Jeremy E Martin

Raphaël Sarr

Lionel Hautier

A dyrosaurid from the Paleocene of Senegal

Keywords:

We describe a partial dyrosaurid skeleton recently prepared out of a limestone block discovered in the 1930s from Danian strata along the Atlantic coast of Senegal. The specimen, from a single individual, comprises nicely preserved elements of the appendicular and axial skeleton from the abdominal and sacral region, which enables us to refine our knowledge on some postcranial characteristics of the Dyrosauridae. Although Dyrosauridae are abundant in early Eocene deposits of North Africa, the present discovery in the Danian of Senegal fills a patchier record early after the K/Pg boundary and provides an important comparative datapoint with the few other Danian dyrosaurid records such as Atlantosuchus from nearby Morocco or Guarinisuchus and Hyposaurus from Brazil and the USA, respectively.

Introduction

The Dyrosauridae represent a monophyletic group of crocodylomorphs that radiated in the marine environment during the Cretaceous. Their earliest record dates from the Late Cretaceous with mentions from the Campanian (Churcher and Russel, 1992;[START_REF] Lamanna | From dinosaurs to dyrosaurids (Crocodyliformes): removal of the post-Cenomanian (Late Cretaceous) record of Ornithischia from Africa[END_REF][START_REF] Kear | Late Cretaceous (Campanian-Maastrichtian) marine reptiles from the Adaffa Formation, NW Saudi Arabia[END_REF][START_REF] Salih | First record of Hyposaurus (Dyrosauridae, Crocodyliformes) from the Upper Shendi Formation of Sudan[END_REF] or from undetermined Late Cretaceous levels [START_REF] Sertich | Rift valley dinosaurs: a new Late Cretaceous vertebrate fauna from Kenya[END_REF]) and perhaps from levels as old as the Cenomanian [START_REF] Buffetaut | Un crocodilien d'un type particulier dans le Cénomanien de Nazaré[END_REF]. The group survived through the end Cretaceous biological crisis and became extinct during the early Eocene, most probably during the Ypresian (Buffetaut, 1978a;[START_REF] Buffetaut | Radiation évolutive, paléoécologie et biogéographie des crocodiliens mésosuchiens[END_REF][START_REF] Hill | Dyrosaurid (Crocodyliformes: Mesoeucrocodylia) fossils from the Upper Cretaceous and Paleogene of Mali: implications for phylogeny and survivorship across the K/T Boundary[END_REF][START_REF] Martin | Sea surface temperature contributes to marine crocodylomorph evolution[END_REF]. Accounts of dyrosaurid remains in the Lutetian [START_REF] Pilgrim | Middle Eocene mammals from North-West India[END_REF][START_REF] Tessier | Contribution à la stratigraphie et à la paléontologie de la partie Ouest du Sénégal (Crétacé et Tertiaire)[END_REF]Buffetaut, 1978a, b) should be re-evaluated in view of recent stratigraphic studies (e.g. O' Leary et al. 2006) before they can be validated or not. Although the fossil record of dyrosaurids is especially abundant with relatively complete specimens from the late Paleocene [START_REF] Buffetaut | Les crocodiliens paléogènes du Tilemsi (Mali) : un aperçu systématique[END_REF][START_REF] Jouve | Taxonomic revision of the dyrosaurid assemblage (Crocodyliformes : Mesoeucrocodylia) from the Paleocene of the Iullemmeden Basin, West Africa[END_REF] and early Eocene [START_REF] Bergounioux | Les reptiles fossiles des dépôts phosphatés sud tunisiens[END_REF]Buffetaut, 1978a;[START_REF] Jouve | A new description of the skull of Dyrosaurus phosphaticus (Thomas, 1893) (Mesoeucrocodylia : Dyrosauridae) from the Lower Eocene of North Africa[END_REF], fewer representatives from the early Paleocene have been reported so far. They include genera such as Hyposaurus from the Danian of the USA [START_REF] Troxell | Hyposaurus, a marine crocodilian[END_REF][START_REF] Denton | The marine crocodilian Hyposaurus in North America[END_REF], Atlantosuchus from the Danian of Morocco [START_REF] Buffetaut | Atlantosuchus coupatezi, n. g., n. sp., un nouveau Dyrosauridé (Crocodylia, Mesosuchia) des phosphates Montiens du Maroc[END_REF][START_REF] Jouve | A long-snouted dyrosaurid (Crocodyliformes, Mesoeucrocodylia) from the Palaeocene of Morocco: phylogenetic and palaeobiogeographic implications[END_REF] and Guarinisuchus from the Danian of Brazil [START_REF] Barbosa | New dyrosaurid crocodylomorph and evidences for faunal turnover at the K-P transition in Brazil[END_REF]). The genus Phosphatosaurus has been reported from Mali and Niger and may also be Danian in age according to Buffetaut (1978a). Other fragmentary finds from Bolivia [START_REF] Buffetaut | Fossil crocodilians from Tiupampa (Santa Lucia Formation, Early Paleocene), Bolivia : a preliminary report[END_REF], Brazil [START_REF] Cope | A contribution to the vertebrate paleontology of Brazil[END_REF], Pakistan [START_REF] Buffetaut | Données nouvelles sur les crocodiliens paléogènes du Pakistan et de Birmanie[END_REF] and Senegal [START_REF] Tessier | Contribution à la stratigraphie et à la paléontologie de la partie Ouest du Sénégal (Crétacé et Tertiaire)[END_REF] have been attributed a Danian or early Paleocene age. Current data indicate that dyrosaurid diversity was lower during the Late Cretaceous than during the Paleocene (6 genera versus 11, respectively). On the other hand, the diversity of dyrosaurids during the Cretaceous may parallel that of the Paleocene depending on phylogenetic resolution within Dyrosauridae [START_REF] Hill | Dyrosaurid (Crocodyliformes: Mesoeucrocodylia) fossils from the Upper Cretaceous and Paleogene of Mali: implications for phylogeny and survivorship across the K/T Boundary[END_REF]. Some authors have proposed that this could be explained by a shift with the extinction of Cretaceous predators such as mosasaurs freeing space for dyrosaurids [START_REF] Jouve | A long-snouted dyrosaurid (Crocodyliformes, Mesoeucrocodylia) from the Palaeocene of Morocco: phylogenetic and palaeobiogeographic implications[END_REF][START_REF] Barbosa | New dyrosaurid crocodylomorph and evidences for faunal turnover at the K-P transition in Brazil[END_REF]. Nevertheless, such diversity counts merge the dyrosaurid fossil record of the entire Paleocene, and dyrosaurid diversity immediately after the K/Pg crisis is poorly known and difficult to evaluate. Records of large vertebrate predators immediately after the K/Pg boundary are equally important as those from the Maastrichtian because they may provide insights into the reorganization of marine foodwebs following a major biotic disruption.

Here, we describe a dyrosaurid specimen collected in the 1930s by two French geologists, Fernand Jacquet and Maurice Nicklès, who both worked for the Service Géologique de l'Afrique Occidentale Française (AOF) in Dakar. Their survey of Paleogene outcrops along the Atlantic coast of Senegal led to the discovery, near the village of Poponguine (Fig. 1), of the presently described specimen, which has been briefly mentioned in several notes (Malavoy, 1934 p.7;1935 p. 7;[START_REF] Jacquet | Sur l'âge éocène inférieur des couches phosphatées du Sénégal[END_REF]. The specimen consists of articulated postcranial elements belonging to a single individual, the bones of which were deposited in a low energy marine environment of Danian age according to associated microfossils.

