by Whit Gibbons

February 12, 2012

I have written recently about two animals, pythons and coyotes, that are known to have attacked and killed people. I have just learned of another that sounds far more terrifying--a species of giant crocodile known as "shieldcroc" with a head five feet long and a total length of 30 feet. Most people will rest easier knowing the species is now extinct, having disappeared about 100 million years ago in northern Africa, and will not take up residency in southern states, as Burmese pythons and coyotes have.

Casey M. Holliday, an anatomist with the University of Missouri School of Medicine, and Nicholas M. Gardner of Marshall University, described the creature based on a specimen from the Royal Ontario Museum in Toronto. The common name, shieldcroc, comes from the thick, shieldlike covering of skin on top of the huge snout. Scientists can only speculate about the function of the massive layer of skin. One suggestion is that it served as a courtship display and attractant to crocodiles of the opposite sex, like the ornamental feathers of a peacock. Some experts have suggested that the shield helped regulate head and body temperature in a tropical habitat. Another guess is that it served as armor to protect the predator when it consumed prey that fought back with large teeth and claws. An entertaining aspect of paleontology is that anyone can speculate about the purpose of an extinct animal's morphological features. Since the creature is long gone, no one is likely to prove your suggestion wrong.

Besides having an unusual covering on the snout, the shieldcroc specimen is notable for being the earliest known direct ancestor of modern crocodilians (which include alligators and caimans). Within the Mesozoic era, the Jurassic geologic period was followed by the Cretaceous, approximately 145 to 65 million years ago. During that time, crocodilians expanded into a wide diversity of terrestrial, freshwater, and saltwater habitats. Today's 23 species of modern crocodilians live primarily in freshwater systems, occasionally making excursions onto land. Saltwater crocodiles of Australia and New Guinea are noted for oceangoing trips but their primary residence is in estuarine or freshwater habitats.

Today's crocodilians are all top-of-the-food-chain carnivores, eating other reptiles, mammals, birds, amphibians, and fishes. The now-extinct crocodilians of the late Mesozoic period had much greater species diversity, and this was reflected in their broad array of food preferences. Some were meat or fish eaters; others were insectivorous, even herbivorous. Does a plant-eating crocodile sound improbable? Consider that a six-foot-long flower-eating iguana lizard is a huge, herbivorous reptile similar in shape to a crocodile. How an animal (or plant) evolves depends not only on its ancestry but also on who the survivors were.

Shieldcrocs must have been formidable creatures, and capturing one would have given today's hunters of American alligators something worth bragging about. In the United States we have two native crocodilians--American crocodiles native to southern Florida and alligators found in much of the Southeast. Fortunately, neither species indulges in the antisocial behavior of eating people for a living. American crocodiles typically behave like alligators, which are usually shy and inoffensive.

In northern Australia, saltwater crocodiles roam the rivers and coastal marshes near where people live. These crocodiles are very different from American alligators with regard to their interactions with humans--they will unhesitatingly attack and eat people. If you want to experience something akin to what an encounter with large scary reptiles might have been like when dinosaurs were on the wane and shieldcrocs possibly held the franchise on ferocious reptile behavior, I suggest a trip to northern Australia. Saltwater crocodiles are the reptile world's equivalent of great white sharks or Bengal tigers. Their maximum size is enormous. Not as big as a shieldcroc, but more than 20 feet long. I have seen a 16-foot-long crocodile and have no doubt that it could easily kill and swallow a full-grown man. If that sort of adventure doesn't appeal to you, then you are probably glad to know that shieldcrocs no longer inhabit the earth.

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