# Monster of Aramberri

The "Monster of Aramberri", also nicknamed in the scientific literature as the Aramberri pliosaur or the Aramberri specimen, is an informal name given to UANL-FCT-R2, a fossil skeleton of a very large pliosaur of which the first remains were discovered during the 1980s near the town of Aramberri, Mexico. The specimen was first interpreted as a dinosaur in 1988, before later inspections re-identified it as a marine reptile belonging to the family Pliosauridae. Initially, two concretions only 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 re-identification of the specimen in 2003. During the early 2000s, a new excavation campaign unearthed several additional fossils of the animal, in which some of them were subsequently 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 15 m (49 ft), with maximum proposals going up to 18 m (59 ft) or even 20 m (66 ft) according to some media. Its former interpretation as a juvenile as well as its incorrect attribution with Liopleurodon—then incorrectly portrayed in the 1999 BBC documentary series Walking with Dinosaurs—would probably have been the origin of the over-exaggeration of its size. Most recent accurate size estimates put fixes the specimen at around 10 and 11 m (33 and 36 ft). Nevertheless, the animal would have an approximately 3 m (9.8 ft) long mandible and the large teeth since lost would have had possesed two sharp edges.
Based on various comparisons and descriptions, the "Monster of Aramberri" is most likely 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 its last prey consumed before its death. 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.

## Research history

During an excursion conducted in the fall of 1985, in the Mexican state of Nuevo León, a geology student of the Autonomous University of Nuevo León reported the discovery of a large concretion of fossils in the Sierra Madre Oriental, in the town of Linares, near the municipality of Aramberri. During the exhumation, a second, equally large concretion was discovered by paleontologists. The inaccessibility of the place and the fossils collected at the time weighing around 200 kg (440 lb) in total, prevented researchers from moving it immediately. The following year, the discovered material was finally moved over 1.24 miles (2.00 km) 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 concerned are all cataloged under the specimen number UANL-FCT-R2. The preparation of the first fossil concretion lasted two months, and when this was finalized, it was fixed vertically on a poured concrete base. The first concretion contained numerous postcranial elements which include, among other elements, seven pectoral vertebrae (transitional vertebrae between the cervical and thoracic regions), fragments of coracoids and a left scapula, ribs as well as gastralia (abdominal ribs). The second concretion contained a rostrum with teeth. These fossils were first mentioned in the scientific literature in 1988 by Walter Hähnel, one of the researchers who participated in their exhumations, which mentioned that they come stratigraphically from the La Casita Formation, a geological formation dating from the Upper Jurassic, and more precisely from 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 10 and 15 m (33 and 49 ft).
In 2001, paleontologists announced via a publication that they had re-identified the specimen as actually coming from an imposing marine reptile of the family Pliosauridae on the basis of its vertebral morphology, while also pointing out that the fossils should be described in more detail later. Still in the announcement, paleontologists give this specimen the informal nickname of the "Monster of Aramberri". From 2002, this specimen received international attention from the media, and the nickname given has since been used by both journalists and scientists. The reported study describing the fossils in more detail was written by Marie-Céline Buchy and her colleagues and was published in 2003. In the 2001 publication, the authors note that the second concretion was not relocated to the university, but it is since the 2003 study where it is noted as lost. The same study also reveals that the specimen was discovered in the contemporary La Caja Formation.
From 2001 to 2007, new expeditions carried out in the type 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 have also been discovered although most of them 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, it seems that 70% of the animal's skeleton is known. In 2003, in order to help paleontologists, the newly elected mayor of Nuevo León sended a helicopter to transport a fossil block weighing a total of 450 kg (990 lb). During his campaign the following year, the mayor 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 in Karlsruhe were 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 a news article published in November 2012, paleontologist Javier Aguilar Pérez expressed 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 of the animal has not received detailed descriptions at present, authors have nevertheless described certain parts of the animal. The various genera of pliosaurids that are regularly compared with the "Monster of Aramberri" are today classified in the Thalassophonea clade, which are characterized by an elongated skull connected to a short neck, unlike many other plesiosaurs, which have a long neck and a small head. Like all plesiosaurs, however, the specimen has 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 years. Initial estimates putted the size at around 15 m (49 ft) long at least, although a maximum estimate of up to 18 m (59 ft) with a body mass of 50 t (49 long tons; 55 short tons) was also proposed. In 2008, Adam S. Smith and Gareth J. Dyke, citing the source of Buchy and colleagues (2003), give a maximum length of 17 m (56 ft). In his thesis published a year later, in 2009, Australian paleontologist Colin McHenry criticizes this interpretation, which he finds very exaggerated. Comparing the Aramberri specimen with the fossil material attributed to Kronosaurus, he reduced its size to between 11.7 and 12.2 m (38 and 40 ft) for a body mass of between 14.9 and 17.8 t (14.7 and 17.5 long tons; 16.4 and 19.6 short tons). In 2014, German paleontologists Eberhard Frey and Wolfgang Stinnesbeck increased the length of the specimen slightly to between 12 and 14 m (39 and 46 ft). In 2021, Frederik Spindler and Martin Mattes further reduced the size of this specimen to between 10 and 11 m (33 and 36 ft) long. In 2024, Ruizhe Jackevan Zhao does not give a precise estimate of the measurements of the specimen. He suggests instead, based on vertebral dimensions, that it would have been similar in size to Pliosaurus funkei, which according to his model was approximately 9.8 m (32 ft) long with a body mass of 12 t (12 long tons; 13 short tons). The same year, Joel H. Gayford and his colleagues estimated that the animal would have reached approximately 10.7 m (35 ft) in length.

