Mosasaurus

Mosasaurus Temporal range: Ma[1][2][3][4] PreꞒ Ꞓ O S D C P T J K Pg N
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
Superfamily:	†Mosasauroidea
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[5][6] 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[5][7][8] 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[9][10] 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[11] Clidastes conodon Cope, 1881

Mosasaurus (

Discovery and identification

The first Mosasaurus fossil known to science was discovered in 1764 in a chalk quarry near Maastricht in the Netherlands in the form of a skull, which was initially identified as a whale.^[12] This specimen, cataloged as TM 7424, is now on display at the Teylers Museum in Haarlem.^[13] Later around 1780,^[a] the quarry produced a second skull that caught the attention of the physician Johann Leonard Hoffmann, who thought it was a crocodile. He contacted the prominent biologist Petrus Camper, and the skull gained international attention after Camper published a study identifying it as a whale.^[16][17][18] This 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 was exaggerated.^[14][18]

MNHN AC 9648, the second skull and holotype of M. hoffmannii, which was nicknamed the "great animal of Maastricht" (left) and Faujas' 1799 interpretation of its excavation (right)

After its seizure, the second skull was sent to the

In 1804, the Lewis and Clark Expedition discovered a now-lost fossil skeleton alongside the Missouri River, which was identified as a 45-foot (14 m) long fish.^[22] Richard Ellis speculated in 2003 that this may have been the earliest discovery of the second species M. missouriensis,^[23] although competing speculations exist.^[24] In 1818, a fossil from Monmouth County, New Jersey became the first North American specimen to be correctly recognized as a Mosasaurus by scientists of the time.^[c][25]

The type specimen of M. missouriensis was first described in 1834 by Richard Harlan based on a snout fragment found along the river's Big Bend.^[22] He coined the specific epithet and initially identified it as a species of Ichthyosaurus^[28] but later as an amphibian.^[29] The rest of the skull had been discovered earlier by a fur-trapper, and it eventually came under the possession of prince Maximilian of Weid-Neuwied between 1832 and 1834. The fossil was delivered to Georg August Goldfuss in Bonn for research, who published a study in 1845.^[30] The same year, Christian Erich Hermann von Meyer suspected that the skull and Harlan's snout were part of the same individual. This was confirmed in 2004.^[22]

The third species was described in 1881 from fragmentary fossils in

The fourth species M. lemonnieri was first detected by Camper Jr. based on fossils from his father's collections, which he discussed with Cuvier during their 1799 correspondence, but Cuvier rejected the idea of another Mosasaurus species.[16]^[34] This species was re-introduced to science and formally described in 1889 by Louis Dollo based on a skull recovered by Alfred Lemonnier from a phosphate quarry in Belgium. Dollo names the species in his honor.^[35][33] Further mining of the quarry in subsequent years uncovered many additional well-preserved fossils, including multiple partial skeletons which collectively represented nearly the entire skeleton of the species. They were described by Dollo in later papers.^[7][36] Despite being the best anatomically represented species, M. lemonnieri was largely ignored in scientific literature. Theagarten Lingham-Soliar suggested two reasons for this neglect. First, M. lemonnieri fossils are endemic to Belgium and the Netherlands, which despite the famous discovery of the M. hoffmannii holotype attracted little attention from mosasaur paleontologists. Second, the species was overshadowed by the more famous and history-rich type species.^[36]

M. lemonnieri is a controversial taxon, and there is debate on whether it is a distinct species or not.

Scientists during the early and mid-1800s initially imagined Mosasaurus as an amphibious marine reptile with webbed feet and limbs for walking. This was based on fossils like the M. missouriensis holotype, which indicated an elastic vertebral column that Goldfuss in 1845 saw as evidence of an ability to walk and interpretations of some phalanges as claws.^[30] In 1854, Hermann Schlegel proved how Mosasaurus actually had fully aquatic flippers. He clarified that earlier interpretations of claws were erroneous and demonstrated how the phalanges show no indication of muscle or tendon attachment, which would make walking impossible. They are also broad, flat, and form a paddle. Schlegel's hypothesis was largely ignored by contemporary scientists but became widely accepted by the 1870s when Othniel Charles Marsh and Cope uncovered more complete mosasaur remains in North America.^[16][43]

