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A mosasaur (from Latin Mosa meaning the 'Meuse', and Greek σαύρος sauros meaning 'lizard'), strictly speaking, is an extinct aquatic lizard with paddle-like limbs within the family Mosasauridae that lived during the Late Cretaceous Period. They also have a long streamlined body with a tail that ends in a downward bend and supports a fin-like fluke on top. Nearly all mosasaurs were wholly marine, though freshwater incursions also occurred. Mosasaur genera belong to one of three mosasauroid groups: the subfamilies Mosasaurinae and Halisaurinae, and the clade Russellosaurina. Mosasaurs evolved from a group of extinct semiaquatic lizards with terrestrial limbs called aigialosaurs during the Turonian (93.9-89.8 mya), mirroring the later evolution of whales from their terrestrial ancestors. It was traditionally believed that all mosasaurs descended from a single origin (monophyletic) and were accordingly classified under the family Mosasauridae. However, emerging discoveries during the 21st century suggests that mosasaurs may not form a natural family, and instead actually represent at least two or three independent lineages that achieved a similar aquatic body plan through convergent evolution (polyphyletic). This multiple-origins hypothesis remains controversial due to the poor fossil record of Turonian mosasauroids.

Mosasaurs were incredibly successful. The three groups collectively attained a cosmopolitan distribution encompassing nearly all latitudes, including polar regions. They were dominant predators in nearly all marine ecosystems, becoming ubiquitous features of Late Cretaceous oceans. Mosasaurs appeared during a period of high global primary productivity sparked by warm oceans and the aftermath of the Cenomanian-Turonian boundary event, and of opened ecological niches following the extinctions of the ichthyosaurs and pliosaurs, which may have supported their rapid worldwide radiation. Within a span of ~30 million years, they diversified into at least 80 unique species and occupied a wide variety of carnivorous niches. The smallest mosasaurs measured about 2 m long, while the largest were apex predators that grew in excess of 14 m. Mosasaurs achieved their peak in diversity towards the very end of the Cretaceous until their sudden extinction during the Cretaceous-Paleogene extinction event 66 mya.

Mosasaurs were the earliest fossil reptiles to be recognized by scientists. The first known remains, belonging to the eponymous Mosasaurus, were discovered in the Netherlands between the 1760s and 80s. Their identification by 1808 as a giant aquatic monitor lizard that no longer exists was key in solidifying the new concept of extinction. The Bone Wars rivalry between American paleontologists Edward Drinker Cope and Othniel Charles Marsh during the late 1800s sparked an explosion of mosasaur research in the United States. Most of the iconic genera were described in this period, and scientific understanding of mosasaur anatomy was perfected by the 1890s. The Bone Wars discoveries also gave rise to the hypothesis that mosasaurs were not monitor lizards but instead close relatives of snakes united under the clade Pythonomorpha. Scientist debate to this day whether mosasaurs are most closely related to monitor lizards or snakes.

Early discoveries
Some of the earliest possible references to mosasaur fossils appear in Native American folklore. The creation myths of the Dakota and Cheyenne nations tell of an ancient age ruled by gigantic water monsters locked in perpetual combat with thunderbirds. These water monsters are said to have been blasted to stone by the thunderbirds' lightning, where they can be found in the ground today. Pawnee and Crow mythology describes similar water monsters with serpentine bodies, crocodilian heads, and sometimes legs or fins that lurked in rivers. The Great Plains are rich mosasaur-bearing outcrops, where encounters of fossil skeletons by these nations when they migrated into the area during the 1600s would have been influential in shaping their legends. For example, the Niobrara Formation of Kansas and Nebraska preserve fossils of giant mosasaurs like Tylosaurus and flying pterosaurs like Pteranodon, which could have served as inspirations for the water monster-thunderbird battles.