### Skull

Relatively few of the animal's cranial bones have been discovered, and some of them have even been lost like the rostrum and teeth. However, using the photographs and the description carried out in 1988 by Hähnel, Buchy and colleagues gave some comments in their first re-examination of the specimen in 2003. The rostrum is 60 cm (24 in) long and have three broken teeth. The teeth are described as massive, reaching about 5.5 cm (2.2 in) in diameter and being bicarinate (possessing two sharp edges). According to a photo taken of this rostrum, the latter would also contain an additional tooth probably coming from an opposite jaw. Therefore, it is uncertain whether the rostrum would 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 estimate that the "Monster of Aramberri" should have a mandible and a skull up to 3 m (9.8 ft) long. The animal's teeth would be conical in shape and the largest ones would have reached 20 cm (7.9 in) in length.

### Postcranial skeleton

Currently, the majority of the postcranial material of the Aramberri pliosaur has not yet been described due to the fact that preparations are not currently finalized. However, much of the material initially discovered and some fossils exhumed and subsequently prepared have been described. The front and back parts of the centra are slightly convex. The latter are pulley-shaped and vary in length from 9 to 10.5 cm (3.5 to 4.1 in). The neural spines of the vertebrae are quadrangular in shape in lateral view, reaching 20 cm (7.9 in) high with a length ranging from 5 to 8 cm (2.0 to 3.1 in). The neural tubes are oval in shape, reaching 8 to 10 cm (3.1 to 3.9 in) high and 2 to 3 cm (0.79 to 1.18 in) wide. The preserved proximal parts of the ribs measure up to 20 cm (7.9 in), but their state of preservation prevents us from saying more about their morphology. The rare ribs associated with the vertebrae have a curved dorsal margin. The femoral head of the "Monster of Aramberri" measures 45 cm (18 in) wide; for comparison, a specimen of Liopleurodon measuring 5 m (16 ft) long has a femoral head which only reaches 14 cm (5.5 in). The gastralia have deep, almost circular grooves that are irregularly spaced. These features are not bite marks, and they could prove to be a diagnostic trait of the animal in future studies.

## Classification

The "Monster of Aramberri" was initially described as a large dinosaur by Hähnel (1988) without more precise classifications, its carnivorous nature suggesting a probable theropod. Paleontologists discovered that it was actually a pliosaur from the family Pliosauridae in the early 2000s. According to Buchy and colleagues (2003), its affiliation is based on the presence of multiple vertebral characteristics unique to this group. These characteristics are the foramina which are present on the ventral side of each vertebra as well as the articulations of the ribs with the latter which are quite particular. The lack of proper cranial diagnosis and remaining undescribed fossils prevent it from being assigned to any European pliosaurid taxon. In 2014, Frey and Stinnesbeck provided a brief description of the Aramberri pliosaur and commented on its hypothetical affinities within the pliosaurid family. The small size of the flippers relative to the body of the animal indicates that the specimen would be a close relative of the Australian pliosaurid Kronosaurus. The group that currently includes Kronosaurus, known as Brachaucheninae, only includes representatives that lived during the Cretaceous, the Aramberri specimen being dated to the Upper Jurassic. In 2009, based on its vertebral morphology, McHenry did not consider the Aramberri specimen to be close to Kronosaurus, casting doubt on the classification later proposed by Frey and Stinnesbeck. Nevertheless, both authors note that the notable anatomy of the gastralia could be diagnostic for the specimen and therefore establish it within a distinct taxon for future studies.

