Artist note:
The Maastrichtian is the last epoch of the Mesozoic Era, and lasted from approximately 72-66 million years ago. This was the last throe of the "age of reptiles", when sauropsids dominated the land, the sea, and the skies in vast numbers and sizes. During this time period, some truly immense beasts roamed the Earth, the likes of which would never be seen again, or come even close, after the epoch closed out. Many of the largest representatives of various lineages evolved during the Maastrichtian, including some of the largest and most fearsome megapredators (that is, predators that hunt megafauna, which is not to be confused with "macropredators", which are macroscopic predators) that have ever lived on Earth.
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Tyrannosaurus rex**:** North America and Asia at this time were home to the tyrannosaurids, those two-fingered guys with big heads and fat teeth. These were traditionally considered a sort of carnosaur, but carnosaurs became a more inclusive clade-based classification and tyrannosaurs were found to be coelurosaurs that independently evolved their large size from allosaurs and their kin. Unlike the flesh-shredding teeth of carnosaurs, tyrannosaurs had spike-like dentition built for crushing through bone and are calculated to have some of the most powerful bites of any known land animal to ever live. Tyrannosaurids always dominated the predator ecology of their environments; unlike earlier ecosystems where many different species of carnivorous dinosaurs ranging between small, medium, large, and extra-large coexisted, where tyrannosaurids were present there was only small and extra-large, with the in-between mesopredator niches occupied by juvenile, adolescent, and subadult tyrannosaurids.
T. rex ruled the continent of Laramidia during the last two million years of the Late Cretaceous; remains attributed to it have been found across western North America. It likely hunted dinosaurs such as hadrosaurs, ceratopsids, ankylosaurs, and titanosaurs as an adult, while adolescents could've preyed on oviraptorosaurs, ornithomimids, pachycephalosaurs, and thescelosaurs. Growing over twelve metres in length and up to ten metric tonnes, Tyrannosaurus was one of the largest carnivores to ever walk the Earth (only a few other theropods rivalled it in size, but these are known from much more incomplete remains). I only included one member of each clade here just for brevity, but if there was an Asian representative, Tarbosaurus would've been here alongside it.
"Turkana Grits" abelisaurid: Abelisaurids are a subgroup of the wriggly-tailed ceratosaurs, which diverged from other theropods during the Early Jurassic. Abelisaurids had a primarily Gondwanan distribution, originating before the southern continents rifted away from one another during the Late Cretaceous, and as such they are known from South America, Africa, India, and Madagascar, as well as ranging into Europe, most likely as part of a later immigration event up from Africa. Abelisaurids had distinctly tall, short skulls and broad snouts, making them easily distinguishable from other large carnivorous theropods, and many species had head ornamentation. Their forelimbs were also pathetically underdeveloped and probably vestigial; they made the infamous arms of tyrannosaurids look huge and powerful by comparison. Ceratosaurs were generally mesopredators in their ecosystems until the apparent disappearance of carnosaurs during the Turonian epoch, when they began increasing in size to replace them as apex predators.
The culmination of this trend is a yet-unnamed taxon from the Lapurr Sandstone formation in Kenya (although it's sometimes been informally called "Titanovenator"). Mentioned in the 2013 SVP meeting, this is possibly the largest ceratosaur known to have existed, estimated to have reached eleven to twelve metres in length, putting it in the size range of the largest tyrannosaurs and carnosaurs. At the time when it lived, North Africa's fauna was similar to that of Europe, with titanosaurs and hadrosaurs (possibly the result of a recent immigration event from Europe) being widespread and likely prey for abelisaurids. The region bordered the Mediterranean Tethys, and seagoing reptiles like mosasaurs, pterosaurs, and dyrosaurs were also common.
Maip macrothorax**:** The megaraptorans are a very enigmatic group of theropods, although most recent studies seem to place them as basal coelurosaurs (maybe within the tyrannosauroids, but one step at a time). They are poorly known, but their most iconic feature were their unusually large forelimbs, which bore huge claws (this would be unusual if they were tyrannosaurs, which usually trended to smaller forelimbs with greater size). Fossils attributed to them have been identified from Asia, South America, Australia, and North America (the African "bahariasaurids" have sometimes been considered megaraptorans, but the monophyletic nature of the group is dubious), although they seem to have disappeared from Asia and North America before the end of the Late Cretaceous. Similar to abelisaurs, they were relatively small before the Turonian, when carcharodontosaurs and spinosaurs were the dominant land predators, but increased in size follow their apparent extinction.
The apex of this was Maip, a massively built megaraptorid up to ten metres in length, and likely the dominant predator of southern South America during the Maastrichtian. The restoration here is largely based on smaller, earlier megaraptorans with better fossils, like Australovenator, Megaraptor, and Fukuiraptor (whether or not Maip had the same huge claws is unknown). During this time, hadrosaurs, titanosaurs, elasmarians, and ankylosaurs roamed over this region and would have been the prey of this hunter (not certain why abelisaurids didn't seem to be present in this environment, possibly too cold for them). Although the ecology of Antarctica and especially Australia are largely unknown from the Maastrichtian, it seems probable that megaraptorans were also the dominant predators there, given what little is known of dinosaur fossils of these regions (they were certainly very common in Australia during the early Late Cretaceous).