One of the earliest depictions of Mosasaurus in

Mosasaurus was a type of derived mosasaur, or a latecoming member with advanced evolutionary traits such as a fully aquatic lifestyle. As such, it had a streamlined body, an elongated tail ending with a downturn supporting 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.^[47][48][49]

Size

The type species, M. hoffmannii, is one of the largest marine reptiles known,^[50][46] though knowledge of its skeleton remains incomplete as it is mainly known from skulls.^[7] Russell (1967) wrote that the length of the jaw equalled one tenth of the body length in the species.^[38] Based on this ratio, Grigoriev (2014) used the largest lower jaw attributed to M. hoffmannii (CCMGE 10/2469, also known as the Penza specimen; measuring 171 centimeters (67 in) in length) to estimate a maximum length of 17.1 meters (56 ft).^[46] Using a smaller partial jaw (NHMM 009002) measuring 90 centimeters (35 in) and "reliably estimated at" 160 centimeters (63 in) when complete, Lingham-Soliar (1995) estimated a larger maximum length of 17.6 meters (58 ft) via the same ratio.^[d][50] No explicit justification for the 1:10 ratio was provided in Russell (1967),^[38] and it has been considered to be probably overestimated by Cleary et al. (2018).^[51] In 2014, Federico Fanti and colleagues alternatively argued that the total length of M. hoffmannii was more likely closer to seven times the length of the skull, which was based on a near-complete skeleton of the related species Prognathodon overtoni. The study estimated that an M. hoffmannii individual with a skull measuring more than 145 cm (57 in) would have been up to or more than 11 meters (36 ft) in length and weighed 10 metric tons (11 short tons) in body mass.^[52]

Isolated bones suggest some M. hoffmannii may have exceeded the lengths of the Penza specimen. One such bone is a quadrate (NHMM 003892) which is 150% larger than the average size, which Everhart and colleagues in 2016 reported can be extrapolated to scale an individual around 18 meters (59 ft) in length. It was not stated whether they applied Russell's 1967 ratio.^[53]

M. missouriensis and M. lemonnieri are smaller than M. hoffmannii but are known from more complete fossils. Based on measurements of various Belgian skeletons, Dollo estimated M. lemonnieri grew to around 7 to 10 meters (23 to 33 ft) in length.^[38][54] He also measured the dimensions of IRSNB 3119 and recorded that the skull constituted approximately one-eleventh of the whole body.^[54] Polcyn et al. (2014) estimated that M. missouriensis may have measured up to 8–9 meters (26–30 ft) in length.^[55][56] Street (2016) noted that large M. missouriensis individuals typically had skulls exceeding lengths of 1 meter (3.3 ft).^[7] A particular near-complete skeleton of M. missouriensis is reportedly measured at 6.5 meters (21 ft) in total length with a skull approaching 1 meter (3.3 ft) in length.^[57] Based on personal observations of various unpublished fossils from Morocco, Nathalie Bardet estimated that M. beaugei grew to a total length of 8–10 meters (26–33 ft), their skulls typically measuring around 1 meter (3.3 ft) in length.^[58] With a skull measuring around 97.7 centimeters (38.5 in) in length, M. conodon has been regarded as a small to medium-sized representative of the genus.^[11]

Skull

The skull of Mosasaurus is conical and tapers off to a short

Fossil skulls of M. conodon (top) and of M. lemonnieri (bottom)

The

The features of teeth in Mosasaurus vary across species, but unifying characteristics include a design specialized for cutting prey, highly prismatic surfaces (enamel circumference shaped by flat sides called prisms), and two opposite cutting edges.^{[11][42][60][61]} Mosasaurus teeth are large and robust except for those in M. conodon and M. lemonnieri, which instead have more slender teeth.^[11][42] The cutting edges of Mosasaurus differ by species. The cutting edges in M. hoffmannii and M. missouriensis are finely serrated,^[5][10] while in M. conodon and M. lemonnieri serrations do not exist.^[f][40] The cutting edges of M. beaugei are neither serrated nor smooth, but instead possess minute wrinkles known as crenulations.^[42] The number of prisms in Mosasaurus teeth can slightly vary between tooth types and general patterns differ between species^[g]—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.^[42]

