Citation:

Schulp, A. S., Polcyn M. J., Mateus O., & Jacobs L. L. (2013).  Two rare mosasaurs from the Maastrichtian of Angola and the Netherlands. Netherlands Journal of Geosciences — Geologie en Mijnbouw. 92(1), 3-10.

Abstract:

We report here the addition of two rare mosasaur taxa to the Maastrichtian marine amniote fauna of Angola. The new specimens include a dentary fragment referable to the large carnivore Prognathodon saturator and an isolated tooth of the small durophage Carinodens belgicus. Both were recovered from Maastrichtian outcrops in southern Angola in 2011. Additionally, a complete posterior mandibular unit of a large mosasaur from the type Maastrichtian of The Netherlands, collected some time prior to 1879 and previously identified as ‘Mosasaurus giganteus’, is described and reassigned here to Prognathodon saturator; historical issues surrounding the taxonomic attribution of this specimen are clarified. The new material extends the known geographic distribution of Prognathodon saturator and Carinodens belgicus.

Introduction

In recent years our knowledge of the marine reptile fauna of Angola has been greatly expanded through the efforts of the PaleoAngola Project (Jacobs et al., 2006; Schulp et al., 2006, 2008; Mateus et al., 2009; Jacobs et al., 2009; Polcyn et al., 2010; Mateus et al., in press). A number of new taxa have been reported (Schulp et al., 2008; Mateus et al., 2009) and skeletal remains of other taxa previously known only from teeth have been described (Polcyn et al., 2010). During the 2011 field season of the PaleoAngola Project, remains of two rare mosasaur taxa were recovered, the extremely large and robust mega-carnivore Prognathodon saturator, and the diminutive durophagous Carinodens belgicus.

Prognathodon saturator Dortangs et al. 2002 was heretofore known only from the holotype material recovered from the type Maastrichtian (~66.1 Ma) of the St. Pietersberg (near Maastricht, Limburg Province, The Netherlands); the holotype is now housed in the collections of the Natuurhistorisch Museum Maastricht (NHMM1998-141). This large globidensine mosasaur was a contemporary of the better known Mosasaurus hoffmanni Mantell, 1829 and Plioplatecarpus marshi Dollo, 1882 (e.g. Kuypers et al., 1998; Mulder, 2003a), but evidenced by the extremely massive and robust skull and dentition, was clearly exploiting a separate and unique feeding niche.

In this contribution, we report two new occurrences of P. saturator; a short fragment of a dentary recovered by the authors from the Maastrichtian of southern Angola in 2011 and a well-preserved posterior mandibular unit (PMU) collected from the type Maastrichtian of The Netherlands sometime prior to 1879. The latter is currently housed in the collections of the Institut Royal des Sciences Naturelles de Belgique, Brussels, Belgium.

Carinodens belgicus Woodward 1891 (see also Dollo, 1913 and Thurmond, 1969) is known primarily from isolated teeth from the Maastrichtian type area of The Netherlands and Belgium (Schulp et al., 2004) and is also known from both shed teeth and relatively complete dentaries from the north African margin, and more complete cranial and postcranial material from Jordan (Mustafa & Zalmout, 2001; Bardet et al., 2005, 2008; Kaddumi, 2009; Schulp et al., 2010). Although previously reported from the Maastrichtian South Atlantic of Brazil (Price, 1957) and from the Congo basin (Polcyn et al., 2010), Carinodens is reported here for the first time from the African Southern Atlantic.

Institutional abbreviations — IRScNB: Institut Royal des Sciences Naturelles de Belgique, Brussels, Belgium; NHMM: Natuurhistorisch Museum Maastricht, Maastricht, The Netherlands; MGUAN-PA: Geological Museum, Universidade Agostinho Neto, Luanda, Angola (PaleoAngola collection); MNHN: Muséum National d’Histoire Naturelle, Paris, France; OCP: Office Chérifien des Phosphates, Khouribga, Morocco; TM: Teylers Museum, Haarlem, The Netherlands.

Systematic palaeontology

Squamata Oppel, 1811

Mosasauroidea Gervais, 1853 (nom. transl. Camp, 1923)

Mosasauridae Gervais, 1853

Mosasaurinae Gervais, 1853 (nom. transl. Williston, 1897a)

Globidensini Russell, 1967 (sensu Bell, 1997)

Prognathodon Dollo, 1889

Prognathodon saturator Dortangs et al., 2002

Holotype: NHMM1998-141

Referred specimen — MGUAN-PA 169 (Fig. 1A-C) is a jaw fragment with one preserved tooth.

