Meridiosaurus, VALLISPARIDISI MONES, 1980

Fortier, Daniel, Perea, Daniel & Schultz, Cesar, 2011, Redescription and phylogenetic relationships of Meridiosaurus vallisparadisi, a pholidosaurid from the Late Jurassic of Uruguay, Zoological Journal of the Linnean Society 163 (5), pp. 644-645 : 644-645

MERIDIOSAURUS VALLISPARIDISI MONES, 1980 ( FIG. 2)

Etymology: Genus from meridionalis, Latin for ‘southern’, and saurus, Greek for ‘lizard’, referring to the southernmost record of a pholidosaurid. Specific epithet from vallis, Latin for ‘valley’, and paradisus, latin for ‘paradise, Eden’, referring to its geographic provenance, the Valle Edén locality.

Holotype: FC-DPV-2322 (formerly SPV-FHC-27-II-78- 1), anterior part of rostrum, mandible fragments, one osteoderm, and unidentifiable fragments. No referred ·

specimens. Housed in the Colección de Vertebrados Fósiles, Faculdad de Ciencias, Montevideo (FC-DPV).

Horizon and locality: The fluviolacustrine sandstone facies of the Batoví Member, Tacuarembó Formation, in the Valle Edén locatily near Tacuarembó city.

Emended diagnosis: Longirostrine mesoeucrocodylian with the following combination of features: (1) lateral constriction of the rostrum at premaxilla–maxilla contact, strong lateral expansion of the premaxilla with the fifth tooth placed in the widest portion; (2) sinusoidal premaxilla–maxilla suture in palatal view, posteromedially directed on its lateral half and anteromedially directed along its medial region; (3) evaginated maxillary alveolar edges forming a discrete collar at each alveolus, lateroventrally oriented; (4) greater number of maxillary teeth (at least 27) with respect to Elosuchus (less than 20 maxillary teeth); (5) nasals do not meet premaxilla dorsally; (6) strong sinusoidal lateral contour of snout in dorsal view, with respect to the similar condition of Elosuchus , forming two waves (‘festooned’); and (7) third, fourth, and fifth alveoli are equally enlarged.

DESCRIPTION

State of preservation: The specimen is very fragmented. Practically all teeth are missing, and even small elements, like an osteoderm, are fragmented. Nevertheless, there is no evidence of water abrasion. The sutures are visible and external ornamentation is preserved, as well the delicate details of a carinated maxillary tooth. There is no evidence of diagenetic distortion.

Gerenal features: The nearly tubular rostrum measures 27.8 cm in length, 2.6–7.6 cm in width, and 1.5 cm in height. The external surface is ornamented with deep pits and grooves, but the evaginated alveolar margins are smooth. The lateral and ventral edges of the rostrum are sinusoidal in dorsal and ventral views, forming waves, constricted anteriorly at the level of premaxilla–maxilla suture in ventral view, and posteriorly at the level of the seventh maxillary teeth. The heart-shaped external naris is longer than wide, not divided, and completely surrounded by the premaxillae. It faces dorsally and is separated from the anterior edge of the rostrum by the premaxillary bar. There is no premaxilla–nasal contact, an autapomorphic feature of Meridiosaurus .

Premaxilla: Both premaxillae were preserved and show fractures of compression oriented dorsoventrally. Each has a maximum length of 7.5 cm. The width for each is 2.6 cm at the level of the premaxilla–maxilla constriction, and the maximum width is 4.2 cm at the level of the fourth tooth. Each premaxilla has five teeth. The alveoli are ventrally long and ventrally oriented. The first two are aligned transversely at the anterior margin of the bone. The other three teeth are positioned in a posteriorly curving arch. Alveoli for the first four teeth enlarge to the fourth tooth. The fifth is the smaller, placed anterolaterally to the maxillary tooth row, and is in close contact with the fourth tooth. In ventral view, there is a deep notch for the first dentary teeth, at the level of the third tooth, and a shallow notch for the second dentary teeth behind it. Dorsally, the acute posterior process extends to the level of the fifth maxillary tooth, and in ventral view to the level of the first maxillary tooth. The distance between the tip of the rostrum and the anteriormost position of the premaxilla–maxilla suture in dorsal view is smaller than the distance between this and the posterodorsal extremity of the premaxilla. The premaxilla–maxilla suture is smooth in dorsal view, and slightly serrated in lateral view. Laterally, the suture is oriented anteroventrally, and ventrally is sinusoidal, with the medial part directed anteriorly and the lateral part directed posteriorly. The premaxilla is constricted at the contact with the maxilla, and expands anteriorly forming a concave notch, similar to the condition in Goniopholis simus ( Salisbury et al., 1999) . It is more expanded than the maxillae until the thirteenth maxillary teeth, where the maxillae become wider. The anterior part is expanded dorsoventrally, and the ventral edge is placed slightly deeper than the maxilla edge, not as far as in Sarcosuchus imperator ( Sereno et al., 2001) or Terminonaris robusta ( Wu et al., 2001) . A small ovate incisive foramen is placed in the middle of the premaxillae in ventral view, and is also visible in dorsal view. Some small to large foramina are located in the ventral view, parallel with the tooth row.