History of the discovery.-Before we borrowed the specimen for preparation and study, the following inscription was visible on the surface of the sediment: "Poponguine -Sénégal, Falaise du Bungalow" (Fig. 3). "La Falaise du Bungalow" is a small seaside cliff subjected to wave erosion and made of an alternating sequence of marls and limestones (see geological section). Nowadays, the Presidential residence is sitting on top of it, having replaced the bungalow of the "Gouverneur général de l'Afrique Occidentale Française".

A note from the head of the Service géologique de l'AOF indicates 1934 as the year of the discovery of the Poponguine specimen, originally identified as a fossil mammal with eight vertebrae, three elongate bones and six ribs [START_REF] Malavoy | Service Géologique de l'Afrique Occidentale Française[END_REF]. The following year, the same specimen was identified as a crocodile from Poponguine [START_REF] Malavoy | Service Géologique de l'Afrique Occidentale Française[END_REF]. [START_REF] Jacquet | Sur l'âge éocène inférieur des couches phosphatées du Sénégal[END_REF], reporting on some debris of a marine crocodylian, provided a brief context for the specimen indicating a provenance from alternating levels of marls and limestones from Poponguine. He attributed the deposits to be early Eocene in age on the basis of their invertebrate content, although more recent studies consider the deposits to be Danian in age (see geological section). Following the tragic loss in 1937 of F. Jacquet during a field survey in Mauritania, M. Nicklès was put in charge of handling the works of his colleague [START_REF] Legoux | Service Géologique de l'Afrique Occidentale Française[END_REF]. A few years later, Arambourg and Joleaud (1943 p. 63) mentioned that M. Jacquet recovered vertebrae and limb bones belonging to Dyrosaurus in the area of Poponguine. [START_REF] Tessier | Contribution à la stratigraphie et à la paléontologie de la partie Ouest du Sénégal (Crétacé et Tertiaire)[END_REF] mentions that this material has been observed by C. Arambourg, citing "1943 Dyrosaurus sp. Arambourg (6), p. 63" and referring to the collection Jacquet of the MNHN (Muséum National d'Histoire Naturelle, Paris). Therefore, the specimen must have been part of the MNHN collections at least until 1952 but it is unclear when the specimen was transferred from Dakar to Paris. From this date, the whereabouts of the specimen become unknown and no mention of it is made in the literature. The specimen re-surfaces much later in Toulouse.

The dyrosaurid specimen was stored in the collections of the Institut Catholique de Toulouse from an unknown date until the mid 90s when it was donated to the Toulouse Museum (MHNT) on September 09 1996 (Y. Laurent, pers. comm.). Understanding how the specimen transited from MNHN Paris to Toulouse is unclear at this stage and we can only speculate on potential links between Camille Arambourg (1885Arambourg ( -1969)), then Professor at MNHN who made the observations on the specimen (see above), and Frédéric-Marie Bergounioux (1900Bergounioux ( -1983)), paleontologist and theologist at the Institut Catholique de Toulouse. Both researchers knew each other, with C. Arambourg having sent to F-M. Bergounioux several fossil turtle specimens for study [START_REF] Bergounioux | Appendice: Chéloniens fossiles des phosphates de Gafsa[END_REF]. It is therefore possible that the dyrosaurid specimen arrived at the collections of Toulouse in this way. However, no mention of the Poponguine specimen is made in Bergounioux' monograph, which extensively describes dyrosaurid material from Tunisia [START_REF] Bergounioux | Les reptiles fossiles des dépôts phosphatés sud tunisiens[END_REF].

Geological setting

Paleocene strata crop out on a narrow littoral stretch between Toubab Dialaw until the southern end of the horst of Diass (Fig. 1). Most of the outcrops are represented by alternating marls and limestones, which take part in the Ndayane Formation. This formation is overlain by discording shelly limestones of the Poponguine Formation, which crop out East of Poponguine. The Ndayane Formation is visible in a cliff, the altitude of which is less than 10 m above sea level. The Ndayane Formation uncomformably overlies the Maastrichtian gritty limestone containing ammonites with a conglomeratic level [START_REF] Castelain | Excursions géologiques dans le Sénégal occidental -Colloque International de Micropaléontologie[END_REF]. A lithological section along the stretch of coastline shows the following units totalling 27 m in thickness:

From the base, grey argillaceous limestone becomes replaced by lenticular limestone (2.5 m thick) with gypsum veins; Most of the cliff (17.5 m thick) is made of limestone beds alternating with yellowish marly limestones with gypsum veins. The limestone beds become thicker in the median section of the cliff and the marly limestone beds become lenticular. Near the summit, limestone beds become thinner whereas marly-limestone beds become thicker. Marls containing calcite rosettes show at their base thin lenticular levels of marly limestones (7m thick).

The microfauna of the Ndayane Formation is rich and diverse and includes planktonic and benthic foraminifers as well as ostracods. These microfaunas have been studied by several authors [START_REF] Castelain | Excursions géologiques dans le Sénégal occidental -Colloque International de Micropaléontologie[END_REF][START_REF] Faye | Contribution à l'étude géologique et hydrogéologique du horst de Ndiass (Sénégal occidental): Thèse doctorat 3 ème cycle[END_REF][START_REF] Sarr | Étude biostratigraphique et paléoenvironnementale des séries d'âge Crétacé terminal à Eocène moyen du Sénégal occidental. Systématique et migration des ostracodes[END_REF][START_REF] Sarr | Contribution à l'étude biostratigraphique de la falaise de Poponguine (Sénégal occidental)[END_REF][START_REF] Sarr | Les ostracodes paléocènes du horst de Diass (Sénégal): biostratigraphie, systématique, paléoenvironnements[END_REF] who established a middle to upper Danian age for the formation on the basis of the following planctonic foraminifer species, also identified here in sediment associated with MHNT.PAL.2012.0.45: Chiloguembelina morsei (Kline), Morozovella inconstans, M. pseudobulloides (Plummer), M. cf. trinidadensis (Bolli), and Planorotalites compressa (Plummer) (Fig. 2). This age is in agreement with that established by [START_REF] Tessier | Contribution à la stratigraphie et à la paléontologie de la partie Ouest du Sénégal (Crétacé et Tertiaire)[END_REF] on the basis of macrofaunas.

The depositional environment corresponds to a middle to external platform with a water depth comprised between 100 and 150 m) with a muddy bottom covered with algae, receiving episodically coarse material from the infralittoral environment [START_REF] Sarr | Les ostracodes paléocènes du horst de Diass (Sénégal): biostratigraphie, systématique, paléoenvironnements[END_REF].

Systematic Paleontology

Crocodylomorpha [START_REF] Hay | Second Bibliography and Catalogue of the Fossil Vertebrata of North America: Carnegie Institute Washington[END_REF]Dyrosauridae De Stefano, 1903 Dyrosauridae indet.

Figures 3-10

Description.-The specimen was preserved as three blocks (Fig. 3.1), which underwent acid preparation (Fig. 3.2). The largest block was prepared so that most elements of the specimen preserve their articulation. Several elements were totally freed from the matrix and could be described individually. The main block preserves the last five thoracal vertebrae (#11 to #15); thoracal vertebra #10 has been freed from the matrix as well as two other thoracal vertebrae (one being heavily weathered and the other only consisting of the centrum) thus totalizing eight thoracic vertebrae. The two sacral vertebrae are preserved on the main block. The second sacral vertebra is split in half and is loose from the main block (Fig. 4). Four caudal vertebrae are also available. The right ischium is firmly connected to a fragmentary ilium (Fig. 5). The right femur (Fig. 6) is preserved and is slightly displaced from the pelvic area.

