Les Polycotylidae sont des plésiosaures à cou court, connus durant le Crétacé et trouvés essentiellement en Amérique du Nord [4], [9], [19] and [20]. La monophylie des Polycotylidae, bien que longtemps mise en doute, est actuellement appuyée par plusieurs travaux [1], [4] and [13]. Classiquement inclus au sein des pliosaures [3] and [18], les polycotylidés sont de nos jours considérés comme étant plus proches des plésiosaures élasmosauridés à long cou [1], [4], [5] and [13], une hypothèse déjà émise il y a près d’un siècle [20].

Polycotylidae is a group of short-necked plesiosaurs exclusive of the Cretaceous. Their remains are essentially known from North America [4], [9], [19] and [20], but more fragmentary material has also been described in Asia [15] and [17], South America [11], Australia and New-Zealand [15]. They stratigraphically range from the Aptian–Albian to the Maastrichtian [15]. The validity of the Polycotylidae is still under debate, but its monophyly has been supported by recent works [1], [4] and [13]. The Polycotylidae currently includes Polycotylus Cope, 1869 [4], [16] and [20], Trinacromerum Cragin, 1888 [4], Dolichorhynchops Williston, 1902 [4], [5] and [19], Georgiasaurus Otschev, 1977 [14] and [17], Sulcusuchus Gasparini & Spalletti, 1990 [11] and, tentatively, Edgarosaurus Druckenmiller, 2002 [9]. Concerning their phylogenetic relationships within the Plesiosauria, polycotylids have been considered for a long time as pliosauroids [3] and [18], but Carpenter [4] and [5] recently reelaborated Williston’s [20] suggestion that polycotylids are more closely related to long-necked elasmosaurids than to “pliosaurs”, a view supported by cladistic analysis [1] and [13].

Abbreviations: MNHGr, « Muséum d’Histoire naturelle de Grenoble », France.

The specimen (MNHGr.PA.11710) comes from the Goulmima region in the southern slope of the High Atlas of Morocco (Fig. 1). Though its exact location remains unknown, fieldwork has allowed us to locate several fossiliferous localities near the villages of Tadirhourst and Asfla, north of Goulmima, Er-Rachidia Province. These sites have yielded a rich fauna of marine vertebrates, including actinopterygians [6], marine reptiles [2], as well as ammonites. The fossils are preserved in ovoid calcareous nodules that occur near the top of a Cenomanian–Turonian limestone bar, a reference level in North Africa [7] and [8]. The fossiliferous nodules are concentrated in Unit 4 of Ferrandini et al. [10], which is Early Turonian in age according to the ammonite association (mainly Mammites). This unit corresponds to an open platform environment related to the maximum of the Cenomanian–Turonian transgressive phase [10].

Plesiosauria de Blainville, 1835

Polycotylidae Cope, 1869

Thililua gen. nov.

Etymology. From Thililua, aquatic god in the Berber mythology [12].

Type species. Thililua longicollis.

Diagnosis. As for type species.

Thililua longicollis sp. nov.

Etymology. The species name alludes to the long neck.

Holotype. MHNGr.PA.11710, a nearly complete skull and mandible, and 37 associated vertebrae, including the entire cervical and pectoral series and the four anteriormost dorsals; preserved articulated in several successive blocks (Fig. 2 and Fig. 3).

Type locality and horizon. Near Goulmima, Er-Rachidia Province, southern Morocco; Unit 4 of the Cenomanian–Turonian limestone bar; Late Cretaceous, Early Turonian [10].

Diagnosis. Polycotylid plesiosaur characterized by premaxillae with swollen lateral processes between the external nares and the frontal foramina; orbits regularly oval without supraorbital processes; dental formula consisting of five premaxillary, at least 22 maxillary and 29 dentary teeth; mandibular symphysis bearing 15 teeth; 30 neck vertebrae; cervical centra nearly as long as high, bearing lateral longitudinal ridges which are paired from the 19th to 22th cervical vertebra.

