Kronosaurus
Template:Short description Template:Use dmy dates Template:Automatic taxobox
Kronosaurus (Template:IPAc-en Template:Respell) 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 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 Template:Convert 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 Template:Convert to more than Template:Convert 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 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 Template:Convert 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 Template:Convert. 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 historyEdit
Initial finds and researchEdit
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.<ref name="Longman1924"/>Template:SfnTemplate:Sfn No information regarding the origin locality of the fossil is known,Template:SfnTemplate:Sfn<ref name="Paleofile">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>Template:Sfn but it seems that it was probably discovered near of Hughenden, Queensland, a town from which Crombie comes.<ref name="Longman1924"/><ref name="Mather1986"/> 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.Template:Sfn The fossil in question, cataloged as QM F1609,Template:SfnTemplate:Sfn consists of a partial mandibular symphysis bearing six conical teeth.<ref name="Longman1924">Template:Cite journal</ref> 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,Template:Sfn which today seems to be placed in the genus Platypterygius.Template:SfnTemplate:Sfn 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.<ref name="Longman1924"/><ref name="Mather1986">Template:Cite book</ref>Template:Sfn 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.<ref name="Hall1985"/>Template:SfnTemplate:Sfn<ref name="Paleofile"/><ref name="Meaning"/> The specific epithet queenslandicus is named after the Queensland, the Australian state where the holotype specimen was most likely discovered.<ref name="Hall1985"/><ref name="Meaning">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="Paleofile"/>
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In August 1929, fifteen more or less partial fossils<ref name="Longman1930">Template:Cite journal</ref> are discovered nearly 3.2 km south of Hughenden.Template:Sfn These same fossils, all catalogued as QM F2137,<ref name="Paleofile"/>Template:Sfn 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.Template:Sfn 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),<ref name="Paleofile"/>Template:Sfn which are analyzed in more detail the following year, and those again by Longman.<ref name="Longman1930"/> 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.<ref name="Longman1932">Template:Cite journal</ref> 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.Template:Sfn 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.<ref name="Longman1935">Template:Cite journal</ref> The most notable specimen, cataloged as QM F2446,Template:Sfn<ref name="Paleofile"/>Template:Sfn 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.Template:Sfn
Harvard expeditionEdit
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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,Template:Sfn and in particular marsupial mammals.<ref name="Pick&Sloan2004"/> 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.<ref name=Gardiner1931>Template:Cite journal</ref><ref name="Hall1985">Template:Cite book</ref><ref name="Pick&Sloan2004">Template:Cite book</ref> 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.<ref name="Mather1986"/> Schevill then ventured into the Rolling Downs geological group, north of the town of Richmond, where he collected two large pliosaur specimens.Template:Sfn These same specimens are collected from the Doncaster Member of the Wallumbilla Formation, dating back approximately 112 million years.Template:Sfn 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.Template:Sfn<ref name="White1935">Template:Cite journal</ref><ref name="Romer&Lewis1959"/>
The story regarding the discovery, exhumation and exhibition of the second specimen, cataloged as MCZ 1285, is much more detailed in many historical sources.<ref name="White1935"/><ref name="Romer&Lewis1959"/><ref name="Hall1985"/><ref name="Mather1986"/>Template:Sfn<ref name="Pick&Sloan2004"/>Template:Sfn This specimen was discovered long before the Harvard Expedition was even launched, by a rancher named Ralph William Haslam Thomas,<ref name="OOKKronosaur"/> in a locality known as Army Downs.<ref name="Longman1935"/><ref name="Romer&Lewis1959">Template:Cite journal</ref>Template:Sfn The latter had been aware for many years of the presence of "something strange coming out of the ground" in a small horse enclosure.<ref name="Mather1986"/><ref name="Pick&Sloan2004"/> These "strange things" were actually a row of vertebrae contained in nodules.