In the 1760s and 1770s, the earliest mosasaur fossils to be scientifically studied were discovered in a subterranean limestone quarry under Mount Saint Peter near Maastricht, The Netherlands. Fossils may have been recovered over centuries as the hill has been continuously excavated since medieval times, but no surviving records of earlier finds are known. The first was a skull found in 1764 or 1766, which was later procured to the Teylers Museum in Haarlem in 1784. Around this time, Mount Saint Peter became an area of interest for fossil collectors. The most notable was retired army surgeon Johann Leonard Hoffmann, who over a period of 25 years accumulated a famous collection of local fossils that included those of mosasaurs. In 1778, a second more complete skull was discovered in the same quarry, which Hoffmann recognized as related to both the Teylers skull and his own finds. Many, including himself, though they belonged to a giant crocodile. At the time, there were no widespread ideas of evolution or extinction, so a living animal was the most sensible identification. Hoffmann intended to publish a essay on the crocodilian identification, but following a correspondence with the anatomist Petrus Camper, who showed him a crocodile jaw to demonstrate its dissimilarity with the fossil skulls, was persuaded not to. Camper argued that that the fossils instead belonged to a unidentified type of sperm whale, which he based using the then-infant method of comparative anatomy. He published his study of the second skull in 1786, which attracted international attention to the giant fossil. In 1794, following the capture of Maastricht during the War of the First Coalition, the French Revolutionary Army seized the second skull and expatriated it to National Museum of Natural History, France.

Bone Wars and late 19th-century research surge
-Leidy 1850s -Cope and Marsh -Merrian (1894) -Louis Dollo (1880s) -Williston (1898) -Nopsca (1900s)

Early-mid 20th century developments
Scientific interest in mosasaurs waned by the 1910s, and in the following decades the vast collections accumulated the century prior became largely forgotten. Publications during this early to mid-20th century period was primarily isolated to sporadic discoveries and occasional new species. Most occurred during European-led expeditions into Africa, such as the documentation of several mosasaurs in Egypt, Morocco, and South Africa between the 1910s and 1930s, and the 1930 discovery of Goronyosaurus in Nigeria. Important fossils were also excavated in California during the latter decade, leading to the discoveries of the evolutionarily advanced genera Plotosaurus and Plesiotylosaurus. The most active research during the wider hiatus was Charles Lewis Camp's work on the anatomy and systematics of mosasaurs. His first 1928 treatise presented a rigorous case for varanoid ancestry of mosasaurs (which became widely accepted for nearly the remainder of the century), while a second 1942 paper reinforced that position with a reconstruction of the neurovascular system inside the mosasaur cranium.

A turning point was reached with Dale Russell's publication of Systematics and Morphology of American Mosasaurs in 1967.

Renaissance period
Russell (1967) seeded a resurgent interest in mosasaurs that came to fruition during the 1980s.

Renewed interest in mosasaur research began in the 1990s

-Bell (1997) -Lingham-Soliar (1990s) -Nicholls (1980s-90s) -Massare (1980s)

Definition
There is no universally agreed definition of "mosasaur." The term evolved over the course of nearly 250 years in attempts to describe large paddle-limbed marine reptiles that resembled monitor lizards. During the late 19th century, fossils that fit these characteristics were classified under the Mosasauridae, thus making "mosasaur" congruent with a scientific family. The traditional sense for both concepts arose by the end of the century, outlined by Williston in 1896 as an "obligately aquatic squamate with paddles." This sense remained unchallenged well into the 21st century. In the advent of cladistics, the Mosasauridae was nuanced to a phylogenetic relationship representing all descendants of a purported single marine radiation from an ancestral mosasaur that fit Williston's definition.

The modern problem of defining "mosasaur" is closely tied to the debate on mosasaur evolution. It arose alongside the convergent evolution hypothesis, which places several, but not all, members of a family of terrestrial-limbed lizards under the mosasaurid tree (see Debate on convergence). This meant that "mosasaur" in the traditional sense is not a natural grouping but an informal evolutionary grade. A review by Caldwell (2012) concluded that "mosasaur" needs to be redefined to accommodate the hypothesis, recommending a new definition uniting only paddle-limbed mosasaurines. Palci et al. (2013) added two suggestions to either synonymize the term with the Mosasauroidea, or abandon it entirely. "Mosasaur" has since been applied inconsistently in academic literature as either the traditional sense or a vernacular term for either mosasaurids broadly or all mosasaurians.