Locality and Horizon — The specimen comes from the Cacoto locality (initially reported as the “Bentiaba 2”-locality in Mateus et al., in press), some 15 km NE of the richly fossiliferous locality of Bentiaba, Namibe Province, Angola (Jacobs et al., 2006). This site exposes strata from the Maastrichtian Mocuio Formation. Inoceramid shell fragments characteristically present in the lower part of the section at nearby Bentiaba are absent in the higher part of the section at both Bentiaba and Cacoto. Inoceramids became extinct in the mid-Maastrichtian (MacLeod, 1996), suggesting this part of the section (and the fossils therein) fall within the upper part of the Maastrichtian and thus MGUAN-PA 169 is part of a slightly younger fauna than the material described so far from the Bentiaba site (e.g. Schulp et al., 2008; Polcyn et al., 2010; Mateus et al., in press).

Description — The specimen is severely weathered, and is composed of portions of the right dentary. It preserves two complete alveoli, multiple isolated bone fragments, a single tooth crown and a few tooth fragments. The teeth are ankylosed to the dentary by a broad tooth base elevated above the dorsal margin of the dentary. The single preserved tooth crown is robust, and measures 68 mm tall. It is missing the enamel, but allows description of diagnostic features. The tooth is bicarinate and only very slightly recurved posteriorly. There is no medial curvature. The tooth is constricted at the base and shows the swelling just above the base that is characteristic of Prognathodon (Bell, 1997; Bardet et al., 2000; Schulp, 2006). This combination of characters allows referral to the genus Prognathodon. The size and robust nature, lack of any apprarent fluting, minimal recurvature, and the nearly circular basal outline elimate most species of Prognathodon including those previously referred to Liodon (Schulp et al, 2008): the crown aspect ratio in lateral view (crown height / crown basal width; compare fig. 10 in Schulp et al., 2008) is at 1,56 and therefore easily differentiated from slender-toothed taxa. The much smaller P. solvayi Dollo, 1889 can be excluded as the teeth are not fluted. Teeth of the slightly smaller P. waiparaensis Welles & Gregg, 1971 are closer in proportions, although the teeth of the holotype and only preserved specimen are more recurved and slightly longer in lateral aspect. Prognathodon currii Christiansen & Bonde, 2002 is large, with minimally recurved teeth that are nearly circular at their bases; however, in P. currii, the teeth are more columnar (much less conical), and nearly domed at their apex eliminating referral to that taxon. The only known late Maastrichtian Prognathodon species that possesses the combination of characters in the Angola specimen is P. saturator (compare Schulp, 2006).

Referred specimen — IRScNB R24 (Fig. 2A-D) is a complete and well preserved, remarkably tall left posterior mandibular unit of a large mosasaur.

Locality and Horizon — The labels filed with the specimen make reference to the previous collection number IG4286 and “registre 1506”. The locality as stated in the collection registration (linked to those previous specimen numbers) is “Montagne St-Pierre Tuffeau de Maestricht” and identified as “Maxillaire inférieur (extrémité postérieur)”. Thus, IRScNB R24 comes from the type Maastrichtian limestones of the St. Pietersberg, SW of Maastricht. At the time of discovery of IRScNB R24 (the second half of the 19^th century; Ubaghs, 1879a, b), the limestone was quarried almost exclusively in underground galleries, in the Nekum Member only, as the near-absence of flint made this member the best choice for building stone. Therefore, we can safely assume that IRScNB R24 is from the Late Maastrichtian Nekum Member (see Schiøler et al., 1997 for an overview of the lithostratigraphy).

Comment — The specimen is clearly a mosasaurine by possession of a surangular buttress of the coronoid. It shares with derived Prognathodon species (e.g. P. overtoni Williston, 1897b; P. waiparaensis Welles & Gregg 1971) contact of the anteroventromedial coronoid with the angular. It shares with P. saturator the nearly complete concealment of the angular in lateral view by the surangular, only exposed anteriorly, and the relatively tall aspect ratio in lateral view (compare Fig. 19 in Schulp, 2006). In addition to the morphological data, the large size, and stratigraphic occurrence is consistent with the referral of this specimen to P. saturator.

