By Hans-Dieter Sues
About 60 million years ago, a group of extinct ground-dwelling birds of often spectacular size and with proportionately enormous heads, the Phorusrhacidae, first appeared in South America, an island continent until about three million years ago. They are informally and memorably known as “terror birds.”
One species, Titanis walleri, crossed into North America after the Isthmus of Panama formed during the Pliocene Epoch, some three million years ago, and made it all the way to Florida.
There are currently 18 recognized species of terror birds, the largest being the middle Miocene Kelenken guillermoi, which attained a height of up to 3 m (10 feet) and a skull length of 71 cm (28 inches–the largest head of any known bird). Their closest living relatives are the (much smaller) seriemas (family Cariamidae), which live in the more southern regions of South America.
Based on their huge heads armed with powerful hooked beaks, terror birds have long been considered predators. However, little was known about their way of life.
A multi-national team of scientists has just published an innovative study in PLoS One on the skull of terror birds, focusing on one particular species, Andalgalornis steulleti. The researchers used CT scanning and advanced methods for structural analysis (originally developed for engineering) to interpret the function of the skull during feeding.
Andalgalornis steulleti thrived in what is now Argentina during the late Miocene and early Pliocene, some five million years ago. A medium-sized terror bird, it stood about 1.4 m (4.6 feet) tall. Its skull is 38 cm (15 inches) long and has a deep, narrow beak that ends in a powerful hook (Figure 1).
FIGURE 1. Skull of terror bird Andalgalornis steulleti in side view, with mouth open and position of the eye reconstructed. Length of skull: 38 cm (15 inches). Click image to enlarge the illustration.
Copyright of Professor Lawrence M. Witmer (Ohio University).
CT scanning of a series of skulls from the collections of the Field Museum in Chicago established that Andalgalornis and other phorusrhacids has unusually rigid skulls. Most present-day birds have zones of flexibility between different parts of the skull. Typically, the upper bill can be raised or depressed relative to the braincase. In terror birds, these joints became inflexible, reinforcing the skull, particularly in the fore-and-aft direction.
Using data from the CT scans, the researchers developed detailed three-dimensional models of the skulls of Andalgalornis and two present-day bird species (an eagle and the red-legged seriema). They employed a method known as Finite Element Analysis to simulate and compare the mechanical behavior of the skull during jaw closure (during prey capture/killing), pulling the head back (when dismembering prey), and shaking the head from side to side (during struggle with larger, actively resisting prey) (Figure 2). The color images generated by these simulations show areas of blue color where stresses are low and red and white patches where stresses increased to dangerously high levels.
FIGURE 2. Stresses on the skull of Andalgalornis under three simulated uses.
Copyright of University of New South Wales/Universidad Nacional de La Plata/National Science Foundation.
These results supported the initial inferences from CT scanning. The structure of the skull of Andalgalornis appears well suited for driving the tip of the beak into the prey and then pulling back. However, shaking its head from side to side would have generated too much stress on the beak.
The researchers suggest that Andalgalornis either preferred prey smaller than itself, which it could quickly subdue and consume, or employed a strategy of repeated attack-and-retreat. It is possible that the large feet of terror birds could have been used to pin down struggling prey but, unlike present-day raptors, their toes do not bear talons.
In order to get baseline data for determining bite forces in birds, two of the researchers collaborated with staff at the La Plata Zoo to measure these forces at the tip of the bill in a red-legged seriema and a black-chested eagle using a bite meter. Using these measurements as well as previously published data, the team calculated that the beak of Andalgalornis only generated a relatively low bite force. The terror bird may have compensated for this weak bite by using its powerful neck muscles to drive the beak into prey like an axe.
Interested readers should check out an animation of the skull of Andalgalornis in the video below:
The animation of the skull of the terror bird Andalgalornis shows CT-scan slices going through the skull (revealing the source data for much of the research), as well as a series of powerful downward bites followed by pulling back of the skull by the neck. Andalgalornis was an extinct, 4.5-feet-tall, flightless predatory bird found as 6-million-year-old fossils in northwestern Argentina. Animation and caption by Ryan Ridgely & Lawrence Witmer, Courtesy of WitmerLab at Ohio University. For more on terror birds and the PLoS ONE article, please visit http://www.ohio.edu/witmerlab
Related National Geographic News stories:
Hans-Dieter (Hans) Sues is a vertebrate paleontologist based at the National Museum of Natural History in Washington, D.C. He is interested in the evolutionary history and paleobiology of vertebrates, especially dinosaurs and their relatives, and the history of ecosystems through time.
A former member of the National Geographic Committee for Research and Exploration, Hans has traveled widely in his quest for fossils and loves to share his passion for ancient life through lectures, writings, and blogging.
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