General taxonomy
Mosasaurids are a member of the order Squamata, which comprises of lizards and snakes. This placement makes them unique among major Mesozoic marine reptiles, as unlike previous and coexisting groups that globally dominated the oceans like ichthyosaurs, sauropterygians, and thalattosuchians, mosasaurids were the only that are not archosauromorphs. The placement of mosasaurids within the squamates remains a long-standing dispute between several competing hypotheses. The two most prominent are the varanoid hypothesis, which holds that mosasaurids are monitor lizards of the superfamily Varanoidea, and the pythonomorph hypothesis, which argues that mosasaurids are close relatives of snakes.

Paleontologists divide mosasaurids into three major groups: the subfamilies Mosasaurinae and Halisaurinae, and clade Russellosaurina. The Russellosaurina is further subdivided into the four subfamilies Tylosaurinae, Plioplatecarpinae, Tethysaurinae, and Yaguarasaurinae. Though traditionally recognized as distinct, Polcyn et al. (2023) suggested based on the discovery of cranial synapomorphies that the latter two should be merged into the Plioplatecarpinae. Within the Mosasaurinae, most paleontologists recognize the tribes Mosasaurini and Globidensini. Some add a third tribe Prognathodontini, though this not universal due to the unstable phylogenies of the taxa it represents. Longrich et al. (2021) also proposed further subdividing Halisaurinae into the two tribes Halisaurini and Pluridensini.

Ancestry
Mosasaurids represent the final culmination of the eponymous Mosasauria, a lineage of aquatic and semiaquatic lizards with contentious origins that likely arose during the Early Cretaceous. The earliest mosasaurians were a small long-bodied group called dolichosaurs. The first to appear in the fossil record was Kaganaias, which inhabited an inland swamp in what is now Japan during the late Barremian shortly before 121 mya. Subsequent dolichosaurs mostly lived in shallow marine habitats. It has been accordingly suggested that ancestral mosasaurians initially adapted to freshwater environments, before entering brackish estuaries and then colonizing marine environments.

During the mid-Cretaceous, a second collection of mosasaurians called the aigialosaurs appeared, from which mosasaurids immediately descend from and together form the superfamily Mosasauroidea. Aigialosaurs do not form a natural group but instead represent a stem assemblage of forms with limbs akin to terrestrial lizards'. Due to the phylogenetic definition of the family, some branches of aigialosaurs may be classified under mosasaurids. It is unclear how they and dolichosaurs are related, as phylogenetic studies conflict on whether the former descends from the latter or the two simply share a common ancestor. The earliest known aigialosaur is Haasiasaurus, from early Cenomanian deposits in Palestine dated to 98 mya. An evolutionary clock by Madzia and Cau (2020) estimated that the group split from the dolichosaurs during the Albian about 109 mya, and so could have emerged earlier. The discovery of Proaigialosaurus in 1950s, a reptile from the Late Jurassic Solnhofen Limestone purported to be an early aigialosaur, may push the origins of the group further back if true. This is unverifiable as the fossils became lost, and some scientists have since suggested the genus is probably an unrelated diapsid. Aigialosaurs were small-sized semiaquatic reptiles, rarely exceeding 1 m in length, that mostly resembled terrestrial lizards with sea snake-like flattened tails and skulls nearly matching primitive aquatic mosasaurids. They were restricted to nearshore habitats in the Mediterranean Tethys Sea during the Cenomanian, mostly concentrated within the Adriatic region of Europe that supported a large shallow carbonate platform at the time. Migration into North America was achieved by the Turonian, with genera like Vallecillosaurus appearing in northern Mexico about 93 mya. Aigialosaurs appeared to have swam in an eel-like anguilliform manner which, being energetically expensive, probably could not support sustained swimming. As a result, they probably adapted as ambush predators with a generalist diet.

