Mosasaurus

Not to be confused with Mesosaurus.

Mosasaurus Temporal range: Campanian-Maastrichtian,
Reconstructed skeleton of M. hoffmannii at the Maastricht Natural History Museum
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Order:	Squamata
Clade:	†Mosasauria
Family:	†Mosasauridae
Tribe:	†Mosasaurini
Genus:	†Mosasaurus Conybeare, 1822
Type species
†Mosasaurus hoffmannii Mantell, 1829
Other species
†M. missouriensis Harlan, 1834 †M. conodon Cope, 1881 †M. lemonnieri Dollo, 1889 †M. beaugei Arambourg, 1952 Species pending reassessment †M. mokoroa Welles & Gregg, 1971 †M. hobetsuensis Suzuki, 1985 †M. flemingi Wiffen, 1990 †M. prismaticus Sakurai et al., 1999
Synonyms
List of synonyms Synonyms of genus Batrachiosaurus Harlan, 1839 Batrachiotherium Harlan, 1839 Macrosaurus Owen, 1849 Drepanodon Leidy, 1856 Lesticodus Leidy, 1859 Baseodon Leidy, 1865 Nectoportheus Cope, 1868 Pterycollosaurus Dollo, 1882 Synonyms of M. hoffmannii Lacerta gigantea von Sömmerring, 1820 Mososaurus hoffmannii Mantell, 1829 Mosasaurus belgicus Holl, 1829 Mosasaurus camperi Meyer, 1832 Mosasaurus dekayi Bronn, 1838 Mosasaurus hoffmanni Owen, 1840 Mosasaurus major De Kay, 1842 Mosasaurus occidentalis Morton, 1844 Mosasaurus meirsii Marsh, 1869 Mosasaurus princeps Marsh, 1869 Mosasaurus maximus Cope, 1869 Mosasaurus giganteus Cope, 1869 Mosasaurus fulciatus Cope, 1869 Mosasaurus oarthus Cope, 1869 Synonyms of M. missouriensis Ichthyosaurus missouriensis Harlan, 1834 Ictiosaurus missuriensis Harlan, 1834 Batrachiosaurus missouriensis Harlan, 1839 Batrachiotherium missouriensis Harlan, 1839 Mosasaurus maximiliani Goldfuss, 1845 Mosasaurus neovidii Meyer, 1845 Pterycollosaurus maximiliani Dollo, 1882 Mosasaurus horridus Williston, 1895 Synonyms of M. conodon Clidastes conodon Cope, 1881 Mosasaurus lemonnieri Dollo, 1889

Mosasaurus (/ˌmoʊzəˈsɔːrəs/; "lizard of the Meuse River") is the type genus of the mosasaurs, an extinct group of aquatic squamate reptiles. It lived from about 82 to 66 million years ago during the Campanian and Maastrichtian stages of the Late Cretaceous. The earliest fossils known to science were found as skulls in a chalk quarry near the Dutch city of Maastricht in the late 1700s, which were initially thought to have been the bones of crocodiles or whales. One particular skull discovered at around 1780, and which was seized during the French Revolutionary Wars for its scientific value and transported to Paris, was famously nicknamed the "great animal of Maastricht". In 1808, naturalist Georges Cuvier concluded that it belonged to a giant marine lizard with similarities to monitor lizards but otherwise unlike any animal known today. This concept was revolutionary at the time and helped support the then-developing ideas of extinction. However, Cuvier did not designate a scientific name for the new animal; this task was completed by William Daniel Conybeare in 1822 when he named it Mosasaurus in reference to its origin in fossil deposits near the Meuse River; the name is accordingly a portmanteau derived from the words Mosa (the Latin translation for the Meuse River that passed along Mount Saint Peter) and saurus (the romanization of the Ancient Greek σαῦρος, meaning "lizard"). The relationships between Mosasaurus and modern reptiles are controversial and scientists continue to debate whether its closest living relatives are monitor lizards or snakes.

Traditional interpretations have estimated the maximum length of Mosasaurus to be up to 17.6 meters (58 ft), making it one of the largest mosasaur genera. Its skull, which was either broad or slender depending on the species, was equipped with robust jaws capable of swinging back and forth and strong muscles capable of powerful bites using dozens of large teeth designed for cutting prey. Its four limbs were shaped into robust paddles to steer the animal underwater. Its tail was long and ended in a paddle-like fluke that bent downwards. Mosasaurus was a predator that had excellent vision to compensate for its poor sense of smell, and a high metabolic rate that suggests it was endothermic ("warm-blooded"), an adaptation only found in mosasaurs among squamates. The classification of Mosasaurus was historically problematic due to an unclear diagnosis of the type specimen. As a result, over fifty different species have been attributed to the genus in the past. A rediagnosis of the type specimen in 2017 helped resolve the taxonomy issue and confirmed at least five species to be within the genus and another five species still nominally classified within Mosasaurus are planned to be reassessed in a future study. Each species was variable with unique anatomical features differentiating them from the robustly-built M. hoffmannii to the slender and serpentine M. lemonnieri.

Fossil evidence suggests that Mosasaurus inhabited much of the Atlantic Ocean and the seaways adjacent to it. Continents that have recovered Mosasaurus fossils include North America, South America, Europe, Africa, Western Asia, and Antarctica. This distribution encompassed a wide range of oceanic climates including tropical, subtropical, temperate, and subpolar climates. Mosasaurus was a common large predator in these oceans and a dominant genus positioned at the top of the food chain. Scientists believe that its diet would have included virtually any animal; it likely preyed on bony fish, sharks, cephalopods, birds, and other marine reptiles including sea turtles and other mosasaurs. It likely preferred to hunt in open water near the surface. From an ecological standpoint, Mosasaurus probably had a profound impact on the structuring of marine ecosystems; its arrival in some locations such as the Western Interior Seaway in North America coincides with a complete turnover of faunal assemblages and diversity. Mosasaurus faced competition with other large predatory mosasaurs such as Prognathodon and Tylosaurus—which were known to feed on similar prey—though they were able to coexist in the same ecosystems through niche partitioning. There were conflicts among them, as an attack on Mosasaurus by Tylosaurus has been documented. Several discovered fossils illustrated deliberate attacks on Mosasaurus individuals by members of the same species. Infighting likely took place in the form of snout grappling, similarly seen in modern crocodiles today.

Research history

See also: Research history of Mosasaurus

MNHN AC 9648, the second skull and holotype of M. hoffmannii

Mosasaurus was first discovered in 1764 in a quarry near Maastricht, the Netherlands in the form of a skull, which was initially identified as a whale. Later around 1780, the quarry produced a second skull that gained international attention after biologist Petrus Camper published a study on it. This fame caught the attention of French revolutionaries, who looted the fossil following the capture of Maastricht during the French Revolutionary Wars in 1794. In a 1798 narrative of this event by Barthélemy Faujas de Saint-Fond, the skull was allegedly retrieved by twelve grenadiers in exchange for an offer of 600 bottles of wine. This story helped elevate the fossil into cultural fame, but historians agree that the narrative is false.

After its seizure, the second skull was sent to the National Museum of Natural History, France at later cataloged as MNHN AC 9648, where it was found by Adriaan Gilles Camper and Georges Cuvier by 1812 to belong to a lizard with affinities to monitor lizards, but an extinct form otherwise unlike any modern animal. The skull became part of Cuvier's first speculations about the conception of extinction, which later led to his theory of catastrophism, a precursor to the theory of evolution. At the time, people did not believe that a species could go extinct, and fossils of animals were often interpreted as some form of an extant species. Cuvier's idea that there existed an animal unlike any today and no longer exists was revolutionary at the time, which led him to proclaim in 1812:

Above all, the precise determination of the famous animal from Maastricht seems to us as important for the theory of zoological laws, as for the history of the globe.

— Georges Cuvier

Although the binomial naming system was well established at the time, Cuvier did not give a scientific name to the animal. Instead, this was done by William Daniel Conybeare, who coined the genus Mosasaurus in 1822, and Gideon Mantell, who added the specific epithet hoffmannii in 1829. Cuvier later designated the second skull as the new species' holotype.

