Spec-Dinovember Day 17: Rexy and Blue, tyrannosaurs and maniraptorans swap niches
The earliest stages of the Cretaceous were a time of transition. The conifer and cycad plant communities of the past were being replaced by newly evolved angiosperms. With the change in plants came a change in herbivores and with that too a change in carnivores. Across Laurasia sauropods were disappearing, stegosaurs were supplanted by ankylosaurs, and the old guard of ceratosaurs, carnosaurs, and megalosaurs were being replaced by new clades of coelurosaurs. Much as in the Early and Middle Jurassic, this reshuffling of niches allowed for brief but interesting role reversals. During the Hauterivian stage in central Siberia the small carnivore guild is populated by coelurosaurs and a few odd ceratosaurs. Among this first group are members of the Proceratosauridae, tiny Tyrannosaurs, who primarily prey upon the many local mammals and the occasional early ceratopsian. The apex predator niche is occupied by a gigantic Eudromaeosaur. At just over a ton in mass, it is large enough to prey upon even the relict stegosaurs of the area.
------------------------------------------------------------------------------
Utahraptor is really odd in that not only is it the largest dromaeosaur, but its also one of the earliest to show up in the fossil record. It's found in the Yellow Cat member of the Cedar Mountain Formation, with essentially only an unconformity and/or the Buckhorn conglomerate separating it from the Jurassic Morrison Formation. Larger carnosaurs don't appear until further up the strat-column in the Ruby Ranch and Mussentuchit Members. This pattern looks like a local extinction of the Jurassic dinosaurs, and then the dromaeosaurs capitalize and quickly fill the large carnivore niche before being outcompeted by a bigger predator that survived somewhere else. So my starting point for this prompt was to take this pattern and put it somewhere that has Early Cretaceous proceratosaurid tyrannosaurs. Most of that clade is Jurassic aged, though Dilong and Yutyrannus are from the Barremian stage. I went with a Siberian location because Kileskus is Siberian (though from the wrong time frame) and there's fragmentary evidence of possible stegosaurs persisting in paleoarctic latitudes there in the middle Cretaceous. So I split the time difference and put all the taxa there. Plus there was probably a period of glaciation during the Hauterivian stage, so this would've been a chilly place and that's neat.