Description — The posterior mandibular units of the holotype of P. saturator NHMM 1998-141 are crushed and only partially exposed (see Schulp, 2006: p. 23), and the medial morphology is largely obscured. IRScNB R24 on the other hand, is well preserved, uncrushed, and completely free of matrix, and thus allows description of these elements and augments previous descriptions of P. saturator (Dortangs et al., 2002; Schulp, 2006). IRScNB R24 is significantly larger than the holotype, measuring 68 cm from the anterior tip of the coronoid to the posterior end of the retroarticular process compared to the (slightly crushed) holotype of P. saturator measuring about 57 cm (i.e. almost 20% longer). All elements are tightly sutured as in the holotype of P. saturator.

In lateral view the surangular broadly overlaps both the angular and the articular. The suture with the articular originates at the posterior terminus of the glenoid and trends posteroventrally, closely paralleling the posterodorsal margin of the articular, extending to the posterior terminus of the PMU and ventrally to a point at which the articular bends sharply medially to form the retoarticular. At that point, the ventral margin turns anteriorly, again closely paralleling the ventral margin of the PMU and exposing only a small portion of the angular in lateral view along the ventral margin. Anteriorly, the surangular ventral margin trends anterodorsally exposing the anterolateral portion of the angular broadly. Dorsal to the angular, the anterior margin of the surangular trends dorsal at a sharper angle, its most dorsal portion underlying the coronoid. Posteriorly, the dorsal margin slopes anterodorsally, buttressing the posterior portion of the coronoid. In lateral view the anterodorsal portion of the coronoid protrudes anteriorly beyond the anterior terminus of the surangular. Its anterior margin trends posteroventrally and is sharply emarginated at the surangular foramen that carries the intramandibular artery, and more posteriorly forms a gently curved ventral margin. The posterior portion of the coronoid is dorsoventrally wide and broadly overlaps the surangular. In medial view the surangular is largely covered on three sides by the other constituent elements of the PMU, but a large central portion is exposed and provided a portion of the insertion of the aductor muscles. The articular is deeply emarginated dorsally by this insertion area just anterior to the glenoid, the anterior portion has parallel sides and is obscured anteriorly by the coronoid. Posteriorly, the articular is sharply deflected medially, forming the retroarticular process. In medial view the coronoid is broad and dorsoventrally deep, the ventral portion overlapping the articular and contacting the angular broadly, forming a posteriorly directed process that is accommodated by a recess in the underlying articular. The posterodorsal portion forms an extensive contact with the medial ascending process of the surangular. The anterior margin is nearly vertical.

Comments — IRScNB R24 was first described and figured by Ubaghs in two publications which saw only limited distribution (1879a: Pl. I; 1879b: Pl. I). In his comparison of IRScNB R24 with the holotype of M. hoffmanni, Ubaghs noted differences in proportion (“On n’a pas oublié que notre échantillon dépasse en longueur celui de Paris [MNHN AC 9648] d’un quart, mais d’à peu près le double en largeur”), but apparently did not consider these differences taxonomically significant. Instead, he attributed the specimen to M. camperi (i.e. the name under which the ‘Paris’ and ‘Haarlem’ specimens of M. hoffmanni were known at that time). His referral was noted by him to be prompted by the fact that the corresponding elements in the Paris specimen (MNHN AC 9648) were only partially exposed, and (mostly) missing in the Haarlem specimen (TM 7424). Because we now know M. hoffmanni from many well-preserved specimens, we can exclude IRScNB R24 from referral to M. hoffmanni. Instead, IRScNB R24 is indistinguishable in all respects which can be compared to the holotype of Prognathodon saturator Dortangs et al., 2002, and can now be confidently referred to that taxon; however, the history of taxonomic issues surrounding the specimen IRScNB R24 and the broader applications of the species name gigantea (or giganteus) are significant, and will be addressed here.

Ubaghs identified the specimen, at that time in his personal collection, as ‘Mosasaurus Camperi H. V. Meyer’. The Ubaghs Collection was later transferred to the museum in Brussels, where the specimen is currently registered as IRScNB R24. Accepting Ubaghs identification, Dollo (1882: p. 62) listed the specimen as ‘Mosasaurus Camperi, v. Meyer’; however, in 1882 he then refers to the specimen as ‘Mosasaurus giganteus Sömmering 1820’ (Dollo, 1882: p. 68) without providing justification for this new referral — nonetheless, identification has not been challenged until now.

The confusion arises in large part from the original construction of the species name given by Von Sömmering (1820) for Late Jurassic crocodilian material from Bavaria, Germany. At that time, he considered his new material conspecific with the holotype of M. hoffmanni, which at that time carried only informal names, referring both the crocodile and mosasaur material to Lacerta gigantea, creating a composite holotype (see Bardet & Jagt, 1996 for a historical background).

