Turonian-Campanian Turnover, 2020
During the late Cretaceous, the world saw a change in the Northern Hemisphere. Up until the Turonian, the apex predators were all primarily members of the family carcharodontosauridae. However, by the Campanian, something had changed. The role of alpha predator was squarely within the claws of the tyrannosauroids. Why this changing of the guard took place is currently unknown and took place at a time in Earth’s history that is rather poorly understood. This paper will explore one possible option; the extinction of sauropods on the northern continents. For the sake of argument, the megaraptorids and the neovenatorids will be grouped alongside the carcharodontosaurs, though their placement in the theropod family tree is up for debate. However, that is an argument for another time.
The fossil record has, for the moment, been rather unhelpful in revealing what the biodiversity was like between the Turonian and Campanian, a span of time that covers roughly 6 million years. Few diagnosable fossils have been found in rock layers that date to around 83 to 89 million years ago. What we do know is the last verified carcharodontosaurids and those closely related to them appeared to have died out by 92 million years ago on the northern continents. Some of the last known species include Siats meekerorum and Shaochilong maortuensis from North America and Asia respectively. Siats, one of the largest predators ever found in North America, appears to have gone extinct by 94 million years ago, with Shaochilong holding out until 2 million years later.
Meanwhile, their eventual successors, the tyrannosauroids were still small and had yet to reach the enormous sizes the group is known for. Some early forms such as Eotyrannus may have exceeded 5 meters on occasion. However these were rather uncommon and did not seem to reach any sizes large enough to truly compete with the carcharodontosaurids. Most species that lived at the time, like Moros intrepidus, Suskityrannus hazelae, and Timurlengia euotica, rarely grew more than 3-4 meters in length. One thing these animals did have up on the carcharodontosaurids were their brains. Adaptability and intelligence may have been a deciding factor when it came down to who would survive and who would go extinct.
Despite the scant fossil record for this time, one change in the food web for the northern hemisphere does seem to stand out. Sauropods, the dominant herbivores for most of the Jurassic and into the early Cretaceous, were on the decline north of the equator. In many cases, it appears the carcharodontosaurids were primarily feeding on the sauropods. However, in North America, there was a noticeable absence of sauropods for a portion of the Cretaceous. The last known species from the continent was Abydosaurus mcintosh from around 104 million years ago and Sonorasaurus thompsoni from between 93 and 112 million years ago. After that, there would be no more sauropods until Alamosaurus appeared at the end of the Cretaceous 36 million years later. In Asia, the fossil record appears to be a little more jumbled, though something similar may have occurred. Sonidosaurus lived somewhere between 80 and 95 million years ago, though the exact time is somewhat unknown. Quaesitosaurus likely lived later, between 70-85 million years ago. Until more fossils are found to prove otherwise, it would seem both North America and Asia experienced a decline and possible extinction in the sauropod populations at around the same time the carcharodontosaurids started to disappear from the Northern Hemisphere.
While the carcharodontosaurids were likely capable of hunting the ornithopods that started replacing the sauropods as the dominant herbivores, the tyrannosauroids appear to have been better equipped for the job. By 80-85 million years ago, species like Alectrosaurus in Asia and Lythronax in North America seem to indicate a change in apex predator. It didn’t take long for the tyrannosauroids to begin increasing in size after the change took place. In short, by around 85-90 million years ago, the sauropods of the northern continents were nearly extinct, leaving the carcharodontosaurids without their primary food source. The tyrannosauroids, with their larger brains and potentially higher adaptability, were poised to exploit the newfound opening by focusing on the faster, more socially complex ornithopods. By the time sauropods started to reappear or at least rebound, the carcharodontosaurids were already extinct.
Let it be known that all of this is largely speculative. The fossil record for this time frame is incredibly incomplete and leaves a lot of room for interpretation. Any number of factors could have contributed to the fall of the carcharodontosaurids on the northern continents. As with everything in paleontology, more fossils are needed.
References
Zanno, L. E.; Makovicky, P. J. (2013). "Neovenatorid theropods are apex predators in the Late Cretaceous of North America". Nature Communications.
Brusatte, S.; Benson, R.; Chure, D.; Xu, X.; Sullivan, C.; Hone, D. (2009). "The first definitive carcharodontosaurid (Dinosauria: Theropoda) from Asia and the delayed ascent of tyrannosaurids". Naturwissenschaften.
Ratkevich, R. (1998). "New Cretaceous brachiosaurid dinosaur, Sonorasaurus thompsoni gen. et sp. nov, from Arizona". Journal of the Arizona-Nevada Academy of Science.
