Elsevier

Cretaceous Research

Volume 82, February 2018, Pages 83-98
Cretaceous Research

New plesiosaurian specimens (Reptilia, Plesiosauria) from the Upper Cretaceous (Turonian) of Goulmima (Southern Morocco)

https://doi.org/10.1016/j.cretres.2017.09.017Get rights and content

Abstract

Two new plesiosaurian specimens coming from lower Turonian deposits of Goulmima in Morocco are described. The three-dimensional digital reconstructions of both specimens provide details about their skull roof, mandible and atlas-axis complex. In addition, computed tomography allows to reconstruct their braincase, which is a part of the skull poorly known among plesiosaurians due to either poor preservation and/or insufficient preparation, but that offers a large number of characters used in phylogenetical analyses. After descriptions and comparisons, the two specimens D1-8213 and MNHN F-GOU14 are assigned to Libonectes morgani and to an undetermined Polycotylidae, respectively. The presence of the North American taxon Libonectes morgani in the deposits of Goulmima confirms a trans-Atlantic faunal connectivity at that time and that Elasmosauridae were able to exploit the open marine environment for dispersion. Polycotylids have already been described from Goulmima; however, the typical preservation of these specimens in nodules prevented their preparation and the access to their internal anatomy. Here, the use of X-ray computed tomography shows the strong interest to use such a technique and provide new anatomical details.

Introduction

Plesiosaurians are extinct predatory marine reptiles that represent one of the longest-ranging groups of Mesozoic marine reptiles, extending stratigraphically from the Upper Triassic to the uppermost Cretaceous (Vincent et al., 2011, Benson et al., 2012). During the Late Cretaceous, they achieved a worldwide distribution, including high-latitude seas surrounding Antarctica (e.g., Gasparini et al., 2003, Vandermark et al., 2006, Novas et al., 2015, Sachs et al., 2015). Compared to in other continents, the fossil record of plesiosaurians is scarce in Africa where only seven valid taxa are known (see Vincent et al., 2011, Vincent et al., 2013, Allemand et al., 2017).

The Turonian deposits (Upper Cretaceous) of the Goulmima area, in the Southern slope of the High Atlas (see Fig. 1A), have yielded a diverse marine fauna including ammonites, chondrichthyans (Underwood et al., 2009), bony fishes (Cavin, 1995, Cavin, 1997, Cavin, 1999, Cavin, 2001, Cavin et al., 2001, Cavin et al., 2010) and large marine reptiles, such as a turtle, the basal mosasauroid Tethysaurus nopcsai Bardet et al., 2003a (Bardet et al., 2003a, Bardet et al., 2008) and three major families of Cretaceous plesiosaurians (Angst and Bardet, 2015). The Elasmosauridae are represented by Libonectes morgani (Carpenter, 1997), the Polycotylidae by Thililua longicollis Bardet, Pereda Suberbiola and Jalil, 2003b and Manemergus anguirostris Buchy, Métayer and Frey, 2005, and the Pliosauridae by Brachauchenius lucasi Williston, 1903 (Bardet et al., 2003b, Buchy, 2005, Buchy et al., 2005, Angst and Bardet, 2015, Sachs and Kear, 2017, Allemand et al., 2017). The taphonomical preservation in Goulmima is particular in that fossils are contained in one or several ovoid calcareous nodules (Cavin et al., 2010), in which the siliceous material prevents complete preparation of the specimens and fossilized bones in the nodules are often completely dissolved, making their extraction from the host rock and their study difficult or impossible. This preservation can often prevent a complete access to the fossil anatomy and the use of computed microtomography represents a suitable solution to circumvent these technical issues.

