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Thylacosmilus atrox, which means “pouch saber” in Greek, lived in South America 3.5 million years ago during the Miocene. Although this animal greatly resembles the large saber-toothed cats, (like the famous saber-toothed tiger) it is in fact not a placental mammal. It is a sparassodont; a sister group to marsupials. Convergent evolution caused these animals to have outwardly similar appearances although they are not as closely related as their morphology would suggest.

Estimates for the size of T. atrox vary from 175 pounds to 330 pounds. Even within this range it was one of the largest carnivorous sparassodonts known.

T. atrox was recognizable by its fantastic dentition, characterized by immense upper canines and square, blunt lower canines. The upper canines grew throughout the life of the animal, which distinguishes it from the saber-toothed cats—whose teeth did not grow continuously. Despite their intimidating appearance, the teeth of T. atrox were fairly fragile.

Stephen Wroe from the Univeristy of New South Wales worked with other scientists on a recent PLOS ONE study aimed at simulating and comparing the usage and force of the jaws of Smilodon fatalis (a true placental cat – the saber toothed tiger) and T. atrox. Using 3D modeling software, the skulls of the two animals were recreated and tested. Relative to S. fatalis, T. atrox had an extremely weak bite force. Wroe et. al estimated it to be comparable to a modern day house cat. This rather surprising result led the authors to study the musculature used in the jaw and neck. They found that the muscles of the neck of T. atrox were highly developed, much more so than the saber-toothed cats.

“Frankly, the jaw muscles of Thylacosmilus were embarrassing. With its jaws wide open this super-predator had a bite less powerful than a domestic cat. On the other hand – its skull easily outperformed that of the placental Smilodon in response to strong forces from hypothetical neck muscles,” Wroe commented.

Additionally, T. atrox has very stout, well-developed forelimbs. These muscular clues led the authors to the conclusion that T. atrox would secure its prey and insert the saber teeth into the neck of the captured animal. This is called a “neck-driven bite” and required very strong neck muscles and a fair amount of precision. This method of prey capture explained the weak jaw muscles and very well developed neck muscles—showing further differences between the pouched saber and its feline relatives.

Read Wroe’s study in PLOS ONE, published June 26, 2013.

The Paleontological Research Institution, Ithaca, New York, is pleased to sponsor Paleontology content for This View of Life. Founded in 1932, PRI has outstanding programs in research, collections, and publications, and is a national leader in development of informal Earth science education resources for educators and the general public.

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Published On: July 25, 2013

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