The large carnivorous dinosaurs of the Mesozoic are inevitably among the most exciting of these extinct animals, but they are also among the hardest to study as they were rare. Anyone who has been on safari will recognise the issue – you can see hundreds of wildebeest or zebra for you set eyes on a lion, and similarly there may have been only one Tyrannosaurus knocking around for every few dozen Triceratops or other big herbivores. As a result, fossil of these are few and far between and reconstructing evolutionary or ecological patterns.

Enter the new dinosaur Siats meekerorum, named today in a paper in Nature Communications. The genus name Siats is from a legendary monster from the Ute tribe of Utah (where the dinosaur was found), with the species name meekerorum a tribute to the Meeker family who support palaeontological research. The fossil is rather incomplete and there’s little more than a partial pelvis and some vertebrae and part of the lower leg, but this is enough to confidently assign this to a new animal, and to work out to which evolutionary group it belongs. Siats is part of a group called the allosauroids, one of the major lineages of large carnivorous dinosaurs with some members becoming true giants that rivaled anything the tyrannosaurs (Tyrannosaurus and its relatives) could produce.

The latter point is a key one – there has long been a suggestion that at various times and places for the Mesozoic dinosaurian carnivores certain groups dominated at certain times. Most notably the large period of the non-avian dinosaurs, the Late Cretaceous saw that large carnivores in North America and Asia were almost exclusively tyrannosaurs, with no other groups getting much, if any, of a look in.

Looking around the world today at the large carnivorous mammals, it is clear that no one group seems to dominate any major geographical location. At least two, and often three, of bears, cats, dog and hyenas are present across most of the world, and often with a number of species of each. North America has several bears, wolves, puma and even jaguar in the south; southern Asia sees wolves, dholes, bears, tiger, leopard, and hyenas. What we don’t really see is a pattern of just big cats, or just wolves and hunting dogs.

However, this may not be true of the Mesozoic dinosaurs, in North America at least, where Siats helps fills in some of the previous gaps in the fossil record of large carnivores. This new animal comes from more or less at the joint between the Early and Late Cretaceous and helps from just before the rise of the really giant tyrannosaurs. In fact the analysis points to a more overarching pattern for North America, in the Late Jurassic, there was a mixture of a number of different dinosaur lineages vying for ‘top spot’ as the largest carnivore, arguably much like mammalian system we see today. Late, in the Early Cretaceous the allosauoids became dominant, with only the odd tyrannosaur for company, or rather, competition, but Siats appears to be one of the last of its kind to be a major player, with the tyrannosaurs taking over after this.

A similar pattern may well have played out is Asia too, with numerous other allosauroids known from the Early Cretaceous and first part of the Late Cretaceous, before the big tyrannosaurs got going, though intriguingly there are some teeth that suggest a giant non-tyrannosaur was still around in China right at the end of the dinosaurs reign. Overall though, the new paper provides some compelling evidence for a faunal turnover of the great carnivores of the Mesozoic and suggestions of a rather different pattern of ecology for these dinosaurs compared to other times and places.

Zanno, L.E. & Makovicky, P.J. 2013. Neovenatorid theropods are apex predators in the Late Cretaceous of North America. Nature Communications. DOI: 10.1038/ncomms3827