The articulated vertebrae are positioned within a mix of ribs, gastralia, and displaced dorsal and ventral osteoderms.

Materials.-MHNT.PAL.2012.0.45, postcranial elements of a single specimen from the abdominal and sacral region.

Thoracal vertebrae.-The anteriormost thoracal vertebra is complete (Fig. 4.1-5) and corresponds to thoracal vertebra #10. In anterior or posterior views, the amphicoelous centrum has a subquadrangular outline, being nearly higher than wide. The posterior margin of the centrum displays a pathological area near its anterolateral corner, which may correspond to an erosive lesion (arrow in Fig. 4.2). The centrum is slightly elongated, being longer than high. In lateral view, the ventral margin of the centrum is faintly concave. The main corpus of the centrum is slightly constricted laterally with thick anterior and posterior margins that define the limits of the anterior and posterior surfaces. The neurocentral suture is visible and runs transversally along the lateral margin of the centrum, in its dorsal half. A distinct and shallow circular pleurocoele spreads over the centrum and neural arch, below the posterior margin of the transverse process. The neural arch is robustly attached to the centrum. The neural spine is relatively short, being nearly as high as the centrum. Its dorsal margin is straight and represents half the length of the centrum. The dorsal surface of the neural spine is vascularized. In anterior and posterior views, the neural arch shows an extensive medial sulcus. The neural canal is heart-shaped in cross-section with its dorsal margin receiving a ventrally projecting lamina at the level of the neural spine. The anterior extent of the prezygapophyses is almost in line with the anterior margin of the centrum, whereas the posterior extent of the postzygapophyses project far beyond the posterior margin of the centrum. The articular faces on the pre-and postzygapophyses are obliquely oriented with respect to the neural spine. The laminar transverse processes are short and comparable in dimensions to the centrum width, but their anteroposterior length is slightly shorter than the length of the centrum. In dorsal view, the distal margin of the transverse processes is unequal with a parapophysis shorter than the diapophysis.

Sacrum.-The anterior part of the first sacral vertebra is connected to the rest of the vertebral column and is partly embedded in the sediment. The second sacral vertebra detached during preparation, revealing a strong sutural surface. Much like Hyposaurus [START_REF] Schwarz | The postcranial skeleton of the hyposaurinae (Dyrosauridae; Crocodyliformes[END_REF], the two sacral vertebrae appear to have not fused together, whereas these bones are fused in Dyrosaurus sp. [START_REF] Schwarz | The postcranial skeleton of the hyposaurinae (Dyrosauridae; Crocodyliformes[END_REF]. The anterior and posterior surfaces of the sacral centra are ovoid in outline, being wider than tall. The sutural surface of the anterior margin of the second sacral vertebra is complex and presents two parallel sulci along its midline (Fig. 4.6). Such similar sulci have previously been reported on the posterior, but not anterior, surface of the second sacral vertebra of D. phosphaticus and of Hyposaurus (YPM 753) by [START_REF] Schwarz | The postcranial skeleton of the hyposaurinae (Dyrosauridae; Crocodyliformes[END_REF]. The second sacral vertebra can be observed in ventral aspect and no obvious shallow sulcus is observed here close to its anterior margin, contrary to the condition in Dyrosaurus sp. and cf. Rhabdognathus [START_REF] Schwarz | The postcranial skeleton of the hyposaurinae (Dyrosauridae; Crocodyliformes[END_REF]. As observed in dorsal view, the first and second sacral ribs are hourglass shaped. The sacral ribs of the first sacral vertebra are about twice as long as the ribs of the second sacral vertebra (Figs. 3,4.8), as is commonly observed among crocodylians. The first sacral rib occupies the anterior two thirds of the lateral surface of the centrum length, whereas the second sacral rib occupies the entire length of the centrum. On the first sacral vertebra, as observed in posterior view, a distinct circular fossa is visible at the sutural intersection of the neural arch, centrum and sacral rib. As in Dyrosaurus sp., the proximal insertion of the sacral rib for the second sacral vertebra is restricted to the corpus of the second sacral centrum [START_REF] Schwarz | The postcranial skeleton of the hyposaurinae (Dyrosauridae; Crocodyliformes[END_REF]) and unlike the condition in Rhabdognathus where the sacral rib overlaps on the centrum of the first caudal vertebra [START_REF] Langston | Dyrosaurs (Crocodilia, Mesosuchia) from the Paleocene Umm Himar Formation, Kingdom of Saudi Arabia[END_REF]. The iliac symphyseal surface of the first rib does not exceed the anterior and posterior margins of the centrum. An anterolateral process is present as in Dyrosaurus sp. and cf. Rhabdognathus [START_REF] Schwarz | The postcranial skeleton of the hyposaurinae (Dyrosauridae; Crocodyliformes[END_REF]. In lateral view, the anterolateral process is thin and shows a concave articular surface; the posterior articular process of the rib is located more medially than the anterolateral process and is nearly as tall as the centrum height, being concave. The iliac symphyseal surface of the second sacral rib is slightly smaller and symmetrical to the first sacral rib, with a tall anterior surface and a thin posterior process. This iliac symphyseal surface exceeds slightly the posterior margin of the second sacral vertebra. The neural spines have weathered out in the described specimen.

Caudal vertebrae.-At least three caudal vertebrae were prepared out of the second block. [START_REF] Schwarz | The postcranial skeleton of the hyposaurinae (Dyrosauridae; Crocodyliformes[END_REF] described shallow bowl-shaped fossae on the anterior and posterior surfaces of the centrum of the caudal vertebrae. Here, such fossae are also observed on both thoracal and caudal vertebrae. The first caudal vertebra possesses no facets for the haemal arches (Figs. 4.14,4.15). The second caudal vertebra possesses a facet for the haemal arches only on the posteroventral margin of its centrum (Figs. 4.19,4.20). The third caudal vertebra possesses articular facets for the haemal arches both on the anteroventral and posteroventral margins of its centrum (Fig. 4.21). This pattern of articulation between caudal centra and haemal arches differs from that described in Dyrosaurus sp. and cf. Rhabdognathus where the facet is present on the anteroventral surface from the second vertebra [START_REF] Langston | Dyrosaurs (Crocodilia, Mesosuchia) from the Paleocene Umm Himar Formation, Kingdom of Saudi Arabia[END_REF][START_REF] Schwarz | The postcranial skeleton of the hyposaurinae (Dyrosauridae; Crocodyliformes[END_REF]. It is, however, similar to Congosaurus bequaerti where this pattern of articulation starts from the third caudal vertebra [START_REF] Schwarz | The postcranial skeleton of the hyposaurinae (Dyrosauridae; Crocodyliformes[END_REF]. The first caudal vertebra has proportions of the centrum similar to the sacral vertebrae; from the second caudal vertebra, the centrum is longer than high with a quadrangular outline of the anterior and posterior margins; the posteroventral margin is also distinctly lowered than the anteroventral margin (compare Figs. 4.15,4.20,4.21).

