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, in Nuevo León, Mexico. In a 1988 study, the specimen was first interpreted as a dinosaur, 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 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. The size of the fossils, its former interpretation as a juvenile and its incorrect attribution with Liopleurodon would likely have been one the cause of its size over-exaggeration in the 1999 BBC documentary series Walking with Dinosaurs. However, the most recent estimates put the size of the specimen at around 10 and 11 m (33 and 36 ft). The Aramberri specimen would have had an approximately 3 m (9.8 ft) long mandible and the large teeth since lost would have had possesed two sharp edges. The gastralia (abdominal ribs) of the Aramberri pliosaur possess traits that could be diagnostic for a distinct pliosaurid lineage that may soon be described.

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. 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.

During an excursion conducted in the fall of 1985^[a] In the Mexican state of Nuevo León, the geology student Walter Hähnel discovered a large concretion of fossils in the Sierra Madre Oriental, in the town of Linares, near the municipality of Aramberri. Stratigraphically, the discovery took place within the La Caja Formation, a geological formation containing numerous fossils dating from the Upper Jurassic, and more precisely from the Upper Kimmeridgian stage.^[3][4] During the exhumation, a second, equally large concretion was discovered by paleontologists.^[3] 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 code name UANL-FCT-R2.^[5][6] The fossils were first described in the scientific literature in 1988 by Hähnel, who identified them as coming from a carnivorous dinosaur.^[7] As the hemipelagic sediments surrounding the region and the anatomy of the specimen contradict this initial interpretation,^[8] followed by a first re-examination led by Marie-Céline Buchy and her colleagues in 2003.^[5] The first concretion contains numerous postcranial elements which include, among other things, 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 which included a rostrum with teeth was however lost and noted as such in 2003,^[9] although it was briefly described by Hähnel in 1988.^[7]

Although the animal had never previously received a scientific name, the large size of the discovered fossils attracted international media attention and the specimen was quickly referred to by the informal nickname "Monster of Aramberri" as well as the from journalists and scientists alike.^[10][8] New expeditions carried out from 2001 to 2007 within 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 suggests that the fossil specimen would have been technically complete before the erosion of the skull and its exhumation for phosphorite.^{[11][12][13][14][15]} In 2003, in order to help paleontologists, the newly elected mayor of Nuevo León sent 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.^[12] 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 kept in Karlsruhe had the preparation finalized, they were returned to the Faculty of Earth Sciences of the Autonomous University of Nuevo León in November 2012, where they have since been stored. However, some of the fossils are on display at the Mexican History Museum in Mexico City.^[16] The newly discovered and prepared fossils consist 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 include 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 for future work.^{[17][11][12][18][19]}

The Aramberri specimen is one of the largest pliosaurs whose fossils discovered are at least well preserved.^[20] Although the entire skeleton of the animal has not received detailed descriptions at present, authors have nevertheless described certain parts of the animal.^[5][21][22] 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, short tail, a massive trunk and two pairs of large flippers.^[23][24]

The "Monster of Aramberri", as its nickname suggests, is also one of the largest pliosaurs identified to date, but estimates of its size have declined considerably over the years. During the 2000s, most media put the size of the Aramberri pliosaurus at between 19 and 20 m (62 and 66 ft) long with a body mass of 50 t (49 long tons; 55 short tons),^[25][26] a weight which has also been fixed by certain researchers who have described the fossils of the animal.^[27] Initial, more precise estimates put the size at around 15 m long at least,^{[28][29][30][14]} although a maximum estimate of up to 18 m (59 ft) was also proposed.^[31][29] In 2008, Adam S. Smith and Gareth J. Dyke, citing the source of Buchy et al. (2003), give a maximum length of 17 m (56 ft).^[32] 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).^[33] 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)^[b]. 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.^[35] 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).^[36]

Relatively few of the animal's cranial bones have been discovered, and some of them have even been lost regarding the rostrum and teeth.^[37][14][38] 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. According to the descriptions, the rostrum would measure 60 cm (24 in) long and contain 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 come from a dentary (one bone of the mandible), a maxilla or premaxilla (two bones of the upper jaw).^[37] 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.^[39][27][14] Some researchers also suggests that the animal's teeth would be conical in shape and that the largest ones would have reached 20 cm (7.9 in) in length.^[27]

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.^[5][21][22] 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 visibly 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.^[37] The femoral head of the "Monster of Aramberri monster" 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).^[13][21] The gastralia have deep, almost circular grooves that are irregularly spaced. As these characteristics cannot be assimilated a bite, these may prove to be a distinctive feature that could prove diagnostic of the animal for future studies.^[31]

The "Monster of Aramberri" was initially described as a large carnivorous dinosaur in 1988,^[7] before paleontologists discovered that it was actually a pliosaur from the family Pliosauridae in the early 2000s.^[1][40][5] According to Buchy et al. (2003), its affiliation is based on the presence of multiple vertebral characteristics unique to this group. These characteristics are the foramens which are present on the ventral side of each vertebrae as well as the articulations of the ribs with the latter which are quite particular.^[41] The specimen was incorrectly assigned to the species Liopleurodon ferox by the media during the 2000s,^[25][26] but 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.^[19] The group that currently includes Kronosaurus, known as Brachaucheninae, only includes representatives that lived during the Cretaceous,^[42] 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.^[43] 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.^[19]

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.^[28] 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.^[44] McHenry shares identical conclusions in his 2009 thesis.^[45] Based on these observations, Frey and Stinnesbeck then interpreted the specimen as being a subadult.^[34] In 2024, Jackevan Zhao, sharing the same observations, considers the specimen as an adult due to its rather imposing size.^[46]

Certain cranial fragments of the Aramberri pliosaur, namely 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.^[12][14][19]

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 statement.^[31]

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.^[47] The presence of this imposing pliosaurid in this region is an argument in favor of the existence of communication between the northern Tethyan domain where most of the fossils were found, and the epicontinental seas which bathed South America at that time.^[48] 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.^[47] The formation contains a number of other marine reptiles from various groups. Among the plesiosaurs, in addition to the "Aramberri monster", two other additional specimens of indeterminate pliosaurids are known.^[49][50] Apart from these three pliosaurids, the only known plesiosaurian is an elasmosaurid also noted as indeterminate.^[51][52] Thalattosuchians include one unattributed specimen^[53][54] and Cricosaurus saltillensis.^[55] The first thalattosuchian cited was also discovered during the excavation of the Aramberri pliosaur.^[56] Ichthyopterygians have been known since 2006^[51][57] but it is from 2009 that ophthalmosaurids have been reported.^[58][59] These ophthalmosaurids are Ophthalmosaurus^[58][59] and Parrassaurus.^[60]

As mentioned previously, the Aramberri specimen was for a time attributed to Liopleurodon.^[25][26] As the specimen was formerly interpreted as a juvenile (therefore suggesting that the supposed adult specimens would have been larger), it is likely that this specimen would have been one of the possible causes of the over-exaggeration of the measurements 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).^[17][11][61] However, the largest specimens of Liopleurodon would have visibly reached 8 m (26 ft) in length with a body mass of 7.8 t (7.7 long tons; 8.6 short tons),^[62] and it is very unlikely that the Aramberri specimen is a representative of this genus.^[17][11][19] Some researchers humorously referred this tendency to over-exaggerate the size of Liopleurodon and the Aramberri specimen as "godzillaisation".^[17][63]

• List of plesiosaur genera
• Timeline of plesiosaur research