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. On the lower jaw, only one type, the dentary teeth, were present. 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 teeth were largely consistent in size and shape with only minor differences throughout the jaws (homodont) except for the smaller pterygoid teeth.^{[9][11][42][62]} The number of teeth in the maxillae, pterygoids, and dentaries vary between species and sometimes even individuals—M. hoffmannii had fourteen to sixteen maxillary teeth, fourteen to fifteen dentary teeth, and eight pterygoid teeth;^[11][46][50] M. missouriensis had fourteen to fifteen maxillary teeth, fourteen to fifteen dentary teeth, and eight to nine pterygoid teeth;^[9][42][63] M. conodon had fourteen to fifteen maxillary teeth, sixteen to seventeen dentary teeth, and eight pterygoid teeth;^[11][42] M. lemonnieri had fifteen maxillary teeth, fourteen to seventeen dentary teeth, and eleven to twelve pterygoid teeth;^[36][11][42] and M. beaugei had twelve to thirteen maxillary teeth, fourteen to sixteen dentary teeth, and six or more pterygoid teeth.^[42] 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.^[62]

The dentition was thecodont (tooth roots deeply cemented within the jaw bone). Teeth were constantly shed through a process where the replacement tooth developed within the root of the original tooth and then pushed it out of the jaw.^[64] 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.^[h][65]

Postcranial skeleton

One of the most complete Mosasaurus skeletons in terms of vertebral representation (Mosasaurus sp.; SDSM 452)

The tail structure of Mosasaurus is similar to relatives like Prognathodon, in which soft tissue evidence for a two-lobed tail is known.

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

Classification

History of taxonomy

Because nomenclatural rules were not well-defined at the time, 19th century scientists did not give Mosasaurus a proper diagnosis during its initial descriptions, which led to ambiguity in how the genus is defined. This led Mosasaurus to become a wastebasket taxon containing as many as fifty different species. A 2017 study by Hallie Street and Michael Caldwell performed the first proper diagnosis and description of the M. hoffmannii holotype, which allowed a major taxonomic cleanup confirming 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.^[j][5] Street & Caldwell (2017) was derived from Street's 2016 doctoral thesis, which contained a phylogenetic study proposing 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 a synonym^[k] of M. hoffmannii.^[l][7]

Systematics and evolution

Mosasaurus is a squamate like monitor lizards and snakes, but scientists still debate which of the two is its closest living relative.

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). Relationships between mosasaurs and living squamates remain controversial as scientists still fiercely debate on whether the closest living relatives of mosasaurs are monitor lizards or snakes.^[48][69] Mosasaurus, along with mosasaur genera Eremiasaurus, Plotosaurus,^[70] and Moanasaurus^[m][72] traditionally form a tribe within the Mosasaurinae variously called Mosasaurini or Plotosaurini.^[38][70][73]

Phylogeny and evolution of the genus

One of the earliest relevant attempts at an evolutionary study of Mosasaurus was done by Russell in 1967.^[73] He proposed that Mosasaurus evolved from a Clidastes-like mosasaur, and diverged into two lineages, one giving rise to M. conodon and another siring a chronospecies sequence which contained in order of succession M. ivoensis, M. missouriensis, and M. maximus-hoffmanni.^[n][o][38] However, Russell used an early method of phylogenetics and did not use cladistics.^[73]

In 1997, Bell published the first cladistical study of North American mosasaurs. Incorporating 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),^[38] Bell also recovered Mosasaurus in a sister relationship with another group which included Globidens and Prognathodon, and M. maximus as a sister species to Plotosaurus. The latter rendered Mosasaurus paraphyletic (an unnatural grouping), but Bell (1997) nevertheless recognized Plotosaurus as a distinct genus.^[73]

Bell's study served as a precedent for later studies that mostly left the systematics of Mosasaurus unchanged,^[7][9] 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,^[70][71][74] with at least one study also recovering M. missouriensis to be the most basal species of the genus instead of M. conodon.^[75] In 2014, Konishi and colleagues expressed a number of concerns with the reliance on Bell's study. 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 species in the genus, were neglected, which affected phylogenetic results.^[7] 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.^[7][9] Third, there was still a lack of comparative studies of the skeletal anatomy of large mosasaurines at the time.^[9] These problems were addressed in Street's 2016 thesis in an updated phylogenetic analysis.^[7]