Description

Artist's impression of M. hoffmannii, one of the largest known mosasaurs

Mosasaurus was a type of derived mosasaur, a latecoming member that had evolved advanced traits such as a fully aquatic lifestyle. As such, Mosasaurus had a streamlined body, elongated tail that ended with a hypocercal downturn that supported a two-lobed fin, and two pairs of flippers. While in the past derived mosasaurs were depicted as akin to giant flippered sea snakes, it is now understood that they were more similar in build to other large marine vertebrates such as ichthyosaurs, marine crocodylomorphs, and archaeocete whales through convergent evolution.

Size

Size range of Mosasaurus compared with a human

The type species, M. hoffmannii, is one of the largest mosasaurs known. Knowledge of its skeleton, however, remains incomplete as it is rarely preserved with articulated postcranial fossils. Because of this, its length is based on rough estimates through extrapolations of incomplete fossils. As the species is well-represented by fossil skulls, the length of the skull or lower jaw can be extrapolated to a hypothetical ratio between it and the total length. A widely known ratio is one recorded by Russell (1967), who wrote that the "given length of jaw equals 10% of body length." Using this ratio and the largest lower jaw attributed to M. hoffmannii (CCMGE 10/2469, also known as the Penza specimen; measuring 1,710 millimetres (67 in) in length), Grigoriev (2014) estimated a maximum length of 17.1 meters (56 ft). Using a smaller partial jaw (NHMM 009002) measuring 900 millimeters (35 in) and "reliably estimated at 1600 mm" when complete, Lingham-Soliar (1995) estimated a larger maximum length of 17.6 meters (58 ft) via the same ratio. No explicit justification for the 1:10 ratio was provided in Russell (1967), and some scientists believe that the actual body proportions of M. hoffmannii were closer to the shorter ratios of related mosasaurine genera. In a 2014 study, Federico Fanti et. al. argued that the total length of M. hoffmannii was more likely closer to seven times the length of the skull by proxy of a near-complete skeleton of the related species Prognathodon overtoni with such proportions. The study estimated that an M. hoffmannii individual with a skull measuring 144 centimeters (57 in) would have measured 11 meters (36 ft) or more.

The Penza specimen, one of the largest known fossils of Mosasaurus

Isolated bones suggest that some M. hoffmannii may have exceeded the lengths of the Penza specimen. One such bone is a quadrate (NHMM 003892) that is 150% larger than the average size; in 2016, Everhart et al. reported that this can be extrapolated to scale an individual around 18 meters (59 ft) in length. It was not stated whether the ratio from Russell (1967) was applied.

M. missouriensis and M. lemonnieri are smaller in size than M. hoffmannii but are known from more complete fossils. Based on measurements of various Belgian skeletons, Dollo estimated that M. lemonnieri grew to around 7 to 10 meters (23 to 33 ft) in length. He also measured the dimensions of IRSNB 3119 and recorded that the skull constituted approximately one-eleventh of the whole body. Polycn et al. (2014) estimated that M. missouriensis may have measured up to 8–9 meters (26–30 ft) in length. Street (2016) noted that large M. missouriensis individuals typically had skulls exceeding lengths of 1 meter (3.3 ft). A particular near-complete skeleton of M. missouriensis is reportedly measured at 6.5 meters (21 ft) in total length; its skull approached 1 meter (3.3 ft) in length. Based on personal observations of various unpublished fossils from Morccoo, Nathalie Bardet estimated that M. beaugei grew to a total length of 8–10 metres (26–33 ft); their skulls typically measures around 1 meter (3.3 ft) in length. Valid explicit size estimates of M. conodon have not been recorded in scientific literature, but with a skull measuring around 97.7 centimeters (38.5 in) in length, it has been regarded as a small to medium-sized representative of the genus.

Skull

Annotated schematic of a M. hoffmannii skull

The skull of Mosasaurus is conical and tapers off to a short and snout that extends a little beyond the frontmost teeth. In M. hoffmannii, this rostrum is blunt, while in M. lemonnieri it is sharp. Above the gum line in both jaws, a single row of small pits known as foramina are lined parallel to the jawline; they are used to hold the terminal branches of jaw nerves. The foramina along the rostrum are of a pattern similar to that found in skulls of Clidastes. The upper jaws in most species are robustly built, broad, and deep except in M. conodon, where they are slender. The disparity is also reflected in the dentary, the lower jawbone. But all species share a dentary that is long and straight. In M. hoffmannii, the top margin is slightly curved upwards; this is also so with the largest specimens of M. lemonnieri, although more typical skulls of the species have a near-perfectly straight jawline. The premaxillary bar, the long portion of the premaxillary bone extending behind the premaxillary teeth, is narrow and constricts near the middle in M. hoffmannii and M. lemonnieri like in typical mosasaurs. However, the bar is uniquely robust and does not constrict in M. missouriensis. The external nostril openings are moderately sized and measure around 21–24% of the skull length in M. hoffmannii. They are placed further toward the back of the skull than nearly all other mosasaurs and begin above the fourth or fifth maxillary teeth, only placed further back in Goronyosaurus. As a result, the rear portions of the maxilla (the main tooth-bearing bone of the upper jaw) lack the dorsal concavity that would fit the nostrils in typical mosasaurs.

The palate, which consists of the pterygoid bones, palatine bone, and nearby bones of other processes, is tightly packed to provide greater cranial stability. The neurocranium housed a brain that was narrow and relatively small compared to other mosasaurs. In contrast, the braincase of the mosasaur Plioplatecarpus marshi provided for a brain around twice the size of that in M. hoffmannii despite being only half the length of the latter. Spaces within the braincase for the occipital lobe and cerebral hemisphere are narrow and shallow, suggesting that such brain parts were relatively small. The parietal foramen in Mosasaurus, which is associated with the parietal eye, is the smallest in the Mosasauridae family. The quadrate bone, which connected the lower jaw to the rest of the skull and formed the jaw joint, is tall and somewhat rectangular in shape, differing from the rounder quadrates found in typical mosasaurs. The quadrate also housed the hearing structures, with the eardrum residing within a round and concave depression in the outer surface called the tympanic ala. The trachea likely stretched from the esophagus to below the back end of the lower jaw's coronoid process, where it split into smaller pairs of bronchi that extended parallel to each other.

Teeth

Closeup of M. hoffmannii teeth, with a replacement tooth developing inside the root of the lower right tooth

The features of teeth in Mosasaurus vary across species, but unifying characteristics include highly prismatic surfaces (prism-shaped enamel circumference), two opposite cutting edges, and a design specialized for cutting prey. Mosasaurus teeth are considered to be large and robust except for those in M. conodon and M. lemonnieri, which instead have more slender teeth. The cutting edges of Mosasaurus can be serrated depending on the species. The cutting edges in M. hoffmannii and M. missouriensis are finely serrated, while in M. conodon and M. lemonnieri serrations do not exist. The cutting edges of M. beaugei are neither serrated nor smooth, but instead possesses minute wrinkles known as crenulations. The number of prisms, or flat sides on a prismatic tooth's circumference, in Mosasaurus teeth can slightly vary between tooth types and general patterns differ between species—M. hoffmannii had two to three prisms on the labial side (the side facing outwards) and no prisms on the lingual side (the side facing the tongue), M. missouriensis had four to six labial prisms and eight lingual prisms, M. lemonnieri had eight to ten labial prisms, and M. beaugei had three to five labial prisms and eight to nine lingual prisms.

