| Prognathodon | |
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| Scientific classification |
Prognathodon is a Mosasaur from late Cretaceous South Dakota and Colorado, USA, Belgium, New Zealand, Morocco, and the Netherlands.
Description[]
Prognathodon constitutes one of the largest-bodied mosasaur genera, with the largest known skull (belonging to P. currii) exceeding 140 cm 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. Though many species were large with sizes approaching or potentially exceeding 10 meters, such as P. currii, P. saturator and P. overtoni, many species were considerably smaller in size. The type species, P. solvayi, was the smallest, barely reaching 5 meters in length.
Other than its size and robust nature, another feature that defines Prognathodon is the form of the sclerotic rings, present in all species of the genus. The function of the scleral ossicles is to maintain the shape of the cornea and support the sclera in the region of Brucke's muscle responsible for affecting accommodation in the lacertilian eye. In the type species, P. solvayi, the sclerotic ring is only partially preserved, consisting of five scleral ossicles in each ring. Similar sclerotic rings are seen in several other mosasaur genera, such as Mosasaurus.
The latest published diagnosis for the genus Prognathodon was provided by Lingham-Soliar and Nolf (1989), and states that the premaxilla lacks a rostrum anterior to the premaxillary teeth. The prefrontal forms a large portion of the posterolateral border of the external nares and the supraorbital wing with heavy triangular ala contacts the postorbitofrontal posteriorly over the orbit medial to the external edge of the frontal. The frontal does not emarginate above the orbits and a median dorsal ridge is either present or absent. The foramina on the parietal are small to moderately large, located anteromedially on a small prominence and are closely embraced on either side by short tongues from the frontal or located on the frontoparietal suture.
The margins of the dorsal parietal surface are parallel to one another and the cranial midline to the posterior base of the diverging suspensorial rami, which forms a rectangular field medially on the parietal. The ventral process of the postorbitofrontal to jugal is indistinctly separated from the moderately well exposed dorsal surface of the postorbitofrontal and the ventroposterior process on the jugal is slightly developed to absent. The squamosal wing to the parietal is large. There is a deep groove present in the floor of the basioccipital for the basilar artery.
The suprastapedial process is fused to the infrastapedial process on the quadrate and the tympanic ala are thick. The stapedial pit is nearly circular to elliptical in form. The posterior process on the dorsal surface of the dentary is present, incipient or absent and the dentary terminates abruptly in front of the first dentary tooth. The dentary itself contains thirteen to fourteen teeth and the pterygoid has seven to eight teeth. The medial wing from the coronoid contacts the angular, the anterior process on the coronoid abrupts over the surangular and makes contact with the posterior process of the dentary or ends with the surangular without contacting the dentary. The retroarticular process is rectangular in outline, medially inflected or laterally lacing. The marginal teeth are stout, bicarinate and smooth or striated. Zygosphenes and zygantra are absent, incipient or large and functional.
History of discovery[]
Prognathodon is noted as having a very robust skull. The skull also shows adaptations towards a very powerful jaw musculature. The ratio between the length of the supratemporal fenestra and the total length of the skull has previously been used as an improvised measurement for mosasaur bite force, and is relatively high in Prognathodon (0.22 in P. overtoni and P. saturator) compared to other genera, such as Mosasaurus (0.19 in M. hoffmannii).
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. Stong jaw musculature combined with a relatively short and tall dentary would have resulted in a very powerful bite.
The skull of the type specimen of Prognathodon saturator is nearly complete, only lacking the anterior portion of the premaxilla and the dentaries. Though most of the anterior marginal teeth are missing, the inclination of the preserved roots suggest that P. saturator had procumbent teeth, a trait also seen in P. solvayi. The dorsal margin of the dentary is concave, whilst the ventral margin of the maxilla is slightly convex. The marginal teeth are massive, smooth and rounded as opposed to most other mosasaur teeth, which are typically facetted and laterally compressed. The mandible of P. saturator is very tall and massive, even more so than in other members of the genus. This is matched by the also massive pterygoid and various other portions of the skull, such as the temporal region and the braincase, which are all stout compared to other species in the genus.
The type specimen of P. lutugini, whilst incomplete, preserves a significantly large portion of the skull. The specimen was originally stated to have preserved a small portion of the premaxilla, though D.V. Grigoriev (2013) noted that said bone could at the moment not be located, and it is potentially missing. Both pterygoids were preserved, though the right one is at the moment almost entirely a restoration made of gypsum, with two original teeth. The left pterygoid was considerably more complete, but lacked all processes other than the basisphenoid process. The posterior alveolar margin was noted for being very small, with the teeth rising from a thin but pronounced vertical ridge. The ventral surface of the basisphenoid process is quite smooth and foramina are visible above the sixth tooth on the lateral surface of the pterygoid as well as above the position between the sixth and seventh teeth on the medial surface. The squamosal bone is only represented by a few fragments, but could be noted for being laterally compressed and tall, as in other species of Prognathodon. Its posteroventral surface is concave for contact with the quadrate. The dentaries are fused with the posterior end of the splenial and the anterior blade of the prearticular and have a tooth count of 13, with at least eight teeth possessing subdental crypts with some replacement teeth having been found in the type specimen. The subdental crypts are positioned postermodeially to the functional teeth. The coronoid is saddle-shaped and has a well-developed posterodorsal process, which gives the dorsal margin of said bone a nearly 110 degree angle between the horizontal anterior end and the subvertical posterior wing. This combination of characteristics aided in determining that P. lutugini was indeed a species of Prognathodon, since some researchers place it in a genus of its own, "Dollosaurus".
Paleobiology[]
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.
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.
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.
Dollo realized that Plioplatecarpus shared characters with the "microrhynchous" group in 1894 and abandoned his previous two family-system, starting to use only one family of mosasaurs, the Mosasauridae, and placing Prognathodon as closely related to Platecarpus and Plioplatecarpus.
Russell (1967) retained Prognathodon within the Plioplatecarpinae, but erected a tribe for the genus and the related Plesiotylosaurus, the Prognathodontini. He considered the mosasaurs within the Prognathodontini to "clearly be of plioplatecarpine derivation", but justified the tribe by that they can be differentiated from other plioplatecarpines by their massive jaws and robust teeth.
Gorden L. Bell Jr. conducted the first major phylogenetic analysis of mosasaurs in 1997, utilizing new methodologies and incorporating further taxa described since Russell's 1967 monograph (particularly basal mosasauroids, such as Aigialosaurus). Bell recovered Prognathodon within the Mosasaurinae, for the first time ever, as a close relative of the genera Globidens and Plesiotylosaurus. The tribe Prognathodontini was synonymized with the Globidensini, another tribe coined by Russell (1967) for Globidens. Bell was also the first to note that his analysis recovered Prognathodon, previously believed to be monophyletic, as paraphyletic.
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. Within Cau and Madzia's analysis (as well as previous work such as Simões et al., 2017), Prognathodon is recovered as a sister group to the Mosasaurini, and the clade Mosasaurini + Prognathodon itself was recovered as forming a sister group to the Globidensini
In the Media[]
- Prognathodon appeared in the movie Sea Rex 3D Journey To A Prehistoric World.
- Prognathodon appeared in the documentary March of the Dinosaurs(The Great Dinosaur Escape).
- Prognathodon appeared in the video game Jurassic World: The Game.
- Prognathodon appeared in Prehistoric Planet, where it is shown pursuing a Zalmoxes.
