Kronosaurus ( ) is an extinct genus of large short-necked pliosaur that lived during the Aptian to Albian stages of the Early Cretaceous in what is now Australia. The first known specimen was received in 1899 and consists of a partially preserved mandibular symphysis, which was first thought to come from an ichthyosaur according to Charles De Vis. However, it was in 1924 that Albert Heber Longman formally described this specimen as the holotype of an imposing pliosaurid, to which he gave the scientific name K. queenslandicus, which is still the only recognized species nowadays. The genus name, meaning "lizard of Kronos", refers to its large size and possible ferocity reminiscent of the Titan of the Greek mythology, while the species name alludes to Queensland, the Australian state of its discovery. In the early 1930s, the Harvard Museum of Comparative Zoology sent an organized expedition to Australia that recovered two specimens historically attributed to the taxon, including a well known skeleton that is now massively restored in plaster. Several attributed fossils were subsequently discovered, including two large, more or less partial skeletons. As the holotype specimen does not present diagnostics to concretely distinguish Kronosaurus from other pliosaurids, these same two skeletons are proposed as potential neotypes for future redescriptions. Two additional species were proposed, but these are now seen as unlikely or belonging to another genus.
Kronosaurus is one of the largest known pliosaurs identified to date. Initial estimates set its maximum size at around long based on the Harvard skeleton. However, this skeleton had been reconstructed with an exaggerated number of vertebrae, so estimates published from the early 2000s reduce the size of the animal from to more than long. Like all plesiosaurs, Kronosaurus has four paddle-like limbs, a short tail and, like most pliosaurids, a long head and a short neck. The largest identified skulls of Kronosaurus dwarf those of the largest known theropod dinosaurs in size. The front of the skull is elongated into a rostrum (snout). The mandibular symphysis, where the front ends of each side of the mandible (lower jaw) fuse, is elongated in Kronosaurus, and contains up to six pairs of teeth. The large cone-shaped teeth of Kronosaurus would have been used for a diet consisting of large prey. The front teeth are larger than the back teeth. The limbs of Kronosaurus were modified into flippers, with the back pair larger than the front. The flippers would have given a wingspan of more than for the largest representatives.
Phylogenetic classifications published since 2013 recover Kronosaurus within the subfamily Brachaucheninae, a lineage which includes numerous pliosaurids that lived during different stages of the Cretaceous. Based on its stratigraphic distribution in the fossil record, Kronosaurus inhabited the Eromanga Sea, an ancient inland sea that covered a large part of Australia during the Early Cretaceous. This inner sea reached cold temperatures close to freezing. Kronosaurus would likely have been an apex predator in this sea, with fossil evidence showing that it preyed on sea turtles and other plesiosaurs. Estimates of its bite force suggest that the animal would have reached between . The skull of a juvenile specimen shows that it would have been attacked by an adult, indicating intraspecific aggression or even potential evidence of cannibalism within the genus. Kronosaurus would have faced interspecific competition with other large predators within this sea, with one attributed specimen showing bite marks from a Cretoxyrhina-like shark.
Research history
Initial finds and research
thumb|left|alt=|[[Queensland Museum|QM F1609, the holotype mandibular symphysis of K. queenslandicus]]
In 1899, a partial fossil of a marine reptile was sent on behalf of a certain Andrew Crombie to the Queensland Museum of Brisbane, Australia, and was received by the zoologist Charles De Vis, who was then the director of the museum during that time. but it seems that it was probably discovered near of Hughenden, Queensland, a town from which Crombie comes. Based on his observations, De Vis considers the fossil to come from a representative of the Enaliosauria, a now obsolete taxon which included plesiosaurs and ichthyosaurs. De Vis initially thought the specimen came from an ichthyosaur, specifically Ichthyosaurus australis, which today seems to be placed in the genus Platypterygius. However, the particular dentition of this specimen quickly makes it change its mind about whether it belongs to this specific genus. The fossil was officially described by De Vis's successor, Albert Heber Longman, in a scientific article published in 1924 by the journal of the Queensland Museum. Longman deduces that the fossil comes from a large pliosaur, to which he gives the genus and species name Kronosaurus queenslandicus. The generic name comes from Kronos, a Titan from the Greek mythology, and from ancient Ancient Greek σαῦρος (saûros, "lizard"), to literally give "lizard of Kronos". Longman would have created this generic name in reference to the imposing size and possible ferocity of the animal, which could recall the story of Kronos, who is known in Greek mythology for having devoured his own children, notably Zeus. are discovered nearly 3.2 km south of Hughenden.
These same fossils, all catalogued as QM F2137, In May and April 1935, a certain J. Edgar Young for the Queensland Museum, collected several fossils from the Toolebuc Formation, more precisely from the Telemon station, about 30 km west of Hughenden. Among all the fossils Young was involved in exhuming are additional remains attributed to Kronosaurus, including the first somewhat more complete cranial parts identified within the genus. In his article published in October 1935, Longman, due to the high number of fossils, suggested that they came from at least two or three individuals. Noting that the fossils were not fully prepared at the time of his description, he describes them preliminary. The most notable specimen, cataloged as QM F2446, The following year, in 1932, it was Schevill who acquired the title of expedition leader, making long journeys and recruiting local help when he could. The Queensland Museum was also invited to participate in this expedition, but this was never approved due to lack of funds and/or interest from the state government. However, Longman, who described the first known fossils of Kronosaurus, nevertheless assisted the expedition, storing specimens as they were sent to him, securing collecting permits, and maintaining correspondence with Schevill. The latter had been aware for many years of the presence of "something strange coming out of the ground" in a small horse enclosure. Noticing his discovery, Thomas therefore informed the members of the Harvard expedition, When the specimen was unearthed, its fossils were then sent to the United States in 86 crates weighing a total of . Two specimens with no specific affiliation were identified in the Bulldog Shale.