<ref name="OOKKronosaur">Template:Cite journal</ref> Noticing his discovery, Thomas therefore informed the members of the Harvard expedition,<ref name="OOKKronosaur"/> and notably Schevill.<ref name="Mather1986"/><ref name="Pick&Sloan2004"/>Template:Sfn The latter then contacts a British migrant trained in the use of explosives, nicknamed "The Maniac"Template:Efn by local residents,<ref name="Mather1986"/><ref name="Thulborn&Turner1993"/>Template:Sfn<ref name="Pick&Sloan2004"/>Template:Sfn in order to extract the specimen of Template:Convert of rock which constitutes its geological matrix.<ref name="Nature1959">Template:Cite journal</ref> When the specimen was unearthed, its fossils were then sent to the United States in 86 crates weighing a total of Template:Convert.<ref name="Hall1985"/><ref name="OOKKronosaur"/><ref name="Pick&Sloan2004"/> According to the export permit, the specimen was transported aboard the SS Canadian Constructor around 1 December 1932.<ref name="OOKKronosaur"/> Once arrived at Harvard, the fossils, which represent approximately 60% of the skeleton, took several years to extract from the limestone<ref name="Pick&Sloan2004"/> because of the lack of money, manpower and space within the museum.<ref name="Hall1985"/> 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.<ref name="Mather1986"/> The researchers then realized that the characters of the holotype (QM F1609) were identical to those of the Harvard specimen (MCZ 1285).<ref name="Nature1959"/> 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.<ref name="Mather1986"/> A first scientific description of the skull was made by Theodore E. White in 1935,<ref name="White1935"/> before it began to be exhibited in the museum four years later, in 1939.<ref name="Hall1985"/>
The rest of the skeleton was kept in the basement of the museum for more than fifteen years. This interim period ended when the fossils attracted the attention of Godfrey Lowell Cabot, a Boston industrialist, philanthropist and founder of the Cabot Corporation. Cabot's family had a history of sighting large sea snakes in the coastal waters around the town he is from. When questioning the museum's director, Alfred Sherwood Romer, about the existence and reports of sea serpents, it occurred to Romer to tell Cabot about the skeleton kept in the museum's basement.<ref name="Hall1985"/>Template:Sfn So Cabot asks about the cost of a restoration and Romer says "about $10,000". Romer may not have been serious, but Cabot clearly was because the check for said sum came shortly after.<ref name="Hall1985"/><ref name="Pick&Sloan2004"/> Given that Romer's primary interest was the study of non-mammalian synapsids, it is possible that he had little regard for the skeleton as a subject of scientific study.Template:Sfn After two years of careful preparations with chisel and acid by Arnold Lewis and James A. Jensen under Romer's direction, their work ultimately cost slightly more than promised by Cabot's base check.<ref name="Hall1985"/><ref name="Pick&Sloan2004"/> The Harvard skeleton was exhibited for the first time on 10 June 1958,<ref name="Hall1985"/> and is followed by a detailed scientific description carried out by Romer and Lewis, which was published the following year by the museum journal.<ref name="Romer&Lewis1959"/>Template:Sfn When the finalization of the specimen was announced in the Australian press, Longman, who is the descriptor of the taxon, was not mentioned. In response, professor and geologist Walter Heywood Bryan sent a message via telegraph informing journalists that it would be regrettable if such an important announcement made no mention of Longman and the interpretation of the initially fragmentary fossil material.<ref name="Mather1986"/> At the age of 93, Thomas, the original discoverer of the specimen, was able to see the mounted skeleton of what he considered "his dinosaur", as well as meet again the leader of the museum's former expedition, each believing that the other had been dead for a long time.<ref name="OOKKronosaur"/>
The arrival of new knowledge in the field of paleontology subsequently calls into question the restoration of the skeleton as proposed by Romer. Indeed, because of many incomplete bones, the latter ordered Lewis and Jensen to add plaster where he deemed it necessary. This latest decision has made it difficult for paleontologists to access real fossils,<ref name="Pick&Sloan2004"/> to the point where some of them use the humoristic nickname "Plasterosaurus" to refer to the specimen.Template:SfnTemplate:Sfn<ref name="Tembe&Siddiqui2014"/>Template:Sfn In addition, it seems that the skeleton was reconstructed with the wrong proportions. According to Australian paleontologist Colin McHenry, the specimen has eight extra vertebrae added to the spine<ref name="Pick&Sloan2004"/> and the skull is not supposed to have a bulbous shaped sagittal crest on top.Template:Sfn In his thesis revising the genus Kronosaurus published in 2009, McHenry called the Harvard skeleton "a rather disappointing restoration of what must have been an excellent fossil specimen".Template:Sfn For this reason, many researchers express their desire to analyze real fossils using CT scans.<ref name="Tembe&Siddiqui2014"/>Template:Sfn
