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A mosasaur (from Latin Mosa meaning the 'Meuse', and Greek σαύρος sauros meaning 'lizard'), strictly speaking,[a] 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 meters (6.6 ft) long, while the largest were apex predators that grew in excess of 14 meters (46 ft). 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.
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 conflict 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.[1]
Scholarly interest in mosasaur fossils began with the discovery of two large partial skulls in 1764 and 1780 respectively from a subterranean limestone quarry near Maastricht, The Netherlands. Locals initially presumed the remains to belong to an unknown type of giant crocodiles. Johann Leonard Hoffmann, a retired surgeon who had been collecting mosasaur bones in the area, contacted his friend, the famed anatomist Petrus Camper, to study the fossils. Camper, via comparative anatomy, found the remains had characteristics not of the crocodile, but of a marine animal. He published his results in 1786 with an alternate identification of a toothed whale. Camper's involvement with the Maastricht fossils brought the attention of French revolutionaries, who confiscated the now-famous second skull in 1794 during the War of the First Coalition. This skull was expatriated to Paris, where it was nicknamed le grand animal de Maestricht[b] and garnered controversy about its identity. Adriaan Gilles Camper, Petrus' son who was also involved in comparative anatomy, re-examined the animal in 1798 in an attempt to defend his father's position. He however realized that his father overlooked some of Hoffmann's fossil vertebrae that, together with the Parisan skull, clearly indicated a reptile related to the modern-day monitor lizard and iguana. Gilles Camper communicated his beliefs to Georges Cuvier, who in 1808 reached the same conclusion. Cuvier furthermore added that le grand animal was a marine lizard of its own kind that no longer exists. However, he was hesitant to declare a more precise identification or scientific name because the Maastricht fossils were too incomplete to provide a full picture of the animal, and left it unnamed for the next twenty years. Despite this, Cuvier correctly speculated that the animal took the likeness of a giant monitor lizard with a long flattened tail and dolphin-like flippers for aquatic propulsion. In 1822, William Conybeare of England suggested Mosasaurus, meaning "lizard of the Meuse"[c] (referring to a river near the Maastricht quarry) as a temporary name, which Cuvier adopted as the formal genus in 1829.
Around the same time, mosasaur fossils began to be discovered across the United States. The first was by the Lewis and Clark Expedition in 1804, where they uncovered a now-lost petrified skeleton of a 45-foot (14 m) long "fish" now believed to likely be of a mosasaur in present-day South Dakota. In 1818, a jaw fragment found in New Jersey became the first to be correctly recognized as affiliated with the Maastricht animal. The most comprehensive find of the early 19th century came again in South Dakota in 1832-33 with the discovery of a partial skeleton with an articulated skull. A study of this specimen by professor Georg August Goldfuss of the University of Bonn in 1845 provided the first glimpse of how a mosasaur's head would have appeared in life. Mosasaur findings also steadily increased throughout Europe. All fossils identified as mosasaurs were initially classified as species of Mosasaurus until the early 1840s, when Richard Owen erected the alternative genera Liodon and Macrosaurus from fragmentary remains in England. Owen also erected the first formal grouping of the mosasaurs, the tribe Natantia under the suborder Lacertilla, for the three genera in 1851.
-Leidy 1850s -Cope and Marsh -Merrian (1894) -Louis Dollo (1880s) -Williston (1898) -Nopsca (1900s)
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.
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)
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.
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.