Closeup of a M. beaugei palate, showing the smaller pterygoid teeth on their namesake bones

Like all mosasaurs, Mosasaurus had four types of teeth, classified based on the jaw bones they were located on. On the upper jaw, there were three types: the premaxillary teeth, maxillary teeth, and pterygoid teeth (a feature present on the palate of all mosasaurs and various modern reptiles). On the lower jaw, only one type, the dentary teeth, were present. The teeth were largely consistent in size and shape with only minor differences throughout the jaws. However, the pterygoid teeth were smaller than the other tooth types. In each jaw row, from front to back, Mosasaurus had: two premaxillary teeth, twelve to sixteen maxillary teeth, and eight to sixteen pterygoid teeth on the upper jaw and fourteen to seventeen dentary teeth on the lower jaw. The number of teeth in the maxillary, pterygoid, and dentary positions vary between species and sometimes even individuals—M. hoffmannii had fourteen to sixteen maxillary teeth, fourteen to fifteen dentary teeth, and eight pterygoid teeth; M. missouriensis had fourteen to fifteen maxillary teeth, fourteen to fifteen dentary teeth, and eight to nine pterygoid teeth; M. conodon had fourteen to fifteen maxillary teeth, sixteen to seventeen dentary teeth, and eight pterygoid teeth; M. lemonnieri had fifteen maxillary teeth, fourteen to seventeen dentary teeth, and eleven to twelve pterygoid teeth; and M. beaugei had twelve to thirteen maxillary teeth, fourteen to sixteen dentary teeth, and six or more pterygoid teeth. One indeterminate specimen of Mosasaurus similar to M. conodon from the Pembina Gorge State Recreation Area in North Dakota was found to have an unusual count of sixteen pterygoid teeth, far greater than in known species.

The roots of Mosasaurus teeth were deeply cemented within the jaw bone. Mosasaurus did not utilize permanent teeth and constantly shed them. Replacement teeth developed within a pit inside the roots of the original tooth called the resorption pit. This is done through an eight-stage process that is distinctly unique to mosasaurs. The first stage was characterized by the mineralization of a small tooth crown developed elsewhere that descended into the resorption pit by the second stage. In the third stage, the developing crown firmly cemented itself within the resorption pit and grew in size; by the fourth stage, it would be of the same size as the crown in the original tooth. Stages five and six were characterized by the development of the replacement tooth's root: in stage five the root developed vertically, and in stage six the root expanded in all directions to the point that the replacement tooth became exposed and actively pushed on the original tooth. In the seventh stage, the original tooth was shed and the now-independent replacement tooth began to anchor itself into the vacancy. In the eighth and final stage, the replacement tooth has grown to firmly anchor itself. Chemical studies conducted on a M. hoffmannii maxillary tooth measured an average rate of deposition of odontoblasts, the cells responsible for the formation of dentin, at 10.9 micrometers (0.00043 in) per day. This was by observing the von Ebner lines, incremental marks in dentin that form daily. It was approximated that it took the odontoblasts 511 days and dentin 233 days to develop to the extent observed in the tooth.

Postcranial skeleton

Life restoration of M. missouriensis

The most complete skeleton of Mosasaurus, whose species-level identification is debated and is on display at the Museum of Geology at the South Dakota School of Mines and Technology under the catalog SDSM 452, has seven cervical (neck) vertebrae, thirty-eight dorsal vertebrae (which includes thoracic and lumbar vertebrae) in the back, and eight pygal vertebrae (front tail vertebrae lacking haemal arches) followed by sixty-eight caudal vertebrae in the tail. All species of Mosasaurus have seven cervical vertebrae, but other vertebral counts vary among them. Various partial skeletons of M. conodon, M. hoffmannii, and M. missouriensis suggest that M. conodon likely had up to thirty-six dorsal vertebrae and nine pygal vertebrae; M. hoffmannii had likely up to thirty-two thoracic vertebrae and ten pygal vertebrae; and M. missouriensis around thirty-three dorsal vertebrae, eleven pygal vertebrae, and at least seventy-nine caudal vertebrae. M. lemmonieri had the most vertebrae in the genus, with up to around forty dorsal vertebrae, twenty-two pygal vertebrae, and ninety caudal vertebrae. Compared to other mosasaurs, the rib cages of Mosasaurus are unusually deep and form an almost perfect semicircle, giving it a barrel-shaped chest. Rather than fused together, extensive cartilage likely connected the ribs with the sternum, which would have facilitated breathing movements and compression when in deeper waters. The texture of the bones is virtually identical with that in modern whales, which implies Mosasaurus enjoyed a high range of aquatic adaptation and neutral buoyancy seen in cetaceans.

The tail structure of Mosasaurus is similar to relatives like Prognathodon, of which soft tissue evidence for a two-lobed tail was first reported in 2013. The centra of the tail vertebrae gradually shorten around the center of the tail and lengthen behind the center, suggesting rigidness around the tail center and excellent flexibility behind it. Like most advanced mosasaurs, the tail bends slightly downwards as it approached the center, but this bend is offset from the dorsal plane at a small degree. Mosasaurus also has large haemal arches located at the bottom of each caudal vertebra that bends near the middle of the tail, which contrasts with the reduction of haemal arches in other marine reptiles such as ichthyosaurs. These and other features support a large and powerful paddle-like fluke in Mosasaurus.

The forelimbs of Mosasaurus are wide and robust. The scapula and humerus are fan-shaped and wider than tall. The radius and ulna are short, but the former is taller and larger than the latter. The ilium is rod-like and slender; in M. missouriensis, it is around 1.5 times longer than the femur. The femur itself is about twice as long than it is wide and ends at the distal side in a pair of distinct articular facets that meet at an angle of approximately 120°. Five sets of metacarpal and phalanges (finger bones) were encased in and supported the paddles, with the fifth set being shorter and offset from the rest. The overall structure of the paddle is compressed, similar to that in Plotosaurus, and was well-suited for faster swimming. In the hindlimbs, the paddle is supported with four sets of digits.

Interactive skeletal reconstruction of M. hoffmannii
(hover over or click on each skeletal component to identify the structure)

Classification

History of taxonomy

See also: Research history of Mosasaurus § History of taxonomy

Fossil skull of the proposed new species M. glycys

Because nomenclature rules were not well-defined at the time, 19th century scientists did not give Mosasaurus a proper diagnosis during its first description, which led to ambiguity in how the genus is defined. This led Mosasaurus to become wastebasket taxon that contained as many as fifty different species. A 2017 study by Street and Caldwell performed the first proper diagnosis and description of the M. hoffmannii holotype, which allowed a major taxonomic cleanup that confirmed five species as likely valid—M. hoffmannii, M. missouriensis, M. conodon, M. lemonnieri, and M. beaugei. The study also held four additional species from Pacific deposits—M. mokoroa, M. hobetsuensis, M. flemingi, and M. prismaticus—to be possibly valid, pending a future formal reassessment. Street and Caldwell (2017) was derived from Street's 2016 doctoral thesis, which contained a phylogenetic study that proposed the constraining of Mosasaurus into four species—M. hoffmannii, M. missouriensis, M. lemonnieri, and a proposed new species M. glycys—with M. conodon and the Pacific taxa belonging to different genera and M. beaugei being synonyms of M. hoffmannii.

Systematics and evolution

As the type genus of the family Mosasauridae and the subfamily Mosasaurinae, Mosasaurus is a member of the order Squamata (which comprises lizards and snakes). Mosasaurus, along with mosasaur genera Eremiasaurus, Plotosaurus, and Moanasaurus traditionally form a tribe within the Mosasaurinae variously called Mosasaurini or Plotosaurini.

Relation with snakes or monitor lizards

See also: Mosasaur § Relation with snakes or monitor lizards

Scientists continue to debate on whether monitor lizards (left) or snakes (right) are the closest living relatives of Mosasaurus.

The specific placement of mosasaurs within the Squamata, and thus the relationship of Mosasaurus with modern reptiles, remains controversial since its inception. Cuvier was the first scientist to deeply analyze the possible taxonomic placement of Mosasaurus. While his original 1808 hypothesis that the genus was a lizard with affinities to monitor lizards remained the most popular, Cuvier was uncertain, even at the time, about the accuracy of this placement. He simultaneously proposed other hypotheses, one suggesting closer affinities with iguanas due to a shared presence of pterygoid teeth. But the issue was most fiercely debated by paleontologists during a 30 to 40 year span during the late 19th to early 20th centuries, during which the two major schools of thought were created: one that supported a monitor lizard relationship and one that supported a closer relationship with snakes. The latter proposition was spearheaded by Cope, who first published such a hypothesis in 1869 by proposing that mosasaurs, which he classified under a group called the Pythonomorpha, was the sister group of snakes. Proponents of the former argued that mosasaurs should be placed within the infraorder Anguimorpha, with lower classifications varying from placing mosasaurs within Varanoidea or its sister taxa, or as true monitor lizards within Varanidae. After a 70-year pause during the 20th century, the debate reemerged beginning in 1997 when a cladistical study recovered mosasaurs in a sister relationship with snakes. This resurrected the previously defunct Pythonomorpha, which contemporary scientists redefined to unify mosasaurs and snakes under one clade. However, there remained little consensus as multiple subsequent cladistical studies yielded contradicting results that supported either major hypothesis. This problem persisted even with the advent of molecular genetics and its combination with fossil data.