Later discoveries and genus validityEdit
Given that the holotype specimen of K. queenslandicus (QM F1609) is fragmentary and does not present any unique characteristics that would qualify the genus as distinct from other pliosaurs, the validity of this taxon has therefore been questioned. As early as 1962, Samuel Paul Welles considered Kronosaurus as a nomen vanum and recommended the designation of a neotype specimen from Harvard University which would preserve the genus validity.Template:Sfn<ref name="Carpenter1996"/>Template:Efn From 1979,<ref name="Mather1986"/> a good number of fossils from large pliosaurs were discovered in various localities in Australia, mainly in the geological strata of the Toolebuc Formation, the formation from which the first fossils attributed to the genus were discovered.Template:Sfn In other formations, only one additional attributed specimen was discovered in the Doncaster Member of the Wallumbilla Formation,Template:Sfn while three specimens, including one attributed to the type species, were discovered in the Allaru Formation.<ref name="Kear2005">Template:Cite journal</ref><ref>Template:Cite journal</ref><ref name="Kear2016">Template:Cite journal</ref><ref name="Holland2018">Template:Cite journal</ref> Two specimens with no specific affiliation were identified in the Bulldog Shale.<ref name="Kear2006">Template:Cite journal</ref><ref name="Kear2016"/><ref name="Holland2018"/> In his 2009 thesis, McHenry describes in detail many fossils attributed to Kronosaurus, including most of the new specimens that he judges to possibly belong to this genus.Template:Efn Of the numerous fossil specimens that he analyzed, McHenry proposed that two partial skeletons, cataloged as QM F10113 and QM F18827, which both come from the Toolebuc Formation, could be candidate neotypes, because they present features that seem to fit with the holotype.Template:Sfn However, no formal ICZN petition to designate a neotype was submitted. In 2022, Leslie Francis Noè and Marcela Gómez-Pérez published a study that revised most of the specimens historically attributed to Kronosaurus. Both authors limit Kronosaurus only to the holotype and consider it a nomen dubium. The holotype specimen does not possess any features allowing a diagnostic, the other attributed fossils are provisionally moved to a new taxon that the two authors name Eiectus longmani, in homage to Longman, the paleontologist who named the original genus. The Harvard skeleton (MCZ 1285) is also designated a holotype of this same genus.<ref name="Noè&Gómez-Pérez2022">Template:Cite journal</ref>
In 2023, Valentin Fischer and colleagues criticized the reassignments even under these circumstances, predicting that they stand contrary to ICZN Articles 75.5 and 75.6Template:Efn 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.<ref name=Fischeretal2023>Template:Cite journal</ref> 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.Template:Sfn 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.Template:SfnTemplate:Sfn 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.Template:Sfn
Species proposed or formerly classifiedEdit
Although the only currently recognized species of Kronosaurus is K. queenslandicus, several authors have suggested the existence of additional species within the genus.Template:Sfn 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.Template:SfnTemplate:SfnTemplate:SfnTemplate:Sfn A study published in 1993 also attributes the specimen under the name Kronosaurus sp., the authors following the same opinion as Molnar.<ref name="Thulborn&Turner1993">Template:Cite journal</ref> 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,<ref name="White1935"/> the real proportions being therefore uncertain.Template:SfnTemplate:Sfn In his 2009 thesis, McHenry nevertheless continues to refer the specimen to K. queenslandicus because of its taphonomic distribution and certain traits which may be consistent with other specimens discovered in the Toolebuc Formation.Template:Sfn To determine whether this statement is true, only a CT scan could reveal the presence of the true notable differences within this reconstructed plaster specimen.<ref name="Tembe&Siddiqui2014">Template:Cite journal</ref>Template:Sfn
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.<ref name="Acostaetal1979">Template:Cite journal</ref> 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.<ref name="Hampe1992">Template:Cite journal</ref> However, these descriptions were made from photographs and remote imaging techniques, in particular because access to the specimen was prohibited by the local community.<ref name="Noè&Gómez-Pérez2022"/> In addition, the state of preservation of the specimen and anatomical characteristics different from those of K. queenslandicus also suggested doubts about the affiliation of this species to Kronosaurus.<ref name="Holland2018"/><ref name="Páramo-Fonsecaetal2018">Template:Cite journal</ref> 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.<ref name="Noè&Gómez-Pérez2022"/>
