Friday, 28 August 2015

The first Tyrannosaurs and how they lived

Everyone knows Tyrannosaurus rex, the most famous dinosaur of all time. It is the only dinosaur commonly referred to by its scientific name(T. rex), the only dinosaur that you can be almost completely certain everyone you ask will have heard of, and one of the most well known large Theropods specimen-wise. But as famous as T. rex is, far fewer people have heard of its ancestors. Indeed, while Tyrannosaurus was a late Cretaceous genus, the Tyrannosaurids go back to the early cretaceous, and the Tyrannosauroid superfamily all the way to the mid Jurassic. It is the latter that we will be talking about today.

Coelurus by Nobu Tamura

The Tyrannosauroids were a wide group, containing many different lineages, among them the more derived late Cretaceous Tyrannosaurids. In this post however I will be covering only the earliest and most basal members of the group, the ones who are either directly ancestral to later species such as T.rex, or at least close to the ancestry. One of these species was Coelurus. While not a particularly impressive dinosaur in its own right, and only known from partial remains, Coelurus has the honor of giving name to the large group called the Coelurosaurids, which includes animals as diverse as Therizinosaurus and the Golden Eagle.  Coelurus itself lived in the late Jurassic of North America, 150 million years ago. It inhabited the Morrison formation, where it shared its habitat not only with stars such as Allosaurus and Brachiosaurus, but also another early Tyrannosauroid, Stokesosaurus, which we will get to later. Given it's small size, at only around 2 meters in length, Coelurus would have been a diminutive part of the Morrison fauna, with a diet most likely consisting of Arthropods and other small animals such as Mammals.

Scene from the Morrison Formation, where Coelurus lived, by Doug Henderson

Another basal Tyrannosauroid was Aviatyrannis, a tiny animal, at less than two meters in length. Its fossils were originally referred to Stokesosaurus, but later made into their own genus when they were found to be distinct. Exactly where Aviatyrannis fits in the phylogeny of the Tyrannosauroids is uncertain, but it is currently viewed as a basal member. The genus is known from very few fossils, with the holotype consisting only of an ilium, which may have belonged to a juvenile. Another possible early Tyrannosauroid is Iliosuchus, though this is controversial, as it's bones are very similar to those of small Megalosaurids. If it was a Tyrannosauroid, it would have been the earliest one yet known.

A pair of Proceratosaurs hunting, by Alexander Lovegrove

Moving on to another lineage of early Tyrannosauroids, the confusingly named Proceratosaurids were a group of primarily small animals, which contained Proceratosaurus, the earliest confirmed Tyrannosauroid. The Proceratosaurids were not in fact ancestral to the later Tyrannosaurids, but instead represent a side branch diverging near the base of the Tyrannosauroid tree. During the mid to late Jurassic they were the most successful Tyrannosauroids we know of, but shortly after reaching truly gigantic sizes with species such as Sinotyrannus, they finally went extinct in the early Cretaceous. The Proceratosaurids were interesting in that they all appear to have possessed elaborate head ornaments, a feature which was either ancestral to all Tyrannosauroids but then lost in most lineages early on, or more probably independently evolved in the Proceratosaurid lineage.

The Proceratosaurid Guanlong by Raul Martin

Proceratosaurus was the earliest known member of its group, and may have represented the basal form of all Proceratosaurids. It was a fairly small animal, about the size of a wolf, and lived in the mid Jurassic of England, 165 million years ago. The reason for the species, and subsequently group's name is that it was initially thought to have been ancestral to the Carnosaur Ceratosaurus, due to what appeared to be a similar nasal crest. More complete finds from other Proceratosaurid species have since showed that the crest was probably much more elongated than that of Ceratosaurus, stretching from the tip of the snout to the back of the head. Proceratosaurus was closely related to another genus named Kileskus, which was a contemporary living in Russia. It probably possessed a similar crest, and was about the same size. More significant was Guanlong, a Chinese relative from the late Jurassic. This animal is known from far better remains than its two relatives, to the point where we have even found fossils of animals from various stages of growth, showing the development of the crest over time, and the age at which the animals matured. 

Juratyrant by Nobu Tamura

The last two Jurassic Proceratosaurids were Juratyrant and Stokesosaurus. The former was a relatively large animal from the late Jurassic of England, 149 mio ago. It may have reached around 5 meters in length, which would have made it a medium sized predator for its time. It may have shared its habitat with Ceratosaurus, which is known from the nearby islands of Portugal. Stokesosaurus was, as mentioned earlier in this post, a North American animal, living in the Morrison formation in what is now Utah, alongside Allosaurus, Stegosaurus, and Coelurus. At the time it lived, Utah had a climate vaguely similar to today, with distinct wet and dry seasons. Many of the bones of larger animals in the formation were carried from the drier uplands into the swampy lowlands by streams and rivers, where they were preserved. Considering its size and the other animals of it's time, Stokesosaurus may have hunted small dinosaurs and primitive birds, which were beginning to appear by then.

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