Conrad uniquely used only M. hoffmannii and M. lemonnieri in his 2008 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 in the same genus.^[76] However, the study used a method unorthodox to traditional phylogenetic studies on mosasaur species because its focus was on the relationships of entire squamate groups rather than mosasaur classification. As a result, some paleontologists caution that lower-order classification results from Conrad's 2008 study such as the specific placement of Mosasaurus may contain technical problems, making them inaccurate.^[74]

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).^[71] The cladogram on the right (Topology B) is modified from Street's 2016 doctoral thesis proposing a revision to the Mosasaurinae, with proposed new taxa and renamings in single quotations.^[7]

Paleobiology

Head musculature and mechanics

In 1995, Lingham-Soliar studied the head musculature of M. hoffmannii. Because soft tissue like muscles do not easily fossilize, reconstruction of the musculature was largely based on the structure of the skull, muscle scarring on the skull, and the musculature in extant monitor lizards.^[50]

In modern lizards, the mechanical build of the skull is characterized by a four-pivot geometric structure in the

Like all mosasaurs, the lower jaws of Mosasaurus could 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. But especially compared to those in M. lemonnieri, the pterygoid teeth in M. hoffmannii are relatively small, which indicates ratchet feeding was relatively unimportant to its hunting and feeding.[50]^[36] Rather, M. hoffmannii likely employed inertial feeding (in which the animal thrusts its head and neck backward to release a held prey item and immediately thrust the head and neck forward to close the jaws around the item^[77]) and used jaw adduction to assist in biting during prey seizure. The magnus adductor muscles, which attach to the lower jaws to the cranium and have a major role in biting function, are massive, indicating M. hoffmannii was capable of enormous 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 it would not have needed the strong magnus depressor muscles (jaw-opening muscles) seen in some plesiosaurs.^[50]

Mobility and thermoregulation

Mosasaurus swam using its tail. The swimming style was likely

The tissue structure of Mosasaurus' bones suggests it had a metabolic rate much higher than modern squamates and its resting metabolic rate was between that of the leatherback sea turtle and that of ichthyosaurs and plesiosaurs.^[79] Mosasaurus was likely endothermic and maintained a constant body temperature independent of the external environment. Although there is no direct evidence specific to the genus, studies on the biochemistry of related mosasaur genera such as Clidastes^[p] 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.^[81] This adaptation would have given several advantages to Mosasaurus, including increased stamina when foraging across larger areas and pursuing prey.^[82] It may have also been a factor that allowed Mosasaurus to thrive in the colder climates of locations such as Antarctica.^{[82][83][84][85]}

Sensory functions

Mosasaurus had relatively large

Brain casts made from fossils of Mosasaurus show that the olfactory bulb and vomeronasal organ, which both control the function of smell, are poorly developed and lack some structures in M. hoffmannii; this indicates the species had a poor sense of smell. In M. lemonnieri, these olfactory organs, although still small, are better developed and have some components lacking in M. hoffmannii. The lack of a strong sense of smell suggests that olfaction was not particularly important in Mosasaurus; instead, other senses like vision may have been more useful.^[50]

Feeding

Paleontologists generally agree that Mosasaurus was likely an active predator of a variety of marine animals.^[50][60] Fauna likely preyed upon by the genus include bony fish, sharks, cephalopods, birds, and marine reptiles such as other mosasaurs^[60] and turtles.^[50] It is unlikely Mosasaurus was a scavenger as it had a poor sense of smell. Mosasaurus was among the largest marine animals of its time,^[50] and with its large, robust cutting teeth, scientists believe larger members of the genus would have been able to handle virtually any animal.^[60] 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.^[50] 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.^[50] Chemical and structural data in the fossils of M. lemonnieri and M. conodon suggests they may have also hunted in deeper waters.^[87]