Ischium.-The nearly complete right ischium is preserved in connection with the ilium in the main block (Figs. 3,5,5.7,5.8,5.9). The left ischium was isolated during preparation; it is eroded on its medial surface and misses its distal blade (Fig. 5.5, 5.6). The ischium measures about 100 mm in its maximum length, the blade representing about 70 percent of this length.

The anterior and posterior margins of the blade are parallel and do not seem to expand significantly near the distal end of the blade, but this area is damaged. The medial surface of the right element is smooth and gently concave. The broken blade of the left element shows that the anterior margin is thinner than the posterior margin.

The proximal portion of the ischium shows the distinct acetabular foramen, which is bordered anteriorly by the forward-projecting anterior iliac process and posteriorly by the posterior iliac process. The anterior margin of the anterior iliac process is vertical and bears in its lower half an anteriorly oriented pubic process. The pubic process sends a short spiny ventral projection, resulting in a markedly concave margin. The facet for accommodating the ilium is flat and obliquely oriented. On the posterior iliac process, the dorsal facet for the ilium is concave, pierced by foramina and bordered by a thin bony rim; its lateral portion is shorter than the medial portion, matching the triangular outline of the connecting ilium.

Anterior to this facet, there is a flat surface that descends within the acetabulum and corresponds to the articular area for the femoral head (in lateral view, this gives to the posterior iliac process, a truncated outline). The anterior region of the acetabulum, at the level of the anterior iliac process, is anterolaterally depressed. In the same area but near the medial border, the margin is faintly crested. As seen in posterior view, the posterior iliac process possesses a short but distinct muscle scar near its proximal end (Fig. 5.9)

Ilium.-The right ilium measures 67 mm in its maximum height, is connected to the ischium in the main block (Fig. 3) but is heavily damaged. The left ilium is complete, although its internal surface is slightly eroded (Figs. 5.1,5.2,5.3,5.4). The anterior process of the ilium is massive and projects dorsally whereas it projects anteriorly in Congosaurus. Just below it, the supraacetabular crest is well defined as a thin ridge delimiting the anterodorsal outer margin of the acetabulum. The anterior peduncle of the ilium is mediolaterally thin with the ischial facet, although eroded, facing ventromedially. The posterior peduncle is massive, D-shaped in ventral view and the ischial facet is here facing fully ventrally. The acetabulum occupies about one third of the lateral surface of the bone and nearly reaches the dorsal surface. The acetabular foramen is large and occupies more space than in Congosaurus. The posterior blade of the ilium is concave on its lateral surface, and in dorsal view, the ilium is sigmoidal.

Except near the anterior spine, the dorsal margin of the blade is widely convex (not waspwaisted), and is indented for muscle attachment. The posteroventral margin of the blade bears strong indentations, especially on the medial surface. The dorsal and ventral margins of the blade merges into a short peduncle projecting posteriorly. The ventral margin between the posterior peduncle and the strong indented area is smooth and concave on the lateral side.

Femur.-The right femur preserves much of the femoral head and a proximal portion of the shaft. The left femur is complete and partly connected to the block (Fig. 6). The femur has a general sigmoid curvature with a strongly medially bent femoral head, a thick shaft, ovoid in section and terminated distally with a pair of massive and ventrally directed condyles. In proximal view, the femoral head is ovoid being wider than high. On its dorsal surface, an anomalous depression filled with sediment is visible (star in Fig. 6.2). The medial most outline of the femoral head is distinctly set apart from the main shaft as originally observed in Hyposaurus [START_REF] Troxell | Hyposaurus, a marine crocodilian[END_REF]. The ventral margin close to the femoral head possesses a small but distinct knob, which sits on the proximal part of an acute ridge that marks the ventrolateral margin of the femoral head. This ridge merges with the lateral margin of the shaft, which is straight and also acute until the level of the fourth trochanter. Dorsal to this ridge, the lateral surface of the femur bears a large depressed zone (Figs. 6.2, 6.7) but is also heavily weathered. At this level, a process is visible on the dorsal surface (Fig. 6.2, 6.7). The fourth trochanter is prominent and is projecting ventrally. Dorsal to it, a shallow ovoid surface corresponds to the caudofemoral flange, which faces fully medially. From this level, and as seen in medial or lateral view, the shaft gently bends ventrally with two massive condyles that project strongly ventrally toward the distal extremity of the shaft. The distal portion of the shaft is largely hidden by an osteoderm. In ventral view, a wide intercondylar fossa is present (Fig. 6.4), which separates a narrow medial condyle from a more massive lateral condyle (Fig. 6.5). The medial surface of the medial condyle is flat and the condyle expands as a narrow outgrowth dorsally, giving to the dorsal surface of the femur an uneven outline. The lateral condyle is sturdy and possesses a slight projection on its convex lateral margin.

Tibia.-The proximal epiphysis of a left tibia is preserved and is bent from the shaft in a posteromedial direction (Fig. 6). The proximal end is 45 mm mediolaterally by 41 mm anteroposteriorly, and the oval shaft, as preserved is much thinner being 20 mm mediolaterally by 18 mm anteroposteriorly. It strongly resembles another indeterminate dyrosaurid tibia described from Pakistan (Fig. 9 in [START_REF] Storrs | A dyrosaurid crocodile (Crocodylia : Mesosuchia) from the Paleocene of Pakistan: Postilla[END_REF]. Its proximal articular surface is deeply depressed in the middle as in Congosaurus bequaerti [START_REF] Schwarz | The postcranial skeleton of the hyposaurinae (Dyrosauridae; Crocodyliformes[END_REF]). This surface possesses two articular facets that are continuous on the medial and posterior margins.

Both facets are separated by a marked posterior cleft (Figs. 6.9, 6.11). The medial articular facet is about twice larger and markedly concave in comparison to the posterior one. The anterior margin of the articular surface is straight and does not show an articular facet. In anterior view, the medial facet is angled relative to the rest of the proximal margin of the tibia, giving to the shaft a strongly concave medial margin. Conversely, the lateral margin is straight and nearly perpendicular with the articular surface (Fig. 6.10). Here, the cnemial crest is not particularly standing out, being distinct only in its most proximal portion where a steplike process accommodates the proximal margin of the fibula (Figs. 6.9, 6.11). A 9 mm long ovoid depression is observed on the anterolateral corner of the tibia (Fig. 6.8), as previously observed in the tibia of Guarinisuchus munizi [START_REF] Sena | New dyrosaurid remains (Crocodyliformes, Mesoeucrocodylia) from the Paleocene of the Paraíba Basin, NE Brazil[END_REF], fig. 3M).

Calcaneum.-A complete left calcaneum is preserved (Fig. 7). The anterior ball is demarcated from the posterior tuber by an extensive medial channel that runs along most of the medial surface of the posterior tuber, just posterior to the medial flange. As observed in medial view, the calcaneal socket, which accommodates the astragalar peg, is deep. Posterior to it, the anterior face of the medial flange is vast and has a gently concave margin that may have limited rotation of the astragalar peg. The anterior ball is convex in its anterior and dorsal parts, and flat in its plantar margin as in extant crocodylians. From a lateral view, the anterior ball shows a vertical wall. Ventrally, it is well demarcated from the posterior tuber by a deep lateroventral channel. The posterior tuber shows on its posterior margin a shallow and rugose groove.

