Terror Birds – Flightlessness Made Terrifying

Article by: Adam Manning
Edited by: Lewis Haller, J. D. Dixon, and Harry Jones

Kelenken guillermoi, an Argentinian Terror Bird from the Miocene. Illustration by Jack Wood.

Name: Phorusrachidae
Age: Paleogene – Neogene
Diet: Carnivore
Size: Ranging from 60 cm to 3 m in height (depending on the species)
Location: South America and Texas (North America)

Terror birds, or phorusrhacids, may have one of the most appropriate names in palaeontology. They were a group of giant, running, flesh-eating birds, first evolving in South America 62 million years (Ma) ago, and only going extinct approximately 2.5 Ma ago. And yet these amazing predators are now extinct, so what caused the downfall of such a terrifying group of animals?

In 1887, Argentine palaeontologist Florentino Ameghino described the first terror bird, found in the Santa Cruz Formation in Patagonia, which dates back to 17 Ma. He named this new fossil Phorusrhacos longissimus. Ameghino and his colleagues initially thought of Phorusrhacos longissimus as being related to modern eagles and hawks, because of their shared carnivorous features, but not everyone agreed with this interpretation. In 1899, Charles W. Andrews resolved this argument of its ancestry concluding that it was most closely related to the modern South American seriema birds. In total, we now know of roughly 20 species of terror birds that became the dominant predators in South America for almost 60 million years. 

Terror birds came in a large variety of morphologies, allowing them to take advantage of many niches as predators within their ecosystem. Some were built for swift running, others for delivering powerful strikes with their massive beaks, but the true method of how they killed is unclear. It has also been discovered that some terror birds, like Mesembriornis milneedwardsi, were capable of using their legs to break apart bones to get at the marrow inside. It may have also used this ability to kill prey as well, similar to secretary birds today. 

However, a unique point of the anatomy of terror birds is that unlike other birds, their skull bones are fused together. This makes their skulls much more rigid and stable, so it was easier for them to use their heads as a weapon in delivering deadly strikes and ripping meat from the bone, rather than using their feet to deliver the killing blow. 

Moreover, terror birds may have used a similar method to modern seriema birds, picking up their prey and smashing it against hard surfaces repeatedly, like the ground, which is quite effective at breaking the bones and tenderising the meat. It is also possible that terror birds used all these methods depending on the circumstance, or different species favoured different methods.

There is evidence to suggest that terror birds were very good at hearing very low-frequency sounds compared to the average hearing capabilities of extant animal species, based on the analysis of their inner ear canals. They could have used this ability to communicate with each other and even detect prey.

So, with all of these amazingly petrifying adaptations, how did the terror birds go extinct? The short answer is that the rest of the world came knocking. South America had been an island continent for millions of years, until plate tectonics slowly but surely brought it together with North America. The land bridge that formed around 5 Ma, the Panamanian Isthmus, allowed the Great American Biotic Interchange to occur, allowing for animals on both continents to travel to the other for the first time. 

On average, North American native animals did better in South America than the other way around, because South America’s only large predators were the terror birds. This lead palaeontologists to initially say that it was competition with mammalian predators (e. g. Smilodon) that killed off the terror birds. Whilst this certainly didn’t help the terror birds, more recent theories suggest it was climate change that led to their extinction due to loss of habitat and prey species.

So in the end, it doesn’t matter how many killer adaptations you have, or how appropriate your name is. You are still at the mercy of the same problem that all species eventually face; when your environment changes, you must adapt to it, or face extinction.

Image References
[1] Kelenken guillermoi, an Argentinian Terror Bird from the Miocene. Illustration by Jack Wood.
[2] The skulls of a terror bird, a golden eagle, and a modern human. Available at: https://www.nationalgeographic.com/news/2010/8/100819-terror-birds-muhammad-ali-ax-hatchet-fought-science-dinosaurs-skulls/.

Information References and Further Sources
[1] Marshall, L. G. (2004). The Terror Birds of South America, Scientific American Special Edition, 14 (2), pp. 82-89. ISSN: 15512991. Available at: http://usuarios.geofisica.unam.mx/cecilia/cursos/TerrorBirds-Marshall94.pdf. Accessed 12th October 2019.
[2] Wilson, N. (2015). ‘The reign of the terror birds’, BBC Earth.
Available at: http://www.bbc.co.uk/earth/story/20150727-the-reign-of-the-terror-birds. Accessed 12th October 2019.
[3] Blanco, R. E., and Jones, W. W. (2005). Terror birds on the run: a mechanical model to estimate its maximum running speed’, Proceedings of the Royal Society B: Biological Sciences, 272 (1574), pp. 1769-1773. doi: https://doi.org/10.1098/rspb.2005.3133. Available at: https://royalsocietypublishing.org/doi/abs/10.1098/rspb.2005.3133. Accessed 12th October 2019.
[4] Chiappe, L. M., and Bertelli, S. (2006). Skull morphology of giant terror birds, Nature, 443 (7114), pp. 929. Available at: https://www.nature.com/articles/443929a. Accessed 12th October 2019.
[5] Degrange, F. J., Tambussi, C. P., Taglioretti, M. L., Dondas, A., and 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), p. e912656. doi:  https://doi.org/10.1080/02724634.2014.912656. Available at: https://www.tandfonline.com/doi/abs/10.1080/02724634.2014.912656. Accessed 12th October 2019.
[6] Alvarenga, H., Chiappe, L., and Bertelli, S. (2011). ‘7. Phorusrhacids: the Terror Birds’, in Dyke, G., and Kaiser, G. Living Dinosaurs: The Evolutionary History of Modern Birds. John Wiley & Sons. pp. 187-208. Available at: https://tinyurl.com/y3gan2s3. Accessed 12th October 2019.
[7] Woodburne, M. O., (2010). The Great American Biotic Interchange: Dispersals, Tectonics, Climate, Sea Level and Holding Pens, Journal of Mammalian Evolution, 17 (4), pp. 245–264. doi: 10.1007/s10914-010-9144-8. Available at: https://link.springer.com/article/10.1007/s10914-010-9144-8. Accessed 12th October 2019.
[8] Anitei, S. (2006) ‘The Largest Terror Bird’, Softpedia News. Available at: https://news.softpedia.com/news/The-Largest-Terror-Bird-38936.shtml. Accessed 29th October 2019.