Maxilla: Both maxillae are present, but fragmented. The right maxilla has 27 alveoli preserved. Each alveolus is surrounded by the unsculptured evaginated maxillary edge. The anterior alveoli are subcircular, and the posterior alveoli are anteroposteriorly oval. Among the first eight alveoli, the third, fourth, and fifth are larger. One complete and nine incomplete teeth are preserved. The teeth have a smooth root (based on the left thirteenth tooth) and a carinated crown (based on the left fourteenth and right third teeth; Fig. 2F). The tooth rows form sinusoidal waves, and at the level of the fifteenth tooth, the maxillae expand posteriorly. In ventral view, there is an acute anterior process, extended to the level of the fourth premaxillary tooth, close to the incisive foramen. The anterior teeth have a large interalveolar space, whereas the posterior alveoli are close to each other. Notches for dentary teeth follow the tooth rows medially, from the eighth tooth to the end of the maxillae. The maxillae are well sutured in the midline. Some small foramina are located in ventral view, medially to the notches for dentary teeth.

Dentition: Alveolar proximodistal diameters vary between 2.6 mm (fifth premaxillary tooth) and 6.3 mm (fourth premaxillary tooth). The maximum interalveolar space is 5.1 mm, between the first and the second maxillary teeth. The teeth have a smooth root and carinated crown, forming ridges ( Fig. 2F). The last maxillary alveoli are lingolabially compressed, oriented parallel with the longitudinal axis of the rostrum. However, all the preserved teeth are circular to subcircular in cross section. No teeth show constriction at the base of the crown, and the width of the root is nearly equal to the width of the crown. No premaxillary alveoli are inturned, as in Sarcosuchus and Terminonaris ( Sereno et al., 2001) . All dentary teeth occlude lingual to maxillary teeth, as suggested by the presence of notches in the premaxillae and the maxillae for occlusion of the dentary teeth. The tooth margins are slightly higher than the medial portion of the palate.

Mandible: Only a post-symphysial dentary fragment was preserved, probably from the right ramus, bearing three incomplete teeth ( Fig. 2G–I). The ornamentation is formed by shallow grooves. The alveolar edges are evaginated, forming discrete evaginations at each alveoli, covering the base of the teeth. All alveoli lie in a 30° angle, postero-anteriorly. As in the maxilla, the dentary teeth have a smooth root and a carinated crown. In lingual view, a thin sheet of the splenial was preserved, forming the ventral edge of a reduced Meckelian groove.

Bone armour: Only one osteoderm was recovered associated with Meridiosaurus remains, from the caudal armour ( Fig. 2J). The osteoderm is strongly ornamented, with ridges, grooves, and punctuations, and is not keeled. It is slightly convex dorsally, and it bears an anteriorly depressed overlap margin for articulation with the preceding osteoderm. It is very similar to the caudal osteoderm of Sarcosuchus ( Sereno et al., 2001) .

Remarks on size and weight: Usually, the ratio between the preorbital length and the total skull length is used in longirostrine crocodylomorphs (e.g. Hua et al., 2007). However, this measure cannot be made for Meridiosaurus . Elosuchus have a ratio between the length of the tip of the snout to the last maxillary tooth, and the total skull length equal to 68%. There is little variation in this percentage among other pholidosaurids, e.g. 65% for Sarcosuchus and 70% for Terminonaris . Considering the last preserved maxillary tooth of Meridiosaurus as its last tooth, we can suppose a skull length of 40 cm. This value is the minimum size of the skull, but it is possible that more teeth were present posterior to the last preserved tooth. Plotting this size in the regression formulae of Sereno et al. (2001), the body length should vary between 2.3 and 2.8 m, which would make it the smallest know pholidosaurid. Using the regression formulae of Farlow et al. (2005), Meridiosaurus weighed between 45 (2.3 m body length) and 85 kg (2.8 m body length). Nothing can be inferred about the age of the specimen.