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Description
Probably the chart people will actually find interesting of the Maastricht formation because mosasaurs are big and lizards and that probably appeals more to my average watcher than a few random fishes.

Mosasaurs were clearly the apex predators of the Maastricht sea 66 million years ago, with several highly adapted species patrolling the waters, with differing niches and habitat preferences to reduce intraspecific competition, though competition would have surely been present, as that is inevitable with so many large aquatic predators. The closest thing to competing with mosasaurs for an apex predatory title in the Maastricht sea were elasmosaurs, reaching body lengths of ~7.1 m. The elasmosaurs failed pretty miserably in this judging by the fact hadrosaur remains are several times more common than elasmosaur remains in this marine formation. So yeah elasmosaurs were only a sporadic faunal component, not very significant here. (See here the elasmosaur chart www.deviantart.com/paleosir/ar… )
Fairly large predatory sharks also occured in these waters, with the largest ones either being Archaeolamna kopingensis or Squalicorax pristodontus.
A.kopingensis is hard to scale due to no close relatives having complete fossils, but it must be somewhere between 3.4 and 4.somemediumdecimal m long. S.pristodontus was typically up to 3 m long but may have reached 4.8 m. Even a 4.8 m shark would pale in comparison to the large predatory mosasaurs.

Mosasaurus hoffmannii;
By far the most popular of the mosasaurs in general and by extension also of those from Maastricht due to it being the first named one, the namesake of the group and easily among the largest (probably the largest, with some *). The holotype is nearly 11 m long through the centra, making it a respectably sized animal. Larger referred specimens exist, most famously the ´Penza specimen` with its 171 cm long lower jaw, indicating a length of 12.9 m. The strong jaws with ziphodont teeth indicate it was hunting relatively large prey, so it probably consumed other mosasaurs regularly, even those of its own species. Sharks were probably a more important source of food because they were more common and generally smaller. The small eyes and relatively poorly developed balance organs indicate M.hoffmannii did not move in very complicated manners and hunted close to the surface during the day.
The Penza specimen has some traits such as swollen tooth bases that may prove to the phylogenetically significant, so it may not be a M.hoffmannii. but instead a large ziphodont ´Prognathodon` (aka a Liodon-morph thingy) More on that later.
Despite being a very large animal in a shallow sea, it was still one of the most common mosasaurs in the Maastricht formation.

Mosasaurus lemonnieri:
A closely related species, M.lemonnieri, is highly distinctive from M.hoffmannii and given its anatomy likely much closer to Plotosaurus. Unlike hoffmannii it has a proportionally tiny skull, resulting in a body length rivalling the king of mosasaurs, despite having a lower jaw a mere 120 cm long.
The teeth are more conical and more recurved than in M.hoffmannii, indicating it was eating slippery prey that had to be restrained and not necessarily cut up into more pieces. The pterygoidal teeth are more pronounced as well, indicating it capitalized more on ratchet feeding where it ´walked´ its jaws over medium sized prey. The longer tail and much longer fins with a higher aspect ratio, perfect for fast pursuit.
It is not very common in the Maastricht formation, but absolutely dominates a certain Belgian marine layer for some reason, where it was insanely succesful.

Cf. Prognathodon saturator
Two referred P.saturator specimens have been described from the Maastricht formation, a partial quadrate and a mandibular ramus. The quadrate has features that are seen only in P.saturator but it is not a perfect fit, so I would not be surprised it it was some closely related but different species (possibly a chronospecies). The quadrate specimen is smaller than the holotype, with a skull ~107 cm long in my scaling.
The ramus is interesting because it looks very M.hoffmannii-like, but has features that are congruent more with Prognathodon. I see a pattern here, same as with Penza....what if it is neither hoffmannii nor saturator. We´ll see. It is also massive just like the Penza specimen. In fact it is the same size as the Penza specimen roughly, or even a few % larger.
P.saturator would have been an offshore predator that likely preferred deeper waters than M.hoffmannii , and certainly different, tougher prey.

´´Liodon`` sectorius:
L.sectorius is a rare species of mosasaur that also possibly preferred deeper water than hoffmannii and Plioplatecarpus, and it was not common in the Maastricht formation (only represented by a dentary and a few isolated teeth - only more common than Cf. P.saturator). The Maastricht material did not come from very gigantic specimens, capping at around 7 m TL. A possible referred and undescribed L.sectorius from the underlying Gulpen fm reaches a length of around 10 m possibly, when going by press reports on skull size (~125 cm).
L.sectorius probably preferred softer prey than P.saturator and smaller prey than M.hoffmannii, probably quite generalized.

Carinodens belgicus:
The smallest mosasaur, and a very specialized one at that. This is a rare species but one with a very wide geographic distribution, basically spanning the Northern hemisphere. All material of it are just isolated teeth and dentaries. Nothing more. This small aquatic lizard had strong crushing dentition in the back of the mouth, and more gripping teeth in the front for manipulating food. Probably ate lots of small clams, crustaceans etc. but also fishes.
Only a few teeth and dentaries are known from the Maastricht formation.

Plioplatecarpus marshi:
One of the more common mosasaurs, roughly tied with M.hoffmannii in the Maastricht formation. It was a medium sized (~5.2 m) mosasaur that was highly adapted for fast and agile aquatic locomotion, with its massive tail and pectoral flippers as well as complex inner ear for terrific sense of movement and large partially forward facing eyes. Given its small skull (62 cm) with extremely recurved teeth it likely caught slippery, fast prey such as fishes and cephalopods among the seagrass beds and reefs of the formation.

special thanks to for making the P.marshi skeletal and credit to for the M.hoffmannii skeletal.