DescriptionEdit
Due to the fact that the holotype specimen of Kronosaurus is non-diagnostic, the majority of anatomical descriptions are based on observations made from more complete fossils later assigned to the genus. The majority of descriptions come from McHenry's thesis published in 2009, although some specimens have been described in other works.<ref name="Holland2018"/>Template:Sfn Kronosaurus has a morphology typical of the pliosaurids of the thalassophonean group, which has a large elongated skull connected to a short neck, unlike many other plesiosaurs, which have a long neck and a small head. Like all other plesiosaurs, Kronosaurus has a short tail, a massive trunk and two pairs of large flippers.Template:SfnTemplate:Sfn<ref name="Holland2018"/>Template:Sfn
SizeEdit
Kronosaurus is one of the largest pliosaurs identified to date,Template:Sfn but several estimates as to its exact size have been proposed during research. As early as 1930, Longman, in his description of propodiums, considered that Kronosaurus would have exceeded in size the imposing Megalneusaurus, a North American pliosaurid dating from the Late Jurassic.<ref name="Longman1930"/>Template:Sfn<ref name="Meaning"/> After the collection of fossils assigned to the genus by the Harvard Expedition, the maximum size of Kronosaurus was generally set at Template:Convert long,based on specimen MCZ 1285.<ref name="Romer&Lewis1959"/>Template:Sfn<ref name="OOKKronosaur"/>Template:Sfn<ref name="Pick&Sloan2004"/> Kronosaurus was then considered as being the largest known marine reptile until 1995, when Theagarten Lingham-Soliar suggested that the Late Cretaceous aquatic squamate Mosasaurus hoffmannii would reach around Template:Convert long,<ref>Template:Cite journal</ref>Template:Sfn the latter having a reduced size to around Template:Convert according to more recent estimates.<ref>Template:Cite journal</ref> Currently, the largest marine reptile identified to date is the Late Triassic ichthyosaur Ichthyotitan, which is thought to have reached around Template:Convert in length.<ref>Template:Cite journal</ref> The Harvard skeleton restoration being erroneous, McHenry gives a smaller size of this specimen between Template:Convert long<ref name="Pick&Sloan2004"/> for a weight of Template:Cvt.Template:Sfn These same measurements are seen as the maximum possible estimates of the genus as a whole.Template:Sfn 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 Template:Convert according to KearTemplate:Sfn at Template:Convert according to Ellis.Template:Sfn In 2024, Ruizhe Jackevan Zhao revises the measurements of MCZ 1285 at Template:Convert.Template:Sfn
Other specimens have been given body estimates although some of these are only known from more limited fossil remains.Template:Efn QM F1609, the holotype specimen, although very fragmentary, would have measured Template:Convert long with a body mass of Template:Cvt. The proposed neotype specimen QM F18827 would have reached a length of Template:Convert with a body mass of Template:Cvt.Template:Sfn The most complete known attributed specimen, QM F10113, would have reached slightly smaller measurements, namely Template:Convert long with a body mass of Template:Cvt.Template:Sfn 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.Template:Sfn Respectively, these two specimens would have reached Template:Convert in length with body masses estimated at Template:Cvt.Template:Sfn
SkullEdit
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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.Template:SfnTemplate:SfnTemplate:Sfn 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 Template:Convert long. Candidate neotype specimens QM F10113 and QM F18827 have cranial lengths reaching Template:Convert, respectively.Template:Sfn The skull of the Harvard skeleton is estimated to be Template:Convert long.<ref name=Bensonetal2013>Template:Cite journal</ref>Template:Efn The cranial measurements of the last three specimens previously cited surpass in size the skull of any known theropod dinosaurs.<ref name="Palaeos">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> The snout and the mandibular rostrum are long and narrow in shape.Template:Sfn 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,Template:Sfn but the skull does not have an anterior interpterygoid vacuity.Template:Sfn
