Tyrannosaurus, Osborn, Osborn, 1905

Longrich, Nicholas R. & Saitta, Evan T., 2024, Taxonomic Status of Nanotyrannus lancensis (Dinosauria: Tyrannosauroidea) - A Distinct Taxon of Small-Bodied Tyrannosaur, Fossil Studies 2 (1), pp. 1-65 : 9-11

3.1. Diversity Patterns of Tyrannosaurs and Apex Predators

The first argument for recognizing Nanotyrannus as a distinct taxon is that tyrannosaurids achieved high diversity in the latest Cretaceous ( Table 3 ) and that well-sampled dinosaurian assemblages typically had several large predator species. This point is far from conclusive, but it is an important starting point in considering the evidence. All else being equal, we should expect multiple tyrannosaurs to exist in the latest Maastrichtian of North America, and arguments for the existence of a distinct taxon should, therefore, be considered carefully.

Tyrannosaurs were diverse in the Late Cretaceous of Laurasia [ 4, 39, 52, 56 – 62]. Small tyrannosaurs of the Cenomanian [ 63] and Turonian [ 64, 65] were replaced by large-bodied tyrannosaurs by the Campanian [ 52, 57]. Multiple lineages evolved, including the gracile Albertosaurinae [ 66] and robust Tyrannosaurinae [ 56]. Tyrannosaurs also show endemicity; distinct taxa occur up and down the Western Interior from Mexico [ 62] and the American Southwest [ 52, 56, 58, 59] north into the Northern Great Plains [ 61, 67] and the High Arctic [ 68], implying high speciation rates.

Strikingly, several well-sampled assemblages supported two distinct tyrannosaurid taxa. The Dinosaur Park Formation assemblage of southern Canada included at least two species, the gracile Gorgosaurus libratus [ 66] and the larger and more robust Daspletosaurus [ 67]; similarly, Gorgosaurus and Daspletosaurus coexist in the Judith River Formation [ 69]. The Nemegt Formation of Mongolia was home to the small and gracile Alioramus [ 70] and the larger, more robust Tarbosaurus [ 53]. Although many formations contain just one species, these formations are generally poorly sampled, producing either a handful of diagnostic remains, or often, a single diagnostic specimen, so their low diversity may be due to sampling. The Horseshoe Canyon Formation is perhaps the only well-sampled formation in western North America to have a single tyrannosaur [ 71]. While our sample of faunas is limited and imperfect, it appears that, as often as not, multiple tyrannosaurs coexisted.

Other theropods show similar patterns. Multiple abelisaurids coexisted in the late Maastrichtian of Morocco [ 72], the Campanian-Maastrichtian La Colonia Formation of Argentina [ 73], and the Maastrichtian Lameta Formation of India [ 74 – 77]. Several large carnivores coexisted in the Cenomanian Kem Kem beds of Morocco [ 78, 79] and the Cenomanian Candeleros Formation of Argentina [ 80, 81]. The Late Jurassic of North America had four medium to giant carnivores— Marshosaurus , Torvosaurus , Allosaurus , and Ceratosaurus . Torvosaurus , Allosaurus , and Ceratosaurus co-occur in the Late Jurassic of Europe, and high diversity is also seen in the Late Jurassic of East Africa [ 82].

Similar patterns occur in mammals and marine vertebrates. Mammalian apex predators are small relative to dinosaurs, but in North America, saber-toothed Smilodon coexisted with American lions, dire wolves [ 83], and the cheetah-like Miracinonyx [ 84], and these would have been joined by puma and jaguars [ 85]. In Europe, saber-toothed cats coexisted with cave lions and hyenas [ 83]. In Africa, as recently as 1.5 Ma, the saber-toothed cats Dinofelis and Homotherium , lions, leopards, cheetahs, and hyenas coexisted [ 86]. The more depauperate predator communities of modern ecosystems are likely due to human-induced [ 87] megafaunal extinctions and give a biased picture of terrestrial predator diversity.

Similarly, marine ecosystems typically have multiple species of apex predators. Several species of large, predatory mosasaurs coexisted in the Maastrichtian [ 88, 89], and the giant shark Otodus megalodon coexisted with the predatory whale Livyatan [ 90]. Modern marine ecosystems, meanwhile, have two different large apex predators, great whites [ 91] and orcas [ 85], as well as smaller apex predators, such as false killer whales and leopard seals [ 85].

It is also common for clades of predators to show size disparity. Felids range from 1.1–1.6 kg (Prionaillurus rubiginosus) to over 300 kg ( Panthera tigris ) [ 85], and canids range from 1.0– 1.5 kg ( Vulpes zerda ) to 80 kg ( Canis lupus ); mustelids range from 25–250 g (least weasel, Mustela nivelis ) to a maximum of 32 kg (wolverine, Gulo gulo ) [ 85]. Among varanids, masses range from 16.3 g (Dampier Peninsula monitor, Varanus sparnus ) to 80 kg (Komodo dragon, Varanus komodoensis ) [ 92]. Among birds [ 93], falcons range from 43 g (black-thighed falconet, Microhierax fringillarius ) to 1.75 kg (Gyrfalcon, Falco rusticolus ); hawks range from 93 g (Pearl Kite, Gampsonyx swainsonii ) to 8.2 kg (Cape Griffon, Gyps coprotheres). Many of these size ranges were still larger in the Pleistocene prior to the elimination of larger members of these clades by humans.

Within predator clades, diversity tends to be higher towards the lower end of the mass range; that is, there are fewer species of big cats than small cats, fewer wolves than foxes, many small weasels and ferrets, and just one wolverine [ 85]. The reasons for these patterns are unknown, but they imply higher speciation rates at low mass, higher extinction at large size, or both. In light of this, one would expect the diversity of small tyrannosaurs to be higher than for large tyrannosaurs.

Extraordinary claims require extraordinary evidence, but the existence of multiple large predators in the late Maastrichtian of North America would be ordinary. It would be extraordinary to find a single, giant predator and no smaller species. In light of other dinosaur faunas, modern mammal communities, and marine faunas, niche partitioning between large predators is the rule. In no known ecosystem—dinosaurian, mammalian, terrestrial, or marine—did a single, giant species of predator dominate. If T. rex was the only tyrannosaurid in the ecosystem, then this leaves a remarkable gap in size between T. rex , approaching [ 103] or exceeding [ 104] 8000–9000 kg in mass, and the dromaeosaurs Dakotaraptor steini [ 105] and Acheroraptor temertyorum [ 106], which were more than an order of magnitude smaller.

Tables

Table 3. Diversity of Tyrannosauroidea in the latest Cretaceous (Campanian-Maastrichtian) of Laramidia, Appalachia, and Asia.

Taxon	Age	Formation	Locality	Describer
Dryptosauridae
Dryptosaurus aquilunguis	Late Maastrichtian	Hornerstown Formation	New Jersey, USA	[ 94]
Appalachiosaurus montgomeriensis (?)	Early Campanian	Demopolis Formation	Alabama, USA	[ 95]