Cuvier (1824) separated the crocodile and the holotype M. hoffmanni by transferring the crocodile material to the newly erected genus Geosaurus, and specifically stated that reference of the species name giganteus to the genus Geosaurus would be inappropriate, but was silent on continued use of it (Cuvier, 1824, p. 343: “Je ne peux lui laisser l’épithète de gigantesque, car, dans le grand genre lacerta, nous avons d’abord l’animal de Maestricht ou mosasaurus qui le surpasse de beaucoup, et nous allons en voir un autre (le mégalosaurus) qui lui est aussi très-supérieur.”).

In 1888, Dollo mistakenly resurrected the species name, identifying IRScNB R24 as Mosasaurus giganteus Sömmering, 1820 (adapting the gender to match masculine Mosasaurus). In his overview published two years later, Dollo (1890) outlined his considerations to maintain the species name giganteus (Dollo, 1890, pp. 160-161), essentially repeating Cuvier’s argumentation. To make matters even more complicated, in 1904, i.e. 16 years after identifying IRScNB R24 as ‘M. giganteus’, Dollo introduced a potential source of confusion by using the species name giganteus in his description of a new species of Prognathodon (holotype IRScNB R106 / formerly 3103; incomplete skull and postcranial skeleton from the upper Campanian / lower Maastrichtian of the Ciply area, southern Belgium; Jagt, 2005); see also Schulp (2006): p. 22.

Although Cuvier’s statement on the assignment of the species name giganteus is understandable in light of the less than gigantic size of Geosaurus, ICZN rules do not allow for exceptions in priority issues because of ‘inappropriateness’ (Article 18: “…are not to be rejected because of a claim that they denote a character or distribution not possessed by the taxon.”; article 23.3.7). Although formal rules governing taxonomy were not published till some decades after the time of Cuvier’s act, in the interest of maintaining a reasonable taxonomy these ICZN rules should also apply retrospectively. In absence of ICZN governance of Cuvier’s act, and in light of his silence on the continued use of the species name, Camp (1942, p. 45) suggested that: “By inference, the specific name gigantea would, from Cuvier’s date, rest with Geosaurus. It should not be applied to the mosasaur as has frequently been done (Kuhn, 1929 [sic!]). Lydekker (1888) restudied von Sömmering’s Bavarian type of Lacerta gigantea and confirmed Cuvier’s reference of that form to the genus Geosaurus. This fixed the name gigantea, and it has subsequently been consistently used.” It should be noted, however, that Camp perhaps missed (he wrote “by inference”) the exact argumentation put forward by Cuvier (1824, p. 343). In any event, the epithet Geosaurus giganteus is currently in use for the metriorhynchid crocodile described and named by Von Sömmering (1820).

Also of note is the fact that, although Mosasaurus hoffmanni Mantell 1829 takes priority over Mosasaurus camperi von Meyer, 1832, Ubaghs (1879a) referred to the Paris (MNHN AC 9648) and Teylers (TM 7424) specimens, as well as the Brussels material (IRScNB R24) discussed here, as Mosasaurus Camperi H. V. Meyer, even though this synonymy had clearly been noted in the literature as early as 1845 (Russell, 1967).

Carinodens Thurmond, 1969

Carinodens belgicus (Woodward, 1891)

Holotype: IRScNB R43

Referred specimen — MGUAN-PA 171 (Fig 1D-H), an isolated tooth crown of a small durophagous mosasaur.

Locality and horizon — The specimen was collected from the upper part of the Bentiaba locality in 2011. The highest levels of this locality are characterized in part by the absence of Inoceramus and is thus likely Upper Maastrichtian (fide MacLeod, 1996).

Description — The enamel on the tooth crown is preserved, with the exception of the worn apical nubbin, a condition commonly seen in durophagous mosasaurs. The tooth is wider than tall in lateral aspect, the apical nubbin located slightly off-centre. The base of the crown is constricted, and minute anastomosing ridges on the thick enamel are present. The outline in labial view of MGUAN-PA 171 is quite similar to tooth position #10 to #13 in Carinodens belgicus (Schulp et al., 2010: fig 4). The aspect ratio in occlusal view (anteroposterior length / labiolingual length) in MGUAN-PA 171 is 1.87, which corresponds closely with the value of tooth position #11 in Carinodens belgicus OCP DEK/GE 454 (1.91). This alone is sufficient to distinguish MGUAN-PA 171 from Carinodens minalmamar Schulp et al., 2010, as the teeth in C. minalmamar are much more laterally flattened. Prominent sulci are present on the lateral aspect of the teeth of C. minalmamar but much less pronounced in MGUAN-PA 171 and other specimens referred to C. belgicus (compare Schulp et al., 2010).

