During the Late Cretaceous period (around 95 million years ago), the Western Interior Seaway isolated western North America (Laramidia) from eastern North America (Appalachia), and occasionally isolated depositional basins from each other.
This led to the development of highly endemic ecosystems in Laramidia; these ecosystems have also roughly been divided into a northern province and a southern province, but such a clean division is contested.
Like many Laramidian dinosaur lineages, the evolutionary history of tyrannosaurids—which are limited in distribution to Asia and Laramidia—is characterized by faunal interchange between the two continents.
The sequence of interchange events which occurred among Laramidian tyrannosaurids is unclear, and the diverse tyrannosauroids which have been discovered in southern Laramidia (including Lythronax, Teratophoneus, and Bistahieversor) have complicated their evolutionary history further.
In particular, an unresolved question is whether Tyrannosaurus originated from Asian tyrannosaurids or from south Laramidian tyrannosaurids.

Based on their phylogenetic results, Zanno and colleagues proposed that the then-unnamed Lythronax displayed features that united tyrannosaurids from southern Laramidia to the exclusion of other genera.
While Loewen and colleagues did not recover a unique group of southern taxa, they did resolve all three as being closely related to each other and basal to a group of larger, later forms.
From these results, Loewen and colleagues suggested that there was significant biogeographic division between northern Laramidian and southern Laramidian forms with limited interchange.
Also, because they found Alioramini to be placed outside Tyrannosauridae, and the Asian genera Tarbosaurus and Zhuchengtyrannus in a group excluding all other tyrannosaurids, Loewen and colleagues proposed that there was only a single interchange of tyrannosaurids from North America to Asia.
They suggested the interchange took place during the late Campanian, when global sea levels fell, Tyrannosaurus being descended from North American forms from before such migration took place.

Due to their differing phylogenetic results, the biogeographic conclusions of Loewen and colleagues were disputed by Brusatte and Carr.
Since Bistahieversor from southern Laramidia was placed outside Tyrannosauridae, and Teratophoneus from Utah nested closest to the Alaskan Nanuqsaurus, Brusatte and Carr instead suggested that there were dynamic and recurrent interchanges of tyrannosaurid fauna between northern and southern Laramidia, and rejected the presence of endemic provinces.
The Asian taxa Tarbosaurus, Zhuchengtyrannus, Qianzhousaurus, and Alioramus were also placed within Tyrannosaurinae, among North American genera.
Brusatte and Carr proposed that at least two continental interchanges occurred, where Tyrannosaurinae originated in Asia and migrated to North America after the divergence of alioramins, and then returned to Asia again with Tarbosaurus and Zhuchengtyrannus.
Another possible scenario suggested by Brusatte and Carr was that two separate migrations to Asia occurred, which separately gave rise to alioramins and larger, later forms.
In both scenarios, Tyrannosaurus, nested among Asian taxa, was an "invasive migrant species that spread across Laramidia" from Asia in the Maastrichtian.

The hypotheses of Asian-North American migration of Brusatte and Carr were supported by a later run of their analysis by Canadian paleontologist Jared Voris and colleagues in 2020.
However, Voris and colleagues amended the original analysis through the additions of the new genera Dynamoterror from southern Laramidia (New Mexico) and Thanatotheristes from northern Laramidia (Alberta), and they were able to replicate the north–south divisions of tyrannosaurids suggested by Loewen and colleagues.
The southern taxa Teratophoneus, Dynamoterror, and Lythronax formed an exclusive group (to the exclusion of Nanuqsaurus, contrary to Brusatte and Carr) of short- and deep-snouted taxa outside a group of more derived northern Laramidia
During the Late Cretaceous period (around 95 million years ago), the Western Interior Seaway isolated western North America (Laramidia) from eastern North America (Appalachia), and occasionally isolated depositional basins from each other. This led to the development of highly endemic ecosystems in Laramidia; these ecosystems have also roughly been divided into a northern province and a southern province, but such a clean division is contested. Like many Laramidian dinosaur lineages, the evolutionary history of tyrannosaurids—which are limited in distribution to Asia and Laramidia—is characterized by faunal interchange between the two continents. The sequence of interchange events which occurred among Laramidian tyrannosaurids is unclear, and the diverse tyrannosauroids which have been discovered in southern Laramidia (including Lythronax, Teratophoneus, and Bistahieversor) have complicated their evolutionary history further. In particular, an unresolved question is whether Tyrannosaurus originated from Asian tyrannosaurids or from south Laramidian tyrannosaurids.
Based on their phylogenetic results, Zanno and colleagues proposed that the then-unnamed Lythronax displayed features that united tyrannosaurids from southern Laramidia to the exclusion of other genera. While Loewen and colleagues did not recover a unique group of southern taxa, they did resolve all three as being closely related to each other and basal to a group of larger, later forms. From these results, Loewen and colleagues suggested that there was significant biogeographic division between northern Laramidian and southern Laramidian forms with limited interchange. Also, because they found Alioramini to be placed outside Tyrannosauridae, and the Asian genera Tarbosaurus and Zhuchengtyrannus in a group excluding all other tyrannosaurids, Loewen and colleagues proposed that there was only a single interchange of tyrannosaurids from North America to Asia. They suggested the interchange took place during the late Campanian, when global sea levels fell, Tyrannosaurus being descended from North American forms from before such migration took place.
Due to their differing phylogenetic results, the biogeographic conclusions of Loewen and colleagues were disputed by Brusatte and Carr. Since Bistahieversor from southern Laramidia was placed outside Tyrannosauridae, and Teratophoneus from Utah nested closest to the Alaskan Nanuqsaurus, Brusatte and Carr instead suggested that there were dynamic and recurrent interchanges of tyrannosaurid fauna between northern and southern Laramidia, and rejected the presence of endemic provinces. The Asian taxa Tarbosaurus, Zhuchengtyrannus, Qianzhousaurus, and Alioramus were also placed within Tyrannosaurinae, among North American genera. Brusatte and Carr proposed that at least two continental interchanges occurred, where Tyrannosaurinae originated in Asia and migrated to North America after the divergence of alioramins, and then returned to Asia again with Tarbosaurus and Zhuchengtyrannus. Another possible scenario suggested by Brusatte and Carr was that two separate migrations to Asia occurred, which separately gave rise to alioramins and larger, later forms. In both scenarios, Tyrannosaurus, nested among Asian taxa, was an "invasive migrant species that spread across Laramidia" from Asia in the Maastrichtian.
The hypotheses of Asian-North American migration of Brusatte and Carr were supported by a later run of their analysis by Canadian paleontologist Jared Voris and colleagues in 2020. However, Voris and colleagues amended the original analysis through the additions of the new genera Dynamoterror from southern Laramidia (New Mexico) and Thanatotheristes from northern Laramidia (Alberta), and they were able to replicate the north–south divisions of tyrannosaurids suggested by Loewen and colleagues. The southern taxa Teratophoneus, Dynamoterror, and Lythronax formed an exclusive group (to the exclusion of Nanuqsaurus, contrary to Brusatte and Carr) of short- and deep-snouted taxa outside a group of more derived northern Laramidia
Lythronax