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
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. No information regarding the origin locality of the fossil is known, but it seems that it was probably discovered near of Hughenden, Queensland, a town from which Crombie comes. Queensland Museum records show that De Vis even sent a letter to Crombie informing him that he had been made aware of the receipt of the material. The fossil in question, cataloged as QM F1609, consists of a partial mandibular symphysis bearing six conical teeth. 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. The specific epithet queenslandicus is named after the Queensland, the Australian state where the holotype specimen was most likely discovered.
In August 1929, fifteen more or less partial fossils are discovered nearly 3.2 km south of Hughenden. These same fossils, all catalogued as QM F2137, are identified as coming from the Toolebuc Formation, dating from the Albian stage of the Early Cretaceous, the holotype having very probably also been discovered in this same locality. The majority of the material recovered is then very incomplete, the only two that can be concretely described being proximal parts of propodials (upper limb bones), which are analyzed in more detail the following year, and those again by Longman. In 1932, in an effort to make the animal's fossils "attractive", Longman published one of the oldest known reconstructions of Kronosaurus. The illustration was drawn in 1931 by a certain Wilfrid Morden, who was inspired in particular by the anatomical features of Peloneustes to fill in the still unknown parts of the animal. 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, consists of a partial middle of the skull which preserves an occipital condyle, the back of the neurocranium, the external nostrils as well as the orbits.
Harvard expedition
In 1931, the Museum of Comparative Zoology sent an expedition to Australia with the dual aim of obtaining specimens of both living and extinct animals, and in particular marsupial mammals. This decision came from the fact that the museum had relatively few Australian animals and therefore wanted to collect more. It was then that the Harvard Australian Expedition began, and was undertaken by a team of six men. The team consisted of coleopterologist P. Jackson Darlington Jr., zoologist Glover Morrill Allen and his student Ralph Nicholson Ellis, chief physician Ira M. Dixon, paleontologist William E. Schevill, and their leader, entomologist William Morton Wheeler. 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. Schevill then ventured into the Rolling Downs geological group, north of the town of Richmond, where he collected two large pliosaur specimens. These same specimens are collected from the Doncaster Member of the Wallumbilla Formation, dating back approximately 112 million years. The first specimen he exhumed, cataloged as MCZ 1284 and discovered on a property called Grampian Valley, consisted of a well-preserved piece of the anterior rostrum closely connected to the entire mandibular symphysis, in addition to several other fragmentary pieces.
The story regarding the discovery, exhumation and exhibition of the second specimen, cataloged as MCZ 1285, is much more detailed in many historical sources. This specimen was discovered long before the Harvard Expedition was even launched, by a rancher named Ralph William Haslam Thomas, in a locality known as Army Downs. The latter had been aware for many years of the presence of "something strange coming out of the ground" in a small horse enclosure. These "strange things" were actually a row of vertebrae contained in nodules. Noticing his discovery, Thomas therefore informed the members of the Harvard expedition, and notably Schevill. The latter then contacts a British migrant trained in the use of explosives, nicknamed "The Maniac" by local residents, in order to extract the specimen of of rock which constitutes its geological matrix. When the specimen was unearthed, its fossils were then sent to the United States in 86 crates weighing a total of . According to the export permit, the specimen was transported aboard the SS Canadian Constructor around 1 December 1932. Once arrived at Harvard, the fossils, which represent approximately 60% of the skeleton, took several years to extract from the limestone because of the lack of money, manpower and space within the museum. One year earlier, in 1934, Schevill asked Longman to send a cast of the holotype mandibular symphysis for comparison with the new specimen. It was then Longman's assistant, a certain Tom Marshall, who took it upon himself to make Schevill's request. The researchers then realized that the characters of the holotype (QM F1609) were identical to those of the Harvard specimen (MCZ 1285). Longman, in his letters to Schevill, suggests that he would have enjoyed seeing the specimen during its preparation in the late 1930s, but he never left Australian territory. A first scientific description of the skull was made by Theodore E. White in 1935, before it began to be exhibited in the museum four years later, in 1939.