In 2023, Valentin Fischer and colleagues criticized the reassignments even under these circumstances, predicting that they stand contrary to ICZN Articles 75.5 and 75.6 and that the aforementioned multiple-species possibility cannot justify a tentative reassignment of all specimens to Eiectus. The authors instead opted to refer to all relevant fossils as Kronosaurus-Eiectus. The same year, Stephen F. Poropat and colleagues maintained K. queenslandicus as a nominally valid taxon that includes all fossils from the Toolebuc and Allaru Formation pending an official ICZN petition, recommending specimen QM F18827 as neotype. The authors also criticize the repurposing of Toolebuc specimens, on the grounds that Noè and Gómez-Pérez presumably ignored the conclusion of McHenry's 2009 thesis that only one species of large pliosaur exists in the formation and that, therefore, all of its specimens can be reliably considered conspecific to the holotype. As for Eiectus, Poropat and colleagues limit it only to MCZ 1285 and the referred specimen MCZ 1284, but their assignment without formal redescription also remains subject to debate, given that the holotype is so massively restored with plaster that all features apparent diagnostics are probably unreliable without comprehensive CT scans.
Proposed and formerly classified species
thumb|left|upright=1.1|alt=Close-up view of the restored skull of a pliosaur|The Harvard skeleton was proposed to belong to another species of Kronosaurus based on suggested cranial differences.
Although the only currently recognized species of Kronosaurus is K. queenslandicus, several authors have suggested the existence of additional species within the genus. In 1982 and again in 1991, Ralph Molnar expressed doubts as to whether the Harvard skeleton (MCZ 1285) belonged to the species K. queenslandicus, given that it was discovered in a locality distinct from that of the first known specimens, namely in the older Wallumbilla Formation. The author therefore suggests that this specimen would belong to another species of Kronosaurus characterized by a deeper and more robust skull than those coming from the Toolebuc Formation. A study published in 1993 also attributes the specimen under the name Kronosaurus sp., the authors following the same opinion as Molnar. However, as White indicates in his description of the specimen in 1935, much of the skull roof is not preserved and is mostly restored in plaster,
thumb|upright=1.1|alt=Fossil skeleton of a pliosaur kept in a museum|Holotype skeleton of [[Monquirasaurus, which was formerly classified as K. boyacensis]]
In 1977, an almost complete skeleton of a large pliosaur was discovered by local residents of the town of Villa de Leyva, Colombia. The specimen, nicknamed "El Fósil" and dating from the Upper Aptian of the Paja Formation, was first provisionally referred to the genus Kronosaurus two years later, in 1979. It was in 1992 that the German paleontologist Olivier Hampe established a second species of the genus under the name of K. boyacensis, the specific name referring to Boyacá, the department surrounding the discovery site. However, these descriptions were made from photographs and remote imaging techniques, in particular because access to the specimen was prohibited by the local community. It was therefore in 2022 that Noè and Gómez-Pérez re-described this specimen and discovered that it belonged to a distinct genus, which they named Monquirasaurus, in reference to Monquirá, the administrative division where the specimen was discovered. the latter having a reduced size to around according to more recent estimates. Currently, the largest marine reptile identified to date is the Late Triassic ichthyosaur Ichthyotitan, which is thought to have reached around in length. The Harvard skeleton restoration being erroneous, McHenry gives a smaller size of this specimen between long for a body mass of . These same measurements are seen as the maximum possible estimates of the genus as a whole. Even before McHenry's thesis was published, paleontologist Benjamin P. Kear and marine biologist Richard Ellis proposed comparable estimates in their respective works both published in 2003, ranging from according to Kear at according to Ellis. In 2026, Ruizhe Jackevan Zhao revises the measurements of MCZ 1285 at in total body length and in body mass.
Other specimens have been given body estimates although some of these are only known from more limited fossil remains. QM F1609, the holotype specimen, although very fragmentary, would have measured long. The proposed neotype specimen QM F18827 would have reached a length of . The most complete known attributed specimen, QM F10113, would have reached slightly smaller measurements, namely long with a body mass of around . The largest specimens of Kronosaurus having been discovered in the Toolebuc Formation, QM F2446 and QM F2454, would have reached measurements almost identical to that of the Harvard skeleton. Respectively, these two specimens would have reached in length.
Skull
thumb|left|upright=1.3|Reconstructed skull
Since the holotype of K. queenslandicus (QM F1609) consists of only a partial mandibular symphysis, very little can be said about it. However, more complete fossil skulls that are assigned to the taxon show unique traits. The skulls of various known specimens of Kronosaurus vary in size. The holotype, which although partial and fragmentary, comes from a skull which would have measured a total of long. Candidate neotype specimens QM F10113 and QM F18827 have cranial lengths reaching , respectively. The skull of the Harvard skeleton is estimated to be long. The cranial measurements of the last three specimens previously cited surpass in size the skull of any known theropod dinosaurs. The snout and the mandibular rostrum are long and narrow in shape. The rostrum in general appears to be arched in shape and is relatively elongated, possessing a distinct median and dorsal crest. The eye sockets face obliquely posteriorly, where they are located laterally on the anterior half of the skull. The temporal fossae (openings in the top back of the cranium) are very large, but the skull does not have an anterior interpterygoid vacuity.
One of the many traits identified as unique in Kronosaurus is that the premaxilla (front upper tooth-bearing bone) has four instead of five or more caniniform teeth.