Carbon isotope studies on fossils of multiple M. hoffmannii individuals have found extremely low values of δ¹³C, the lowest in all mosasaurs for the largest individuals. Mosasaurs with lower δ¹³C values tended to occupy higher trophic levels, and one factor for this was 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.^[60]

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 with dismembered and punctured remains of a 1 meter (3.3 ft) long fish in its gut. This fish was much longer than the length of the mosasaur's skull, which measured 66 centimeters (26 in) in length, confirming that M. missouriensis

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 by a 2004 study by Kauffman concluded that the mosasaurs were either Mosasaurus or Platecarpus. The positioning of both bite marks are at the direction the nautiloid's head would have been facing, indicating it was incapable of escaping and was thus already sick or dead during the attacks; it is possible this phenomenon was from a parent mosasaur teaching its offspring about cephalopods as an alternate source of prey and how to hunt one. An alternate explanation postulates the bite marks as from one individual mosasaur that lightly bit the nautiloid at first, then proceeded to bite again with greater force. However, there are differences in tooth spacing between both bites which indicate different jaw sizes.^[88]

Behavior and paleopathology

Intraspecific combat

There is fossil evidence that Mosasaurus engaged in aggressive and lethal combat with others of its kind. One partial skeleton of M. conodon bears 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 piercing through the quadrate bone. No injuries on the fossil show signs of healing, suggesting that the mosasaur was killed by its attacker by a fatal blow in the skull.^[89] Likewise, an M. missouriensis skeleton has 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, implying survival of the incident.^[57] Takuya Konishi suggested an alternative cause of this example being head-biting behavior during courtship as seen in modern lizards.^[57][90]

Attacks by another Mosasaurus are a possible cause of physical pathologies in other skulls, but they could have instead arisen from other incidents like attempted biting on hard turtle shells. In 2004, Lingham-Soliar observed that if these injuries were indeed the result of an intraspecific attack, then there is a pattern of them concentrating in the skull region. Modern crocodiles commonly attack each other by grappling an opponent's head using their jaws, and Lingham-Soliar hypothesized that Mosasaurus employed similar head-grappling behavior during intraspecific combat. Many of the fossils with injuries possibly attributable to intraspecific combat are of juvenile or sub-adult Mosasaurus, leading to the possibility that attacks on smaller, weaker individuals may have been more common.^[91] However, the attacking mosasaurs of the M. conodon and M. missouriensis specimens were likely similar in size to the victims.^[57][89] In 2006, Schulp and colleagues speculated that Mosasaurus may have occasionally engaged in cannibalism as a result of intraspecific aggression.^[92]

Diseases

There are some M. hoffmannii jaws with evidence of infectious diseases as a result of physical injuries. Two examples include IRSNB R25 and IRSNB R27, both having fractures and other pathologies in their dentaries. 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. IRSNB R27 has two fractures: one had 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. Lingham-Soliar described this pit as resembling a tooth mark from a possible attacking mosasaur. 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. The dentaries ahead of the fractures in both specimens are in good condition, suggesting that the arteries and trigeminal nerves had not been damaged; if they were, those areas would have necrotized due to lack of blood. The dentaries' condition suggests that the species may have had an efficient process of immobilizing the fracture during healing, which helped prevent damage to vital blood vessels and nerves. This, along with signs of healing, indicates that the fractures were not imminently fatal.^[91]

In 2006, Schulp and colleagues published a study describing a quadrate of M. hoffmannii with multiple unnatural openings and an estimated 0.5 liters (0.13 U.S. gal) of tissue destroyed. This was likely a severe bone infection initiated by septic arthritis, which progressed to the point where a large portion of the quadrate was reduced to abscess. Extensive amounts of bone reparative tissue were also present, suggesting the infection and subsequent healing process may have progressed for a few months. This level of bone infection would have been tremendously painful and severely hampered the mosasaur's ability to use its jaws. The location of the infection may have also interfered with breathing. Considering how the individual was able to survive such conditions for an extended period of time, Schulp and colleagues speculated it switched to a foraging-type diet of soft-bodied prey like squid that could be swallowed whole to minimize jaw use. The cause of the infection remains unknown, but if it were a result of an intraspecific attack then it is possible one of the openings on the quadrate may have been the point of entry for an attacker's tooth from which the infection entered.^[92]

