Terrific Tetanurae #13: Mirischia asymmetrica

The forest seems to whisper as the dry season pours into the land, stripping trees of their leaves and animals of their color. We’re in Cretaceous Brazil, and the land is dominated by forest. In these woods, every animal must fight to survive, from the smallest of fishes to the largest of dinosaurs. One animal in particular is suffering from the dry climate. The animal in question is an old Irritator bull. The animal is already at a disadvantage, having lost part of his left arm in a battle with another spinosaur. The wound has become infected, and the old bull’s immune system is not presently strong enough to fight off the bug. For now, the bull must rest and hope for better weather. Unfortunately, another predator has set its sights on the spinosaur, and not as a rival, but as food.

A month passes, and the old bull’s health has improved. The infection, however, has taken its toll on the animal, and the Irritator has become weak. He must find food or die. In the corner of his eye he spots movement. He moves closer, and observes a small coelurosaur chasing a juvenile pterosaur. This is Mirischia asymmetrica, the most common predator in these woods. The small coelurosaur, noticing the gigantic spinosaur, hisses and spreads its frill of feathers, which are colored bright blue to daze predators. The little tetanuran darts into the bushes. The spinosaur, knowing not to expend lots of energy on such a small catch, lumbers away. He’ll have to wait another day to eat.

Mirischia asymmetrica is a tyrannoraptoran from the Cretaceous of Brazil. This coelurosaur was assigned to compsogathidae (Rauhut et. al., 2010), a group of small, gracile meat eating dinosaurs. In fact, Mirischia asymmetrica is the only known compsognathid from the Americas. That depends on whether Mirischia is actually a compsognathid. In his book Tetrapod Zoology, Darren Naish suggested that M. asymmetrica might be a tyrannosauroid. Of course, this idea has been explored by the wonderful community of paleoartists.

Mirischia most likely hails from the Santana Formation of Brazil (Martill et. al., 2000), and, if so, would have shared its environment with many other archosaurs, including the spinosaurid Irritator (Weishampel et. al., 2004). This spinosaur may have been the apex predator of the Santana ecosystem, while smaller predators like Mirischia foraged for prey on the forest floor. Another small theropod, Santanaraptor, also called the Santana Formation home (Weishampel et. al., 2004). In time, the ancient forests of South America would loose the bloodline of Mirischia as other predators colonized the landscape.

During the Cenomanian, new groups of theropods started to take the throne of top predator. During the Jurassic and Early Cretaceous, the megalosaurs and allosauroids reigned as the top predators of ecosystems across the globe. During the Late Cretaceous, however, new groups of killer dinosaurs rose up the ranks. To the north, the members tyrannosauroid group, of which Mirischia might belong to (and, if not, was closely related to) developed larger sizes to go after larger prey.

The skull of T. rex, a tyrannosaurid tyrannosauroid dinosaur. Photo by the author, 2015.

The skull of T. rex, a tyrannosaurid tyrannosauroid dinosaur. Photo by the author, 2015.

In the south, another group of carnivorous dinosaurs rose to the top. These were the abelisaurids, relatives of the Jurassic Ceratosaurus. In the Late Cretaceous, these blunt-faced, stubby-armed marauders would conquer the ecosystems of the southern hemisphere, leaving other large carnivores of other predatory groups few and far between.

The skull of Majungasaurus crenatissimus, an abelisaurid. Photo by the author, 2015.

The skull of Majungasaurus crenatissimus, an abelisaurid. Photo by the author, 2015.

In this way, the tyrannosauroids might have been prevented from conquering the south, and the domain of Mirischia fell to a rival group of killer dinosaurs.

Mirischia   by the author. Pencils on paper, 2015.

Mirischia by the author. Pencils on paper, 2015.


1. Rauhut, O. W. M.; Milner, A. C.; Moore-Fay, S. 2010. “Cranial osteology and phylogenetic position of the theropod dinosaur Proceratosaurus bradleyi (Woodward, 1910) from the Middle Jurassic of England.” Zoological Journal of the Linnean Society 158(1):155-195. 

2. Naish, Darren. 2010. Tetrapod Zoology Book One. Bideford: CFZ Press. p. 235. 

3. Weishampel, D. B; Dodson, P.; Osmólska, H. 2004. “Dinosaur distribution (Early Cretaceous, South America).” In: Weishampel, David B.; Dodson, Peter; and Osmólska, Halszka (eds.): The Dinosauria, 2nd Edition, Berkeley: University of California Press. Pp. 563-570.

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