Description. The skull and mandible were found associated and articulated together with 37 vertebrae, including the entire cervical and pectoral series, and the anteriormost dorsals (Fig. 2). Both the cranium and vertebrae have suffered strong lateral compression. The specimen is assumed to be adult as some of the cranial sutures are difficult to trace and the neural arches are all fused to the vertebral centra. Based on comparisons of the skull plus neck length to total body length ratio from reconstruction of Dolichorhynchops [19], the estimated overall length of Thililua was about 5.5–6 m. For measurements, see Table 1 and Table 2.

Skull. The skull of Thililua is long and gracile (Fig. 3). The preorbital segment represents approximately 60% of the total skull length (Table 1). The premaxillae bear five pairs of teeth and extend up to the posterior margin of the orbit to contact the parietal. The premaxillae contribute to the dorsal border of both the external nares and the frontal foramina. Between these pairs of foramina, they are well expanded laterally. Posteriorly, the premaxillae taper to a point located just posterior to the orbital margin. The maxilla contributes to the ventral border of the naris and to the ventral rim of the orbit, extending far posterior to it to contact the squamosal. It bears at least 22 teeth. The external nares are oval-shaped and larger than the frontal foramina. The prefrontal is triangular-shaped and forms the posterolateral margin of the naris and the anterolateral margin of the orbit. Its suture with the frontal is difficult to observe; it seems to be located just anterior to the frontal foramen. The shape of the frontals is unclear. They are separated along the dorsal midline by the premaxillae. The frontals form most of the dorsal rim of the orbit and lack a lateral process. The orbits are regularly oval. Due to crushing, the maxillae are artificially displaced dorsally and the anteroventral rim of the orbits is distorted. The postfrontal and postorbital region is poorly preserved. The right jugal is preserved as a horizontal rectangular bone, while the left one is crushed. It contributes to the posteroventral margin of the orbit and to the anteroventral margin of the temporal fenestra. It contacts the maxilla along a straight suture. The suture with the squamosal is unclear. The temporal fenestrae are larger than the orbits. The right temporal fenestra is preserved and the left one is crushed and reduced to a narrow groove. The parietal apparently bears a low sagittal crest, but this is probably due to poor preservation. There is no evidence of a pineal foramen. The suspensorium is vertically oriented. The squamosal forms the posterolateral margin of the temporal fenestra. Its suture with the quadrate is clearly exposed laterally. The palate is not available. The occipital region is hidden by the anteriormost cervical vertebrae, which are preserved in articulation with the skull.

Mandible. The mandible is complete but the retroarticular processes (Fig. 3). The mandibular rami are very close to each other due to crushing, so that the medial surfaces are not available. The dentary represents three quarters of the total length of the lower jaw. It bears 29 teeth. The symphysis is very long, nearly half the mandible length (Table 1), and includes 15 pairs of teeth. The dentary row extends posteriorly to the level of the middle part of the orbit. Just in front of the top of the coronoid eminence, the suture with the surangular is nearly vertical. Fragments of bone preserved dorsally to the coronoid process could represent ossified cartilage remains of the coronoid, as suggested for Libonectes [5]. The surangular is short and subrectangular. Posterior to the coronoid eminence, its dorsal margin descends to the glenoid fossa (articular). The jaw articulation lay slightly lower than the tooth row. The angular is narrow and extends anteriorly ventral to the dentary and surangular. Its suture with the splenial is straight and located just anterior to the level of the external naris. In ventral view, the splenial extends anteriorly to the tenth pair of teeth, taking part into the symphysis.

Dentition. Only the first three pairs of premaxillary teeth are well preserved. The remainder teeth are damaged and most of the crowns have been partially reconstructed, but the tooth count can be established. There are five premaxillary, at least 22 maxillary and 29 dentary teeth. There is no evidence of caniniform teeth. The dentition is homodont, with the eleventh and twelfth maxillary teeth slightly larger than the others. The teeth are conical, long and slender. A weak anterior carina is present. The enamel crown is ornamented with very fine striations on the posterolingual side, whereas it is nearly smooth on the labial one. The occlusion of the upper and lower tooth rows is deeply interdigitate.