Emergence
Aquatic mosasaurids begin to appear in the fossil record during the Turonian between 94 to 90 mya. Their emergence was characterized by a push towards rapidly increasing aquatic adaptations in aigialosaurs within a span of about 15 million years until they became fully obligate aquatic swimmers. This was achieved through a reorganization of the postcranial body. The sacrum ("plesiopelvis") was gradually lost through detachment of the pelvis' ilium from the ribs ("hydropelvis"). This eliminated the hip's ability to support the animal's body weight on land but in turn freed the region for further aquatic enhancement. The limbs transitioned from those built for terrestrial locomotion ("plesiopedal") to stiff and broad paddle-like appendages optimized towards swimming through a process similar to that achieved in cetaceans ("hydropedal"). The arm and leg bones were shortened and widened, the finger and toe bones were elongated and increased in quantity, and connective tissue was developed to maintain cohesion of all digits as a singular hydrodynamic unit in a manner comparable to wearing scratch mitts. A significantly denser physical environment of the water in itself is often considered the primary driver of rapid selection towards the derived mosasaurid build. Cross et al. (2022) hypothesized that selective pressure may have in particular applied towards increasing efficiency in ambush hunting, as coupled with postcranial adaptations was also the gradual elongation of the jaws. Such would optimize biting speed and thus capture of faster-moving prey. As swimming ability improved, bone microstructure remodeled from a thick and densely mineral-filled architecture for maintaining neutral buoyancy in poor-swimming aigialosaurs to a spongy architecture of tightly-packed parallel fiber networks similar to in those in cetaceans to maintain energy efficiency and structural integrity as active swimming took over the role of buoyancy control. This may have also led to the development of warm-bloodedness to meet the metabolic demands of the new vascularization.

The russellosaurines were the first group to appear with the fully aquatic bodyplan. They probably arose somewhere along the northern margins of Gondwana, with transitional forms like Tethysaurus and Sarabosaurus present in North Africa and the the Western Interior Seaway (WIS) of North America respectively around 94-93 mya. The earliest known mosasaurids with fully aquatic limbs to appear in the fossil record are Angolasaurus and Tylosaurus, both about 92-91 mya in the WIS. Fully aquatic mosasaurines emerged later in the WIS as Clidastes-like forms around 86.4 mya. The earliest halisaurine fossils appeared around the same time in the same region through Eonatator between 87-78.5 mya.

Debate on convergence
The details of how mosasaurids arose from aigialosaurs is controversial. The traditional belief held that all descent from a single aquatic origin, that is, only one aigialosaur lineage evolved the mosasaur bodyplan. But evidence emerged during the 21st century suggesting that mosasaurids represent multiple aigialosaur lineages that obtained the aquatic form independently through convergent evolution. The possibility of multiple origins was understood by Russell (1967), who speculated of two distinct Cenomanian Clidastes-like and Platecarpus-like lines. The modern hypothesis was conceived by Bell and Polcyn (2005) through their description of Dallasaurus, an aigialosaur recovered as a basal mosasaurine. Their phylogenetic analysis also recovered other aigialosaur taxa as separate basal members of each of the three mosasaur groups. Follow-up phylogenetic studies in subsequent years recovered topologies consistent with convergent evolution, mostly as two independent mosasaurine and russellosaurine-halisaurine lineages. The discovery of a neurocranial synapomorphy within Tethysaurus (a plesiopedal genus) and derived plioplatecarpines by Polcyn et al. (2023) may indicate that the tylosaurines and plioplatecarpines also evolved the aquatic bodyplan independently. Amelia Zietlow of the American Museum of Natural History commented that multiple convergent evolutions is not unprecedented in squamates, noting that limblessness developed independently in at least seven modern groups.

A competing hypothesis maintains that aquatic mosasaurs emerged only once, and that the recovery of plesiopedal taxa at the bases of mosasaur groups is the result of evolutionary reversal. Though first discussed as a possible alternative to convergent evolution by Dutchak and Caldwell (2009), statistical evidence was first presented by Simões et al. (2017). Through ancestral state reconstruction of phylogenetic characters using a modified Markov model on several phylogenies, they found that a common origin of plesiopelvic and plesiopedal traits with reversal in at least one plesiopedal group (Tethysaurinae) is more likely than convergent evolution. The models excluded some aigialosaurs used in Bell and Polcyn (2005) like Haasiasaurus and yielded ambiguous results on the evolutionary situation of Dallasaurus. However, the authors opined that such ambiguity opens the possibility that Dallasaurus is actually not a mosasaurine but incidentally appears as one due to incomplete fossil representation. Cross et al. (2022) performed another ancestral state reconstruction using quantitative data obtained from a principal component analysis of mosasauroid limb morphometrics. Their results were inconclusive regarding the number of hydropedal origins due to complications related to Dallasaurus, but noted that most of their models reconstructed the genus as an evolutionary reversal from ambiguous ancestral limbs to firmly aigialosaur-like ones. The study accordingly predicted that their results would clearly indicate a single origin of hydropedality if Dallasaurus turns out to not be a mosasaurine.