The present study examines two new plesiosaurian specimens (D1-8213 and MNHN F-GOU14) from the area of Goulmima in Southern Morocco (Fig. 1). Although their exact location remains unknown, the calcareous matrix surrounding the specimens is nevertheless consistent with the vertebrate-bearing nodules known from the Unit T2a of the Akrabou Formation (Ettachfini and Andreu, 2004). These nodules are concentrated near the top of a Cenomanian-Turonian calcareous succession and considered as early Turonian in age based on the ammonite assemblage (mainly Mammites) (Cavin, 1995, Cavin, 1997, Cavin, 1999, Cavin, 2001, Cavin et al., 2001, Cavin et al., 2010, Bardet et al., 2003a, Bardet et al., 2003b, Ettachfini and Andreu, 2004, Buchy, 2005, Buchy et al., 2005, Kennedy et al., 2008, Allemand et al., 2017). Both specimens are preserved in incompletely prepared nodules and we used X-ray computed microtomography in order to reveal anatomical details that are not otherwise directly observable.

D, Musée de Rhinopolis, Gannat, France; MNHN, Muséum National d'Histoire Naturelle, Paris, France; NHMUK, Natural History Museum, London, UK; ROM, Royal Ontario Museum, Toronto, Ontario, Canada; SGU, Saratov State University, Saratov, Russia; SMNK, Staatliches Museum für Naturkunde Karlsruhe, Germany; SMNS, Staatliches Museum für Naturkunde, Stuttgart, Germany; SMU SMP, Shuler Museum of Paleontology, Southern Methodist University, Dallas, U. S. A.

Section snippets

Material and method

The two specimens, MNHN F-GOU14 and D1-8213 (Fig. 2), are respectively housed in the Muséum National d'Histoire Naturelle (Paris, France) and the Rhinopolis Museum (Gannat, France) collections. These two specimens are preserved in a nodule in two pieces that fit perfectly. The complete nodules enclosing MNHN F-GOU14 and D1-8213 measure 36, 12, 11 and 41, 15, 12 cm long, wide and high respectively. The specimens are incompletely prepared. In both cases, the anterior half of the nodule shows

Systematic palaeontology

  • SAUROPTERYGIA Owen, 1860

  • Order PLESIOSAURIA de Blainville, 1835

  • Super Family PLESIOSAUROIDEA Welles, 1943 (sensu Ketchum and Benson, 2010)

  • Family ELASMOSAURIDAE Cope, 1869

  • Genus LIBONECTES Carpenter, 1997

  • Type species: ELASMOSAURUS MORGANI Welles, 1949

  • Synonym. Libonectes atlasense Buchy, 2005 (sensu Sachs and Kear, 2017).

  • Holotype. SMU SMP 69120, skull and mandible, atlas-axis complex, 48 successive cervical vertebrae, fragmentary thoracic ribs, gastralia and associated gastroliths (Welles, 1949,

Diagnostic features and paleobiological implications

According to Sato et al. (2011), it is usually difficult in plesiosaurians to determine whether a particular feature of the braincase is diagnostic to a species or shared within a higher taxonomic rank because of the difficulties to access this region. Here, the description of two plesiosaurian specimens provides additional knowledge about their braincases. We considered their variability and, as in the study performed by Sato et al. (2011), to focus on their potential phylogenetic signal and

Conclusion

The plesiosaurian braincase is a part of the skull poorly known due to either poor preservation and/or insufficient preparation. Here, the use of computed microtomography provides new anatomical information about the braincase and the palate of two Moroccan plesiosaurian specimens difficult to study by direct observation because of their particular mode of preservation. This contribution emphasizes the importance to have access and consider these regions in order to identify plesiosaurian taxa,

Acknowledgments

This work was supported by a grant from the Agence Nationale de la Recherche under the LabEx ANR-10-LABX-0003-BCDiv, in the program “Investissements d'avenir” ANR-11-IDEX-0004-02. We are grateful to the Rhinopolis Museum for the loan of the specimen D1-8213. We thank the AST-RX platform at the MNHN for access to the CT-scan, M. Garcia Sanz (MNHN, UMS 2700 OMSI) for producing the CT scans and F. Goussard (MNHN, UMR 7207 CR2P) for assistance with 3D digital reconstructions and imaging based on CT

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