Discussion and conclusions

Stratigraphic range and geographic distribution of P. saturator — Previously, the temporal and geographic occurrence of P. saturator was restricted to a single specimen from the Maastrichtian type area (Dortangs et al., 2002) from the upper Lanaye Member (Gulpen Formation; late Maastrichtian, Belemnitella junior Cephalopod Zone). IRScNB R24 comes from the same area as the holotype of P. saturator. With the holotype from the upper Lanaye Member (Gulpen Formation) and IRScNB R24 from the Nekum Member (Maastricht Formation), the minimum range of P. saturator can now be defined from the top of the Gulpen Formation throughout most of the Maastricht Formation. The Bentiaba site in Angola (e.g. Jacobs et al., 2006) is extraordinarily rich in fossil marine reptiles, and in the past six years, the lower reaches of the site have been extensively sampled in five field seasons by the PaleoAngola team. The lower reaches, particularly the middle Maastrichtian Mocuio Formation (see Mateus et al., in press), did not yield any large Prognathodon teeth, whereas the smaller globidensine mosasaur Prognathodon kianda is present (Schulp et al., 2008), along with Globidens phosphaticus (Polcyn et al., 2010), which in turn disappears in the higher part of the section along with inoceramids. Pending more detailed stratigraphic logging, we infer the part of the Cacoto section that yielded MGUAN-PA-169 to represent slightly younger strata, because of the disappearance of inoceramid shells (MacLeod, 1996), indicating the Angolan Prognathodon saturator is mid-Maastrichtian or younger. A tooth mentioned and illustrated by Antunes (1964: plate 26, fig 4) from Cabinda may represent a large Prognathodon. Bardet et al., 2000 ‘tentatively’ identified the specimen as P. giganteus, but given the size and proportions, referral to P. cf. saturator would be more conservative on morphological grounds. The teeth of the much older Campanian species P. giganteus Dollo 1904, preserved in the holotype IRScNB R 106 (see also Lingham-Soliar, 1994 and Bardet et al., 2000) are considerably smaller, and differ in having a more pronounced posterior curvature and a more circular cross-section (Lingham-Soliar, 1994: fig. 1e-f). However, it should also be noted that the diagnostic value of the posterior curvature in Prognathodon is relatively limited, and in the holotype of P. saturator (NHMM 1998-141), curvature of the posterior carina differs from almost straight to considerably curved from one tooth position to the next in a single individual.

Stratigraphic range and geographic distribution of Carinodens — Our knowledge of the distribution and morphology of Carinodens has expanded considerably in recent years (e.g. Kaddumi, 2009; Schulp et al., 2010 and references therein). In addition to the records from Europe, the north African margin, and Middle East, Carinodens has also been reported from the South Atlantic Maastrichtian of Brazil (Price, 1957) and the Congo (Polcyn et al., 2010), both at a paleolatitudes of approximately 13°S. This report extends the range of Carinodens belgicus to approximately 25°S paleolatitude, mirroring similar occurrences from north Africa and the Middle East.

Acknowledgements

Many thanks to Annelise Folie for her help in providing access to the collections in her care, and to John Jagt and Louis Verding (both Natuurhistorisch Museum Maastricht) for valuable feedback on an earlier version of this manuscript. Fieldwork in Angola has been made possible by support from LS Filmes / Vida Foundation, the Petroleum Research Fund, National Geographic, ISEM at SMU, the Royal Netherlands Embassy in Luanda and TAP Portugal airlines, and their support is gratefully acknowledged here.

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Figure captions

Figure 1. Prognathodon saturator Dortangs et al., 2002, right dentary fragment MGUAN-PA-169, in medial view (A), tooth crown in occlusal (B) and lateral (C) view; Carinodens belgicus (Woodward, 1891), isolated tooth crown MGUAN-PA-171, in buccal (D), lingual (E), occlusal (F), basal (G) and posterior (H) view. Scale bars equal 1 cm.

Figure 2. Prognathodon saturator Dortangs et al., 2002, posterior mandibular unit IRScNB R24, in dorsal (A), medial (B), anterior (C) and lateral (D) view. Scale bar equals 10 cm.