North America saw the rise and fall of numerous members of Theropoda. Many held the role of apex predator at different points in time. This study focuses on the alpha predators during the Cretaceous (namely Acrocanthosaurus atokensis, Siats meekerorum, Lythronax argestes, and Tyrannosaurus rex) and how they superseded each other in the changing environment.
The end of the Jurassic saw the extinction of a number of large theropods (Ceratosaurus sp., Allosaurus sp., etc). However, it appears to have taken some time for a new large theropod to arise after the Jurassic period ended roughly 145 million years ago. The first fairly large theropod to appear in North America at the start of the Cretaceous was the large dromaeosaur Utahraptor ostrommaysorum. Adult specimens are estimated at 5-7 meters long and close to 450-700 kilograms. However, Utahraptor didn’t appear until some 126 million years ago. What filled this gap is unknown at the moment. Considering the success of allosauroids during the late Jurassic, it’s possible some, currently unknown, species of that group continued to hold the role of top predator as a relic population.
Reemergence of Giants
After Utahraptor came the arrival of the first truly giant theropod since the Jurassic. Appearing on the scene between 110-116 million years ago was the carcharodontosaur Acrocanthosaurus atokensis. Far larger than the dromaeosaurs that preceded it, Acrocanthosaurus stretched up to 11.5 meters in length and likely exceeded 4900 kilograms in weight. As with most members of the group, Acrocanthosaurus exhibits traits ideal for hunting giant prey (namely sauropods). Depending on the region, sauropods suitable for predation included: Astrodon johnstoni, Brontomerus mcintoshi, and possibly even Sauroposeidon proteles(though this species may have exceeded 28 meters and 45000 kilograms). Evidence for this exists in the form of footprints found in Texas that show what was likely an Acrocanthosaurus running alongside a large sauropod. Other contemporary animals include the ornithopod Tenontosaurus sp. and the dromaeosaur Deinonychus antirrhopus.
Evidence suggests Acrocanthosaurus was the sole giant predator specialized in hunting sauropods. This specialization may have contributed to the downfall of the species. Sauropods were becoming increasingly rare. With the exception of species like Abydosaurus mcintoshi around 104.5 million years ago, sauropods would be absent from the continent until the appearance of giant titanosaurs roughly 68 million years ago.
Rise of a New Killer
After the extinction of Acrocanthosaurus, the role of alpha predator was open. The related megraptorids were quick to produce their largest member, Siats meekerorum roughly 98.5 million years ago. The holotype specimen of this theropod was already comparable to Acrocanthosaurus in size(11.9 meters long and 3600 kilograms), despite being immature at the time of death. Adults may have exceeded 12 meters and 5000 kilograms, though there is no direct fossil evidence for this as of yet. The absence of sauropods on the continent meant Siats was forced to focus on different prey. Ornithopods like Protohadros byrdi and Eolambia caroljonesa would have made for suitable options. Less suitable were the armored ankylosaurs of the time, represented by species like Nodosaurus textilis and Animantarx ramaljonesi.
The reign of Siats did not last forever though. New theropods, better adapted at hunting ornithopods and ankylosaurs, were on the rise. The introduction of ceratopsians also ushered in a new era of dominant herbivores. Siats was less equipped than the competition at tackling such prey. As a result, it was outcompeted and rendered obsolete.
Originating in Asia and spreading across the northern hemisphere, the tyrannosauroids were quickly establishing dominance throughout their range. As carnosaurs like Siats continued to decline, tyrannosauroids grew in both size and number. The time immediately following Siats is a bit of a mystery. However, the recently discovered Timurlengia euotica from Uzbekistan gives insight into the rise of tyrannosauroids. Though still fairly small(~3-4 meters), Timurlengia exhibits an increase in brain size and sophistication. This particular species lived around 90 million years ago. It seems likely a North American form arose around the same time and represented the first steps being taken by tyrannosauroids to take on the role of apex predator. By 80 million years ago, North American variants were large enough to officially dethrone the local carnosaurs. One of the earliest large species was Lythronax argestes. Estimated at almost 8 meters long and 2500 kilograms, Lythronax was a prime example of size increase in tyrannosaurs. Better equipped to hunt well armed and sophisticated prey, Lythronax and its kind were perfect for the role of apex predator.
Over the next 10 million years, tyrannosaurs continued to spread and increase in size. Two lines represented the apex predators in western North America(more primitive forms persisted along the east coast). One was the slender, long legged albertosaurinae. The other, the more robust tyrannosaurinae. Over time, the tyrannosaurinae began replacing their albertosaurinae cousins. Eventually, the tyrannosaurinae culminated in the largest, most advanced member of the group. Evidence suggests this one species may have outcompeted all other tyrannosaurs, placing it squarely at the top of the food chain.
King of the Dinosaurs
Roughly 68 million years ago, a new predator arrived on the scene. The origins of this carnivore are still debated, with possible roots in North America and Asia. The most recent analysis suggests an Asian origin. Whatever the case, Tyrannosaurus rex quickly established its dominance in western North America. Finds indicate a range stretching as far north as Canada and as far south as Texas. Adults could exceed 12 meters in length and weighed in at over 7000 kilograms, arguably the heaviest theropod ever known. Tyrannosaurus had little competition. The next largest carnivore in the area was the large dromaeosaur Dakotaraptor steini. Though large for a dromaeosaur, Dakotaraptor still didn’t exceed 5.5 meters in length around 300 kilograms in weight. This is roughly half the overall length of an adult Tyrannosaurus.
Numerous species of potential prey existed at the time for Tyrannosaurus. Ceratopsians included Triceratops sp., Torosaurus sp., and Eotriceratops xerinsularis. Hadrosaurs were also present in the forms of Edmontosaurus sp., Hypacrosaurus sp., and others. Various kinds of ankylosaurs lived alongside Tyrannosaurus as well, though their armor would’ve made attack fairly difficult. The largest animal in the environment was Alamosaurus sanjuanensis, the first sauropod to appear since Abydosaurus. However, since adults could reach nearly 30 meters and exceed 67 tons in weight, large individuals were probably safe from attack(though juveniles and injured specimens were likely fair game). Ornithomimids, pachycephalosaurs and small ornithopods likely represented potential prey for juvenile tyrannosaurs.
Extinction of Giants
Around 65.5 million years ago, the reign of giant theropods was cut short due to an invader from space. Had the asteroid not struck, these efficient and adaptable carnivores likely would have continued to diversify and rule the continent of North America. The tyrannosaurs seem best equipped for this, but as the fossil record shows, extinction can topple even the greatest of dynasties.
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