Radiale.-The right radiale is preserved but misses most of its proximal portion (Figs. 8.1, 8.2) and is heavily eroded on its posterior surface. A portion of the facet for the ulnare is preserved on the lateral margin and distinctly projects laterally from the proximal portion of the bone. The shaft is wasp-waisted. The distal articular surface is concave. Hyposaurus [START_REF] Schwarz | The postcranial skeleton of the hyposaurinae (Dyrosauridae; Crocodyliformes[END_REF]. The distal condyles are well individualized and possess marked pits on their medial and lateral edges. The right or left metacarpal V (Fig. 8.11) is partly weathered at both extremities, has a rod-like shaft and is 38 mm long.

Phalanx.-A single carpal phalanx is preserved . It is 18 mm long, waspwaisted with a proximal portion nearly as wide as the distal portion. In palmar view, the distal condyles are well demarcated by a marked intercondylar groove.

Osteoderms.-Several osteoderms are associated with the skeleton (Figs. 3,9). They are all displaced and therefore no clear configuration of the dorsal or ventral shield can be deduced.

All dorsal osteoderms lack an external keel (Figs. 3,9.1, 9.2) a condition identical to that of various hyposaurine dyrosaurids [START_REF] Schwarz | The postcranial skeleton of the hyposaurinae (Dyrosauridae; Crocodyliformes[END_REF]. The dorsal osteoderms attributed to the paravertebral shield are square to trapezoidal in outline being slightly wider than long.

Their anterior and posterior margins are slightly vaulted and thin in comparison to the thick median body. As observed in dorsal view, the anterior margin is concave in its median portion and its dorsal surface shows a smooth facet covering the entire width of the osteoderm. A short anterolateral process emerges from this smooth facet. Just posterior to the anterior facet, a row of nearly circular and well-aligned cupules is present. This row is distinct from the rest of the dorsal surface, which is slightly raised and ornamented with slightly larger and randomly distributed cupules. The posterior margin is gently convex. The median and lateral margins are nearly straight. As for the lateral margin, some incipient indentations are sometime visible and may have contacted a more lateral osteoderm. The ventral surface is perfectly smooth and sometime exhibits a few foramina. Isolated ventral osteoderms were also identified and present a wide V-shaped margin and a peg on the opposite margin (Fig. 9.3). Part of the ventral shield is still articulated but sediment hides much of the organization. A suite of four articulated osteoderms is visible on one side of the specimen and is positioned ventral to the gastralia (Figs. 9.4, 9.5). Ventral osteoderms are smaller than dorsal osteoderms and are distinctly wider than long. Their margins possess interdigitations for suturing to other osteoderms. The medial margin is strongly concave whereas the lateral margin possesses a pointed process that strongly projects laterally (Fig. 9.3). The external surface is ornamented with a few barely visible shallow circular pits, which do not spread near the margins of the osteoderm. Those ventral osteoderms are gently vaulted, being nearly flat.

Ribs, gastralia and other structures.-Several elongated elements are preserved in the main block and are intertwined with the locally disarticulated skeletal parts. Their encasing in the sediment and the hiding of articular portions render their identification tentative. Thin, straight rod-like structures with a perfectly circular cross section spread to the left of the thoracic region from the eleventh to thirteenth thoracal vertebrae (Fig. 3). They are less than 5 mm in diameter. Thicker elements, about 10 mm in diameter, correspond to ribs that are also scattered in this area and offer sigmoid outlines. Another type of morphology is represented by two nearly complete structures observed in this same area (Fig. 3) as well as by some fragments recovered during acid preparation (Fig. 10). These are wide, flat on one side, convex on the other, and gently curved rib-like structures with an articular facet on one end.

Remarkably, they display numerous ovoid pits on both sides that range in diameter from 1 to 3 mm. These pits are similar to those observed on the external surface of osteoderms.

Gastroliths.-At least five pebbles (1 to 3 cm large) are scattered adjacent to the thirteenth thoracal vertebra (Fig. 3). Another one is observed near the eleventh vertebra. Another one was freed from the block. The nature of these stones is pure quartz and therefore different to the embedding carbonate matrix. The pebbles are not perfectly rounded and possess smooth angular edges.

Discussion

Taxonomic identification and comparison.-The material was initially identified as a suite of vertebrae and limb bones belonging to the genus Dyrosaurus in [START_REF] Arambourg | Vertébrés fossiles du bassin du Niger: Bulletin de la direction des mines[END_REF]. The unprepared block permitted observations of weathered bone surface only (Fig. 3), and although no details are given in [START_REF] Arambourg | Vertébrés fossiles du bassin du Niger: Bulletin de la direction des mines[END_REF], some osteoderms were obviously identifiable. Since Thevenin (1911a,b), dyrosaurids were recognized as part of the Crocodylia with ample descriptions available, for example Hyposaurus [START_REF] Troxell | Hyposaurus, a marine crocodilian[END_REF] or Congosaurus [START_REF] Swinton | On Congosaurus bequaerti Dollo[END_REF]. As noted by [START_REF] Brochu | A dyrosaurid crocodyliform braincase from Mali[END_REF], dyrosaurid taxonomy has been largely established on characters of the rostrum and lower jaw. Most recently erected dyrosaurid species have also been established on the basis of complete skulls [START_REF] Barbosa | New dyrosaurid crocodylomorph and evidences for faunal turnover at the K-P transition in Brazil[END_REF][START_REF] Hastings | A new small shortsnouted dyrosaurid (Crocodylomorpha, Mesoeucrocodylia) from the Paleocene of Northeastern Colombia[END_REF][START_REF] Hastings | A new longirostrine dyrosaurid (Crocodylomorpha, Mesoeucrocodylia) from the Paleocene of north--eastern Colombia: biogeographic and behavioural implications for New--World Dyrosauridae[END_REF][START_REF] Martin | A sebecosuchian in a middle Eocene karst with comments on the dorsal shield in Crocodylomorpha[END_REF]. Isolated skull remains or partially articulated postcranial elements are difficult to assign to a given genus or species, despite extensive description of the postcranial anatomy for Hyposaurinae (e.g. [START_REF] Schwarz | The postcranial skeleton of the hyposaurinae (Dyrosauridae; Crocodyliformes[END_REF]). Although the presently described skeleton from Senegal, MHNT.PAL.2012.0.45, lacks those diagnostic characters established on cranial morphology, it is assigned to the Dyrosauridae on the basis of the following postcranial characters in association: (1) amphiplatyan vertebral centra; (2) femoral head being twisted along the shaft by about 30° from the distal condyles; (3) femur with strong beak-like projection of the proximal head [START_REF] Troxell | Hyposaurus, a marine crocodilian[END_REF]; (4) strong posterior cleft on the proximal surface of the tibia; (5) ischium with forward-projecting anterior iliac process; (6) elongated anterior ball and narrow posterior tuber of the calcaneum; (7) absence of external keel on dorsal osteoderms; (8) presence of ventral osteoderms being twice as wide as long. A last character, (9) deeply pitted rib-like structures, may not be used in the diagnosis of Dyrosauridae, but is discussed herein due to a single previous report in a dyrosaurid specimen. These characters are detailed below and anatomical comparisons are made with Alligator mississippiensis, which represents an extant eusuchian.

Amphiplatyan vertebrae widely occur in non-eusuchian crocodylomorphs. In the post-Cretaceous fossil record, only two groups of crocodylomorphs possess such a type of vertebrae: Sebecosuchia represented by taxa living exclusively in the continental environment, and Dyrosauridae sharing the marine habitat with longirostrine eusuchians, for which vertebral centra cannot be confounded due to their procoelous intervertebral articulation.

