The "Monster of Aramberri" is an informal name given to a fossil skeleton of a very large pliosaur since numbered as UANL-FCT-R2, of which the first remains were discovered in 1985 near the town of Aramberri, Mexico. In scientific literature, some authors also refer to it as the Aramberri pliosaur or the Aramberri specimen. Initially interpreted as a dinosaur in 1988, it was formally reidentified as a marine reptile of the family Pliosauridae in a short 2001 publication. Initially, only two concretions containing the animal's fossils were discovered, with one of the two—containing the fossils of a rostrum and teeth—later noted as lost in the first in-depth study conducted on the specimen in 2003. During the 2000s, a new excavation campaign unearthed several additional fossils of the animal. Subsequently, most of these fossils were sent to Karlsruhe State Museum of Natural History, Germany, to be prepared, before returning them in 2012 to the Autonomous University of Nuevo León, where they are mainly stored. Another significant portion of the fossils are currently stored in the Desert Museum of Saltillo.
The "Monster of Aramberri" is one of the largest pliosaurs ever discovered, but estimates of its size have dropped considerably over the years. Initial estimates set a length of around based on the assumption that it represents a juvenile, with maximum proposed length estimates up to . Its informal attribution to Liopleurodon by the BBC was linked with the massively over-sized, long depiction of this genus in the BBC documentary series Walking with Dinosaurs, though the specimen itself wasn't part of the inspiration behind the series' portrayal. More recent, accurate size estimates have since suggested that the specimen likely measures around long in total. Nevertheless, the animal would have an approximately long mandible, and the large teeth, since lost, would have possessed two sharp edges.
The specimen is viewed as a representative of the Thalassophonea, a derived clade of pliosaurids characterized by a short neck and a large, elongated skull. The gastralia (abdominal ribs) of the Aramberri pliosaur possess traits that could be diagnostic for a distinct pliosaurid lineage that may soon be described. In the trunk, the Aramberri pliosaur preserves fossils of what appears to be an ichthyosaur, suggesting that this was prey consumed before it died. Two known cranial fragments of the animal also preserve bite marks that would have been made by another, more imposing pliosaur. The La Caja Formation, where the "Monster of Aramberri" was discovered, contains abundant marine fossils from a shallow environment dating from the Kimmeridgian stage of the Late Jurassic. It shared its habitat with a variety of other animals, including invertebrates, fish, thalattosuchians, ichthyosaurs, and other plesiosaurs.
History of discovery
In the fall of 1985, a geology student from the Autonomous University of Nuevo León discovered a large concretion of fossils during an excursion in the Sierra Madre Oriental, approximately northwest of Aramberri, Nuevo León. During the exhumation, a second, equally large concretion was discovered by paleontologists. The site's inaccessibility and the weight of the collected fossils prevented researchers from moving them immediately. The following year, the discovered material was finally moved over via fairly complex technical processes before reaching a road facilitating transport. Subsequently, the two large concretions were sent to the Faculty of Earth Sciences of the Autonomous University of Nuevo León, where the fossils are cataloged under the specimen number UANL-FCT-R2. The preparation of the first fossil concretion lasted two months, and upon completion, it was fixed vertically on a poured concrete base. The first concretion contained numerous postcranial elements, including seven pectoral vertebrae (transitional vertebrae between the cervical and thoracic regions), fragments of coracoids and a left scapula, ribs as well as gastralia. The second concretion contained a rostrum with teeth. These fossils were first mentioned in the scientific literature in 1988 by German geologist Walter Hähnel, one of the researchers who participated in their excavation. In his paper, he indicated that the discovery had been made stratigraphically within the La Casita Formation, a Late Jurassic geological formation more precisely dated to the late Kimmeridgian. Despite the fact that the specimen was discovered in marine sediments, Hähnel identified it as a carnivorous dinosaur with a length ranging between .
The main fossils of the specimen were rediscovered during the autumn of 2000 and, based on their vertebral morphology, were reidentified as belonging to a large marine reptile of the family Pliosauridae. The discovery was announced in September 2001 by German paleontologist and colleagues in a brief publication, in which they gave this specimen the informal nickname of the "Monster of Aramberri". On 28 December 2002, the German magazine Der Spiegel published an article about the discovery, adopting this nickname and highlighting the completeness and remarkable size of the marine reptile. This coverage drew the attention of international media, to the point that the nickname has since been widely used by both journalists and scientists. A detailed study of the fossils, led by French paleontologist Marie-Céline Buchy and colleagues, was published in 2003. In the 2001 publication, the authors noted that the second concretion was not relocated to the university, and the 2003 study later noted it as lost. Based on the stratigraphic position of various ammonite specimens at the site, the authors of the latter study determined the Aramberri pliosaur was discovered in the La Caja Formation, which dates to the "middle" Kimmeridgian.
