Skull

The skull of Monquirasaurus is large and blunt, 2.45 m (8.0 ft) long from the snout tip to the posterior end of the right squamosal (2.65 to the posterior end of the retroarticular process of the mandible) and 1.2 m (3.9 ft) wide across the lateral margin of the quadrates. When viewed from the side the cranium is elongated and low, however it is still severely crushed. The jaws of the holotype are tightly closed and most of the dentition is preserved in situ. The cranium does not only suffer from crushing, but most of the dorsal surface is also heavily weathered and many bones are missing, making it almost impossible to observe most of the skull sutures. The skull has undulating margins, with 3 lateral expansions visible in dorsal view. The first is present along the lateral margins of the premaxilla, followed by an expansion around the large anterior caniniforms and one just beneath the orbits. A 4th expansion was probably also present in the posterior area of the skull, around the lateral margins of the temporal fenestra. Both anterior expansions are immediately followed by a medial constriction of the cranium. The nares are located 1.13 m (3.7 ft) from the snout tip, anterior to the orbits and on the same level as the 11th maxillary tooth. The mandible is preserved in better condition than the cranium, largely resisting the crushing that affected the latter. There is however still some dorsoventral compression present around the mandibular rami.^[1]

Size compared to a human

The mandible is robust and resembles classic pliosaurid morphology with large caniniform teeth housed at the anterior margin of the bone. The anterior portion is slightly upturned and expanded as in Simolestes and Acostasaurus. The teeth of Monquirasaurus are round in cross section with the largest being the caniniforms. The anterior dentition is markedly anisiodont and interlocking, while more posterior areas of the skull have tooth rows overlapping. The anterior caniniforms of the maxilla for instance obscure the dentary teeth of that region, while further posterior the dentary teeth form a pronounced underbite obscuring the posterior maxillary teeth. All of this makes it difficult to determine the original tooth count of the animal. The anterior expansion of the premaxilla contains 4 teeth on each side, with premaxillary tooth 2 being the largest, extending halfway down the mandible. While the 3rd and 4th tooth are both large in their own right, they are still smaller than the preceding tooth. Some uncertainty surrounds the presence of a small tooth just behind the 4th premaxillary. This area is almost entirely obscured by the 4th dentary caniniform, however there is a hint of a small alveolous in that area. While the absence of visible sutures makes the separation of premaxillary and maxillary dentition uncertain, Noè and Gómez-Pérez suggest that, if a small tooth would have been present, it would most likely been a maxillary.^[1] They reason that all pliosaurids of the Paja Formation are united through the presence of 4 premaxillary teeth, while furthermore pointing out that the first maxillary tooth in pliosaurs is oftentimes much smaller and followed by a large caniniform (such as the next preserved tooth in Monquirasaurus). Between the last preserved premaxillary and first maxillary caniniform extends a toothless gap or diastema, equivalent in diameter to the area taken up by a large tooth. The first preserved (potentially second overall) maxillary tooth is the largest in the jaw, although broken on either side this tooth is inferred to have reached beyond the ventral margin of the mandible. Following this tooth are continuously smaller caniniforms followed by many much smaller maxillary teeth that progressively grow smaller towards the posterior. The anterior mandibular teeth are large and interlocking with the premaxillary teeth. The largest of these is the 4th dentary, which is still smaller than the later maxillary caniniforms, but larger than any of the premaxillary teeth. The dentary teeth after the small 5th tooth are obscured by the maxillary caniniforms, however most likely also rather small. Larger dentary teeth are preserved posterior to this overlap. Overall Monquirasaurus may have had a minimum of 19 teeth in its upper jaw whilte the mandibles preserve 19 tooth positions on the right and 23 on the left.^[1]

Skeletal diagram and size estimation of the pliosaur Monquirasaurus boyacensis, with the preserved elements in white, and the reconstructed elements on grey.

Postcranial skeleton

Hind view of the holotype

The body of Monquirasaurus was 4.65 metres (15.3 ft) long along the vertebral column, not including the skull and missing tail. As preserved, the body was broad, however in life it would have been narrower. The vertebral column is largely preserved in articulation, with some gaps towards the posterior end of the body. The neck of Monquirasaurus was short, with 8 articulated vertebrae preserved excluding the atlas-axis complex, which is obscured by the cranium (the presence of a third vertebra obscured by the cranium is possible, but not certain). The cervical series is followed by 3 pectoral, 22 dorsal and 3 sacral vertebrae. Out of these vertebrae, the last dorsal and first 2 sacrals are only preserved as impressions. The preserved neural arch of the 17th and 18th vertebrae (dorsals 5 and 6) are dorsally subtriangular, narrow anteriorly and flattened posteriorly, suggesting a "tongued-and-grooved" interlocking arrangement. This would make the back stiff and rigid, as in other plesiosaurians. Most of the tail is missing, with only 3 preserved caudal vertebrae articulating with the sacrals. The appendicular skeleton is only partly preserved, with all limbs missing the distal end of the flippers and girdles obscure by the overlaying torso.^[1]