The year is 1993. I am a little 3-year-old terror. My little sister had only just been born and my parents needed to get rid of me for a few hours. Their plan? To take me to see a silly dinosaur movie called "Jurassic Park". The result? A life-long fascination with dinosaurs and an introduction to my favourite dinosaur - Tyrannosaurus rex. It's probably THE most famous dinosaur - along with Triceratops, Stegosaurus and Brontosaurus (although the latter is no longer valid, people still call Sauropods "Brontosaurs").
Tyrannosaurus rex was described in 1905 by the American palaeontologist Henry Fairfield Osborn. Osborn noted that it was the largest carnivorous dinosaur that had been described up to that point (Osborn, 1905). The fossil itself was discovered in 1902 by Barnum Brown and Richard Lull from Montana. The most distinctive characteristic of Tyrannosaurus, according to Osborn, was a very large humerus, however, he does not go into any detail, because not all of the skeleton was accessible to him. However, a year later, Osborn provided a more thorough description and a longer list of characters.
Restoration from Osborn (1905)
Firstly, the skull is short, with two large openings in front of the eyes (called antorbital fenestrae) and the bone at the back of the skull (the squamosal) has a horizontal bar facing forwards. It has 13 teeth in the maxillaries (upper jaw bone) and 12-13 teeth in the dentary (the main lower jaw bone). There is a pair of reduced cutting teeth at the front of the dentary. The teeth are oval in cross-section, being wider from left-to-right than front-to-back and have serrated edges. The tooth sockets (called dental alveoli) are expanded into triangular supporting plates on the inner side of the jaws.
Tyrannosaurus skull from Osborn (1906) The dotted lines indicate missing material and were based on the skull of Allosaurus, a rather different Theropod.
In terms of the vertebrae (backbones), Osborn was unsure of the number of presacral vertebrae (the backbones in front of the hip) but he guessed 23. It had nine cervical vertebrae (neck bones) with broad neural spines (the part of the vertebra that sticks up) and 5 sacral (hip) vertebrae with the spines fused together to form one continuous ridge. The atlas (first cervical vertebra) is composed of four parts: the hypocentrum (a horseshoe-shaped mass of bone on the underside), two neurapophyses (a pair of arches at the top) and a pleurocentrum or odontoid (a protuberance on the top). The axis, the second cervical vertebra, also consists of a hypocentrum as well as a centrum (which is more of a club-shaped structure).
Tyrannosaurus cervical vertebrae
Tyrannosaurus sacral vertebrae
The shoulder girdle has a greatly reduced scapula (shoulder blade) and humerus (upper arm bone). The large humerus Osborn was talking about in 1905 appears to be an error.
A=Allosaurus scapula, B=Tyrannosaurus scapula
Tyrannosaurus humerus in A=front view, B=rear view
Tyrannosaurus also possessed abdominal ribs, unusual for Dinosaurs, which Osborn likened to the Tuatara Hatteria (now a synonym of Sphenodon).
Tyrannosaurus abdominal ribs
The pelvic girdle (hip) consists of an elongate and compressed ilium (Il.) with an elongate horizontal plate extending from the anterior (front) portion. The pubes (P.) are firmly fused in the middle and also at the far end. Finally, the ischia (Is.) are reduced at their ends.
Tyrannosaurus pelvic girdle
All the limb and main girdle bones are hollow. The hind limbs are greatly elongated (Osborn appears to love that word based on how often he used it), with large, hollow cavities. The femur (thigh bone) is longer than the tibia (shin). Three of the main metatarsals (foot bones) are partly fused and the hallux (toe) is reduced.
And that's the description of Tyrannosaurus according to Osborn (1906). More has been discovered and the description has changed since then, so let's look at the current diagnosis from the second edition of The Dinosauria (THE authority for dinosaurs). The relevant chapter on Tyrannosauroids is authored by Thomas Holtz, an expert on Theropods. Tyrannosaurus is the last and also the largest known Tyrannosaurid with a skull length of 1.53m (5ft) and a femur length of 1.38m (4ft 6in) (Holtz, 2004). In addition to this, Tyrannosaurus had the narrowest nostrils and the widest skull of any known Tyrannosaur.
And that's it. That's the description and anatomy of Tyrannosaurus done. Part 2 will look at its systematics. But that won't be for a while yet. Next will be a brief discussion on geochronology.
See also:
References
Holtz, T. R. (2004) 'Tyrannosauroidea' in Weishampel, D. B., Dodson, D. and Osmólkska, H. (eds), The Dinosauria (2nd Edition), pp. 111-136, London: University of California Press
Holtz, T. R. (2004) 'Tyrannosauroidea' in Weishampel, D. B., Dodson, D. and Osmólkska, H. (eds), The Dinosauria (2nd Edition), pp. 111-136, London: University of California Press
Osborn, H. F. (1905) 'Tyrannosaurus and other Cretaceous carnivorous dinosaurs', Bulletin of the American Museum of Natural History, 21, pp.249-265 Click to access paper
Osborn, H. F. (1906) 'Tyrannosaurus, Upper Cretaceous carnivorous dinosaur. (Second communication.)', Bulletin of the American Museum of Natural History, 22, pp.281-296 Click to access paper
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