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.Template:EfnTemplate:SfnTemplate:Sfn<ref name="Holland2018"/>Template:Sfn The frontal bones (bones bordering the eye sockets) do not come into contact with the margin of the eye sockets due to the connection between the postfrontal and prefrontal bones. The frontal bones also do not come into contact with the middle part of the skull roof due to the connection between the parietal bones and posterior facial processes of the premaxillae. The prefrontals are large and contact the anteromedial part of the eye sockets as well as the posterior border of the nostrils. The lacrimal bones (bones bordering the lower front edges of the eye sockets) are present in small specimens, but tend to be fused in adults. The dorsal surface of the median dorsal crest is formed by the premaxillae and nasal bones (bones bordering the external nares), which in adults are fused.Template:Sfn The hyoid bones are robust.Template:Sfn
The mandibular symphysis of Kronosaurus is elongated and spatulate (spoon-shaped), and like its close relatives Brachauchenius and Megacephalosaurus, it contains up to six pairs of teeth.Template:SfnTemplate:Sfn<ref name="Holland2018"/> Each dentary (the tooth-bearing bone in the mandible) has up to 26 teeth. The mandibular glenoid (socket of the jaw joint) is kidney-shaped and angled upwards and inwards.<ref name="Holland2018"/> The main autapomorphy of Kronosaurus teeth is that they are conical in shape, roughly ridged, and lacking distinct carinae.Template:Sfn<ref name="Kear2006"/><ref name="Holland2018"/> The dentition of Kronosaurus is heterodont, that is, it has teeth of different shapes. The larger teeth are caniniform and located at the front of the jaws, while the smaller teeth are more sharply recurved, stouter, and located further back.Template:Sfn<ref name="Holland2018"/>
Postcranial skeletonEdit
The Harvard skeleton historically attributed to Kronosaurus received a study detailing its postcranial anatomy by Romer and Lewis in 1959.<ref name="Romer&Lewis1959"/> However, as the latter was massively restored in plaster, it is currently difficult to discern the real fossil material.Template:Sfn Additionally, the specimen is temporarily referred to Eiectus; CT scans may in time reveal whether or not the specimen belongs to Kronosaurus.Template:Sfn Many Kronosaurus specimens preserve postcranial material.Template:Sfn The most complete specimen known, catalogued as QM F10113, preserves an important part of the postcranial anatomy which could reveal important information for a more in-depth diagnosis of the taxon.Template:Sfn<ref name="Holland2018"/> This same specimen should also be described in more detail in a future study.Template:Sfn Some features concerning the postcranial anatomy of the genus have however been noted, both in McHenry's thesis and in other articles.Template:SfnTemplate:Sfn<ref name="Knutsenetal2012"/>
Based on the different specimens analyzed, McHenry estimates that Kronosaurus would have had at least 35 presacral vertebrae, including thirteen cervical and five pectoral vertebra.Template:Sfn Unlike Pliosaurus, the cervical centra (vertebral bodies) are wider than the dorsals.<ref name="Knutsenetal2012"/> The anterior dorsal vertebrae are higher than wide.Template:Sfn The zygapophyses would have been visibly absent from the anterior dorsal vertebrae and in the caudal vertebrae.Template:Sfn In the thoracic region, the ribs would have been robust, as suggested by the transverse processes which are equally robust.Template:Sfn The ribs would also been single-headed.Template:Sfn Although the tail of Kronosaurus is unknown from articulated specimens,Template:Sfn the end of the caudal vertebrae would have supported a small caudal fin like in other plesiosaurs.<ref name="Massare1988"/><ref name="Smith2013"/> The coracoid and pubis are both elongated from front to back.Template:Sfn The hindlimbs of Kronosaurus are longer than its forelimbs, with the femur being longer and more robust than the humerus.Template:Sfn This suggests that the largest representatives of Kronosaurus would have rear flippers which would have formed a wingspan exceeding Template:Convert.Template:Sfn
Classification and evolutionEdit
De Vis initially suggested that the Kronosaurus holotype specimen belonged to an ichthyosaur. However, when Longman described the taxon in 1924, he assigned it to the family Pliosauridae based on multiple anatomical features,<ref name="Longman1924"/> an affiliation which will be mainly recognized throughout the 20th century as well as in the 21st century by the scientific community.Template:Sfn However, some alternative classifications have been proposed throughout research. For example, in 1962, Welles suggested that Kronosaurus possibly belonged to the family Dolichorhynchopidae.Template:Sfn<ref name="Acostaetal1979"/> However, this family is today recognized as polyphyletic (unnatural grouping) and is seen as invalid.Template:Sfn