## Paleobiology
### Ontogenety

In their 2003 study, Buchy and his colleagues noted that the seventh neural arch is not fused with the vertebrae, the other neural arches being incomplete to allow similar observations. Plesiosaur specimens with neural arches not fused to the vertebrae are generally seen as juveniles, and the authors therefore consider the Aramberri specimen to be one as well. However, Buchy questioned this interpretation in 2007, citing that very few pliosaur fossils have been found with neural arches fused to the vertebrae, and that these were most likely juvenile traits carried over into adulthood. McHenry shares identical conclusions in his 2009 thesis. Based on these observations, Frey and Stinnesbeck then interpreted the specimen as being a subadult. In 2024, Jackevan Zhao, sharing the same observations, considers the specimen as an adult due to its rather imposing size.

### Bite marks

Some cranial fragments of the Aramberri pliosaur, the jugal and the pterygoid, show bite marks showing that the specimen would have been attacked at the back of the head. The bite mark present on the pterygoid shows that it would have been made by a tooth whose crown is estimated according to sources at between 4 and 7 cm (1.6 and 2.8 in), suggesting a total length of the tooth of approximately 30 cm (12 in). Despite the significant injury, the pterygoid shows the presence of a callus, suggesting that the animal may have survived the incident. Conversely, the jugal would have been perforated by a tooth which would have reached two-thirds of that which had touched the pterygoid, but which would have probably been fatal because there are no signs of healing. The animal that would have injured or even killed the Aramberri pliosaur would probably have been larger, but the authors did not give estimates of its size in order to avoid speculations.

### Stomach contents

At the trunk level, the Aramberri specimen shows what appear to be poorly preserved bones with etching traces from a possible ichthyosaur. The ichthyosaur in question would likely have been the specimen's last meal before its death, but further studies are needed to confirm this.

## Paleoecology

The La Caja Formation, where the "Monster of Aramberri" was discovered, would have been a calm marine environment of shallow depth estimated between 150 and 300 m (490 and 980 ft), although the proximity of an island is attested by plant deposits probably torn away during rare storms. The presence of this imposing pliosaurid in this region is an argument in favor of the existence of a connection between the northern Tethyan domain where most of the fossils were found, and the epicontinental seas which covered South America at that time. The formation, dating from the late Kimmeridgian, contains numerous invertebrate fossils, ammonites being the most abundant. Other invertebrates present include belemnites, bivalves, brachiopods, serpulids, radiolarians and calpionellids. Fossil remains of fish are also known. The formation contains a number of other marine reptiles from various groups. Among the plesiosaurs, in addition to the "Monster of Aramberri", two other additional specimens of indeterminate pliosaurids are known. Apart from these three pliosaurids, the only known plesiosaurian is an elasmosaurid also noted as indeterminate. Thalattosuchians include one unattributed specimen and Cricosaurus saltillensis. The first thalattosuchian cited was also discovered during the excavation of the Aramberri pliosaur. Ichthyopterygians have been known since 2006, but ophthalmosaurids have only been reported from 2009. These ophthalmosaurids are the type genus Ophthalmosaurus and Parrassaurus.

## In popular culture

Most media put the size of the Aramberri pliosaur at between 19 and 20 m (62 and 66 ft) long and consider it as a representative of Liopleurodon. The depiction of Liopleurodon in the 1999 BBC documentary series Walking with Dinosaurs, where it is depicted as reaching 25 m (82 ft) in length and weighing 150 t (150 long tons; 170 short tons), and its former juvenile interpretation (thus suggesting that adult specimens contemporaries would have been way larger), are possible causes of these erroneous estimates and attributions. However, the largest known specimen of Liopleurodon would have reached 8 m (26 ft) in length with a body mass of 7.8 t (7.7 long tons; 8.6 short tons), and it is admitted that the Aramberri specimen is not a representative of this genus. McHenry humorously referred this tendency to over-exaggerate the size of Liopleurodon and the Aramberri specimen as "godzillaisation".
In 2017, a life-sized restoration of the "Monster of Aramberri" was carried out on the basis of the dimensions of the preserved fossils. It has since been exhibited at the Papalote Museo del Niño, in Monterrey.