November 25. Tyrannosaurus rex

Obviously we can’t leave the Cretaceous without at least a mention of Tyrannosaurus rex. The tyrant lizard king – that’s what the name means – only lived during a relatively brief span of geologic time, a million years from 67 to 66 million years ago, just before the end of the Cretaceous. It was one of the largest land predators of all time, at 12 meters or 40 feet in length.

Henry Fairfield Osborn described the species from specimens collected at Hell Creek, Montana, in 1902 and 1908. Since then fossils from more than 50 individuals have been found. There’s enough individual variation that there are also specimens originally assigned to other species that were reassigned to Tyrannosaurus rex after further study, but there are also other species within the overall group of tyrannosaurids.

Photo of Sue, the most complete T. rex specimen known,
in the Chicago Field Museum,
by Connie Ma, used under Creative Commons license.  

They were bipedal carnivores, and the debate continues over whether they were active predators, scavengers, or both. Most researchers today seem to agree that Tyrannosaurus rex was an opportunist that both scavenged and caught live prey. It had a really strong bite that would force its teeth – up to 12 inches long – into pretty much whatever it wanted to bite.

Another current debate is whether T. rex was warm-blooded or cold-blooded. As near as I can tell, the jury is still out on that question.

The Hell Creek Formation, the rocks in which many T. rex fossils have been found, was laid down on the margin of the Cretaceous Interior Seaway that we talked about a few days ago. The setting was one of coastal flood plains and deltas, with rivers and some swampy areas. The climate must have been subtropical or tropical and clearly supported a wide range of life. There were lush forests of angiosperms, conifers, gingkoes, and cycads. Besides the famous T. rex, the Hell Creek Formation contains mammals, birds, pterosaurs, lizards, snakes, turtles, crocodilians, and a wide range of dinosaurs large and small. The more marine sediments contain fish, plesiosaurs, ammonites, sharks and rays, and there were snails, oysters, and clams in the marine and fresh waters. Lots of life, a long, tall food chain with carnivores like Tyrannosaurus rex at or near the top.

Tyrannosaurs only represent about 4% of the dinosaur fossils in the Hell Creek. The most numerous group were the Ceratopsians, horned dinosaurs, of which the most famous member is probably Triceratops, which we’ll talk about a little more in a few days.

There is of course a vast amount of information readily available about Tyrannosaurus rex. I have links below to a video and a podcast, both with Tyrannosaur expert Dr. Thomas Holtz who is at the University of Maryland, for more information.

—Richard I. Gibson

Video: The Life and times of Tyrannosaurus rex with Thomas Holtz 

Podcast link: New understanding 


Photo of Sue, the most complete T. rex specimen known, in the Chicago Field Museum, by Connie Ma, used under Creative Commons license.