The femur from Poponguine is identical to that of any dyrosaurid in possessing a beak-like projecting head. As such, a right femur with a "strong beak-like curve of the head and the weak fourth trochanter" was described in Hyposaurus [START_REF] Troxell | Hyposaurus, a marine crocodilian[END_REF]. The right femur of Congosaurus bequaerti was described by [START_REF] Jouve | Congosaurus bequaerti, a Paleocene dyrosaurid (Crocodyliformes ; Mesoeucrocodylia) from Landana (Angola)[END_REF] and by [START_REF] Schwarz | The postcranial skeleton of the hyposaurinae (Dyrosauridae; Crocodyliformes[END_REF]. A flattened femur is known for Acherontisuchus guajiraensis, for which "the fourth trochanter is large for the family and the paratrochanteric fossa is deeper than the shallow pit seen in C. bequaerti" [START_REF] Hastings | A new longirostrine dyrosaurid (Crocodylomorpha, Mesoeucrocodylia) from the Paleocene of north--eastern Colombia: biogeographic and behavioural implications for New--World Dyrosauridae[END_REF]. [START_REF] Sena | New dyrosaurid remains (Crocodyliformes, Mesoeucrocodylia) from the Paleocene of the Paraíba Basin, NE Brazil[END_REF] described the proximal portion of a right femur for Guarinisuchus munizi. A right dyrosaurid femur was described by [START_REF] Arambourg | Les vertébrés fossiles des gisements de phosphates (Maroc, Algérie, Tunisie)[END_REF], who noticed that the proximal head is in line with the distal condyles (Arambourg, 1952, fig. 62), a condition also identified in the specimen from Poponguine (measuring about 30°). On the other hand, the proximal head is arranged perpendicular to the distal condyles in Congosaurus bequaerti and modern crocodylians [START_REF] Arambourg | Les vertébrés fossiles des gisements de phosphates (Maroc, Algérie, Tunisie)[END_REF]Schwarz et al. 2006 report 140° in Congosaurus bequaerti). According to previous works, the morphology of the femur is variable within the family and the specimen from Poponguine most resembles Hyposaurus in sharing with it a weakly developed fourth trochanter and with the dyrosaurid femur described by [START_REF] Arambourg | Les vertébrés fossiles des gisements de phosphates (Maroc, Algérie, Tunisie)[END_REF] in sharing with it a femoral head in line with the distal condyles.

The tibia of dyrosaurids is distinctive in possessing a marked posterior cleft as well as a strong embayment underlining the cnemial crest, both features being well expressed at the proximal extremity of the bone. Although these features are present in Alligator too, they are not as marked. Overall, the tibia of MHNT.PAL.2012.0.45 is similar to other tibia described for dyrosaurids [START_REF] Langston | Dyrosaurs (Crocodilia, Mesosuchia) from the Paleocene Umm Himar Formation, Kingdom of Saudi Arabia[END_REF][START_REF] Schwarz | The postcranial skeleton of the hyposaurinae (Dyrosauridae; Crocodyliformes[END_REF][START_REF] Sena | New dyrosaurid remains (Crocodyliformes, Mesoeucrocodylia) from the Paleocene of the Paraíba Basin, NE Brazil[END_REF].

The acetabular foramen is completely enclosed by both the ischium and the ilium, which contribute to two-third and one third of it, respectively. No clear difference is noticeable between the Senegalese ilium and that of extant eusuchians. The ischium of MHNT.PAL.2012.0.45 is comparable to other dyrosaurid ischia as described for Hyposaurus (Troxell, 1925, fig. 13), for an indeterminate Pakistanese dyrosaurid [START_REF] Storrs | A dyrosaurid crocodile (Crocodylia : Mesosuchia) from the Paleocene of Pakistan: Postilla[END_REF], for Dyrosaurus sp. [START_REF] Schwarz | The postcranial skeleton of the hyposaurinae (Dyrosauridae; Crocodyliformes[END_REF], for Dyrosaurus maghribensis (Jouve et al. 2006) and for Acherontisuchus guajiraensis [START_REF] Hastings | A new longirostrine dyrosaurid (Crocodylomorpha, Mesoeucrocodylia) from the Paleocene of north--eastern Colombia: biogeographic and behavioural implications for New--World Dyrosauridae[END_REF], which all display a strongly projecting anterior ischium process, an overhanging projection of the ischium process that holds the dorsoventrally elongate facet for the pubis, a narrow to blade-like shaft with nearly parallel edges and a distal process that does not particularly widen distally. These characters are also visible in some notosuchians such as Yacarerani boliviensis (Leardi et al., 2015, fig. 13) or Stratiotosuchus maxhechti (Riff and Kellner, 2011, fig. 12). This is contrary to the situation observed in thalattosuchians (e.g. Herrera et al. 2013, fig. 5), in extant eusuchians (e.g. Mook, 1921, fig. 17) or in coelognathosuchians such as the pholidosaurids Terminonaris robusta (Fig. 7 in [START_REF] Wu | Terminonaris (Archosauria: Crocodyliformes): new material from Saskatchewan, Canada, and comments on its phylogenetic relationships[END_REF]), Oceanosuchus boecensis (Hua et al. 2007, fig. 4) or the goniopholidid Anteophthalmosuchus (Martin et al. 2016, fig. 15). The shaft in the continuity of the ischiac blade of MHNT.PAL.2012.0.45 is wide but not as wide as that of Dyrosaurus sp. [START_REF] Schwarz | The postcranial skeleton of the hyposaurinae (Dyrosauridae; Crocodyliformes[END_REF]) and the overhanging anterior ischiac process is not as much constricted against the shaft as that seen in Dyrosaurus sp. (compare with [START_REF] Schwarz | The postcranial skeleton of the hyposaurinae (Dyrosauridae; Crocodyliformes[END_REF], fig. 10C).

The only other described calcaneum for a dyrosaurid belongs to Rhabdognathus sp. from the Paleocene of Saudi Arabia (Storrs, 1995), which is preserved in connection with an astragalus and is similar in its proportions to MHNT.PAL.2012.0.45. The calcaneum of MHNT.PAL.2012.0.45 differs from that of the extant Alligator in possessing a longer than wide anterior ball and a taller than wide posterior tuber. The calcaneum of Alligator possesses a lateral projection arising from the lateral surface of the posterior tuber; in the dyrosaurid MHNT.PAL.2012.0.45, this process is absent. All other characters described in MHNT.PAL.2012.0.45 are similar to that of Alligator.