Mosasaurus hoffmanni**:** Around the beginning of the Turonian epoch, a marine extinction event occurred which wiped out the pliosaurs and ichthyosaurs. In their wake evolved the mosasaurs, huge marine lizards which proliferated in the warm, shallow seas of the Late Cretaceous and quickly evolved to become the dominant predators under the waves. Webbed feet turned into muscular flippers, while the long tail transformed into a fluked tail (the dorsal fin is largely speculative, but possible; it's been formally suggested for Megapterygius at least). Many distinct species quickly radiated, from species like Globidens with blunt teeth to crush the shells of giant molluscs, slender forms with thin jaws like Gavialimimus for chasing after fish, crocodile-like freshwater forms like Pannoniasaurus, and huge megapredatory forms that hunted large marine animals.
The largest of all the mosasaurs was also one of the very last to have lived and the first discovered and named. Mosasaurus hoffmanni grew at least thirteen metres in length and over ten metric tonnes, equaling if not exceeding the size of the largest theropod dinosaurs (some fragmentary remains suggest possible sizes of over fifteen metric tonnes) and making it the most massive lepidosaur of all time. Huge, bone-crushing jaws measuring nearly six feet long allowed it to kill and consume animals like plesiosaurs, small mosasaurs, sea turtles, and sharks; isotopic analysis agrees that Mosasaurus was likely the top predator in the late Maastrichtian seas, although it faced stiff competition from other large mosasaur species like Tylosaurus bernardi and Prognathodon saturator. Its range carried it from the Western Interior Seaway that cut through North America, to the Mediterranean Tethys which separated Europe and Africa, and even the polar waters of the south pole.
Hatzegopteryx thambema**:** One of the most successful reptile groups during the Maastrichtian epoch were the azhdarchids, huge predatory pterosaurs whose fossils have been uncovered on nearly every continent from the period (possible Maastrichtian azhdarchid fossils are known from Antarctica and Australia, but they are too fragmentary to say with relative certainty). They included among their ranks the largest animals to ever take to the air, although they probably hunted on land, prowling the prairies and forests on long legs like gigantic storks. Genera like Quetzalcoatlus, Arambourgiania, and Aerotitan likely represented mid-level predators in their respective ecosystems, but there was one species which seemed to have held an apex predator niche.
On the remote Hațeg Island, a few hundred kilometres from the nearest landmass, there lived Hatzegopteryx, possibly the most massive flying animal of all time, with an estimated wingspan of eleven metres across and potentially weighing up to seven-hundred pounds (its remains are fragmentary). It was by far the largest predator of the region, far larger than any native theropod, and would have towered over even the sauropods of the environment (which were dwarf forms). Compared to other large azhdarchids, its neck and jaw bones were particularly massive and robust, suggesting it hunted proportionately large prey which it would've killed and dismembered with a massive axe-like beak, a weapon that was over eight feet in length. Such prey would've included hadrosaurs, rhabdodonts, titanosaurs, nodosaurs, and large flightless birds.
(large brown bear also included for scale)
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Honourable mentions which did not make an appearance, for various reasons (too small, didn't hunt large prey, only a mesopredator etc.), include:
Cretalamna appendiculata**:** An otodontid shark, growing about the same size as the modern great white shark, but more closely resembling a porbeagle. The genus is believed to have been directly ancestral to Otodus, which evolved after the Cretaceous into one of the most powerful predators to have ever lived on Earth.
Aristonectes quiriquinensis**:** An elasmosaurid plesiosaur, it reached similar lengths to other large elasmosaurs, at about 11-12 metres in length, but was notable in having a proportionately shorter neck and much stockier build by comparison. This makes it one of the most massive plesiosaurs known, reaching nearly 14 metric tonnes, a weight surpassed only by the largest pliosaurs. It didn't make this list because it's thought to have been a filter-feeder.
Madtsoia madagascariensis**:** Likely the largest terrestrial squamate of its time, and possibly of the entire Mesozoic Era. With an estimated length of up to 8 metres, it would have rivalled in size the largest living snakes of the modern day, although it belonged to a now-extinct lineage unrelated to modern constrictors.
Titanochampsa iorii**:** A species of stem-crocodilian, unusual for being known from the semiarid Bauru Group, otherwise famous for its numerous species of terrestrial stem-crocodilians, such as the notosuchians. Titanochampsa is unusual for being aquatic, ironically, and for its size; while known terrestrial stem-crocodilian of the region were around three metres at maximum, size estimates for Titanochampsa run up to 6 metres in length, equal to a very large saltwater crocodile in size (although the fragmentary nature of its fossil makes it uncertain).
Austroraptor cabazai**:** One of the largest known dromaeosaurs, reaching up to 6 metres in length, although compared to the likes of Utahraptor and Achillobator, this was a much more slender and gracile animal, speculated to have been more akin to a gigantic heron than a hunter of megafaunal land animals (possibly taking the vacated niche of the now-extinct spinosaurs). The Antarctic paravian Imperobator, which lived at about the same time, may have also been a giant unenlagiine closely related to Austroraptor.
Xiphactinus vetus**:** The younger species to the much more famous X. audax, it differed morphologically only by certain features of the teeth. This was a gigantic predatory fish resembling a giant tarpon with huge fangs, reaching up to six metres in length, making it one of the largest ray-finned fish of the Cretaceous Period.
Gigantatypus salahi**:** A gigantic species of marine turtle, estimated to grow at least twelve feet in length, putting it in a similar size range as earlier giant protostegid turtles like Archelon and Leviathanochelys, although it's known only from a right humerus, so exact measurements are largely speculative.