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The exact phylogenetic positioning of Kronosaurus within the Pliosauridae has also been debated. In 1992, Hampe proposed to classify Kronosaurus with its close relative Brachauchenius in the proposed family Brachaucheniidae.<ref name="Hampe1992"/> Kenneth Carpenter agreed with Hampe in 1996, although noting some notable cranial differences between the two genera.<ref name="Carpenter1996">Template:Cite journal</ref>Template:Efn The family Brachaucheniidae was originally erected in 1925 by Samuel Wendell Williston to include only Brachauchenius.<ref>Template:Cite book</ref>Template:SfnTemplate:Sfn In 2001, F. Robin O'Keefe revised the classification of Pliosauridae and classified Kronosaurus as a basal representative distantly related to Brachauchenius.Template:Sfn In 2008, two studies and a thesis proposed alternative classifications for Kronosaurus. Patrick S. Druckenmiller and Anthony P. Russell classified Kronosaurus as a derived pliosaurid, Hilary F. Ketchum still classifying it as a sister taxon of Brachauchenius in this family.Template:Sfn Adam S. Smith and Gareth J. Dyke reclassify both genera within the Brachaucheniidae, but the family is seen as the sister taxon of the Pliosauridae.<ref>Template:Cite journal</ref> McHenry suggests that if Ketchum's proposal is proved as valid, then it would be preferable to relegate Brachaucheniidae as a subfamily of the Pliosauridae, therefore being renamed Brachaucheninae.Template:Sfn McHenry nevertheless maintains the name Brachaucheniidae in his thesis detailing in more detail Kronosaurus pending further phylogenetic results.Template:Sfn In 2013, Roger B. S. Benson and Druckenmiller named a new clade within Pliosauridae, Thalassophonea. This clade included the "classic", short-necked pliosaurids while excluding the earlier, long-necked, more gracile forms. The authors thus move the family Brachaucheniidae as a subfamily, renaming it Brachaucheninae, and classify many Cretaceous pliosaurids there, including Kronosaurus. Within this subfamily, Kronosaurus appears to be one of the most derived representatives, being generally placed in a clade including Brachauchenius and more recently Megacephalosaurus.<ref name="Benson&Druckenmiller2013">Template:Cite journal</ref> Subsequent studies have uncovered a similar position for Kronosaurus.<ref name="Bensonetal2013"/><ref name="Fischeretal2015">Template:Cite journal</ref><ref name="Fischeretal2017">Template:Cite journal</ref><ref name="Madziaetal2018">Template:Cite journal</ref><ref name="Fischeretal2023"/>
The cladogram below is modified from Madzia et al. (2018):<ref name="Madziaetal2018"/>
The Brachaucheninae subfamily brings together the majority of pliosaurids dating from the Cretaceous, with phylogenetic analyzes often uniting them within this clade. However, it is possible that this is not the only lineage of thalassophoneans to have survived after the Jurassic. Indeed, Lower Cretaceous pliosaur teeth, displaying characteristics distinct from the Brachaucheninae, suggest that at least one other lineage crossed the Jurassic-Cretaceous boundary.<ref name="Holland2018"/><ref name="Madziaetal2018"/><ref name="Zverkovetal2018">Template:Cite journal</ref>Template:Sfn Members of the Brachaucheninae are variable and only one uniting characteristic between all is known; the possession of somewhat circularly-shaped teeth rather than full or somewhat trihedral-shaped teeth seen in some Jurassic pliosaurs. Some characteristics that are shared by most brachauchenines like Megacephalosaurus includes skull features (such as an elongated snout, gracile rostrum, and consistently sized teeth) that are better adapted for a general evolutionary shift towards smaller prey. However, there are notable exceptions such as Kronosaurus, which has teeth that are each shaped differently. Kronosaurus is one of the few representatives of this group who not share any of these traits, having differently shaped teeth.<ref name="Zverkovetal2018"/> This type of dentition therefore indicates that Kronosaurus was a genus specialized in hunting large prey, unlike most other representatives of this group.<ref name="Zverkovetal2018"/><ref name="Holland2018"/>
PaleobiologyEdit
Plesiosaurs were well-adapted to marine life.<ref name="Fleischleetal2018"/><ref name="Fleischleetal2019"/> They grew at rates comparable to those of birds and had high metabolisms, indicating homeothermy<ref name="Houssaye2013">Template:Cite journal</ref> or even endothermy.<ref name="Fleischleetal2018">Template:Cite journal</ref> The possibility of endothermy is also very probable in plesiosaurs that lived in Australia, including Kronosaurus, the southernmost areas having had particularly cold temperatures.<ref name="Kear2006"/><ref name="Fleischleetal2018"/> A 2019 study by palaeontologist Corinna Fleischle and colleagues found that plesiosaurs had enlarged red blood cells, based on the morphology of their vascular canals, which would have aided them while diving.<ref name="Fleischleetal2019">Template:Cite journal</ref> The short tail, while unlikely to have been used to propel the animal, could have helped stabilise or steer the plesiosaur.<ref name="Massare1988">Template:Cite journal</ref><ref name="Smith2013">Template:Cite journal</ref>