Mosasaurids represent the final culmination of the eponymous Mosasauria, a lineage of aquatic and semiaquatic lizards with contentious origins[2] 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[3][4] during the late Barremian shortly before 121 mya.[5] 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.[3]
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'.[6] Due to the phylogenetic definition of the family,[7] some branches of aigialosaurs may be classified under mosasaurids.[8] 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.[2] The earliest known aigialosaur is Haasiasaurus, from early Cenomanian deposits in Palestine dated to 98 mya.[9][10] 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.[7] 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.[10] Aigialosaurs were small-sized semiaquatic reptiles, rarely exceeding 1 m (3.3 ft) in length,[11] that mostly resembled terrestrial lizards with sea snake-like flattened tails[10] and skulls nearly matching primitive aquatic mosasaurids.[12] They were restricted to nearshore habitats in the Mediterranean Tethys Sea during the Cenomanian, mostly concentrated within the Adriatic region of Europe[13] that supported a large shallow carbonate platform at the time.[14] Migration into North America was achieved by the Turonian, with genera like Vallecillosaurus appearing in northern Mexico by 93 mya[15][16][17] and earlier.[18] 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.[19]
All mosasaurid subfamilies had diverged no later than the start of the Turonian around 94 mya.[d][20] Aquatic mosasaurids subsequently begin to appear in the fossil record throughout the age from 94 to 90 mya.[20] 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.[10] 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.[21] 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.[19] As swimming ability improved, bone microstructure remodeled from a thick and densely mineral-filled architecture for maintaining neutral buoyancy in poor-swimming aigialosaurs[10] 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.[22][23] This may have also led to the development of warm-bloodedness to meet the metabolic demands of the new vascularization.[23]
The russellosaurines were the first group to appear with the fully aquatic bodyplan. They probably arose somewhere along the margins of the North Atlantic,[24] with transitional forms like Tethysaurus and Sarabosaurus present in North Africa and the Western Interior Seaway (WIS) of North America respectively around 94 to 93 mya.[20][25] The earliest mosasaurid with fully aquatic limbs to appear in the fossil record is a yet-undescribed Russellosaurus-like species about 93 mya in the WIS.[26] Larger russellosaurines including Angolasaurus[27] and Tylosaurus[23][24][28] shortly appeared between 92 to 91 mya.[17] Fully aquatic mosasaurines emerged later in the WIS as Clidastes-like forms around 86.4 mya.[29][30] The earliest halisaurine fossils appeared around the same time in the same region through Eonatator between 87 to 78.5 mya.[25]
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, with the plesiopedal Tethysaurus and primitive halisaurines represented as transitional forms. But this paradigm became challenged by the description of the aigialosaur Dallasaurus by Bell and Polcyn (2005), whose phylogenetic analysis recovered it as a basal mosasaurine and Tethysaurus a basal russellosaurine. This suggested that the three major mosasaurid groups actually represent multiple aigialosaur lineages that obtained the aquatic form independently through convergent evolution.[31] Subsequent phylogenetic studies in the following decade recovered topologies consistent with at least two independent aquatic origins between the russellosaurines and mosasaurines or more. The discovery of a neurocranial synapomorphy within Tethysaurus 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 ple
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.[e] Though discussed as a possible alternative by Dutchak and Caldwell (2009), statistical evidence was first presented by Simões et al. (2017) via ancestral state reconstruction of phylogenetic characters 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[f] and yielded ambiguous results on the evolutionary situation of Dallasaurus. However, the authors opined the latter could suggest that Dallasaurus is not actually a mosasaurine but erroneously recovers as one due to incomplete fossil representation.[32] Cross et al. (2022) performed another ancestral state reconstruction using quantitative data from 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 outputs 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 is not a mosasaurine.[19]
Aquatic mosasaurs arose shortly after the Cenomanian-Turonian anoxic event (OAE 2), a period when global oceans experienced severe depletions of dissolved oxygen. This precipitated a mass extinction that eliminated over one quarter of marine invertebrate diversity and major vertebrates like the ichthyosaurs, and subsequently facilitated a global restructuring of marine ecosystems. Mosasaurs however did not experience an immediate adaptive radiation as would be expected for a group filling in an ecological vacuum. Instead, they appeared to have diversified gradually during their early history. The post-OAE 2 Turonian ocean remained occupied by pliosaurs, polycotylids, sharks, and teleosts. Competition from these predators likely constrained the available niche space. Early mosasaurs therefore may have evolved via opportunism by occupying limited niches opened up following OAE 2 and into the Turonian. In particular, tylosaurines may have taken advantage of the mid-Turonian extinctions of the pliosaurs and of most polycotylids to rapidly evolve into giant apex predators. As the only to have developed fully aquatic adaptations by this time, the russellosaurines established themselves as the dominant mosasaur group, a position held for the next 15 million years.[33] They dispersed quickly from their North Atlantic cradle to across much of the globe so that by the early Coniacian stage around 88 mya, they achieved a widespread distribution stretching from the Russian North Pacific to the Angolan South Atlantic and Tethyan West Australia.