Chure, D.; Britt, B.; Whitlock, J.A.; Wilson, J.A. (2010). "First complete sauropod dinosaur skull from the Cretaceous of the Americas and the evolution of sauropod dentition". Naturwissenschaften
Xu Xing; Zhang Xiaohong; Tan Qingei; Zhao Xijin; Tan Lin (2006). "A new titanosaurian sauropod from Late Cretaceous of Nei Mongol, China". Acta Geologica Sinica
Kurzanov, S. and Bannikov, A. (1983). "A new sauropod from the Upper Cretaceous of Mongolia". Paleontologicheskii Zhurnal
Mader, B. J.; Bradley, R. L. (1989). "A redescription and revised diagnosis of the syntypes of the Mongolian tyrannosaur Alectrosaurus olseni". Journal of Vertebrate Paleontology.
Zanno, L.E.; Loewen, M.A.; Farke, A.A.; Kim, G.-S.; Claessens, L.P. A.M.; McGarrity, C.T. (2013). "Late Cretaceous theropod dinosaurs of Southern Utah". In Titus, A.L.; Loewen, M.A. (eds.). At the Top of the Grand Staircase: The Late Cretaceous of Southern Utah. Bloomington: Indiana University Press
Zanno, Lindsay E.; Tucker, Ryan T.; Canoville, Aurore; Avrahami, Haviv M.; Gates, Terry A.; Makovicky, Peter J. (February 2019). "Diminutive fleet-footed tyrannosauroid narrows the 70-million-year gap in the North American fossil record". Communications Biology.
See Gradstein et al. (2004) for a detailed description of the ICS' timescale
Stephen L. Brusatte, Alexander Averianov, Hans-Dieter Sues, Amy Muir and Ian B. Butler (2016). "New tyrannosaur from the mid-Cretaceous of Uzbekistan clarifies evolution of giant body sizes and advanced senses in tyrant dinosaurs". Proceedings of the National Academy of Sciences of the United States of America.
Fowler, D. W.; Sullivan, R. M. (2011). "The First Giant Titanosaurian Sauropod from the Upper Cretaceous of North America". Acta Palaeontologica Polonica
Wolfe, D.G.; McDonald, A.T.; Kirkland, J.I.; Turner, A.H.; Smith, N.D.; Brusatte, S.L.; Loewen, M.A.; Denton, R.K.; Nesbitt, S.J. (6 May 2019). "A mid-Cretaceous tyrannosauroid and the origin of North American end-Cretaceous dinosaur assemblages" (PDF). Nature Ecology & Evolution.
The fossil record has, for the moment, been rather unhelpful in revealing what the biodiversity was like between the Turonian and Campanian, a span of time that covers roughly 6 million years. Few diagnosable fossils have been found in rock layers that date to around 83 to 89 million years ago. What we do know is the last verified carcharodontosaurids and those closely related to them appeared to have died out by 92 million years ago on the northern continents. Some of the last known species include Siats meekerorum and Shaochilong maortuensis from North America and Asia respectively. Siats, one of the largest predators ever found in North America, appears to have gone extinct by 94 million years ago, with Shaochilong holding out until 2 million years later.
Meanwhile, their eventual successors, the tyrannosauroids were still small and had yet to reach the enormous sizes the group is known for. Some early forms such as Eotyrannus may have exceeded 5 meters on occasion. However these were rather uncommon and did not seem to reach any sizes large enough to truly compete with the carcharodontosaurids. Most species that lived at the time, like Moros intrepidus, Suskityrannus hazelae, and Timurlengia euotica, rarely grew more than 3-4 meters in length. One thing these animals did have up on the carcharodontosaurids were their brains. Adaptability and intelligence may have been a deciding factor when it came down to who would survive and who would go extinct.
Despite the scant fossil record for this time, one change in the food web for the northern hemisphere does seem to stand out. Sauropods, the dominant herbivores for most of the Jurassic and into the early Cretaceous, were on the decline north of the equator. In many cases, it appears the carcharodontosaurids were primarily feeding on the sauropods. However, in North America, there was a noticeable absence of sauropods for a portion of the Cretaceous. The last known species from the continent was Abydosaurus mcintosh from around 104 million years ago and Sonorasaurus thompsoni from between 93 and 112 million years ago. After that, there would be no more sauropods until Alamosaurus appeared at the end of the Cretaceous 36 million years later. In Asia, the fossil record appears to be a little more jumbled, though something similar may have occurred. Sonidosaurus lived somewhere between 80 and 95 million years ago, though the exact time is somewhat unknown. Quaesitosaurus likely lived later, between 70-85 million years ago. Until more fossils are found to prove otherwise, it would seem both North America and Asia experienced a decline and possible extinction in the sauropod populations at around the same time the carcharodontosaurids started to disappear from the Northern Hemisphere.
