Most people today know that modern birds are the evolutionary descendants of dinosaurs, and survived the great mass extinction at the end of the Cretaceous period that killed off all the non-avian dinos. But few people know that, for a brief time in what is now the Americas, after the dinosaurs were wiped out, it was birds that assumed their position as super-predators at the top of the food chain.
The example of the evolutionary transition from dinosaurs to birds, as represented by the fossil Archaeopteryx, is without a doubt the most famous in the world, and nearly every biology textbook that discusses evolution cites Archaeopteryx as an example. Six specimens of Archaeopteryx lithographica have been found. In appearance, they resemble the skeletons of small theropod dinosaurs—it is only the unmistakable imprints of feathers surrounding the fossil bones which indicate that we are dealing with a bird. In fact, the resemblance is so close that one of the skeletons was misidentified for several decades as a small theropod and another was misidentified as a pterodactyl, mistakes not corrected until someone noticed the very faint impressions of feathers.
Our picture of the evolution between dinosaurs and birds became even clearer in the late 1990s, with the discovery of a number of remarkable fossils in China. In 1996, Chinese paleontologists discovered a typical small theropod skeleton with a surprise: it was covered in fine downy thread-like fibers, which accorded perfectly with incipient feathers. The dinosaur, named Sinosauropteryx (“Chinese lizard-bird”), probably used the feather-like covering as insulation. Today, there is a whole series of excellently-preserved fossils showing a smooth evolutionary transition from small theropod dinosaurs to modern feathered birds.
The age of the dinosaurs came to an abrupt end 65 million years ago, when a comet hit the Earth near present-day Yucatan and kicked off a mass extinction that wiped out virtually every animal that was larger than a dog. But one group of small dinosaurs—the birds—managed to eke through. They are still with us today, and number around 10,000 species.
It was, however, the mammals—hairy descendants of a group of reptiles called therapsids—which would go on to dominate the Earth after the dinosaurs. At the time of the Cretaceous extinctions, mammals were tiny nocturnal insectivores, not much bigger than rats, who lived at the edges of the dinosaurian ecosystems. With the big dinos gone, however, the mammals underwent an evolutionary explosion, growing in size and adapting rapidly to fill all of the ecological niches that had been vacated by the dinosaurs.
But in one geographic area, the “Age of Mammals” took a temporary detour, and became the “Age of Birds”.
In the time after the Cretaceous extinctions (a period known as the Tertiary), sea levels were much higher than they are now, so most of what is now Panama was underwater, and North and South America were separated from each other and went their separate evolutionary ways. The mammals appeared and diversified, and it was assumed that they had inherited the mantle of dominance from the dinosaurs.
But a surprise came in 1887, when an Argentinian paleontologist named Florentino Ameghino uncovered something strange while digging in 60-million year old Tertiary deposits in Patagonia. He uncovered a piece of what looked like a lower jaw, very large in size. From the shape, he deduced that it belonged to a mammal, and named it Phorusrhacos longissimus. Additional bones that were found three years later, however, demonstrated that it was a gigantic bird, and Ameghino then concluded that it was a predator akin to a flightless hawk or eagle. It was the largest predator to be found in these deposits.
Over the decades, new discoveries were made, and today there are about 20 species of Phorusrhacids that have been described. They range in size from Psilopterus lemoinei, which stood about three feet tall, to Kelenken guillermoi, which towered over ten feet.
The “Terror Birds”, as they were dubbed, bore a remarkable resemblance to dinosaurs. They were all flightless and had tiny vestigial wings, but were capable of running on two muscled legs and three-toed feet with long talons. The beaks were outsized, making up over half the skull, and bore a sharp recurved hook at the end similar to those found in modern raptor birds like eagles, leading paleontologists to conclude that they were carnivorous predators who likely ran down their prey in a manner similar to theropod dinosaurs. Most authorities today attribute the South American Seriema, a three-foot tall long-legged predatory South American bird also known as the Crested Screamer, as the closest living relative of the extinct Phorusrhacids.
All of the earliest known Terror Birds were found in what is now South America, indicating that they evolved here not long after the dinosaurs died out. At this time, most of South America’s mammals were marsupial herbivores, pouched animals like Kangaroos or Opossums. The ancestor of the Phorusrhacid birds was apparently a long-legged runner descended from theropod dinosaurs, which managed to make its way through the Cretaceous extinction event and then dominate the apex predator niches on the South American plains before the mammals could. The Terror Birds then held on to this ecological role for the next 50 million years.
Then about 3 million years ago, ocean levels dropped and tectonic forces pushed the land up to form the Isthmus of Panama, connecting the two continents. The result was what paleontologists refer to as the Great American Interchange, when wildlife from each continent now had a land route to the other. Deer, horses, camels, tapirs, bears and cats went south, while sloths, armadillos, and giant rodents went north.
At least one species of Terror Bird, Titanis walleri, also went north. The first bones of this species were discovered in 1961 by two scuba divers who were exploring an underground limestone cave along the Santa Fe River in northern Florida and were examined by a paleontologist at the University of Florida. The find was at first mistakenly described as a relative of the Ostrich or Rhea, but was then attributed to the Phorusrhacids in 1963. Today there are 41 fossil specimens from Florida and a single toe bone from Texas.
It’s not clear just how Titanis got here, though. There are no fossil Terror Birds found so far in Central America, or between Texas and Florida. And the earliest Titanis fossil in Florida dates to 5 million years ago—before the time that the Central American land route is believed to have opened up 3 million years ago. It remains a mystery.
While not the largest of the Phorusrhacids, Titanis stood over six feet tall (though some adult skeletons measured only a bit over four feet, perhaps indicating a size difference between the sexes). Study of the skull indicates that these birds had limited side-to-side mobility, but had powerful muscles that were capable of propelling the sharp beak up and down. Titanis may have killed its prey by striking it with the beak or by bashing it on the ground in a manner similar to that of the Seriema. Or it may have attacked prey with its taloned feet, like the modern Secretary Bird, before pulling it apart with its hooked beak.
However, there is a minority view among paleontologists which argues that the Terror Birds were actually herbivores who ate large fruits and seeds with a parrot-like hooked beak. This was supported by a calcium isotope study carried out in 2013 in Germany, which concluded that the isotope ratios in the bone were more typical of a herbivore than a carnivore. This finding is still being disputed, however, and most paleontologists still conclude that the Terror Birds were carnivorous hunters.
For a time, one of the Florida finds was reported as having been dated to around 15,000 years ago, perhaps existing during the time when humans first entered Florida. However, later work has revised that date, and now it is considered that no Titanis fossil dates younger than 1.8 million years.
That is also about the time that the Terror Birds, including the Floridian species, disappear from the fossil record. The most popular hypothesis for their extinction concludes that the giant birds were unable to successfully compete with the large mammal predators, like Sabertooth Cats, Jaguars, American Lions, and Wolves, which the birds encountered in Florida and which also moved south to invade Phorusrhacid territory in South America during the Great Interchange. Other hypotheses point to changes in climate (the beginning of the last Ice Age) as a factor.
Other fossil finds–a femur in Algeria named Lavocatavis and some bones named Eleutherornis in France–have been attributed as early Phorusrhacids, indicating that the Terror Birds may have inhabited Africa and Europe, but this is still controversial. It has even been hypothesized that the Terror Birds may have evolved in Europe or Africa and crossed to South America via a now-submerged chain of islands that stretched across the Atlantic, but this has not been widely accepted.
Today, the Florida Natural History Museum displays some of the original Titanis bones as well as a life-sized reconstructed skeleton.