Phylogeny and evolution of the genus

Skull of M. conodon

One of the earliest relevant attempts at an evolutionary study of Mosasaurus was done by Russell (1967), who proposed that Mosasaurus evolved from a Clidastes-like mosasaur, and diverged into two lineages: one gave rise to M. conodon and another sired a chronospecies sequence which contained in order of succession M. ivoensis, M. missouriensis, and M. maximus-hoffmanni. However, Russell used a primitive method of phylogenetics since cladistics had yet to be widely established.

In 1997, Bell published the first cladistical study of North American mosasaurs. Incorporated the species M. missouriensis, M. conodon, M. maximus, and an indeterminate specimen (UNSM 77040), some of his findings agreed with Russell (1967), such as Mosasaurus descending from an ancestral group containing Clidastes and M. conodon being the most basal of the genus. Contrary to Russell (1967), Bell also found that Mosasaurus formed a sister relationship with another group that included Globidens and Prognathodon, and that M. maximus was a sister species to Plotosaurus. The latter rendered Mosasaurus paraphyletic, but Bell (1997) nevertheless recognized Plotosaurus as a distinct genus.

Bell's study served as a precedent for later studies that have mostly left the systematics of Mosasaurus unchanged, although some later studies have recovered the sister group to Mosasaurus and Plotosaurus to instead be Eremiasaurus or Plesiotylosaurus depending on the method of data interpretation used, with at least one study also recovering M. missouriensis to be the most basal species of the genus instead of M. conodon, This led to a number of issues. First, the genus was severely underrepresented by incorporating only the three North American species M. hoffmannii/M. maximus, M. missouriensis, and M. conodon; by doing so, others like M. lemonnieri, which is one of the most completely known in the genus, were neglected, which affected phylogenetic results. Second, the studies relied on an unclean and shaky taxonomy of the Mosasaurus genus due to the lack of a clear holotype diagnosis, which may have been behind the genus's paraphyletic status. Third, there was still a lack of comparative studies of the skeletal anatomy of large mosasaurines at the time. These problems were addressed in Street (2016), who also performed an updated phylogenetic analysis.

Skeleton of M. beaugei

Well-preserved fossil of M. missouriensis

Skull of M. lemonnieri

Conrad (2008) uniquely utilized only M. hoffmannii and M. lemonnieri in his phylogenetic analysis, which recovered M. hoffmannii as basal to a multitude of descendant clades containing (in order of most to least basal) Globidens, M. lemonnieri, Goronyosaurus, and Plotosaurus. This result indicated that M. hoffmannii and M. lemonnieri are not related by genus. However, the study utilized a method unorthodox to traditional phylogenetic studies on mosasaur species because its focus was on higher rather than lower classification. As a result, some paleontologists caution that lower-classification results from Conrad (2008) such as the specific placement of Mosasaurus may contain technical problems that can make it inaccurate.

The following cladogram on the left (Topology A) is modified from a maximum clade credibility tree inferred by a Bayesian analysis in the most recent major phylogenetic analysis of the Mosasaurinae subfamily by Madzia & Cau (2017), which was self-described as a refinement of a larger study by Simões et al. (2017). The cladogram on the right (Topology B) is modified from Street's 2016 doctoral thesis that proposes a revision to the Mosasaurinae. Because Street (2016) is not a peer-reviewed publication, it is not cited in Madzia & Cau (2017).

Topology A: Maximum clade credibility tree by Madzia & Cau (2017)

Mosasaurinae

Dallasaurus turneri Clidastes liodontus Clidastes moorevillensis Clidastes propython Mosasaurini Prognathodon overtoni Prognathodon rapax Prognathodon saturator Prognathodon currii Prognathodon solvayi Prognathodon waiparaensis Prognathodon kianda Eremiasaurus heterodontus Plesiotylosaurus crassidens Mosasaurus conodon Mosasaurus missouriensis Mosasaurus hoffmannii Plotosaurus bennisoni Globidens alabamaensis Globidens dakotensis

Topology B: Proposed revision by Street (2016)

Mosasaurinae

Prognathodon solvayi Clidastes propython Clidastes liodontus Clidastes moorevillensis Globidens alabamaensis Globidens dakotensis Prognathodon kianda Eremiasaurus heterodontus Prognathodon overtoni Prognathodon saturator Prognathodon currii Prognathodon rapax Plesiotylosaurus crassidens Marichimaera waiparaensis nov. comb Mosasaurini Amblyrhynchosaurus wiffeni nov. gen., nov. sp. Moanasaurus hobetsuensis nov. comb. Moanasaurus mangahouange Moanasaurus longirostis nov. sp. Mosasaurus missouriensis Mosasaurus lemonnieri Mosasaurus hoffmannii Mosasaurus glycys nov. sp. Antipodinectes mokoroa nov. comb. Umikosaurus prismaticus nov. comb. Aktisaurus conodon nov. comb. Plotosaurus bennisoni

Paleobiology

Head musculature and mechanics

The skull of M. hoffmannii was adapted to withstand powerful bites.

Much of the knowledge on the musculature and mechanics of the head of Mosasaurus are largely based on Lingham-Soliar's 1995 study on M. hoffmannii skulls. Because soft tissue like muscles do not easily fossilize, reconstruction of the head musculature is largely based on the structure of the skull, the nature of muscle scarring on the skull, and the musculature in extant monitor lizards.

In modern lizards, the mechanical build of the skull is characterized by a four-pivot geometric structure in the cranium that allows flexible movement of the jaws, possibly to allow the animal to better position them and prevent deflection (the positioning of a prey in a way that increases its chance of escape from a predator) when hunting. However, the skull of M. hoffmannii is characterized by a rigid three-pivot geometric cranial structure, which indicates that its jaw mechanics were different than modern lizards; these cranial structures are united by strong interlocking sutures that can resist compression and shear forces caused by a downward thrust of the lower jaw muscles or an upward thrust of prey. This rigid but highly shock-absorbent structure of the cranium likely allowed a powerful bite force during prey seizure.

Like all mosasaurs, the lower jaws of Mosasaurus were capable of adduction, allowing it to swing forward and backward. In many mosasaurs like Prognathodon and M. lemonnieri, this function mainly served to allow ratchet feeding, in which the pterygoid and jaws would "walk" captured prey into the mouth like a conveyor belt. However, especially compared to that in M. lemonnieri, the pterygoid teeth in M. hoffmannii are relatively small, which indicates ratchet feeding was relatively unimportant when hunting and feeding. Rather, M. hoffmannii likely swallowed its prey whole and used jaw adduction to assist in hard biting during prey seizure. The magnus adductor muscles, one of the muscles attaching the lower jaws to the cranium and which has a major role in biting function, are massive, indicating that M. hoffmannii was capable of enormously powerful bite forces. The long, narrow, and heavy nature of the lower jaws and attachment of tendons at the coronoid process would have allowed quick opening and closing of the mouth with little energy input underwater, which also contributed to the powerful bite force of M. hoffmannii and suggests that it would not have needed the strong magnus depressor muscles (jaw-opening muscles) seen in some plesiosaurs.

Mobility and thermoregulation

Reconstruction of an M. hoffmannii forelimb

Mosasaurus swam using its tail. The swimming style was likely of a sub-carangiform swimming style, which is best seen today in mackerels. Its elongated paddle-like limbs functioned as hydrofoils for maneuvering the animal. The paddles' steering function was supported by large muscle attachment from the outwards-facing side of the humerus to the radius and ulna and an enhanced ability of pronation allowed by modified joints. However, the powerful forces resulting from utilization of the paddles may have sometimes resulted in bone damage, as evidenced by a M. hoffmannii ilium with significant separation damage from the bone's head from the stem likely caused by frequent shearing forces at the articulation joint.

