.-S., N.-E.J. and N.B.; writing—original draft preparation and writing—review and editing, N.R.L., X.P.-S., N.-E.J. and N.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** The research of XPS is supported by the Spanish Ministry of Science, Innovation and Universities, the European Regional Development Fund (project PID2021-122612OB-I00 MCIN), and by the Basque Government (research group IT1485-22).

**Data Availability Statement:** All data are available in the main body of the paper.
**Acknowledgments:** All work was done within the framework of the collaboration agreement between the University of Bath and the Université Cadi Ayyad Marrakech. We thank B. Bougadir and M. Ghamizi for help for the management and conservation of the paleontological collections acquired within the framework of this agreement. The authors also thank Mustapha Meharich of Oued Zem for help in Morocco.

**Conflicts of Interest:** The authors declare no conflicts of interest.

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