Avascular necrosis has been reported by many studies to be present in every examined specimen of M. lemonnieri and M. conodon.^[60][93][94] In examinations of M. conodon fossils from Alabama and New Jersey and M. lemonnieri fossils from Belgium, Rothschild and Martin in 2005 observed that the condition affected between 3-17% of the vertebrae in the mosasaurs' spines.^[93] Avascular necrosis is a common result of decompression illness; it involves bone damage caused by the formation of nitrogen bubbles from inhaled air decompressed during frequent deep-diving trips, or by intervals of repetitive diving and short breathing. This indicates that both Mosasaurus species may have either been habitual deep-divers or repetitive divers. Agnete Weinreich Carlsen considered it the simplest explanation that such conditions were a product of inadequate anatomical adaptation. Nevertheless, fossils of other mosasaurs with invariable avascular necrosis still exhibit substantial adaptations like eardrums that were well-protected from rapid changes in pressure.^[94]

Unnatural fusion of tail vertebrae has been documented in Mosasaurus, which occurs when the bones remodel themselves after damage from trauma or disease. A 2015 study by Rothschild and Everhart surveyed 15 Mosasaurus specimens from North America and Belgium and found cases of fused tail vertebrae in three of them.^[q] Two of these cases displayed irregular surface deformities around the fusion site caused by drainage of the vertebral sinuses, which is indicative of a bone infection. The causes of such infections are uncertain, but records of fused vertebrae in other mosasaurs suggest attacks by sharks and other predators as a possible candidate. The third case was determined to be caused by a form of arthritis based on the formation of smooth bridging between fused vertebrae.^[95]

Life history

It is likely that Mosasaurus was

Mosasaurus had a transatlantic distribution, with its fossils having been found in marine deposits on both sides of the Atlantic Ocean. These localities include the

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 extended 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. The two mosasaurs Mosasaurus and Prognathodon appear to have been the dominant taxa, being widespread and ecologically diversified throughout the seaway.^[101]

The northern Tethyan margin was located around the paleolatitudes of

The southern Tethyan margin was located along the equator between 20°N and 20°S, resulting in warmer tropical climates. Seabeds bordering the cratons in Africa and Arabia and extending to the Levant and Brazil provided vast shallow marine environments. These environments were 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'^[r][101] were also common mosasaurs alongside Globidens.^[101] 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.^[5][101] 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 the 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.^[101]

Western Interior Seaway

Many of the earliest fossils of Mosasaurus were found in

The biogeography of the region has been subdivided into two Interior Subprovinces characterized by different climates and faunal structures, and their borders are separated in 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.^[102] The fossil assemblages throughout these regions suggest a complete faunal turnover when M. missouriensis and M. conodon appeared at 79.5 Ma, indicating that the presence of Mosasaurus in the Western Interior Seaway had a profound impact on the restructuring of marine ecosystems.^[111] The faunal structure of both provinces was generally much more diverse prior to the appearance of Mosasaurus, during a faunal stage known as the Niobraran Age, than it was during the following Navesinkan Age.^{[111][102][112]}

In what is now Alabama within the Southern Interior Subprovince, most of the key genera including sharks like

Antarctica

Mosasaurus is known from late Maastrichtian deposits in the

López de Bertodano Formation in Seymour Island.^[84] Located within the polar circle at around 65°S,^[103] temperatures at medium to large water depths would have been around 6 °C (43 °F) on average, while sea surface temperatures may have dropped below freezing and sea ice may have formed at times.^[83][121] Mosasaurus appears to be the most diverse mosasaur in the Maastrichtian Antarctica. At least two species of Mosasaurus have been described, 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.^[84] M. sp. has also been described. However, it is possible that such specimens may actually represent Moanasaurus, although this depends on the outcome of a pending revision of the genus.^[37] At least four other mosasaur genera have been reported in Antarctica, including Plioplatecarpus, the mosasaurines Moanasaurus and Liodon,^[84] and Kaikaifilu. The validity of some of these genera is disputed as they are primarily based on isolated teeth.^[122] Prognathodon and Globidens are also expected to be present based on distribution trends of both genera, although conclusive fossils have yet to be found.^[84] Other Antarctic marine reptiles included elasmosaurid plesiosaurs like Aristonectes and another indeterminate elasmosaurid.^[123] The fish assemblage of the López de Bertodano Formation was dominated by Enchodus and ichthyodectiformes.^[124]