The tissue structure of Mosasaurus' bones suggests that it had a metabolic rate much higher than modern squamates and its resting metabolic rate was between the leatherback sea turtle and the ichthyosaurs and plesiosaurs. In order to keep up with its high metabolic requirements, Mosasaurus was likely endothermic and maintained a constant warm-blooded temperature independent of the external environment. Although there is no direct evidence specific to the genus, studies on the biochemistry of related endothermic mosasaur genera such as Clidastes suggests that endothermy was likely present in all mosasaurs. Such a trait is unique among squamates, the only known exception being the Argentine black and white tegu, which can maintain partial endothermy. This adaptation would have given several advantages to Mosasaurus, including increased stamina when foraging in larger areas and pursuing prey. It may have also been a factor that allowed Mosasaurus to thrive in the colder climates of locations such as Antarctica.

Sensory functions

Sclerotic ring of Mosasaurus

Mosasaurus had relatively large eye sockets with large sclerotic rings occupying much of the sockets' diameter, which is correlated with eye size and suggests that it had good vision. The eye sockets were located at the sides of the skull, which created a narrow field of binocular vision at around 28.5° but alternatively allowed excellent processing of a two-dimensional environment. This may have been particularly useful for Mosasaurus, which likely lived near the surface in open waters where three-dimensional environments are virtually nonexistent.

Part of the inner ear of Mosasaurus was described from a fossil by Grigoriev (2016), but the remains are incomplete. The inner ear is an important structure that plays a significant role in the mobility, sensitivity, and balance of animals. Mosasaur inner ears have been studied in the past, but exclusively with non-mosasaurine species. In Mosasaurus, the endpoints of the rear and middle semicircular canals, which helps control the sense of balance, are separated from each other; in other mosasaurs, the ends are fused together. However, this feature may be a result of incomplete preservation. The middle semicircular canal is large, measuring 25.3 millimeters (1.00 in) in a Mosasaurus of ~14 meters (46 ft) in length. However, this may not have caused a difference in inner ear sensitivity, as would happen in modern cetaceans when the canal size changes, as the proportions between the canal and skull lengths are very similar with modern monitor lizards and smaller mosasaurs like Platecarpus.

Brain casts made from the cranium of Mosasaurus skulls show that the olfactory bulb and vomeronasal organ, which control the function of smell, are poorly developed and lack a number of components in M. hoffmannii; this indicates that the species had an extremely poor sense of smell. In M. lemonnieri, these olfactory organs, although still small, are better developed and has some components that M. hoffmannii lack. The lack of a strong sense of smell suggests that olfaction was not particularly important in Mosasaurus; instead, other senses like a well-developed sense of vision may have been more useful.

Feeding

Restoration of M. hoffmannii preying on a sea turtle

While there is little direct evidence of the feeding habits of Mosasaurus, paleontologists generally agree that it was likely an active predator that preyed on a variety of marine animals. Fauna that was likely preyed on by the genus include bony fish, sharks, cephalopods, birds, and marine reptiles such as other mosasaurs and turtles. It is unlikely that Mosasaurus was a scavenger as it had a poor sense of smell. Mosasaurus was among the largest marine animals of its time, and with its large, robust cutting teeth, scientists believe that larger members of the genus would have been able to handle virtually any animal. Lingham-Soliar (1995) suggested that Mosasaurus had a rather "savage" feeding behavior as demonstrated by large tooth marks on scutes of the giant sea turtle Allopleuron hoffmanni and fossils of re-healed fractured jaws in M. hoffmannii. The species likely hunted near the ocean surface as an ambush predator, using its large two-dimensionally adapted eyes to more effectively spot and capture prey. Chemical and structural data in the fossils of M. lemonnieri and M. conodon suggests that they may have also hunted in deeper waters.

Carbon isotope studies on fossils of multiple M. hoffmannii individuals have found extremely low values of δ¹³C, the lowest in all mosasaurs. There are several implications for δ¹³C levels in the feeding ecology of mosasaurs. The relationship between δ¹³C levels in mosasaurs and their trophic levels are found to be negatively correlated; mosasaurs with lower δ¹³C values tended to occupy higher trophic levels. One factor for this is dietary; a diet of prey rich in lipids such as sea turtles and other large marine reptiles can lower δ¹³C values. M. hoffmannii's low δ¹³C levels reinforces its likely position as an apex predator.

Currently, there is only one known example of a Mosasaurus preserved with stomach contents: a well-preserved partial skeleton of a small M. missouriensis dated about 75 million years old (Ma) with dismembered and punctured remains of a 1 meter (3.3 ft) long fish in its gut. This fish is much longer than the length of the mosasaur's skull, which measured 66 centimeters (26 in) in length, confirming that M. missouriensis consumed prey larger than its head by dismembering and consuming bits at a time. The presence of other large mosasaurs like Prognathodon, which specialized in robust prey, coexisting with M. missouriensis strongly suggests that the species likely specialized more on prey best consumed using cutting-adapted teeth in an example of niche partitioning.

There is a possibility that Mosasaurus may have taught their offspring how to hunt, as supported by a fossil nautiloid Argonautilus catarinae with bite marks from two conspecific mosasaurs, one being from a juvenile and the other being from an adult. Analysis of the tooth marks have concluded that the mosasaurs were either Mosasaurus or Platecarpus. The positioning of both bite marks are at the direction that the nautiloid would have been facing, indicating that it was incapable of escaping and was thus already sick or dead during the attacks; it is possible that this phenomenon was from a parent mosasaur teaching its offspring that cephalopods were an alternate source of prey and how to hunt one. An alternate explanation is that the bite marks are from one individual mosasaur that lightly bit the nautiloid at first, then proceeded to bite again with greater force, but differences in tooth spacing between both bites indicate different jaw sizes, which makes the first hypothesis more likely.

Intraspecific combat

Like modern crocodiles, Mosasaurus likely grappled their opponent's head during infighting.

There is fossil evidence that Mosasaurus engaged in aggressive and lethal combat with other individuals of its kind. One particular fossil is of a partial M. conodon skeleton bearing multiple cuts, breaks, and punctures on various bones, particularly in the rear portions of the skull and neck, and a tooth from another M. conodon individual piercing through the quadrate bone. No injuries on the fossil show signs of healing, signifying that the mosasaur was killed by its attacker through a fatal blow in the skull. Another example is of a M. missouriensis skeleton with a tooth from another M. missouriensis embedded in the lower jaw underneath the eye. In this case, there were signs of healing around the wound, signifying that the victim survived the incident. However, Konishi suggested an alternative cause of this example being head-biting behavior during courtship as seen in modern lizards.

There are multiple other known fossils of Mosasaurus skulls that show signs of severe injuries, some likely fatal or leading to infections. They were likely perpetrated by an attack by another Mosasaurus, although another possible explanation for some of these injuries includes attempted biting on hard turtle shells. Lingham-Soliar suggests that if these injuries were indeed the result of an intraspecific attack, then it is notable that many of the injuries are concentrated in the skull. Modern crocodiles commonly attack other crocodiles by grappling their opponent's head using their jaws, and it has been hypothesized that the concentration of such injuries in the skull indicates that Mosasaurus also employed head-grappling during intraspecific combat. Many of the fossils with injuries possibly attributable to intraspecific combat are of juvenile or sub-adult Mosasaurus, giving the possibility that attacks on smaller, weaker individuals may have been more common. However, the attacking mosasaurs of the M. conodon and M. missouriensis specimens were likely similar in size to the victims. Some scientists have speculated on the possibility that Mosasaurus may have even occasionally engaged in cannibalism as a result of intraspecific aggression.