Diversity reached a low point during the Coniacian with few taxa comprised of generalists. While this coincides with the disappearance of transitional forms, it is likely that the period is also marred by sampling bias. After the Coniacian, however, ecomorphological diversification began to accelerate. The Santonian marked the appearance of more specialized russellosaurines like Ectenosaurus and Selmasaurus, while pre-existing genera like Tylosaurus continued to increase in body size. With the simultaneous decline of the giant shark Cretoxyrhina, mosasaurs were cemented as the undisputed dominant predators of their ecosystems. Cross et al. (2022) proposed that by that period, mosasaurs became their own main competitors, shaping their own evolution and spurring innovation into new niches in an arms race. An alternate hypothesis by Polcyn et al. (2014) suggests that the rise of mosasaur diversity should be instead attributed to physical drivers, namely a warmer climate and higher sea levels that opened vast epicontinental seas, and an expansion of coastal upwelling, though statistical test by Cross et al. (2022) found that physical processes played a minimal role for ecomorphological diversity. By the early Campanian stage around 83 mya, russellosaurines expanded into the South Pacific and Antarctica, achieving a global distribution.
A major disturbance occurred with an intercontinental extinction event from 80[34] to 77[35] mya known as the Niobraran-Navesinkan turnover, also called the Middle Campanian Crisis. The causes of this event remains uncertain, although hypotheses range from global cooling and sea level fall[36][37] to a meteor impact.[38][39] During this period, a disproportionate number of mosasaur species went extinct in North America and Europe (with similar evidence of major community changes in South America and Pacific Ocean). In the northern Gulf of Mexico, the extinction rate was as high as 63.3%[40] and seemed to have affected mainly generalist and smaller taxa. Coinciding with this disturbance was the wider emergence of the mosasaurines. Previously restricted to a few small-sized taxa isolated to North America, they had already begun to diversify, evolve larger sizes, and cross the previously-inaccessible Atlantic into Europe during the early Campanian. In the wake of the Niobraran-Navesinkan turnover, they replaced communities previously dominated by generalist russellosaurines, and the global mosasaurid composition shifted from a russellosaurine-dominated to a mosasaurine-dominated regime.
MacLaren et al. (2022) observed that global ecomorphological disparity (γ-disparity) increased across the early to late Campanian transition. This was likely driven by the extinction of generalist russellosaurines, but accelerated diversification due to increased isolation of local populations amidst oceanographic changes such as regression of the WIS during the Laramide Orogeny may have also contributed.
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.[24] 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 meters (9.8 ft) in body length each.[20][25] The group evolved rapidly; evidence of fully paddle-limbed plioplatecarpines such as Angolasaurus debuted in the WIS fossil record by 92 mya,[27][17] while by 91-90 mya tylosaurines like Tylosaurus already achieved large body sizes and specialized projectile-like snouts,[24][17][41] and around the same time Yaguarasaurus radiated into South America with body lengths exceeding 5 meters (16 ft).[42] By the early Coniacian stage around 88 mya, russellosaurines achieved a trans-Atlantic dispersal[25] stretching from present-day England[43] to Angola.[44] Mosasaurine mosasaurs later emerged in North America, with Clidastes-like forms appearing in the WIS[45] around 86.4 mya.[29][30] The group remained entirely endemic to the continent until the early Campanian stage.[45] The halisaurines also appeared in fossil record around the same time as the mosasaurines through Eonatator in the WIS between 87-78.5 mya.[25]
The following cladogram synthesizes multiple phylogenetic studies to represent nearly all valid genera, excluding aigialosaur taxa.
[46].