Vertebrae. Thirty cervical, three pectoral and the four anteriormost dorsal vertebrae are preserved in articulation (Fig. 2). The vertebrae are very compressed laterally, and consequently the width of the centra is about half the height. The centra are slightly amphicoelous and bordered by a rugose rim of bone. The lateral surfaces are concave and the ventral ones bear a longitudinal keel that split the nutritive foramina. The zygapophyses are long and robust. The neural spines, as preserved, are low and the dorsal border is nearly horizontal. The ribs are single-headed. From the atlas-axis complex, only the atlas and axis neural spines, the axis centrum and intercentrum are visible. The general shape is similar to that of Dolichorhynchops [19], but the facet for the axial rib is confined to the axis centrum. The remainder cervical vertebrae are mainly characterised by centra whose average length is approximately 85–90% of the height (Table 2). The lateral processes are narrow and horizontal from cervical 3 to 27 and become broad and vertically oval in the posteriormost cervicals. The anterior cervical ribs are hatchet-shaped. From cervical 9 to 23, there is a longitudinal ridge located high on the lateral surface of the centrum. This ridge is split into two from the cervical 19 to 22. In the three pectorals, the rib facets are oval to reniform, with a ventral small tubercle, as in Trinacromerum [19]. The lateral surface of the anteriormost dorsal centra is excavated and pierced by highly located nutritive foramina. Small bony fragments are present ventrally between the articular surfaces of the posteriormost cervical, pectoral and dorsal centra, deforming them. These structures may be pathological.

Thililua longicollis is referred to the Polycotylidae (sensu [4], [5] and [13]) on the basis of the following combination of characters (available on the specimen): the dorsal process of premaxillae extend posteriorly and separates the frontals (paralleled in pliosaurids); maxilla forms an expanded posterior contact with the squamosal; jugal forms a subrectangular horizontal bar (as in elasmosaurids); splenial is included in the mandibular symphysis (as typically in longirostrine plesiosaurs).

Thililua longicollis also shares with all other polycotylids but Edgarosaurus muddi (provisionnally reported to polycotylids, see [9]) a slender long-snouted skull whose preorbital segment is about 60% of the total length, and a uniform in size dentition, without caniniform teeth. Moreover, the premaxillae contact the parietal and there is no evidence of a pineal foramen, as in Dolichorhynchops osborni, Trinacromerum bentonianum and Sulcusuchus erraini (unlike Edgarosaurus) [4], [9] and [11]. Finally, Thililua exhibits a frontal foramen above the orbit and a long mandibular symphysis, about half the total length, as in Dolichorhynchops and Trinacromerum [5]. The latter character is also present in Polycotylus latipinnis [16]. In Edgarosaurus, there is no evidence of a frontal foramen and the mandibular symphysis is shorter [9]. Pending a complete revision of the group and in the absence of a consensus diagnosis (compare [4] and [9]), the above-mentioned features are here provisionally considered as characteristic for the Polycotylidae or less inclusive clades.

Thililua has long and slender teeth, with fine striae as in Dolichorhynchops and Sulcusuchus, unlike the robust, coarsely striated teeth of Polycotylus, Trinacromerum and Edgarosaurus [4], [9] and [11].

Thililua differs from Dolichorhynchops and Trinacromerum in the lack of a supraorbital process, in a premaxillae–parietal contact located posterior to the orbits (instead of above the orbits) and in the facet for the axial rib located on the axis centrum (instead of located on both the centrum and the intercentrum of the axis) [4], [5] and [19]. In addition, it is different from Trinacromerum in that its suspensorium is vertical (instead of posteriorly inclined) [4]. Unlike Sulcusuchus, Thililua lacks a longitudinal groove on the lateral surface of both the maxilla and dentary, and a deep notch on the lateral surface of the squamosal [11]. Comparisons with Georgiasaurus penzensis are not possible because of the lack of homologous material [14] and [17].

Thililua longicollis is unique among polycotylids in showing the following autapomorphies: •

the premaxillae have swollen lateral processes between the external nares and the frontal foramina (absent at least in Dolichorhynchops and Trinacromerum [4] and [5]);

From a palaeobiogeographical point of view, Thililua is the first polycotylid plesiosaur hitherto discovered in Africa and under subtropical palaeolatitudes.