28 million years ago, South America was an island continent much like Australia. Marsupial lions stalked the land as a top predator, but big birds evolved that rivaled and probably surpassed them in ferocity. The phorashacids grew to over 6 feet tall and weighed over 300 pounds. They sported large hatchet like bills and long claws which they respectively used to bludgeon and tear apart small to medium-sized mammals, reptiles, and other birds. 5 million years ago, the continents of North and South America drifted close together, but even before the landbridge joined them, a species of phorashacid, known as Titanis walleri, island-hopped north and colonized the southern half of the continent.
Two different illustrations of the terror bird. The top picture is from google images. The bottom is from the June 1997 issue of Discover magazine by Steven Kirk. Note the stiff T-rex-like arms. Scientists aren’t sure, if their arms were actually like this, because no fossil arm bones have ever been found. They assume this based on shoulder bone. Terror birds were a kind of evolutionary throwback to their velociraptor ancestors, but they differed in some notable ways. They had no teeth and they didn’t have a long tail.
Terror birds likely found abundant food in North America such as small pronghorns, juvenile horses, deer fawns, rabbits, rodents, reptiles, as well as some carrion and plant foods. They stalked their prey, perhaps blending in color with thickets where they hid and waited for unsuspecting animals to wander. Then, with a burst of speed (maybe up to 40 mph) they’d run at the victim and stike them on the back with their hatchet-like bill, paralyzing the prey. Using dinosaur-like arms, they held the struggling prey down and tore chunks of flesh from the suffering animal. They probably picked up and dashed smaller prey on the ground before swallowing them whole, not unlike a method used by their closest living relative–the seriamas.
Not a bill from Sesame Street’s friendly big bird. This is a fossil replica of a terror bird skull. According to wikipedia, a complete skull has never been found.
Scientists were surprised to find Titanis walleri fossils in Florida. For birds Titanis has an unusual bone structure. Most bird bones are hollow, making them light and conducive to flight. But terror birds couldn’t fly. Its bones were thick and heavy, like a large mammal’s. Many of the first bones discovered were labled as horse bones, stuck in drawers in museum basements, and forgotten. At first scientists in Florida didn’t realized they’d excavated locally what was formerly thought to be a family of birds restricted to South America. Pierce Brodnorb formally named Titanis walleri a new species in 1963. Most terror bird fossils were excavated from the Santa Fe River in Florida, but one specimen was discovered in Texas. (The first to find fossil material of the bird in Florida was Benjamin Waller, a scuba diver.) In both fossil localities Pleistocene-age fossils are mixed with Pliocene-aged fossils because rivers are eroding through two different aged deposits. So for awhile, scientists were uncertain how recently the terror bird lived. Eventually, scientists discerned different chemical signatures from the fossil bones, using an analysis of the ratios of rare earth elements which change over time. (See my blog entry–”The Fossil Rich Region of Tunica Hills, Louisiana,” for a more detailed explanation of how rare earth elements are used to bracket the age of fossils. http://markgelbart.wordpress.com/2011/08/28/the-fossil-rich-region-of-tunica-hills-louisiana/) None of the terror bird fossils were younger than 2 million years, suggesting it became extinct early in the Pleistocene.
I wonder why such an awesome predator became extinct. I doubt colder climate was the cause of extinction–there were mini-Ice Ages during the Pliocene. Giant tortoises survived the harsher Pleistocene Ice Ages, and if they survived these, the terror bird should have been able to as well. The floral composition of the environment probably didn’t change significantly between the late Pliocene and early Pleistocene. I suspect it was a new cavalcade of fauna that doomed the terror birds. Black bears (Ursus americanus) crossed the Bering landbridge about this time. Pleistocene black bears grew as big as grizzlies and were probably more ferocious than today’s bruins. Maybe they were able to drive terror birds off their nests. Maybe wolves and big cats evolved to become more adept at hunting the chicks. And maybe prey such as rabbits and small ungulates evolved to run in circles and avoid capture. Terror birds could run fast in straight lines but are thought not to have been efficient at changing directions while pursuing prey.
This is the terror bird’s closest living relative. A red-legged seriama is bashing a snake against a rock. They’re an inhabitant of South American grasslands. They’re much smaller than the phorashacids, growing to only 31 inches long.
A nature documentary showing terror bird hunting behavior can never be made. However, their closest living relatives, seriamas (Cariama cristata and Chunga burmeisteri), still inhabit the grasslands of South America where they prey on insects, snakes, lizards, small mammals, and other birds. They kill their prey by slamming them against rocks as depicted in the photo above and the link to the youtube video below.
The Flock is a pretty good sci-fi thriller about terror birds. The book may be made into a movie. The cover of my copy of the book has an alternative illustration.