Mosasaurs attained a wide range of sizes. They can be grouped into three size classes of small (1–4 meters (3.3–13.1 ft), medium (4.1–8 meters (13–26 ft)), and large (>8 meters (26 ft)) species.[50][51] The smallest known mosasaur was Carinodens at about 2.6 meters (8.5 ft) long.[52] 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 meters (39–52 ft).[53][54] Its M. hoffmannii counterpart is the "Penza" specimen (CCMGE 10/2469), whose total length was traditionally estimated to be 17.1 meters (56 ft).[55] However, the latter has been considered an overestimate,[56] and a 2014 study suggested body proportions[57] that would indicate a length closer to 12 meters (39 ft). Some fragmentary bones from both species suggest even larger sizes exceeding 14 meters (46 ft).[54][58] Everhart speculated that it would have been possible for some very old individuals to grow up to 20 meters (66 ft), though stressed the lack of fossil evidence.[58]
Paleontologists compared mosasaurs to a Komodo dragon with flippers.[59][60][61] 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[62] with both pairs of limbs reduced and flattened into paddle-like flippers.[63] The tail was flattened and tapered off into a downward curve that supported a two-lobed fin resembling an upside-down shark's tail.[64][65] These features provided a superficial similarity to other aquatic tetrapods such as basal ichthyosaurs, marine crocodiles, and archaeocete whales through convergent evolution.[62] 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.[66]
The generic shape of the mosasaur skull is that of a long blunt cone compressed at the lateral sides with a corner at the base removed by a transection perpendicular to it. Structurally, it is composed of nine independent functional units of either individual bone or tightly articulated groups of bones--seven in the upper skull and two in the lower jaws--that each articulate with neighboring units through joints (except for the stapes). As lizards, the mosasaur skull retains the basic anatomical configuration of the group. It retains the primitive diapsid condition, where two large holes in the temporal region behind the eyes on each side of the skull are present. The upper hole, the supratemporal fenestra, is completely encircled by bone and served as the insertion point for muscles that close the lower jaw. The lower hole, or infratemporal opening, in contrast has an expansive opening along the cheek, where what used to be the quadratojugal bone in ancestral diapsids was replaced by a ligament (quadromaxillary ligament[12]) in squamates. This is a derived condition that may have developed to increase cranial mobility in ancestral representatives. The morphology of mosasaur skulls most closely resembles that of Varanus among extant reptiles, and so the genus are often used by paleontologists as an analogue in anatomical and phylogenetic studies.
The muzzle forms the largest functional unit. It holds the teeth-bearing upper jaws, the bones encircling the external nares and orbits (eye sockets), and two long bones (postorbitofrontal and squamosal) that extend posteriorly to separate the supratemporal fenestrae and infratemporal openings. The muzzle extends nearly the whole skull length, but makes up the near-entirety of only the anterior half of the upper skull or more. The upper jaw bones are the maxilla, which bears eight to seventeen or more teeth per side depending on the species, and premaxilla, which articulates anterior to the maxilla and always bears two teeth per side. The premaxilla also forms the bony rostrum, which in some groups like the tylosaurines elongates anterior to the frontmost teeth, and a thin long extension called the premaxillary bar[g] that runs along the midline at the top of the skull to articulate with the frontal bone at the skull roof. Both bones also support a long internal network of trigeminal nerves with terminal branches exiting as foramina randomly scattered along the ventral margins above the gum line. The premaxillary bar and upper edges of the maxilla also form most the borders of the external nares; the opening is also bordered at the posterior end by the frontal and, in some genera, prefrontal bones. Both bones also form part of the skull roof. The frontal is a large solidly-built plate roughly shaped like an isosceles triangle with the apex facing the anterior end and base articulating with the parietal functional unit. The anterior sections of the legs articulate with the prefrontals. The posterior sections of the frontal's legs also articulate with the postorbitofrontals, which in turn posteriorly articulate with the squamosals. The orbits, which are large and occur just behind the anteroposterior midline of the skull, are bordered by the maxilla, prefrontal, postorbitofrontal, and frontal (except in some genera, where the prefrontal and postorbitofrontals articulate instead) bones at the dorsal and anterior edges and L-shaped jugal bones, whose ends articulate with the maxillae and postorbitofrontals. The posterior corner of the intersection between both end of the jugal formed the attachment point for the quadromaxillary ligament, which in some species shows a distinct serif-like extension. In some genera, the prefrontal possess a distinct brow ridge along its border with the orbit. ~~
that in turn articulate with neighboring functional units through joints. The upper skull holds seven of these units. The largest is the muzzle, which holds the teeth-bearing upper jaws and bones encircling the nasal cavities and orbits (eye sockets), and makes up the entire front half of the upper skull or more. At the rear ends, the muzzle articulates with three other function units. Towards the top is the parietal unit, consisting of only the parietal bone, which alongside the muzzle's frontal bone forms the skull roof. Below articulates the basal unit, which includes the pterygoids and ectopterygoids. This unit is capable of independent mobility to varying degrees depending on the species and bears teeth. In-between is the occipital unit, which includes the many bones that form the braincase. The basal unit is also connected to the epipterygoid unit, a thin elongated bone that links the pterygoid center to connective tissues between the parietal and occipital units near the skull roof. The rear ends of the occipital unit articulates with the quadrate unit, which is a robust bone of either rectangular, oval, or fishhook shape that articulates with the lower below below and holds a dual function of mediating the opening and closing of the jaws and sheltering the ears. The stapes unit, a small bone located between the occipital and quadrate units, is not articulated with any other bone and functions in transmitting sound vibrations from the eardrum to brain.