Life history

Fragmentary skull of a juvenile Mosasaurus (NHMM 200793) from Geulhem, Netherlands

It is likely that Mosasaurus was viviparous (giving live birth) like modern mammals today. There is no evidence for live birth in Mosasaurus itself, but it is known in a number of other mosasaurs; examples include a skeleton of a pregnant basal mosasauroid Carsosaurus, a Plioplatecarpus fossil associated with fossils of two mosasaur embryos, and fossils of newborn Clidastes from pelagic (open ocean) deposits. Such fossil records, along with a total absence of any evidence suggesting external egg-based reproduction, indicates the likeliness of viviparity in Mosasaurus. Microanatomical studies on bones of juvenile Mosasaurus and related genera have found their bone structures are comparable to adults and did not exhibit the bone mass increase found in juvenile primitive mosasauroids to support buoyancy and associated with a lifestyle in shallow water, signifying that Mosasaurus was precocial: they were already efficient swimmers and lived fully functional lifestyles in open water at a very young age, and did not require nursery areas to raise their young. However, a number of localities in Europe and South Dakota have yielded concentrated assemblages of juvenile M. hoffmannii, M. missouriensis and/or M. lemonnieri. These localities are solely shallow ocean deposits, suggesting that juvenile Mosasaurus may have still utilized shallow waters.

Paleopathology

M. hoffmannii specimen IRSNB R25, with an infected fracture to the left dentary (seen between the two middle tooth crowns in the back)

With its evidently "savage" lifestyle, there are a number of known fossils of M. hoffmannii that exhibit severe physically-inflicted damage. Two examples include IRSNB R25 and IRSNB R27 from the Royal Belgian Institute of Natural Science, both having fractures and other pathologies in their dentary bones. These conditions were described by Lingham-Soliar in a 2004 study. IRSNB R25 preserves a complete fracture near the sixth tooth socket. Extensive amounts of bony callus almost overgrowing the tooth socket are present around the fracture along with various osteolytic cavities, abscess canals, damages to the trigeminal nerve, and inflamed erosions signifying severe bacterial infection. There are two finely ulcerated scratches on the bone callus, which may have developed as part of the healing process. Specimen IRSNB R27 has two fractures: one has almost fully healed and the other is an open fracture with nearby teeth broken off as a result. The fracture is covered with a nonunion formation of bony callus with shallow scratch marks and a large pit connected to an abscess canal. Both specimens show signs of deep bacterial infection alongside the fractures; some bacteria may have spread to nearby damaged teeth and caused tooth decay, which may have entered deeper tissue from prior post-traumatic or secondary infections. However, the dentaries ahead of the fractures in both specimens are in good condition, indicating that the arteries and trigeminal nerves had not been damaged; if they were, those areas would have necrotized due to a lack of blood. The dentaries' condition suggests that the individuals may have had an efficient process of immobilizing the fracture during healing, which likely helped prevent damage to vital blood vessels and nerves. This, along with signs of healing, also signifies that the fractures were not imminently fatal. The cause of these injuries cannot be determined for certain, but two possibilities exist. One possibility may have been collateral damage from a bite on a hard surface such as a turtle shell, which would have caused intensified stress on the jawbones; another possibility is damage inflicted by another individual during intraspecific combat. The pit in IRSNB R27 has been described as resembling a tooth mark, which gives the possibility that it was the location of an attack by another mosasaur.

In 2006, Schulp et al. published a study describing a fossil quadrate of M. hoffmannii with a massive chronic infection. The bone was extensively damaged, had multiple unnatural openings, and an estimated half-liter (0.5 kg) of bone tissue was destroyed. It is likely that this was the result of a severe case of bone infection initiated by septic arthritis, which progressed to the point that a large portion of the quadrate was reduced to abscess. Extensive amounts of bone reparative tissue were also present, suggesting that the infection and subsequent healing process may have progressed for a few months. This level of bone infection would have likely been tremendously painful and severely hampered the mosasaur's ability to use its jaws. The location of the infection may have also interfered with respiration. Considering that the individual was able to survive such conditions for an extended period of time, it is likely that it switched to a foraging-type diet of soft-bodied prey such as squid that could be swallowed whole to minimize jaw usage. The cause of the infection remains unknown, but if it were a result of an intraspecific attack then it is possible that one of the openings on the quadrate may have been the point of entry for an attacker's tooth from which the infection entered.

Avascular necrosis has been reported by many studies to be present in every examined specimen of M. lemonnieri and M. conodon. In examinations of M. conodon fossils from Alabama and New Jersey and M. lemonnieri fossils from Belgium, Rothschild and Martin (2005) observed that between 3-17% of the vertebrae in the mosasaurs' spines were affected by this condition. Avascular necrosis is a common result of decompression illness, caused by bone damage from the formation of gaseous nitrogen bubbles within inhaled air decompressed during deep or repetitive diving. This indicates that both Mosasaurus species may have either been habitual deep-divers or repetitive divers. Agnete Weinreich Carlsen commented that it would be frugal to consider the appearance of such conditions as being due to non-adaptation of the animal's anatomy, as fossils of other mosasaurs that also invariably suffer avascular necrosis show evidence of developed eardrums that were well-protected from rapid changes in pressure.

Paleoecology

Distribution, ecosystem, and ecological impact

Mosasaurus inhabited the Western Interior Seaway of North America and Mediterranean Tethys of Europe and Africa.

Excluding the Pacific species unassessed by Street and Caldwell (2017) and identified as separate genera in Street (2016), Mosasaurus was a transatlantic mosasaur with its fossils having been found in marine deposits on both sides of the Atlantic Ocean. These localities include the Midwestern and East Coast of the United States, Canada, Europe, Turkey, Russia, the Levant, the African coastline from Morocco to South Africa, Brazil, Argentina, and Antarctica. During the Late Cretaceous, the aforementioned regions made up the three seaways inhabited by Mosasaurus: the Atlantic Ocean, the Western Interior Seaway, and the Mediterranean Tethys. Multiple oceanic climate zones encompassed the seaways including tropical, subtropical, temperature, and subpolar climates. The wide range of oceanic climates yielded a large diversity of fauna that coexisted with Mosasaurus.

Mediterranean Tethys

The Mediterranean Tethys during the Maastrichtian stage was located in what is now Europe, Africa, and the Middle East. In recent studies, the confirmation of paleogeographical affinities extends this range to areas across the Atlantic including Brazil and the East Coast state of New Jersey. It is geographically subdivided into two biogeographic provinces that respectively include the northern and southern Tethyan margins. From an ecological view, the two mosasaurs Mosasaurus and Prognathodon appear to be the dominant taxa in the entire seaway, being very widespread and ecologically diversified throughout the Mediterranean Tethys. The northern Tethyan margin was located around the paleolatitudes of 30–40°N, consisting of what is now the European continent, Turkey, and New Jersey.

At the time, Europe was a scattering of islands with most of the modern continental landmass being underwater. The northern Tethyan margin provided a warm-temperate climate with habitats dominated by mosasaurs and sea turtles. M. hoffmannii and Prognathodon sectorius were the dominant species in the northern province. However, other Mosasaurus species such as M. lemonnieri have been found to be the dominant species in certain areas such as Belgium, where its occurrences greatly outnumber that of other large mosasaurs. Other mosasaurs that have been found in the European side of the northern Tethyan margin include smaller genera such as Halisaurus, Plioplatecarpus, and Platecarpus; the shell-crusher Carinodens; and larger mosasaurs of similar trophic levels including Tylosaurus bernardi and four other species of Prognathodon. Sea turtles such as Allopleurodon hoffmanni and Glyptochelone suickerbuycki were also prevalent in the area and other marine reptiles including indeterminate elasmosaurs have been occasionally found. Marine reptile assemblages in the New Jersey region of the province are generally equivalent with those in Europe; the mosasaur faunae are quite similar but exclude M. lemonnieri, Carinodens, Tylosaurus, and certain species of Halisaurus and Prognathodon. In addition, they exclusively feature M. conodon, Halisaurus platyspondylus and Prognathodon rapax. Many types of sharks such as Squalicorax, Cretalamna, Serratolamna, and sand sharks, as well as bony fish such as Cimolichthys, the saber-toothed herring Enchodus, and the swordfish-like Protosphyraena are represented in the northern Tethyan margin.