FeedingEdit
Due to its imposing size, morphology and distribution, Kronosaurus would most likely have been the apex predator of the ancient Eromanga inland sea.Template:Sfn<ref name="Holland2018"/> Stomach contents have been found in some Kronosaurus specimens.Template:Sfn The most notable of these is specimen QM F10113, the most complete known, which contains the remains of a sea turtle. The position of the turtle at the skeletal level indicates that the specimen died of suffocation after swallowing its prey.Template:Sfn The fossil remains are too fragmentary to determine what genus this turtle belongs to, but its measurements are similar to the protostegid Notochelone,Template:Sfn which is the most widespread sea turtle of the Albian strata of Queensland.Template:SfnTemplate:Sfn In 1993, Tony Thulborn and Susan Turner analyzed the severely crushed skull of an elasmosaurid,<ref name="Thulborn&Turner1993"/> which is today recognized as belonging to Eromangasaurus.Template:Sfn In their study, the authors discovered the presence of multiple bite marks made by large teeth. These same traces correspond to the dentition of the specimens referred to its contemporary Kronosaurus, proving its predation towards this animal. This is also the first reported evidence of a pliosaur attack on an elasmosaurid.<ref name="Thulborn&Turner1993"/>Template:Sfn Elasmosaurids having a very elongated neck and a small head, the injuries found in Eromangasaurus suggest that Kronosaurus would have regularly attacked this region of the body. Although no direct fossil evidence of feeding is known, the animal would likely also have preyed on leptocleidids.Template:Sfn
Intraspecific combatEdit
The smallest specimen attributed to Kronosaurus, cataloged as QM F51291, shows bite marks on its skull.<ref name="Holland2018"/>Template:Sfn In his 2009 thesis, McHenry highlights that the maximum possible size of Kronosaurus is Template:Convert, and suggests that the three known specimens not reaching the minimum size of Template:Convert represent juveniles or subadults.Template:Sfn After analysis, he therefore suggests that this specimen would have been a juvenile which would have been fatally killed by the bite of an adult, indicating an intraspecific aggression or even cannibalism in Kronosaurus. He supports this hypothesis on the basis of common observations of many adult crocodilians not hesitating to attack juveniles. However, McHenry suggests that it is also possible that the bites would have been made shortly after the specimen died of another cause.Template:Sfn
Bite forceEdit
A large part of McHenry's 2009 thesis is dedicated to the bite force of Kronosaurus using biomechanical analyses. Using these techniques, McHenry discovered that Kronosaurus exceeded the bite force of any living animal, itself being only slightly surpassed in some estimates by the well-known theropod dinosaur Tyrannosaurus.Template:Sfn Based on specimen QM F10113, the bite force of Kronosaurus is estimated to be between Template:Convert.Template:Sfn Still based on the same specimen, a 2014 Foffa et al. (2014) reestimates the bite force at between Template:Convert, corresponding to its close Jurassic relative Pliosaurus kevani. The estimates of this study regarding the bite force of these two pliosaurids exceed that of the predatory placoderm fish Dunkleosteus but are far from equaling that of the megalodon, to which the latter would have reached between Template:Convert.<ref name="Foffaetal2014">Template:Cite journal</ref>
PaleoecologyEdit
Contemporaneous biotaEdit
All the geological formations from which fossils attributed to Kronosaurus have been discovered are located in the Great Artesian Basin (GAB).Template:Sfn During the Lower Cretaceous, this geographical area was flooded by an inland sea known as the Eromanga Sea.<ref name="Rey2013">Template:Cite journal</ref><ref name="Voiculescu-Holvad2018">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> The sedimentary record shows that this sea was relatively shallow, muddy and stagnant.<ref name="Rey2013"/> Temperatures in this sea would have been particularly cold, approaching near freezing,<ref name="Rey2013"/><ref name=Lurio1999>Template:Cite journal</ref> and seasonal ice may have formed in some areas.<ref name=Alley2019>Template:Cite journal</ref> Sea temperatures during the Albian would nevertheless have been warmer than during the Aptian.<ref name="Day1969">Template:Citation</ref>