quadratomaxillary ligament: https://pmc.ncbi.nlm.nih.gov/articles/PMC6956442/
Main difference between varanus and mosasaur skull is that the mosasur skulls muzzle unit is much longer
Another major feature is the loss of cranial kinesis. Most squamates, including aigialosaurs[67], are capable of rotating parts of their skull via three intracranial movements: cranium rotation at a parietal-supraoccipital joint (metakinesis), upper jaw rotation at a parietal-frontal joint (mesokinesis), and horizontal swinging of the lower jaws via joints at both ends of the quadrate (streptostyly). Primitive mosasaurs including Eonatator and Clidastes were also capable of these movements to a degree.[68] But in derived mosasaurs, the joints are immobilized via extensive suturing or overlap of connected bones, yielding a rigid akinetic skull.[69] It was previously hypothesized that the shift reflected adaptation towards handling larger prey or greater bite forces,[70] but the presence of akinesis in weaker-biting mosasaurs such as Plotosaurus and Plioplatecarpus has cast doubt on this.[71] Another hypothesis instead holds that akinesis is a universal adaptation for feeding in a viscous medium like water; namely by simplifying jaw movement to overcome the fluid mechanical problem of water filled inside an open mouth expelling when the mouth is closed, which can deflect any captured prey with it.[72] Some advanced mosasaurs nevertheless appeared to remain capable of kinesis around a joint between the angular and splenial at the lower jaw (intramandibular kinesis), which may have assisted in suction feeding.[73]
The mosasaur brain was mostly encased within a bony cavity (endocranium) formed by the braincase and skull roof. This has allowed reconstructions of what the brain may have roughly appeared like through endocasts from three-dimensionally preserved skulls. A partial rubber cast of the Platecarpus endocranium by Camp (1942) showed a softly S-shaped build with organizations similar to that of a juvenile Nile monitor. However, it has been suggested this reconstruction could be biased as Camp created the endocast by assembling dissociated skull bones which may not represent the condition in life. Endocasts from CT scanning of two Tethysaurus skulls by Allemand et al. (2024) found a more unique morphology. It was morphometrically most similar to snakes compared to anguimorphs (including monitor lizards) and iguanians but also exhibited a unique combination of features characteristic of all three groups. The overall build was narrow to a degree only seen in some snakes and was not as curved as in Platecarpus. The forebrain's olfactory bulb and peduncle, which control smell, matched the long and slender form of the iguanians and some anguimorphs (including monitor lizards) unlike the short structures in snakes. The midbrain's optic tectum, which coordinates motor responses to visual stimuli, was modified into a shape and position seen in anguimorphs but not iguanians or snakes.
All anguimorphs and snakes possess a forked tongue, so phylogenetic bracketing infers that mosasaurs likely had them too.
Like many lizards, mosasaurs possess a parietal eye on the skull roof's midline, evidenced by a prominent foramen located either on the parietal bone or between the frontal-parietal suture. The parietal eye is an photoreceptive organ used by extant ectothermic lizards to gather information about environmental light levels to mediate various physiological cycles such as thermoregulation and seasonal metabolism. But as endothermic animals can maintain a consistent metabolism without environmental light information, they have little need for such function. However, there is no evidence that the organ became vestigial or progressively disappeared in mosasaurs, as is shown across the fossil history of extant endotherms. Instead, they appeared to have remained functional or even specialized as seen in the enlarged organs in genera like Plioplatecarpus and Phosphorosaurus. Holmes (2024) noted that nearly all lizards that live in darkness (such as arboreal or nocturnal species) lost their parietal eyes, except for burrowing lizards where it is instead repurposed to modulate vertical movement from the underground to the surface. He hypothesized that mosasaurs may have adapted an analogous function, such as in monitoring light-correlated prey cycles like diel vertical migration or seasonal migrations across different foraging grounds.[74]
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