Restoration of M. beaugei, which is known from Morocco and Brazil

The southern Tethyan margin was located along the equator between 20°N and 20°S, resulting in warmer tropical climates. Located around what is now Africa, Arabia, the Levant, and Brazil, seabeds bordering the cratons in Africa and Arabia provided vast shallow marine environments. These environments were also dominated by mosasaurs and marine side-necked turtles. Of the mosasaurs, Globidens phosphaticus is the characteristic species of the southern province; in the African and Arabian domain, Halisaurus arambourgi and 'Platecarpus ptychodon' (a dubious taxon that may represent various mosasaurs such as Gavialimimus or Platecarpus somenensis) were also the common mosasaurs. Mosasaurus was not well-represented: the distribution of M. beaugei was restricted to Morocco and Brazil and isolated teeth from Syria suggested a possible presence of M. lemonnieri, although M. hoffmannii also had some presence throughout the province. Other mosasaurs from the southern Tethyan margin include the enigmatic Goronyosaurus, the shell-crushers Igdamanosaurus and Carinodens, Eremiasaurus, four other species of Prognathodon, and various other species of Halisaurus. Other marine reptiles such as the marine monitor lizard Pachyvaranus and sea snake Palaeophis are known there. Aside from Zarafasaura in Morocco, plesiosaurs were scarce. As a tropical area, bony fish such as Enchodus and Stratodus and various sharks were common throughout the southern Tethyan margin.

Western Interior Seaway

Mosasaurus coexisted with bony fish such as Xiphactinus, sea turtles like Protostega and plioplatecarpine mosasaurs in North America.

Many of the earliest fossils of Mosasaurus were found in Campanian stage deposits in North America, including what was once the Western Interior Seaway, an inland sea that flowed through what is now the central United States and Canada and connected the Arctic Ocean to the modern-day Gulf of Mexico. The region was rather shallow for a seaway and had a depth of about 800–900 meters (2,600–3,000 ft) at its deepest. Extensive drainage from the neighboring continents, Appalachia and Laramidia, brought in vast amounts of sediments. Together with the formation of a nutrient-rich deepwater mass from the mixing of continental freshwater, Arctic waters from the north, and warmer saline Tethyan waters from the south, this created a warm and highly-productive seaway that supported a rich diversity of marine life.

The biogeography of the region was generally subdivided into two Interior Subprovinces characterized by different climates and faunal structures, which bordered around modern-day Kansas. The oceanic climate of the Northern Interior Subprovince was likely a cool temperate one, while the Southern Interior Subprovince had warm temperate to subtropical climates. The fossil assemblages throughout these regions suggest a complete faunal turnover by the time M. missouriensis and M. conodon appeared, implying that the presence of Mosasaurus in the Western Interior Seaway had a profound impact on the restructuring of marine ecosystems. The faunal structure of both provinces prior to the appearance of Mosasaurus were generally much more diverse, and scientists have classified these periods of diversity as the Niobraran Age. During this age, the Northern Interior Subprovince was dominated by plesiosaurs, hesperornithid seabirds, and the mosasaur genus Platecarpus; and the Southern Interior Subprovince, which was much more diverse than the north in all groups, was dominated by sharks, turtles, and a large diversity of mosasaurs including Tylosaurus and Clidastes.

The appearance of M. missouriensis and M. conodon in the Western Interior Seaway around 79.5 Ma coincided with the transition to the succeeding Navesinkan Age, which coincided with the collapse of the Niobraran faunal order and a complete turnover of marine faunal structure. In what is now modern-day Alabama within the Southern Interior Subprovince, most of the key genera including the mosasaurs Clidastes, Tylosaurus, Globidens, Halisaurus, and Platecarpus and sharks such as Cretoxyrhina disappeared and were replaced by Mosasaurus. The diversity of marine reptiles as a whole significantly declined and by then Mosasaurus dominated the entirety of the region, accounting for around two-thirds of all mosasaur diversity with Plioplatecarpus and Prognathodon sharing the remaining third. The Northern Interior Subprovince also saw a restructuring of mosasaur assemblages by the beginning of the Navesinkan Age, characterized by the disappearance of mosasaurs like Platecarpus and their replacement by Mosasaurus and Plioplatecarpus. However, Niobraran genera such as Tylosaurus, Cretoxyrhina, hesperornithids, and plesiosaurs including elasmosaurs such as Terminonatator and polycotylids like Dolichorhynchops maintained their presence until around the end of the Campanian, during which the entire Western Interior Seaway started receding from the north. Mosasaurus continued to be the dominant genus in the seaway until the end of the Navesinkan Age at the end of the Cretaceous. There were still a diversity of fauna that coexisted with Mosasaurus. These additional genera included sea turtles such as Protostega and Archelon; many species of sea birds including Baptornis, Ichthyornis, and Halimornis; crocodilians such as Deinosuchus; and many genera of fish including sharks such as the mackerel sharks Cretalamna, Squalicorax, Pseudocorax, and Serratolamna, the goblin shark Scapanorhynchus, the sand tiger Odontaspis, and the saw shark Ischyrhiza; and bony fish such as Enchodus, Protosphyraena, Stratodus, and the ichthyodectids Xiphactinus and Saurodon.

Antarctica

Mosasaurus fossils were found in the Seymour Island of Antarctica, which once provided cool temperate waters.

Mosasaurus is known from Late Maastrichtian deposits in the Antarctic Peninsula, specifically the López de Bertodano Formation in Seymour Island. This locality is estimated to have been located at around ~65°S latitude during the Maastrichtian. Being within the Antarctic polar circle, the Seymour Island locality likely provided a rather unique climate. Chemical studies on oxygen-18 isotopes found in shells and benthic foraminifera have calculated intermediate-depth and deep-sea ocean temperatures at a mean average of 6 °C (43 °F) with fluctuations of 4–12 °C (39–54 °F) throughout the Maastrichtian; one of the same studies has also suggested that sea surface temperatures may have been colder, possibly dropping below freezing and forming sea ice at times. Alternatively, a study using data acquired from ancient bacterial membrane lipids yielded a slightly warmer temperature of 12 ± 5 °C (54 ± 9 °F) around 66 Ma. Nevertheless, these estimated climates characterize primarily cool temperate environments with possible subpolar and warm episodes.

At least two species of Mosasaurus have been described in Seymour Island, but the true number of species is unknown as remains are often fragmentary and specimens are described in open nomenclature. These species include one comparable with M. lemonnieri and another that appears to be closely related to M. hoffmannii. A number of Mosasaurus fossils known in the locality are considered too fragmentary to be identified to the species level. Nevertheless, the genus appears to be the most taxonomically diverse in the Maastrichtian Antarctica. Mosasaurus is not the only mosasaur from Seymour Island; at least four other genera have been reported in similar or same deposits. These include Plioplatecarpus, the mosasaurines Moanasaurus and Liodon, and Kaikaifilu. However, many of these genera are primarily based on isolated teeth, and studies on the dental variability of Kaikaifilu demonstrated the ease of misidentification when examining Antarctic mosasaur teeth, so it is possibile that more genera were reported than there actually was. Prognathodon and Globidens are also expected to be present based on distribution trends of both genera, although conclusive fossils have yet to be found. Other marine reptiles included elasmosaurid plesiosaurs like Aristonectes and another indeterminate elasmosaurid. The fish assemblage of the López de Bertodano Formation was dominated by Enchodus and ichthyodectiformes, accounting for 21.95% and 45.6% of local fish diversity respectively. Of the remaining percentages, sand sharks made up 10.5%, the cow shark Notidanodon 6.8%, chimaeras 3.9%, saw sharks 2.7%, various other teleost fish 2.4%, and the remaining 6% were shared between other sharks like Paraorthacodus, frilled sharks, Protosqualus, and Cretalamna.

Habitat preference

Mosasaurus inhabited offshore ocean habitats of various depths.

A traditional method of determining the habitat preference of fossil animals is by determining the habitat represented by the deposits they were from. Known fossils of Mosasaurus were typically recovered from deposits representing nearshore habitats during the Cretaceous period, with some fossils coming from deeper water deposits. Lingham-Soliar (1995) elaborated on this, finding that Maastrichtian deposits in the Netherlands with M. hoffmannii occurrences represented nearshore waters around 40–50 metres (130–160 ft) deep. Changing temperatures and an abundance in marine life were characteristic of these localities. The morphological build of M. hoffmannii, nevertheless, was best adapted for a pelagic surface lifestyle. It likely resided near the surface and exploited the locality's rich assemblages.

