Prognathodon

Prognathodon was first described by Louis Dollo in 1889 based on specimens gathered in Belgium. There is some confusion over the correct generic name for the taxon. Dollo first mentioned the taxon as "Prognathodon" in some preliminary notes and provided a provisional diagnosis, but replaced the name Prognathodon with "Prognathosaurus" and used Prognathosaurus in all of his subsequent papers mentioning the genus. The first later use of "Prognathodon" was by Dale A. Russell in a comprehensive monograph on North American mosasaurs in 1967, where the priority of Prognathodon was made apparent. Russell also revised the species assigned to Prognathodon from North America, but only briefly commented on the Belgian specimens. . A very large specimen found in Israel was for some time informally named "Oronosaurus", but eventually described as a new species of Prognathodon, P. currii. Two specimens of Prognathodon overtoni described in 2011 from the early late Campanian (c. 74.5 Ma) Bearpaw Formation in Alberta, Canada provided the first fully articulated skeletons of the genus. Detailed studies of these and previously discovered specimens allowed several characters to be established that distinguishes Prognathodon from closely related genera like Liodon and long-snouted mosasaurines. The preserved teeth and gut contents also allowed studies into the inferred paleoecology of the genus. Also described in 2009 by Kaddumi from the same locality was P. primus, based on an articulated upper skull, with the name referencing it being the first near-complete mosasaur skull from the Middle East. The specimen is nicknamed Carlo, after the ENCI worker who discovered it. An anatomical review of South African mosasaurs by Woolley et al. (2022) reclassified T. capensis, originally assigned to Tylosaurus and later to Taniwhasaurus, as a chimera of two different mosasaur genera (cf. Prognathodon and cf. Taniwhasaurus), but not identifiable at the species level. ==Description==

Prognathodon constitutes one of the largest-bodied mosasaur genera, with the largest known skull (belonging to P. currii) exceeding in length. Despite its massive size, remains of the genus are often fragmentary and incomplete. To date, very few specimens are known with articulated skulls and none with an entire skeleton. Considering this 2002 study, Schupl and colleagues estimated that P. saturator would have reached in length. . Though many species (such as P. currii, P. saturator and P. giganteus) were large with sizes approaching or potentially exceeding , P. hudae is estimated to have reached in length. P. sectorius would have approached the same size, about in length. Similar scleral rings are seen in several other mosasaur genera, such as Mosasaurus. In 1998, Kase and colleagues estimated that P. overtoni had an anterior bite force of and a posterior bite force of . The quadrates of Prognathodon, similarly to the genus Globidens, have fused suprastapedial and infrastapedial processes, which is possibly an adaptation to counteract the strong forces experienced by the bone during biting. Strong jaw musculature combined with a relatively short and tall dentary would have resulted in a very powerful bite. The tail fluke is clearly asymmetric. The lower fin lobe follows the caudal vertebrae and would have had a streamlined cross-section in life, based on the proportions of the axial skeleton and the other soft tissues. The upper fin lobe is unsupported by the skeleton and is preserved as a small, almost wing-like, structure above the last few caudal vertebrae. The shape of the tail fluke is similar to that of the carcharhinid sharks, though turned upside down, with a small upper lobe and large lower lobe. The proportions of the soft tissue structures and their relation to the skeletal elements of the specimen can be used to infer the shape and size of the fins in other species of Prognathodon and potentially in other mosasaur genera as well. In their description of the specimen, Lindgren et al. (2013) noted that the specimen is strangely small by Prognathodon standards compared to some of the larger specimens in the genus. Thus, it was assumed that ERMNH HFV 197 was a juvenile specimen. That tail fin would grow to account for the increased weight of bigger individuals is rather logical, and can be observed in other groups that possess tail fins, such as extant sharks and extinct ichthyosaurs. For this reason, it is likely that adult Prognathodon, particularly of the larger species, would have had larger tail fins relative to their body size. Lindgren et al. (2013) especially noted that the upper fin lobe likely would have grown to be proportionally larger in bigger individuals. == Classification and species ==

Modern phylogenetic analyses continually place Prognathodon within the Mosasaurinae subfamily, despite this Prognathodon has historically been seen as a genus sharing close relations with Platecarpus and the Plioplatecarpinae. Louis Dollo was one of the earliest researchers to work on mosasaur systematics, initially placing them as a distinct lizard suborder and dividing the group into two families, the Mosasauridae and the "Plioplatecarpidae". In this early taxonomy, the Mosasauridae contained the genera Clidastes, Mosasaurus, Platecarpus, Halisaurus and Tylosaurus and the Plioplatecarpidae was monotypic, only containing Plioplatecarpus. In 1890, following further mosasaur discoveries (including that of Prognathodon), Dollo revised his taxonomy, dividing the Mosasauridae into three groups. These groups were based on how developed the rostrum was on the premaxilla, the size of the suprastapedial process of the quadrate and if the haemal arches were fused to the centra of the caudal vertebrae. Prognathodon was placed alongside Platecarpus in a "microrhynchous" group. The two other groups