While the carcharodontosaurids were likely capable of hunting the ornithopods that started replacing the sauropods as the dominant herbivores, the tyrannosauroids appear to have been better equipped for the job. By 80-85 million years ago, species like Alectrosaurus in Asia and Lythronax in North America seem to indicate a change in apex predator. It didn’t take long for the tyrannosauroids to begin increasing in size after the change took place. In short, by around 85-90 million years ago, the sauropods of the northern continents were nearly extinct, leaving the carcharodontosaurids without their primary food source. The tyrannosauroids, with their larger brains and potentially higher adaptability, were poised to exploit the newfound opening by focusing on the faster, more socially complex ornithopods. By the time sauropods started to reappear or at least rebound, the carcharodontosaurids were already extinct.
Let it be known that all of this is largely speculative. The fossil record for this time frame is incredibly incomplete and leaves a lot of room for interpretation. Any number of factors could have contributed to the fall of the carcharodontosaurids on the northern continents. As with everything in paleontology, more fossils are needed.
References
Zanno, L. E.; Makovicky, P. J. (2013). "Neovenatorid theropods are apex predators in the Late Cretaceous of North America". Nature Communications.
Brusatte, S.; Benson, R.; Chure, D.; Xu, X.; Sullivan, C.; Hone, D. (2009). "The first definitive carcharodontosaurid (Dinosauria: Theropoda) from Asia and the delayed ascent of tyrannosaurids". Naturwissenschaften.
Ratkevich, R. (1998). "New Cretaceous brachiosaurid dinosaur, Sonorasaurus thompsoni gen. et sp. nov, from Arizona". Journal of the Arizona-Nevada Academy of Science.
Chure, D.; Britt, B.; Whitlock, J.A.; Wilson, J.A. (2010). "First complete sauropod dinosaur skull from the Cretaceous of the Americas and the evolution of sauropod dentition". Naturwissenschaften
Xu Xing; Zhang Xiaohong; Tan Qingei; Zhao Xijin; Tan Lin (2006). "A new titanosaurian sauropod from Late Cretaceous of Nei Mongol, China". Acta Geologica Sinica
Kurzanov, S. and Bannikov, A. (1983). "A new sauropod from the Upper Cretaceous of Mongolia". Paleontologicheskii Zhurnal
Mader, B. J.; Bradley, R. L. (1989). "A redescription and revised diagnosis of the syntypes of the Mongolian tyrannosaur Alectrosaurus olseni". Journal of Vertebrate Paleontology.
Zanno, L.E.; Loewen, M.A.; Farke, A.A.; Kim, G.-S.; Claessens, L.P. A.M.; McGarrity, C.T. (2013). "Late Cretaceous theropod dinosaurs of Southern Utah". In Titus, A.L.; Loewen, M.A. (eds.). At the Top of the Grand Staircase: The Late Cretaceous of Southern Utah. Bloomington: Indiana University Press
Zanno, Lindsay E.; Tucker, Ryan T.; Canoville, Aurore; Avrahami, Haviv M.; Gates, Terry A.; Makovicky, Peter J. (February 2019). "Diminutive fleet-footed tyrannosauroid narrows the 70-million-year gap in the North American fossil record". Communications Biology.
See Gradstein et al. (2004) for a detailed description of the ICS' timescale
Stephen L. Brusatte, Alexander Averianov, Hans-Dieter Sues, Amy Muir and Ian B. Butler (2016). "New tyrannosaur from the mid-Cretaceous of Uzbekistan clarifies evolution of giant body sizes and advanced senses in tyrant dinosaurs". Proceedings of the National Academy of Sciences of the United States of America.
Fowler, D. W.; Sullivan, R. M. (2011). "The First Giant Titanosaurian Sauropod from the Upper Cretaceous of North America". Acta Palaeontologica Polonica
Wolfe, D.G.; McDonald, A.T.; Kirkland, J.I.; Turner, A.H.; Smith, N.D.; Brusatte, S.L.; Loewen, M.A.; Denton, R.K.; Nesbitt, S.J. (6 May 2019). "A mid-Cretaceous tyrannosauroid and the origin of North American end-Cretaceous dinosaur assemblages" (PDF). Nature Ecology & Evolution.