The dermal armor of dyrosaurids is composed of both a dorsal and a ventral shield (e.g. [START_REF] Schwarz | The postcranial skeleton of the hyposaurinae (Dyrosauridae; Crocodyliformes[END_REF], and the specimen from Poponguine preserves disarticulated dorsal elements as well as some ventral elements in articulation (Fig. 9). The dorsal elements of MHNT.PAL.2012.0.45 are quadrangular and lack a median keel, a condition seemingly unique to Dyrosauridae [START_REF] Schwarz | The postcranial skeleton of the hyposaurinae (Dyrosauridae; Crocodyliformes[END_REF][START_REF] Martin | A sebecosuchian in a middle Eocene karst with comments on the dorsal shield in Crocodylomorpha[END_REF]. Among Dyrosauridae, the osteoderms of Anthracosuchus balrogus are different from those described here or from any other dyrosaurids in being thick, unpitted and devoid of an imbricating surface [START_REF] Hastings | A new blunt-snouted dyrosaurid, Anthracosuchus balrogus gen. et sp. nov. (Crocodylomorpha, Mesoeucrocodylia)[END_REF]. Thick osteoderms have also been described in Congosaurus bequaerti [START_REF] Jouve | Congosaurus bequaerti, a Paleocene dyrosaurid (Crocodyliformes ; Mesoeucrocodylia) from Landana (Angola)[END_REF][START_REF] Schwarz | The postcranial skeleton of the hyposaurinae (Dyrosauridae; Crocodyliformes[END_REF]) but these are not as thick as in A. balrogus. The dorsal osteoderms of the median series in D. maghribiensis exhibit a laterally projecting peg on their anterolateral margin (Jouve et al., 2006). None of these characters are seen in the Poponguine specimen, which osteoderms morphology is typical of hyposaurine taxa [START_REF] Schwarz | The postcranial skeleton of the hyposaurinae (Dyrosauridae; Crocodyliformes[END_REF]. As stated previously and contrary to Dyrosauridae, the osteoderms of Goniopholididae and Pholidosauridae share a wider than long morphology and the possession of an anterolateral keel [START_REF] Martin | A sebecosuchian in a middle Eocene karst with comments on the dorsal shield in Crocodylomorpha[END_REF]. In MHNT.PAL.2012.0.45, a series of four elements of the ventral shield are preserved in articulation (Figs. 9.4, 9.5). These wider than long elements are not comparable to the hexagonal ventral elements described in goniopholidids (Martin et al. 2016) or in the pholidosaurid Terminonaris robusta [START_REF] Wu | Terminonaris (Archosauria: Crocodyliformes): new material from Saskatchewan, Canada, and comments on its phylogenetic relationships[END_REF].

Massive, flat and deeply pitted rib-like structures are present in the Poponguine specimen (Fig. 10). Similar bony structures were briefly described and figured for Rhabdognathus sp. by [START_REF] Langston | Dyrosaurs (Crocodilia, Mesosuchia) from the Paleocene Umm Himar Formation, Kingdom of Saudi Arabia[END_REF] and identified as costal cartilages. At present, these elements cannot be identified with certainty. Their morphology is intriguing and attention should be kept in the future on such elements, whether they could be pathological or more widespread among dyrosaurids.

The present description of dyrosaurid postcranial elements, although not permitting an identification beyond Dyrosauridae indet., brings some additional knowledge on the morphology of this particular group with details often obscured by compression or articulation when relatively complete specimens are available. A close relationship between dyrosaurids and outgroup taxa such as the Pholidosauridae, Thalattosuchia or Gavialoidea has often been proposed in phylogenetic works and problems related to branch attraction in phylogenies have been discussed [START_REF] Pol | Skull anatomy of Dakosaurus andiniensis (Thalattosuchia: Crocodylomorpha) and the phylogenetic position of Thalattosuchia[END_REF]Martin et al. 2016). Updating phylogenetic codings with new morphological details as those presented here, may help tease apart such problems. For example, the morphology of osteoderms, both dorsal and ventral, differs between these groups of longirostrine forms. Current efforts of describing the postcranial anatomy of various crocodylomorph lineages (e.g. Pol et al. 2011;[START_REF] Herrera | Postcranial skeleton of Cricosaurus araucanensis (Crocodyliformes: Thalattosuchia): morphology and palaeobiological insights[END_REF][START_REF] Leardi | The postcranial anatomy of Yacarerani boliviensis and the phylogenetic significance of the notosuchian postcranial skeleton[END_REF]Martin et al. 2016) will, among other phylogenetic problems, permit to identify what characters are independent of longirostry.

Presence of gastroliths.-The presence and significance of gastroliths or stomach stones in marine reptiles [START_REF] Taylor | Stomach stones for feeding or buoyancy? The occurrence and function of gastroliths in marine tetrapods[END_REF] and more generally among vertebrates [START_REF] Wings | A review of gastrolith function with implications for fossil vertebrates and a revised classification[END_REF] has been discussed at length. [START_REF] Denton | The marine crocodilian Hyposaurus in North America[END_REF] interpreted the presence of gastroliths in the stomachal area of the dyrosaurid Hyposaurus to serve as ballasts, both for diving or to prevent rolling in the waves. Our observations on the Senegalese specimen does not support this view because despite their sizeable dimensions, the total mass of the gastroliths is estimated at about 100 grams and is insufficient to account for a role as ballast in comparison to the total weight of the animal. Indeed, the animal total weight cannot be precisely reconstructed here but may have certainly surpassed 50 kg, which is a highly imprecise estimation. It is nevertheless highly unlikely that 0.2% of this weight would have a ballast effect. Furthermore, previous studies including crocodiles fed experimentally with stones and experimental models revealed that gastrolith masses accounting for less than 2% of body mass do not play a role in buoyancy or stability because the studied crocodiles compensate for added weight by augmenting lung volume [START_REF] Kirshner | Buoyancy control in the estuarine crocodile, Crocodylus porosus Schneider[END_REF][START_REF] Henderson | Effects of stomach stones on the buoyancy and equilibrium of a floating crocodilian: a computational analysis[END_REF][START_REF] Grigg | Biology and evolution of crocodylians[END_REF].

One hypothesis that requires further attention invokes an improvement of dive time thanks to gastrolith weight due to the indirect consequence of increasing lung volume [START_REF] Seymour | Physiological adaptations to aquatic life[END_REF]discussion in Gregg and[START_REF] Grigg | Biology and evolution of crocodylians[END_REF]. Observations on extant crocodiles have shown that pebbles are deliberately swallowed with some animals going into specific areas to collect gastroliths [START_REF] Cott | Scientific results of an inquiry into the ecology and economic status of the Nile Crocodile (Crocodilus niloticus) in Uganda and Northern Rhodesia[END_REF]. The gastroliths preserved in the stomachal cavity of MHNT.PAL.2012.0.45 are clearly allochthonous, the depositional environment where the specimen was recovered consisting exclusively of fine-grained limestone.

Dyrosaurids had enhanced capacities for swimming [START_REF] Schwarz | The postcranial skeleton of the hyposaurinae (Dyrosauridae; Crocodyliformes[END_REF]) and may have travelled long oceanic distances. Therefore, the origin of the gastroliths is unknown and their precise function, if any, remain unknown.

Fossil record of Danian dyrosaurids.-The Senegalese dyrosaurid described in the present work adds to the limited record of Danian dyrosaurids from Africa. Originally presented as Montian (an equivalent to the Selandian) by [START_REF] Buffetaut | Atlantosuchus coupatezi, n. g., n. sp., un nouveau Dyrosauridé (Crocodylia, Mesosuchia) des phosphates Montiens du Maroc[END_REF], the taxon Atlantosuchus coupatezi from Sidi Chenane in the Oulad Abdoun Basin of Morocco has been attributed a Danian age based on the selachian assemblage recovered in the sedimentary matrix [START_REF] Jouve | A long-snouted dyrosaurid (Crocodyliformes, Mesoeucrocodylia) from the Palaeocene of Morocco: phylogenetic and palaeobiogeographic implications[END_REF]. Atlantosuchus coupatezi is known from a mandibular symphysis [START_REF] Buffetaut | Atlantosuchus coupatezi, n. g., n. sp., un nouveau Dyrosauridé (Crocodylia, Mesosuchia) des phosphates Montiens du Maroc[END_REF] and from skull elements including a nearly complete skull as well as some cervical vertebrae [START_REF] Jouve | A long-snouted dyrosaurid (Crocodyliformes, Mesoeucrocodylia) from the Palaeocene of Morocco: phylogenetic and palaeobiogeographic implications[END_REF]. There is no anatomical overlap with the material from Poponguine so no comparison can be made. Another African dyrosaurid of possible Danian age is Phosphatosaurus sp. reported on the basis of a portion of symphysis from Gadamata in the Adar Doutchi of Niger as well as from fragmentary remains collected by René Lavocat in Mali (Buffetaut, 1978a). Again, no anatomical overlap exists with the Senegalese specimen.

