Research: The “Terror Bird:” Paleobiology of a Fierce Bird

by Federico “Dino” Degrange

Federico hunting for terror bird fossils in Patagonia in 2015
Federico hunting for terror bird fossils in Patagonia in 2015

Editor’s note: Dr. Degrange received his B.S. degree in Biology with orientation in Paleontology from the University of La Plata and his Ph.D. from the same university. His main professional interest is centered on the evolution of zoophagy in Neornithes birds, with emphasis on the study of the paleobiology of extinct South American carnivorous Neornithes and on their interaction with each other and with non-carnivorous birds.

During most of my childhood, my grandfather used to tell me stories about how he had fought in his younger years against dragons with his Don Rodrigo Diaz de Vivar (the “Cid Campeador”) sword replica (of course, he would say that it was the real one!). Obviously, it sounds crazy to think that such creatures could ever exist, at least to grown-ups. However, years later, when I started my PhD, I analyzed a fascinating group of birds that clearly defies the concept of what a bird should look like. Particularly, the unusual skull and the way it functions is kind of strange and unique… like mythological dragons.

Poetry aside, these non-flying predatory birds are the Phorusrhacidae, a group of extinct birds related to extant seriemas (Cariamidae). Phorusrhacids were first discovered by the Ameghino brothers in the 1800s. The first species, Phorusrhacos longissimus, was unearthed from southern Patagonia in Argentina, published in 1887, and initially thought to be a mammal. A long time has passed since the first species was found and, over time, scientific discoveries have unearthed much more material (especially in Argentina) and led to the description of different, new species. However, it was only in 1978 that Larry Marshall nominated the group as the “terror birds” due to the fierce appearance he thought these birds may have had. The name stuck and phorusrhacids are popularly known around the globe by that name. The concept of the terror bird is easy to imagine: big, fierce, sanguinary, brutal, flightless, running birds … but does this image resemble the 18 phorusrhacid species actually known? The answer may seem disappointing: NO.

Truth be told, phorusrhacids comprise one of the most characteristic and diverse groups of birds from the South American Cenozoic, including larger species (some almost 3 m tall!), but also much smaller ones. Smaller representatives (some only 90 cm high) are not the image that the phrase “terror bird” conjures up in popular thought. Nevertheless, this is not disappointing at all! It tells us that terror birds were morphologically diverse, which is exciting! So, this may raise another question… if not all phorusrhacids were terrifying, does this mean that some species were not predators? Again: NO. All phorusrhacids were predators and they were one of the dominant predators in the food chain in South America during the Cenozoic. Regardless of size, they all seem to have had a unique technique to subdue prey which seems kind of mythological. Although these flightless birds had extremely powerful hind limb muscles attached to a long and slender pelvis which enabled them to chase prey, when the victim was at close range, phorusrhacids killed by striking using their large beaks as axes, with precise repeated vertical strikes! (Degrange et al., 2010).

This hypothesis was published based upon the species Andalgalornis steulleti, the same one that Marshall used for the “terror bird” concept. The unusual hunting technique was possible thanks to an exclusive feature that all phorusrhacids share: the inability to move the beak in relation to the brain cage. This is contrary to what happens in the rest of the Neognathae (the group of living birds that includes all birds except for tinamous and ratites), which includes phorusrhacids! And what is especially curious is that larger phorusrhacids had even more rigid skulls.

In 2012, we analyzed the morphology and function of the neck of Andalgalornis and concluded that the neck was adapted to perform this rare attack technique (Tambussi et al., 2012). Other than this paper and the 2010 publication (and my unpublished PhD thesis), there are only a few studies of these birds’ paleobiology on topics such as running speed estimations.

Note: Click on the photo for a closer view!