Many invertebrates are known from the fossil record dating from the Late Aptian to Late Albian of the GAB, mainly represented by molluscs. Free-swimming organisms include cephalopods, which include many ammonites, belemnites, and squids. Benthic zones are mainly dominated by bivalves, with gastropods and scaphopods being less diverse. Other types of invertebrates are known, such as crinoid echinoderms, decapod crustaceans, brachiopods, polychaete annelids and one species of glass sponge.<ref name="Day1969"/>Template:Sfn The diversification of fish within the Eromanga Sea seems to vary according to geological periods, since they are not very present in the Albian strata but are abundant in the Aptian archives, particularly in the Upper Aptian.Template:Sfn These include actinopterygians such as Australopachycormus, Richmondichthys Flindersichthys, Cooyoo and Pachyrhizondontus. The only known sarcopterygians are the lungfish Ceratodus and Neoceratodus.Template:Sfn Chondrichthyans are also present, represented by Archaeolamna, Carcharias, Cretolamna, Cretoxyrhina, Edaphodon, Echinorhinus, Leptostyrax, Microcorax, Notorynchus, Pseudocorax, Pristiophorus, Scapanorhynchus and several species of orectolobiforms and palaeospinacids.Template:Sfn These fish include surface-dwelling, midwater, and benthic varieties of various sizes, some of which could get quite large. They filled a variety of niches, including invertebrate eaters, piscivores, and, in the case of Cretoxyrhina, large apex predators.Template:Sfn
The Eromanga Sea is known for its great diversification of marine reptiles.<ref name="Kear2016"/> Identified marine turtles include the protostegids Cratochelone, Bouliachelys and Notochelone,Template:SfnTemplate:Sfn<ref name="Kear2016"/> this latter being the most diverse within the inland sea.Template:SfnTemplate:Sfn Several ichthyosaur fossils have been discovered in Queensland and were historically assigned to several different genera.Template:Sfn We now know that these fossils probably belong to the species Platypterygius australis,Template:Efn which is one of the youngest ichthyosaurs known in the fossil record.Template:SfnTemplate:Sfn<ref name="Kear2016"/> Other fossils attributable to this species have been discovered in other formations of the GAB, notably in the Bulldog Shale, but they prove to be too fragmentary to determine a clear diagnostic.Template:Sfn Several plesiosaurians have been identified, but most fossils are either too fragmentary or non-diagnostic for them to be assigned to a specific genus or species.Template:SfnTemplate:Sfn<ref name="Kear2016"/> Kronosaurus is stratigraphically the most widespread plesiosaurian in Australia,<ref name="Kear2016"/> and would be the only large representative of a pliosaurid known to date in the country,Template:Sfn if we exclude the proposed genus Eiectus.<ref name="Noè&Gómez-Pérez2022"/>Template:Sfn The only known cryptoclidid is Opallionectes.<ref name="Kear2006"/><ref name="Kear2016"/>Template:Sfn Elasmosaurids include Eromangasaurus and numerous interminate representatives.Template:Sfn<ref name="Kear2006"/><ref name="Kear2016"/>Template:Sfn Some representatives of the Leptocleidia clade, which includes Leptocleididae and Polycotylidae, are known. Leptocleids include Leptocleidus, Umoonasaurus, and a few specimens with undetermined attributions.Template:Sfn<ref name="Kear2016"/>Template:Sfn Polycotylids are only known from undetermined or not yet described specimens, the most notable of them the Richmond specimen.Template:Sfn<ref name="Kear2006"/>Template:Sfn<ref name="Kear2016"/>Template:Sfn
Some archosaurs from various groups have also been identified in the fossil record of the Eromanga Sea. Numerous fragmentary remains of dinosaurs from specimens that probably perished after drowning in the waters of Eromanga are known, these being identified as coming from the sauropod Austrosaurus, the ankylosaurian Minmi and the ornithopod Muttaburrasaurus. In addition to dinosaurs, many pterosaur fossils are known, and these could have been predators comparable to many modern-day seabirds. However, theirs fossils are often fragmentary, and few taxa have been named.Template:Sfn<ref name="Voiculescu-Holvad2018"/> Among the erected genera, there are Aussiedraco, Mythunga and Thapunngaka.Template:Sfn
Interspecific competitionEdit
Despite its status as an apex predator, Kronosaurus was sometimes attacked by other contemporary predators. Indeed, a mandible cataloged as KK F0630, possibly representing a large subadult or a small adult specimen, shows bite marks which would have been made by lamniform sharks belonging to the Cretoxyrhinidae family. Injuries of this type are not unlikely, as several sharks attributed to this family have been identified in various geological formations where Kronosaurus is known. The grooves showing the bite marks being surrounded by aberrant raised osseous growth indicate that the specimen would have healed during its lifetime.<ref name="Holland2018"/>
See alsoEdit
NotesEdit
ReferencesEdit
BibliographyEdit
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