Habitat preference

Known fossils of Mosasaurus have typically been recovered from deposits representing nearshore habitats during the Cretaceous period, with some fossils coming from deeper-water deposits.^[87][125] Lingham-Soliar (1995) elaborated on this, finding that Maastrichtian deposits in the Netherlands with M. hoffmannii occurrences represented nearshore waters around 40–50 meters (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.^[50]

δ¹³C is also correlated with a marine animal's feeding habitat as isotope levels deplete when habitat is farther from the shoreline, so some scientists interpreted isotope levels as a proxy for habitat preference. Separate studies involving multiple Mosasaurus specimens have yielded consistently low δ¹³C levels of tooth enamel, indicating that Mosasaurus fed in more offshore or open waters. It has been pointed out how δ¹³C can be influenced by other factors in an animal's lifestyle, such as diet and diving behavior.^[87][125] To account for this, a 2014 study by T. Lynn Harrell Jr. and Alberto Perez-Huerta examined 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 deeper than 50 meters (160 ft).^[125]

Interspecific competition

niche partitioning

.

Mosasaurus lived alongside other large predatory mosasaurs also considered apex predators, most prominent among them being the tylosaurines and Prognathodon.^[50][60] Tylosaurus bernardi, the only surviving species of the genus during the Maastrichtian, measured up to 12.2 meters (40 ft) in length^[126] while the largest coexisting species of Prognathodon like P. saturator exceeded 12 meters (39 ft).^[60] These three mosasaurs preyed on similar animals such as marine reptiles.^[9][50][60]

A study published in 2013 by Schulp and colleagues 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 the Maastrichtian-age Maastricht Formation showed that while there was some convergence between certain specimens, the average δ¹³C values between the two species were on average 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 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.^[60]

Another case of presumed niche partitioning between Mosasaurus and Prognathodon from the Bearpaw Formation in Alberta was documented in a 2014 study by Konishi and colleagues. 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 helped maintain resource partitioning between the two mosasaurs.^[9]

Nevertheless, 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 T. bernardi was the only known coexisting animal likely capable of causing such damage, using its robust arrow-like elongated snout. 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.^[127]

Extinction

By the end of the Cretaceous, mosasaurs were at the height of their

Cretaceous-Paleogene extinction event which also wiped out the non-avian dinosaurs. Mosasaurus fossils have been found less than 15 meters (49 ft) below the boundary in the Maastricht Formation, the Davutlar Formation in Turkey, the Jagüel Formation in Argentina, Stevns Klint in Denmark, Seymour Island, and Missouri.^[130]

M. hoffmannii fossils have been found within the K-Pg boundary itself in southeastern Missouri between the

Chicxulub asteroid that catalyzed the K-Pg extinction event.^[128] As well as physical destruction, the impact also blocked out sunlight^[131] leading to a collapse of marine food webs.^[128] Any Mosasaurus surviving the immediate cataclysms by taking refuge in deeper waters would have died out due to starvation from a loss of prey.^[128]

One enigmatic occurrence of Mosasaurus sp. fossils is in the Hornerstown Formation, a deposit 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 Mosasaurus and its associated fauna survived the K-Pg extinction. According to one hypothesis, the fossils may have originated from an earlier Cretaceous deposit and were reworked into the Paleocene formation during its early deposition. Evidence of reworking typically comes from fossils worn down due to further erosion during their exposure at the time of redeposition. Many of the Mosasaurus fossils from the Main Fossiliferous Layer consist of isolated bones commonly abraded and worn, but the layer also yielded better-preserved Mosasaurus remains. Another explanation suggests the Main Fossiliferous Layer is a Maastrichtian time-averaged remanié deposit, which means it originated from a Cretaceous deposit with winnowed low-sediment conditions. A third hypothesis proposes that the layer is a lag deposit of Cretaceous sediments forced out by a strong impact by a tsunami, and what remained was subsequently refilled with Cenozoic fossils.^[2]