Description
Probably the chart people will actually find interesting of the Maastricht formation because mosasaurs are big and lizards and that probably appeals more to my average watcher than a few random fishes.

Mosasaurs were clearly the apex predators of the Maastricht sea 66 million years ago, with several highly adapted species patrolling the waters, with differing niches and habitat preferences to reduce intraspecific competition, though competition would have surely been present, as that is inevitable with so many large aquatic predators. The closest thing to competing with mosasaurs for an apex predatory title in the Maastricht sea were elasmosaurs, reaching body lengths of ~7.1 m. The elasmosaurs failed pretty miserably in this judging by the fact hadrosaur remains are several times more common than elasmosaur remains in this marine formation. So yeah elasmosaurs were only a sporadic faunal component, not very significant here. (See here the elasmosaur chart www.deviantart.com/paleosir/ar… )
Fairly large predatory sharks also occured in these waters, with the largest ones either being Archaeolamna kopingensis or Squalicorax pristodontus.
A.kopingensis is hard to scale due to no close relatives having complete fossils, but it must be somewhere between 3.4 and 4.somemediumdecimal m long. S.pristodontus was typically up to 3 m long but may have reached 4.8 m. Even a 4.8 m shark would pale in comparison to the large predatory mosasaurs.

Mosasaurus hoffmannii;
By far the most popular of the mosasaurs in general and by extension also of those from Maastricht due to it being the first named one, the namesake of the group and easily among the largest (probably the largest, with some *). The holotype is nearly 11 m long through the centra, making it a respectably sized animal. Larger referred specimens exist, most famously the ´Penza specimen` with its 171 cm long lower jaw, indicating a length of 12.9 m. The strong jaws with ziphodont teeth indicate it was hunting relatively large prey, so it probably consumed other mosasaurs regularly, even those of its own species. Sharks were probably a more important source of food because they were more common and generally smaller. The small eyes and relatively poorly developed balance organs indicate M.hoffmannii did not move in very complicated manners and hunted close to the surface during the day.
The Penza specimen has some traits such as swollen tooth bases that may prove to the phylogenetically significant, so it may not be a M.hoffmannii. but instead a large ziphodont ´Prognathodon` (aka a Liodon-morph thingy) More on that later.
Despite being a very large animal in a shallow sea, it was still one of the most common mosasaurs in the Maastricht formation.

Mosasaurus lemonnieri:
A closely related species, M.lemonnieri, is highly distinctive from M.hoffmannii and given its anatomy likely much closer to Plotosaurus. Unlike hoffmannii it has a proportionally tiny skull, resulting in a body length rivalling the king of mosasaurs, despite having a lower jaw a mere 120 cm long.
The teeth are more conical and more recurved than in M.hoffmannii, indicating it was eating slippery prey that had to be restrained and not necessarily cut up into more pieces. The pterygoidal teeth are more pronounced as well, indicating it capitalized more on ratchet feeding where it ´walked´ its jaws over medium sized prey. The longer tail and much longer fins with a higher aspect ratio, perfect for fast pursuit.
It is not very common in the Maastricht formation, but absolutely dominates a certain Belgian marine layer for some reason, where it was insanely succesful.

Cf. Prognathodon saturator
Two referred P.saturator specimens have been described from the Maastricht formation, a partial quadrate and a mandibular ramus. The quadrate has features that are seen only in P.saturator but it is not a perfect fit, so I would not be surprised it it was some closely related but different species (possibly a chronospecies). The quadrate specimen is smaller than the holotype, with a skull ~107 cm long in my scaling.
The ramus is interesting because it looks very M.hoffmannii-like, but has features that are congruent more with Prognathodon. I see a pattern here, same as with Penza....what if it is neither hoffmannii nor saturator. We´ll see. It is also massive just like the Penza specimen. In fact it is the same size as the Penza specimen roughly, or even a few % larger.
P.saturator would have been an offshore predator that likely preferred deeper waters than M.hoffmannii , and certainly different, tougher prey.

´´Liodon`` sectorius:
L.sectorius is a rare species of mosasaur that also possibly preferred deeper water than hoffmannii and Plioplatecarpus, and it was not common in the Maastricht formation (only represented by a dentary and a few isolated teeth - only more common than Cf. P.saturator). The Maastricht material did not come from very gigantic specimens, capping at around 7 m TL. A possible referred and undescribed L.sectorius from the underlying Gulpen fm reaches a length of around 10 m possibly, when going by press reports on skull size (~125 cm).
L.sectorius probably preferred softer prey than P.saturator and smaller prey than M.hoffmannii, probably quite generalized.

Carinodens belgicus:
The smallest mosasaur, and a very specialized one at that. This is a rare species but one with a very wide geographic distribution, basically spanning the Northern hemisphere. All material of it are just isolated teeth and dentaries. Nothing more. This small aquatic lizard had strong crushing dentition in the back of the mouth, and more gripping teeth in the front for manipulating food. Probably ate lots of small clams, crustaceans etc. but also fishes.
Only a few teeth and dentaries are known from the Maastricht formation.

Plioplatecarpus marshi:
One of the more common mosasaurs, roughly tied with M.hoffmannii in the Maastricht formation. It was a medium sized (~5.2 m) mosasaur that was highly adapted for fast and agile aquatic locomotion, with its massive tail and pectoral flippers as well as complex inner ear for terrific sense of movement and large partially forward facing eyes. Given its small skull (62 cm) with extremely recurved teeth it likely caught slippery, fast prey such as fishes and cephalopods among the seagrass beds and reefs of the formation.

special thanks to for making the P.marshi skeletal and credit to for the M.hoffmannii skeletal.