Monquirasaurus

Credit: Fabio Alejandro.

*Monquirasaurus* is an extinct genus of pliosaurid that lived in Columbia during the Early Cretaceous.

## History

The holotype of *Monquirasaurus* is preserved as 7.3 meters long. It is an articulate young adult discovered by Samuel Argas, Enrique and German Zubieta flrom Tito Hurtado. Excavations were conducted by the Instituto Colobiano de Geología y Minería (now Servicio Geológico Colombiano), the Instituto Colombiano de Antropología e Historia and the Universidad Nacional de Colombia. The specimen is known as El Fósil, and was named *Kronosaurus boyacensis* by Hampe (1992). Years prior it had been referred to as a *Kronosaurus*. It was described via images and remote imaging techniques, since the local community and the Junta de Acción Comunal did not allow for in-person viewing. This created a lacking and untrustworthy description. In 2021, Noè and Gómez-Pérez viewed the specimen in-person and described it as *M. boyacensis*. Many had suspected that *"K." boyacensis* was distinct years before[*citation needed*]. In the original type locality, a museum called Museo El Fósil was erected around the holotype. The specific epithet refers to the Boyacá Department.

## Description

*Monquirasaurus* has a large, blunt, 2.45 meter-long, 1.2-meter-wide at the largest point skull. It is elongate and low from the side, but very crushed in the holotype. Most of the holotype's dentition is articulated and tightly closed in death. Most of the dorsal side is weathered and incomplete. This makes it impossible to examine the cranial sutures. Undulating margins and 3 lateral expansions are seen in the dorsal (the first along the lateral margins of the premaxilla, one around the large anterior caniniforms, another beneath the orbits and a possible 4th likely at the lateral margins of the temporal fenestra). Anterior expansions are followed by medial constriction at the cranium. The nares are 1.13 meters from the rostral tip, anterior to the orbits and at the same level as the 11th maxillary tooth. The mandible is more intact and less crushed (except for some dorsoventral compression at the rami). The mandible is classically pliosaurid. There are premaxillary teeth that increase in size, then large caniniform teeth that increase in size. Then, smaller teeth line the jaws, which gradually increase and then decrease in size. When interlocked, these teeth formed a diastema. An absent space may have been filled by a small tooth that may have been a maxillary. Monquirasaurus had a minimum of 19 teeth in the upper jaw and 19 sockets on the right of the mandible with 23 on the left.

*Monquirasaurus* had a body 4.65 meters long (not including the skull and tail). The body was broad in preservation but slimmer in life. The spine is articulated, but there are some gaps towards the posterior of the body. It had a short neck with 8 vertebrae excluding the axis/atlas, since this is obscured by the skull. It is possible a 3rd vertebra is also obscured by the crania but it is not certain. There are 3 pectorals, 22 dorsals and 3 sacrals; only the first and last sacrals are preserved as impressions. The neural arch of dorsals 5 and 6 are dorsally subtriangular, narrow in the anterior and flattened in the posterior, which suggests a "tongued-and-grooved" interlocking system that would make the back stiff. Only 3 caudals are preserved articulated with the sacrals. The appendicular skeleton is partial, with all limbs missing the distal end and the girdles are obscured by the torso. The precaudal length is 6.74 meters (not including the missing parts of the tail). This makes Monquirasaurus about 8 meters long. The holotype is sexually mature; a young adult.

## Classification

The 2021 study did not conduct a phylogenetic analysis so it is unknown where exactly *Monquirasaurus* fits, but it is definitely a pliosaurid and unlikely to be a brachauchenine. They suggest it may be in a Lower Cretaceous lineage of non-brachauchenines alongside *Sachicasaurus* and *Acostosaurus* that had gone unrecognized. Testing is needed to confirm or deny this.

## Paleoecology

*Monquirasaurus* is part of the Arcillolitas abigarrades member of the Paja Formation of the Late Aptian. This formation is a shoreface-lower shoreface environment containing elasmosaurids, marine turtles, crocodylomorphs, ichthyosaurs and other large pliosaurs (*Acostasaurus*, *Stenorhynchosaurus* and *Sachicasaurus*).

## References

* https://www.sciencedirect.com/science/article/abs/pii/S0195667121003700?via%3Dihub
* https://doi.org/10.1016%2Fj.cretres.2021.105122
* Hampe, O. 1992. Ein großwüchsiger Pliosauride (Reptilia:Plesiosauria) aus der Unterkreide (oberes Aptium) von Kolumbien. *Courier Forschungsinstitut Senckenberg* 145. 1–32.
* Acosta, C.E.; G. Huertas, and P.M. Ruiz. 1979. Noticia preliminar sobre el hallazgo de un presunto *Kronosaurus* (Reptilia: Dolichorhynchopidae) en el Aptiano superior de Villa de Leiva, Colombia. *Lozania (Acta Zoologica Colombiana)* 28. 1–7.