At about the same latitude as Senegal, dyrosaurids are also known from Danian deposits in Brazil [START_REF] Cope | A contribution to the vertebrate paleontology of Brazil[END_REF] and from the early Paleocene of Bolivia [START_REF] Buffetaut | Fossil crocodilians from Tiupampa (Santa Lucia Formation, Early Paleocene), Bolivia : a preliminary report[END_REF].

Given the proximity of coastlines during the Eocene of the recently opened North Atlantic Ocean, faunal exchanges between these two provinces may have been easily covered by marine dwelling organisms such as dyrosaurids. Reports from the Danian of Brazil including Hyposaurus derbianus [START_REF] Cope | A contribution to the vertebrate paleontology of Brazil[END_REF] concerned fragmentary specimens from the Maria Farinha Formation (see review in [START_REF] Hastings | A new small shortsnouted dyrosaurid (Crocodylomorpha, Mesoeucrocodylia) from the Paleocene of Northeastern Colombia[END_REF]. Guarinisuchus munizi is known from a complete skull and few postcranial elements, also from the Maria Farinha Formation of Brazil [START_REF] Barbosa | New dyrosaurid crocodylomorph and evidences for faunal turnover at the K-P transition in Brazil[END_REF] and recently, other mandibular and cranial remains have been assigned to this species [START_REF] Sena | New dyrosaurid remains (Crocodyliformes, Mesoeucrocodylia) from the Paleocene of the Paraíba Basin, NE Brazil[END_REF].

Finally, Danian material from Alabama, USA has been revised and assigned to Hyposaurus rogersii by [START_REF] Denton | The marine crocodilian Hyposaurus in North America[END_REF]. The material includes various vertebrae, osteoderms, an ischium, part of a femur and other postcranial fragments (see list in [START_REF] Denton | The marine crocodilian Hyposaurus in North America[END_REF]. Although anatomical overlap with the Poponguine material exists, the Alabama specimens have not been described in detail. Moreover, their attribution to Hyposaurus rogersii needs to be re-evaluated by emending the diagnosis of the species with postcranial material. Presently, the most diagnostic specimen of the species comprises one complete skull from the Maastrichtian of New Jersey [START_REF] Buffetaut | Une nouvelle définition de la famille des Dyrosauridae de Stefano, 1903 (Crocodylia, Mesosuchia) et ses conséquences: inclusion des genres Hyposaurus et Sokotosuchus dans les Dyrosauridae[END_REF][START_REF] Denton | The marine crocodilian Hyposaurus in North America[END_REF].

A revision of H. rogersii is potentially important to better understand dyrosaurid survival across the K/Pg boundary. As presently understood, four species of dyrosaurids (perhaps five if Phosphatosaurus is included) are known from Danian deposits worldwide and include Atlantosuchus coupatezi, Guarinisuchus munizi, Hyposaurus derbianus and Hyposaurus rogersii. Previous studies have merged post-K/Pg dyrosaurid diversity (i.e. Danian, Selandian, Thanetian) in a whole Paleocene time bin (e.g. [START_REF] Jouve | A long-snouted dyrosaurid (Crocodyliformes, Mesoeucrocodylia) from the Palaeocene of Morocco: phylogenetic and palaeobiogeographic implications[END_REF], which may overestimate diversity counts and not reflect the effects of the K/Pg mass extinction on dyrosaurid survival directly after the event. Therefore, focusing on the Danian diversity through collecting efforts will certainly improve this aspect. [START_REF] Jouve | A long-snouted dyrosaurid (Crocodyliformes, Mesoeucrocodylia) from the Palaeocene of Morocco: phylogenetic and palaeobiogeographic implications[END_REF] and [START_REF] Barbosa | New dyrosaurid crocodylomorph and evidences for faunal turnover at the K-P transition in Brazil[END_REF] concurred with the idea that dyrosaurids may have been under competition with the diverse mosasaurs of the Maastrichtian. Whether the extinction of mosasaurs at the K/Pg boundary freed some ecological space for dyrosaurids during the Paleocene is possible but still untested [START_REF] Martin | Calcium Isotopic Evidence for Vulnerable Marine Ecosystem Structure Prior to the K/Pg Extinction[END_REF]. It is equally possible that dyrosaurid diversity only started to increase later in the Paleocene but admittedly, the dating constraints on the Paleocene dyrosaurid record remains imprecise, despite the availability of some stratigraphic studies, notably from Mali [START_REF] Hill | Dyrosaurid (Crocodyliformes: Mesoeucrocodylia) fossils from the Upper Cretaceous and Paleogene of Mali: implications for phylogeny and survivorship across the K/T Boundary[END_REF].

Conclusions

Although the early Paleocene marine deposits of Senegal are less fossiliferous with respect to vertebrate remains than other terrains in northern Africa such as the phosphate deposits of Morocco, they are potentially important for the fossil record of the Dyrosauridae and other marine reptiles that survived the end Cretaceous mass extinction events. The present description of a partial dyrosaurid skeleton from Danian deposits adds to the limited record of dyrosaurids of that age worldwide. Thanks to its state of preservation, the specimen also provides an update on the postcranial morphology of the Dyrosauridae and therefore adds new insights into potential characters to consider in future phylogenies.

  Fig. 8.8). Immediately medial to it, an extensive flat facet is inclined toward the medial

FIGURE CAPTIONS FIGURE 1 -

 CAPTIONS1 FIGURE CAPTIONS

FIGURE 2 -

 2 FIGURE 2-Planktonic foraminifers recovered from the sediment of MHNT.PAL.2012.0.45,

FIGURE 3 -

 3 FIGURE 3-Main block containing the partially articulated dyrosaurid skeleton

FIGURE 4 -

 4 FIGURE 4-Selected vertebrae of MHNT.PAL.2012.0.45 from the Danian of Poponguine,

FIGURE 5 -

 5 FIGURE 5-Elements of the sacrum of MHNT.PAL.2012.0.45 from the Danian of

FIGURE 6 -

 6 FIGURE 6-The hind limb of MHNT.PAL.2012.0.45 from the Danian of Poponguine,

FIGURE 7 -

 7 FIGURE 7-The left calcaneum of MHNT.PAL.2012.0.45 from the Danian of Poponguine,

FIGURE 8 -

 8 FIGURE 8-Elements of the manus of MHNT.PAL.2012.0.45 from the Danian of Poponguine,

Acknowledgments

Several persons provided insights and information that helped us retrace part of the lost history of the specimen described in this work. We express our sincere thanks to Y. Laurent and F. Duranthon (MHNT, Toulouse), E. Buffetaut (ENS Paris), J. Claude (ISEM, Montpellier), E. Robert and S. Passot (Univ. Lyon) and S. Charbonnier (MNHN, Paris). The specimen was prepared by S. Unal and S. Jiquel (ISEM, Montpellier). We thank S. Jouve, A.

Hastings and E. Frey for their time and effort in reviewing a previous version of this work.