An (almost) complete skeletonn of Llallawavis (the new terror bird species) Photo courtesy of M. Taglioretti and F. Scaglia
An (almost) complete skeletonn of Llallawavis (the new terror bird species) Photo courtesy of M. Taglioretti and F. Scaglia

However, a recently discovered new species has enriched our paleobiological knowledge about these birds. Llallawavis scagliai was published this year (Degrange et al., 2015). Represented by the most complete skeleton of a phorusrhacid ever found (almost 95% of the total skeleton), the beauty of the material recovered brought new insights into the phylogeny and sensory capabilities of this fascinating group of birds. Based on the material, a more updated phylogenetic proposal was achieved, revealing that there are two types of phorusrhacids: 1) the “psilopterine” type (the “lesser terrifying” phorusrhacids), characterized by smaller sizes (although some species could reach 70 kg), slender bodies, and poor to non-flying capabilities; and 2) the “terror bird” type (the “truly terrifying” phorusrhacids) characterized by larger body masses (up to 180 kg) and the total absence of flying capabilities, and with even more rigid skulls. This new phylogenetic hypothesis shows us that phorusrhacids were more diverse by the late Pliocene (~3 million years ago) than previously thought, which obliges us to re-evaluate the possible causes of extinction of these fierce birds.

Skull of Llallawavis readied for CT scanning Photo © Federico Degrange
Skull of Llallawavis readied for CT scanning Photo courtesy Federico Degrange

Additionally, Llallawavis revealed a new aspect of phorusrhacids’ paleobiology. Through the use of CT scans, it was possible to reconstruct the bird’s inner ear anatomy. Inner ear anatomy provides two basic pieces of information – one related to head orientation and agility, and the other related to hearing capabilities. Llallawavis’ inner ear anatomy revealed that it was a very agile predator, capable of fast head movements, which are indispensable to gaze stabilization during pursuit of prey (because you need to keep your eyes fixed on the prey). Moreover, phorusrhacids were able to hear low-frequency sounds (~2,300 Hz). Unfortunately, this doesn’t mean that we can reconstruct the song of a phorusrhacid, but it does allow us to make some interesting paleobiological inferences. If low-frequency sounds could be heard, it seems reasonable to suggest that phorusrhacids could have also produced such sounds. Low-frequency sounds are transmitted over long distances with little attenuation, thereby enabling long distance communication, as well as the ability to hear prey that are out of sight.

There is a long path left to walk before we have a more holistic idea about the paleobiology and evolution of phorusrhacids. We do not yet comprehend how this predator interacted with other predators (phorusrhacids or not). However, it seems fair to state that we are closer to understanding what a phorusrhacid is: not just a fierce bird, but a fascinating one!

Contact information:

Dr. Federico Javier Degrange

Centro de Investigaciones en Ciencias de la Tierra (CICTERRA), CONICET and Universidad Nacional de Córdoba, Avenida Vélez Sársfield 1611, X5016GCA, Córdoba, Argentina. [email protected]

For further reading:

Degrange, F.J.; Tambussi, C.P.; Moreno, K.; Witmer, L.M. & Wroe, S. 2010. Mechanical Analysis of Feeding Behavior in the Extinct “Terror Bird” Andalgalornis steulleti (Gruiformes: Phorusrhacidae). PLoS ONE 5 (8): e11856. ISSN: 1932-6203.

Press release for article on the phylogeny and sensory capabilities of terror birds.

Degrange, F.J.; Tambussi, C.P.; Taglioretti, M.L.; Dondas, A. & Scaglia, F. 2015. A new Mesembriornithinae (Aves, Phorusrhacidae) provides new insights into the phylogeny and sensory capabilities of terror birds. Journal of Vertebrate Paleontology 35(2): e912656. DOI: 10.1080/02724634.2014.912656

Descriptions of the above paper on livescience

Interview with Federico and description of the above paper

Tambussi, C.P.; De Mendoza, R.S.; Degrange, F.J. & Picasso, M.B.J. 2012. Flexibility along the neck of the neogene terror bird Andalgalornis steulleti (Aves Phorusrhacidae). PLoS ONE 7 (5): e37701. ISSN: 1932-6203.