were the "megarhynchous" (including Tylosaurus and Hainosaurus) and the "mesorhynchous" (including Mosasaurus and Clidastes) groups. The view of the relationships of the genus to other mosasaur genera has changed little since 1997, it is routinely recovered as within the Mosasaurinae as well as paraphyletic. Cau and Madzia (2017) noted that the inclusion of Prognathodon and Plesiotylosaurus within the Globidensini would suggest a closer relationship between the genera than the reality of the situation. Though Prognathodon and Plesiotylosaurus are routinely recovered as sister genera, Cau and Madzia (2017) did not resurrect the tribe Prognathodontini in their list of mosasaur clades and their preferred definitions, offering no comment as to why not. pointed out that robust and conical tooth crowns with blunt, serrated carinae and smooth enamel are routinely assigned to the genus, despite the generic type species, P. solvayi deviating from that description since it exhibits markedly labio-lingually compressed and gently facetted marginal teeth. Species seen as valid and within Prognathodon, as recovered by Simões et al. (2017), • Prognathodon solvayi Dollo, 1889 — Maastrichtian, Belgium and Spain, tentatively from the United States (North Carolina). • Prognathodon giganteus Dollo, 1904 — Maastrichtian, Belgium, Jordan and Syria. • Prognathodon lutugini Yakovlev, 1901 — Late Campanian, Ukraine. :Though not the largest, Prognathodon lutugini was a large species of Prognathodon at an estimated 8 meters in length. and may thus belong to P. lutugini as well. :Prognathodon lutugini differs from all other species of Prognathodon in its splenial-angular surface having distinct horizontal tongues and grooves, the shape of the vertebral condyle (being slightly depressed with a height to width ratio of 0.75) and the length proportions of the vertebrae (cervical vertebrae being almost equal in size to the longest vertebrae of the column). P. lutugini also differs from all other species except for P. solvayi by having 13 teeth in its dentary, though can be excluded from being sunked into P. solvayi due to its smooth enamel surface. :The absence of a dentary anterior projection differentiates P. lutugini from P. kianda, as well as P. lutugini having larger anterior pterygoid teeth. The lack of a medial striation on its tooth surfaces differentiates it from P. waiparaensis and P. solvayi. Additionally, the presence of carinae serrations on the teeth separates it from P. kianda and P. currii. The shape of the vertebral condyle (not being higher than they are wide) separates it from P. saturator and P. rapax. :P. lutugini was originally named as a species of Clidastes by Yakovlev in 1901 based on the type specimen CNIGR 818, an incomplete skull and skeleton. Considering the combination of functionary accessory articulations and large pterygoid teeth to be unique, Yakovlev erected a new genus in 1905, Dollosaurus, named in honor of the belgian paleontologist Louis Dollo. and Simões et al. (2017), recover said species as outside the genus Prognathodon, most of them thus potentially representing genera on their own. • Prognathodon kianda Schulp et al., 2008 — Maastrichtian, Angola. :P. kianda is known from the Maastrichtian of Angola and is unique in possessing a high marginal tooth count and relatively small pterygoid teeth. This species seemingly share close relations with the dubious genus Liodon and is regularly found to be outside of Prognathodon (and recovered as a far more basal mosasaurine) in most phylogenetic analyses. :P. overtoni can be distinguished from other species, including P. solvayi, by the smooth enamel of its teeth and their non-procumbant nature. Three species of the dubious genus Liodon (L. sectorius, L. compressidens and L. mosasauroides), two with a slender snout morphology, were assigned to Prognathodon in 2008 due to exhibiting similarity in the aspect ratios of their marginal dentition along the jaw margin to P. kianda. Palci et al. (2014) however suggested the possibility that these three species, and perhaps also the type species of Liodon (L. anceps) were closer to Mosasaurus than to Prognathodon and that Liodon should perhaps instead by synonymized with Mosasaurus (though no formal proposal was made). The species P. stadtmani was redescribed as the type species of its own genus, Gnathomortis, in 2020. == Paleobiology ==

Metabolism δ18O analysis of the enamel of a tooth attributed to cf. Prognathodon from South Africa suggests that Prognathodon maintained an internal body temperature higher than that of the coeval Squalicorax. Its internal body temperature was likely higher the surrounding seawater as well. Paleoecology The discovery of well-preserved specimens of Prognathodon overtoni in the Campanian Bearpaw Formation of Alberta, Canada allowed detailed studies of the gut contents (including fragments of a large and a small fish, a sea turtle and potentially a cephalopod) and dentition which allowed speculation into the ecology of Prognathodon. As with most mosasaurs, the teeth of these specimens are carinate, with the carinae aligned roughly parallel to the jaw. On unworn teeth, the apex is acute but blunt, and has fine, wavy, anastomosing ridges for as much as 25% of the crown height. Such ornamentation could potentially strengthen the teeth. The blunt tip and roughened surface suggest a tooth that was used for capturing fairly hard prey, and the presence of turtle bones as gut contents lends support to the hypothesis that Prognathodon was adapted to crush through hard-shelled prey. The teeth are, however, quite high relative to the size of the skull, which suggests that they were used for impaling prey rather than for crushing or grasping it. Many of the fully erupted teeth have crenulations on the carinae that produce a fine serration. The