From 2001 to 2007, new expeditions carried out in the original locality by Mexican, French and German paleontologists helped by residents of the city, made it possible to exhume the caudal part of the specimen, and two thirds of the skeleton. Cranial fragments were also discovered, although most are unidentifiable. Field data even suggested that the fossil specimen would have been technically complete before the erosion of the skull and its exhumation for phosphorite. Ultimately, approximately 70% of the skeleton has been recovered. In 2003, in order to help paleontologists, the newly elected governor of Nuevo León sent a helicopter to transport a fossil block weighing a total of . During his campaign the following year, the governor approved and even completed the construction of a road leading to the excavation site. Once exhumed, most of the fossils were transferred to the Natural History Museum in Karlsruhe, Germany, for preparation. The Karlsruhe Museum could not accommodate more fossils due to the size of the animal, so the remaining material was transferred to the Desert Museum in Saltillo, Coahuila. When the material sent to Karlsruhe was prepared, they were returned to the Faculty of Earth Sciences of the Autonomous University of Nuevo León in 2012, where they have since been stored. Some fossils were temporarily exhibited in 2007 at the Mexican History Museum in Mexico City. The newly discovered and prepared fossils consisted of nine cervical vertebrae preserved on three blocks, additional dorsal vertebrae, ribs and gastralia, a femoral joint head, as well as an almost complete pelvic girdle. The cranial elements identified included a pterygoid, a jugal and part of a maxilla preserving a dental alveolus. The new fossils are still being prepared and should receive much more detailed descriptions in future work. In 2012, Mexican paleontologist Javier Aguilar Pérez suggested that the specimen should receive a skeletal mount formed from the fossils once their preparations are completed. Furthermore, he also expressed that new cranial fragments should be found in the field where the specimen was discovered, but no expedition has apparently been sent since.
Description
The Aramberri specimen is one of the most complete large pliosaurs ever discovered. Although the entire skeleton has not been described in detail, certain parts have received targeted descriptions. Plesiosaurs are usually categorized as belonging to the small-headed, long-necked "plesiosauromorph" morphotype or the large-headed, short-necked "pliosauromorph" morphotype, with the "Monster of Aramberri" belonging to the latter category. Like all plesiosaurs, it had a short tail, a massive trunk and two pairs of large flippers.
Size estimates The "Monster of Aramberri", as its nickname suggests, is also one of the largest pliosaurs identified to date, but its size estimates have declined considerably over the years. Initial estimates placed the size at around long at least, although maximum estimates of up to with a body mass of were also proposed. In 2008, British paleontologist Adam S. Smith and his Irish colleague Gareth J. Dyke, citing the 2003 paper, gave a maximum length of . In his 2009 thesis, Australian paleontologist Colin McHenry criticized the initial interpretations, which he found very exaggerated. Comparing the Aramberri specimen with the fossil material attributed to the Australian pliosaurid Kronosaurus, he reduced its size to between for a body mass of between . In 2014, Frey and Wolfgang Stinnesbeck increased the length of the specimen slightly to between . In 2021, German paleontologists Frederik Spindler and Martin Mattes further reduced the size of this specimen to between long. In 2024, Chinese paleontologist Ruizhe Jackevan Zhao did not give a precise estimate of the measurements of the specimen. He suggested instead, based on vertebral dimensions, that it would have been similar in size to Pliosaurus funkei, which according to his model was approximately long with a body mass of . The same year, Australian biologist Joel H. Gayford and his colleagues estimated that the animal would have reached approximately in length.
Skull Relatively few of the animal's cranial bones have been discovered, and some of them have been lost, such as the rostrum and teeth. However, using one of the photographs presented in Hähnel's 1988 publication, Buchy and colleagues gave some comments in their first re-examination of the specimen in 2003. The rostrum is long and has three broken teeth. The teeth are described as massive, reaching about in diameter and being bicarinate (possessing two sharp edges). The fossil also contains an additional tooth probably coming from an opposite jaw. Therefore, it is uncertain whether the rostrum represents a dentary (one bone of the mandible), a maxilla or premaxilla (two bones of the upper jaw). Based on a mandible of a large pliosaur preserved at the Oxford University Museum of Natural History in England, the authors estimated that the "Monster of Aramberri" had a mandible and a skull up to long. The animal's teeth were conical in shape and the largest ones would have reached in length.
Postcranial skeleton