A newer approach is through a biogeochemical analysis, one being the measurement of δ¹³C levels in tooth enamel. Another known correlation with δ¹³C levels shows that δ¹³C typically depletes as the foraging habitat of the animal is farther from the shore, meaning that lower levels of the isotope can be correlated with feeding habitation in more open waters and vice versa. This was tested on multiple Mosasaurus fossils in multiple studies which have yielded consistent results signifying that Mosasaurus fed in more offshore or open waters. However, it has been pointed out that measuring δ¹³C levels may not be the most accurate method of determining the preferred habitat of Mosasaurus due to influence by other factors in the animal's lifestyle. In M. hoffmannii, one example is diet while the Bohr effect through diving behavior may have been another in M. lemonnieri and M. conodon. As a result, isotope levels can misrepresent the actual habitat preferences of Mosasaurus. As a solution, paleontologists T. Lynn Harrell Jr. and Alberto Perez-Huerta conducted a 2014 study specifically examining the concentration ratios of neodymium, gadolinium, and ytterbium in M. hoffmannii and Mosasaurus sp. fossils from Alabama, the Demopolis Chalk, and the Hornerstown Formation. Previous studies demonstrated that ratios of these three elements can act as a proxy for relative ocean depth of a fossil during early diagenesis without interference from biological processes, with each of the three elements signifying either shallow, deep, or fresh waters. The rare earth element ratios were very consistent throughout most of the examined Mosasaurus fossils, indicating consistent habitat preference, and clustered towards a ratio representing offshore habitats with ocean depths between or deeper than 50–150 metres (160–490 ft). A few outliers existed that instead represented shallower waters 50 metres (160 ft) deep or less.

Interspecific competition

Mosasaurus was able to coexist with other large predatory mosasaurs like Prognathodon through niche partitioning.

Mosasaurus lived alongside other large predatory mosasaurs also considered apex predators, most prominent among them being the tylosaurines and Prognathodon. Tylosaurus bernardi, the only surviving species of the genus during the Maastrichtian, measured up to 12.2 meters (40 ft) in length while the largest coexisting species of Prognathodon like P. saturator exceeded 12 meters (39 ft). These three mosasaurs converged in a diet on on similar animals such as marine reptiles.

A study published in 2013 by Schulp et al. specifically tested how mosasaurs such as M. hoffmannii and P. saturator were able to coexist in the same localities through δ¹³C analysis. The scientists utilized an interpretation that differences in isotope values can help explain the level of resource partitioning because it is influenced by multiple environmental factors such as lifestyle, diet, and habitat preference. Comparisons between the δ¹³C levels in multiple teeth of M. hoffmannii and P. saturator from Maastricht Formation showed that while there was some convergence between certain specimens, the average δ¹³C values between the two species were generally different. This is one indication of niche partitioning, where the two mosasaur genera likely foraged in different habitats or had different specific diets to coexist without direct competitive conflict. The morphological builds of the two species add to the context of this finding. The teeth of P. saturator are much more robust than those of M. hoffmannii and were specifically equipped for preying on robust prey like turtles. While M. hoffmannii also preyed on turtles, its teeth were built to handle a wider range of prey less suited for P. saturator.

Another case of presumed niche partitioning between Mosasaurus and Prognathodon from the Bearpaw Formation in Alberta was documented in a 2014 study by Konishi et al. The study found a dietary divide between M. missouriensis and Prognathodon overtoni based on stomach contents. Stomach contents of P. overtoni included turtles and ammonites, providing another example of a diet specialized for harder prey. In contrast, M. missouriensis had stomach contents consisting of fish indicative of a diet specialized in softer prey. It was hypothesized that these adaptations reinforced resource partitioning between the two mosasaurs.

However, competitive engagement evidently could not be entirely avoided. There is also evidence of aggressive interspecific combat between Mosasaurus and other large mosasaur species. This is shown from a fossil skull of a subadult M. hoffmannii with fractures caused by a massive concentrated blow to the braincase; Lingham-Soliar (1998) argued that this blow was dealt by a ramming attack by Tylosaurus bernardi, as the formation of the fractures were characteristic of a coordinated strike (and not an accident or fossilization damage) and that T. bernardi was the only known coexisting animal likely capable of such damage using its robust projectile-like elongated rostrum. This sort of attack has been compared to the defensive behavior of bottlenose dolphins using their beaks to kill or repel lemon sharks, and it has been speculated that T. bernardi dealt the offensive attack via an ambush on an unsuspecting Mosasaurus.

Extinction

Mosasaurus went extinct as a result of the K-Pg extinction event; its last fossils were found at or close to the boundary, which is represented by the thick dark band separating the lighter and darker layers of this cliff.

By the end of the Cretaceous, mosasaurs like Mosasaurus were at a height of evolutionary radiation, and their extinction was a sudden event. During the Late Maastrichtian, global sea levels dropped, which drained the continents of their nutrient-rich seaways and altered circulation and nutrient patterns, reducing the number of available habitats for Mosasaurus. The genus endured by accessing new habitats in more open waters. The geologically youngest fossils of Mosasaurus, which includes those of M. hoffmannii and indeterminate species, occur up to the Cretaceous-Paleogene boundary (K-Pg boundary). The demise of the genus was likely a result of the Cretaceous-Paleogene extinction event that also wiped out the dinosaurs. Localities where Mosasaurus fossils have been found 15 meters (49 ft) below the boundary or less include the Maastricht Formation, the Davutlar Formation in Turkey, the Jagüel Formation in Argentina, Stevns Klint in Denmark, Seymour Island, and Missouri.

M. hoffmannii fossils have been found within the K-Pg boundary itself between the Paleocene Clayton Formation and Cretaceous Owl Creek Formation in southeastern Missouri. Vertebral fossils from the layer were found with fractures formed after death. The deposition of the layer itself was likely a tsunamite, alternatively nicknamed the "Cretaceous cocktail deposit", formed as a result of a combination of catastrophic seismic and geological disturbances, mega-hurricanes, and giant tsunamis as direct consequences of the impact of the Chicxulub asteroid that catalyzed the K-Pg extinction event. Aside from the physical destruction of these events, the impact also had subsequent environmental reverberations like blocking out sunlight that led to a collapse of marine food webs. Any Mosasaurus that may have survived the immediate cataclysms by taking refuge in deeper waters would have eventually died out due to starvation from a total loss of prey structure.

One enigmatic occurrence is of Mosasaurus sp. fossils found in the Hornerstown Formation, a deposit that is typically dated to be from the Paleocene Danian age, which was immediately after the Maastrichtian age. The fossils were found in association with fossils of Squalicorax, Enchodus, and various ammonites within a uniquely fossil-rich bed at the base of the Hornerstown Formation known as the Main Fossiliferous Layer. This does not mean that Mosasaurus and its associated fauna may have survived the K-Pg extinction; there are several alternate explanations as to why these animals have been found in nominally Cenozoic deposits. One argument proposes the fossils actually originated from an earlier Cretaceous deposit that was reworked into the Paleocene formation during its early deposition. Evidence of reworking typically comes from fossils that are worn down due to further erosion during its exposure at the time of its redeposition. Many of the Mosasaurus fossils from the Main Fossiliferous Layer consist of isolated bones that are commonly abraded and worn; however, the layer also yielded better-preserved Mosasaurus remains. Another explanation suggests that the Main Fossiliferous Layer is a Maastrichtian time-averaged remanié deposit, which means that it originated from a Cretaceous deposit with little sedimentation and was gradually winnowed into the overlying deposits. A third hypothesis proposes that the layer is a lag deposit of Cretaceous sediments that were forced out by a strong impact event such as a tsunami and was subsequently refilled with Cenozoic fossils.

See also

• Paleontology portal

External links

• Media related to Mosasaurus at Wikimedia Commons
• Data related to Mosasaurus at Wikispecies
• Oceans of Kansas