presence of serrated carinae would suggest that Prognathodon instead was an opportunistic predator comparable to modern killer whales, rather than particularly adapted to crush its prey. Such a predator can not only feed on very large vertebrate prey, but also feed upon a variety of other prey. However, P. overtoni teeth lack the pointed tip that is otherwise characteristic for opportunistic predators with "cutting" teeth. As such, the teeth of Prognathodon seemingly show adaptations not usually found together. ) and a typical "cutting" mosasaur tooth (right, Mosasaurus). Teeth of Prognathodon'' seem to contain characteristics of both. It is worth noting that P. overtoni displays heterodonty similar to other mosasaurines, such as Globidens and Carinodens. For instance, the anterior teeth are more incurved and slender than those posterior to them with a gradual change in shape along the tooth row. The anterior teeth have a ratio of crown length to basal crown width of 2.0 to 2.5, whereas teeth in the middle of the tooth row have ratios in the range of 1.7 to 2.0. These ratios are consistent with both mosasaurs with "cutting" and "crushing" teeth. Though robust, the teeth of Prognathodon are nowhere near as broad as those of typical "crushing"-teeth mosasaurs, such as Globidens. The posteriormost teeth are sharply curved and short and were thus unlikely to have been used for prey capture or food processing. Teeth on the maxilla and dentary of both examined specimens show considerable wear. Crown apices are unusually smooth and polished, this breakage and subsequent polishing is likely due to prolonged contact with food. The tooth breakage is not severe and nearly horizontal, which is unlike typical predators with "cutting" teeth. The teeth may have been robust enough to prevent extensive breaking, or perhaps the curvature limited it. Many teeth are worn uniformly, which suggests a third possibility; that it represents a gradual grinding down of the teeth as a result of handling food. Somewhat similar wear is found on teeth of Globidens schurmanni, known to have fed on inoceramid bivalves. It is clear that the wear on the teeth does not represent simple breakage, since that would result in different amounts of wear on different teeth. In contrast to the marginal teeth, the pterygoid teeth, though unusually large for a mosasaur, do not exhibit any wear. This suggests that the marginal and pterygoid teeth had different functions, perhaps the pterygoid teeth were used to grip the prey before swallowing it. The large anterior pterygoid teeth that characterize Prognathodon likely provided an effective grip on large food items, indicating that the genus was capable of swallowing prey in large pieces. One of the Alberta specimens, TMP 2007.034.0001, is the first Prognathodon specimen with preserved gut contents. These contents include the remains of a very large (1.6 meter) fish, a smaller fish, a sea turtle and the possible remains of a cephalopod. These prey items are quite different from one another and would normally be prey items for different niches of predators, but are together consistent with a large and opportunistic apex predator. Though seemingly able to feed on prey typical for "cutting"-teeth mosasaurs, such as the large fish, the robust teeth suggest that somewhat harder prey, such as the sea turtle, was also commonly devoured. Prognathodon overtoni, likely similar in ecology to other species of the genus, was thus likely an opportunistic predator capable of feeding upon nearly anything in the Western Interior Seaway. Taphonomy The type specimen of Prognathodon saturator preserved several details that are taphonomically interesting. The likely cause of death of the specimen was age or disease, due to marine predators large enough to kill something of its size being unknown from the Maastrichtian. The degree of articulation of the specimen suggests that the animal reached the sea floor moments after its death, where it was scavenged by sharks prior to being buried by the sediments. Evidence of shark scavenging include the finds of associated teeth of shark genera Squalicorax and Plicatoscyllium among the bones of the mosasaur. It is possible to exclude the possibility of the sharks having been eaten by the mosasaurs prior to its death (and thus representing stomach contents) due to them showing no evidence of having been attacked by stomach acids. The number of shark teeth (not to mention the consistent size and color of the teeth) is also too high to be attributed to background abundance. The skeleton itself also preserves a variety of bite marks, providing direct evidence of shark scavenging. Pathology A Prognathodon specimen known as NHMM 2012 (often nicknamed as "Carlo"), described by Bastiaans et al. in the journal Cretaceous Research in 2020, was discovered in the Netherlands near Maastricht, and was shown to have severe facial deformities including a devastating partial amputation of the premaxilla. The ''Prognathodon's injuries are believed to be the result of a fight with another mosasaur (likely another Prognathodon''). This specimen shows signs of regrowth of bone around the injury as well as subsequent infections that were ongoing at the time of the mosasaur's death, which may have been partially caused by the injuries inflicted. The specimen stands as one of the few clearly confirmable cases of intra-specific combat between mosasaurs. It also revealed a very atypical immune response to the infection. Unlike modern reptilians where solid fibrous masses are produced to contain infections, this mosasaur showed a much more mammal-like response, including liquid pus. This is one of the first cases in which immunological responses in fossil taxa have been studied in such detail and where extant relatives differ to extinct taxa. ==References==