Rise to dominion
Following their appearance, the russellosaurine mosasaurs evolved rapidly. Tylosaurines evolved particularly rapidly during the Turonian and later Coniacian stages; the earliest fossils of Tylosaurus dated 92-91 mya already demonstrated a large body length of up to 6 m and the subfamily's signature projectile-like elongated snout.

The first mosasaurs to achieve the aquatic bodyplan were in the russellosaurines. The group probably arose during the early Turonian somewhere along the northern margins of Gondwana. Intermediate forms such as Tethysaurus and Sarabosaurus were already present in North Africa and the WIS around 94-93 mya and outsized coexisting aigialosaurs at 3 m in body length each. The group evolved rapidly; evidence of fully paddle-limbed plioplatecarpines such as Angolasaurus debuted in the WIS fossil record by 92 mya, while by 91-90 mya tylosaurines like Tylosaurus already achieved large body sizes and specialized projectile-like snouts,  and around the same time Yaguarasaurus radiated into South America with body lengths exceeding 5 m. By the early Coniacian stage around 88 mya, russellosaurines achieved a trans-Atlantic dispersal stretching from present-day England to Angola. Mosasaurine mosasaurs later emerged in North America, with Clidastes-like forms appearing in the WIS around 86.4 mya. The group remained entirely endemic to the continent until the early Campanian stage. The halisaurines also appeared in fossil record around the same time as the mosasaurines through Eonatator in the WIS between 87-78.5 mya.

Early russellosaurine mosasaurs evolved during a unique transitional period shortly after the Cenomanian-Turonian anoxic event (OAE 2) that occurred 93.9 ± 0.15 mya.

Diversity
The following cladogram synthesizes multiple phylogenetic studies to represent nearly all valid genera, excluding aigialosaur taxa. .

Size
Mosasaurs attained a wide range of sizes. They can be grouped into three size classes of small (1-4 m, medium (4.1-8 m), and large (>8 m) species. The smallest known mosasaur was Xenodens at about 1 m long. The largest species on record is either Tylosaurus proriger or Mosasaurus hoffmannii, but their maximum sizes are debated. The largest reliable T. proriger specimen is the "Bunker" specimen (KUVP 5033), estimated to measure between 12-15.8 m.  Its M. hoffmannii counterpart is the "Penza" specimen (CCMGE 10/2469), whose total length was traditionally estimated to be 17.1 m. However, the latter has been considered an overestimate, and a 2014 study suggested body proportions that would indicate a length closer to 12 m. Some fragmentary bones from both species suggest even larger sizes exceeding 14 m. Everhart speculated that it would have been possible for some very old individuals to grow up to 20 m, though stressed the lack of fossil evidence.

A common method of size estimation from incomplete specimens is by measuring the lower jaw, from which the total length can be extrapolated through a jaw-to-body ratio. Typical mosasaurs have a ratio between 1:6.5 and 1:8 (~12-15% of total length), but some species have unusually small skulls for their body. The longest ratio is held by Mosasaurus lemonnieri at ~1:11.

General morphology
Paleontologists compared mosasaurs to a Komodo dragon with flippers. Their bauplan resembled modern-day varanoids but, as secondary aquatic lizards, were modified for a wholly marine lifestyle. Their bodies were streamlined into a fusiform shape with both pairs of limbs reduced and flattened into paddle-like flippers. The tail was flattened and tapered off into a downward curve that supported a two-lobed fin resembling an upside-down shark's tail. These features provided a superficial similarity to other aquatic tetrapods such as basal ichthyosaurs, marine crocodiles, and archaeocete whales through convergent evolution. The proportion between the tail and the rest of the mosasaur body varied between groups. Mosasaurine tails were shorter than the torso section that lays between the skull and hind limbs, while in halisaurines they were about the same length. Russellosaurine tails were longer than the torso, in total comprising of approximately half of the entire body length.