See also

• Plotosaurus
• Eremiasaurus
• Moanasaurus

Notes

1. ^ The exact year is not fully certain due to multiple contradicting claims. An examination of existing historical evidence by Pieters et al., (2012) suggested the most accurate date would be on or around 1780.^[14] More recently, Limburg newspapers reported in 2015 that Ernst Homburg discovered a Liège magazine issued in the October 1778 reporting in detail a recent discovery of the second skull.^[15]
2. ^ hoffmannii was the original spelling used by Mantell, ending with -ii.^[21] Later authors began to drop the final letter and spelled it as hoffmanni, as became the trend for specific epithets of similar structure in later years. Recent scientists argue that the special etymological makeup of hoffmannii cannot be subjected to International Code of Zoological Nomenclature Articles 32.5, 33.4, or 34, which would normally protect similar respellings. This makes hoffmannii the valid spelling, although hoffmanni continues to be incorrectly used by many authors.^[9]
3. ^ Because the genus Mosasaurus was not coined at the time, the original identifier, Samuel L. Mitchill, described the fossil as a lizard monster or saurian animal resembling the famous fossil reptile of Maestricht [sic]."^[25] Cuvier doubted whether the two specimens were related. The congeneric relationship was eventually confirmed by James Ellsworth De Kay in 1830,^[25] and the New Jersey fossil was named Mosasaurus dekayi in his honor.^[26] The taxon was declared a nomen dubium in 2005,^[2] and other fossils attributed to it were reidentified as M. hoffmannii.^[27]
4. ^ Lingham-Soliar may have misapplied the ratio. His calculations interpreted "body length" as the length of the postcranial body, not the total length of the animal as demonstrated in Russell (1967), This erroneously inflated the estimate by 10%.^[38][50]
5. ^ Also known as the internarial bar^[50]
6. ^ One specimen traditionally attributed to M. lemonnieri has serration-like features in its cutting edges. Scientists believe this specimen likely belongs to a different species.^[40]
7. ^ The number of prisms in M. conodon and number of lingual prisms in M. lemonnieri are uncertain.^[42]
8. ^ This study was conducted on only one tooth and may not represent the exact durations of dentinogenesis in all Mosasaurus teeth.^[65]
9. ^ The number of caudal vertebrae is not fully certain for M. conodon and M. hoffmannii. At least ten have been documented in M. conodon, while the count is completely unknown in M. hoffmannii.^[11]
10. ^ Street & Caldwell (2017) also included M. dekayi as a potentially valid species without addressing^[5] its dubious status.^[27]
11. ^ Street & Caldwell (2017) revised this assessment of M. beaugei and found it to be a distinct species based on additional anatomical distinctions.^[5]
12. ^ As the proposal remains restricted to a PhD thesis, it is defined as an unpublished work per Article 8 of the ICZN and therefore is not yet formally valid.^[67][68]
13. ^ Some studies such as Madzia & Cau (2017) also recover Prognathodon and Plesiotylosaurus within the Mosasaurini.^[71]
14. ^ M. maximus is a North American taxon Russell (1967) recognized as a distinct species.^[38] It is now generally recognized as a junior synonym of M. hoffmannii, although some scientists maintain the taxon is a distinct species.^[5][7]
15. ^ maximus-hoffmannii was the wording used in Russell (1967); this is in recognition of the belief of a close relationship between the two species.^[38]
16. ^ The 2018 MS thesis of Cyrus Green disputes the notion that Clidastes was an endotherm based on the skeletochronology of the genus, finding its growth rates to be too low to be endothermic and instead similar to ectotherms. The dissertation argued that the high body temperatures calculated in pro-endotherm studies were a result of gigantothermy. However, only four specimens were studied.^[80]
17. ^ Two of the 15 surveyed fossils were reported from the Niobrara Formation,^[95] a deposit that Mosasaurus was previously thought but is no longer recognized to be present in.^[7][38]
18. ^ A dubious taxon that may represent various mosasaurs such as Gavialimimus or Platecarpus somenensis^[106]

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External links

Wikimedia Commons has media related to Mosasaurus.

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