It began in the 1880s with a toothless jaw. And then some leg and hip bones and a vertebra were found. The leg bones, comparable in size to those of African ostriches, also bore similarities to fossils of the unrelated, giant, flightless Gastornis birds of Europe. Across the 1880s and 1890s, these discoveries slowly led archaeologists to realise they were dealing with a hitherto unknown group of giant, fearsome birds (1). With more complete fossil specimens subsequently discovered and clues provided by their unique morphologies, it did not take long for paleontologists to realise that all members of the “terror birds”, or Phorusrhacids, were carnivores, and that some were apex predators. 

Through isotopic dating of sediments in which terror bird fossils were found, paleontologists concluded that this taxonomic family existed from 43 million years ago (mya) – possibly even earlier – until their extinction 100,000 years ago (although no single species of Phorusrhacids survived this long) (2,3). Various fossils have since been found in South America and deemed to belong to Phorusrhacids species. Though most fossils have been found in Argentina, they have also been found in Brazil, Uruguay, Chile, Bolivia, Peru, the Southern United States and most recently, in Colombia. Throughout South America, there are various more fossils currently being discovered, some of which are being assigned to new species. At the moment, there are at least 18 characterised species, with some fossil-described species in contention of belonging to Phorusrhacids (4). 

Although size differed between species, there are morphological features common to all Phorusrhacids. Species such as Kelenken guillermoi, Phorusrhacos longissimus, and a few individuals of the North American Titanis walleri were giants at least 2 meters tall, weighing more than 100kg. Meanwhile, the shorter North American Titanis walleri was 1.4 to 1.9m in height and weighed an impressive 150kg (5,6). At the other extreme, the comparatively tiny Psilopterus bachmanni weighed only 4.5kg (7)! Smaller Phorusrhacids preyed on small vertebrates and invertebrates, with some species perhaps capable of short flight durations, filling a different predator niche than their larger counterparts (7).

Though the prehistoric South American environment, unlike today's, was generally grasslands and woodlands, different Phorusrhacids species lived in distinct habitats. These differences include variation in aridity, as well as differences in the large and small prey present in different localities (8). Furthermore, Earth’s overall climate also varied during the more than 40 million years in which terror birds were present, such that the habitats of different terror bird species living in different periods of geologic time also differed. Reconstruction of some specifics of each locality’s prehistoric environment is not always possible (9). Lastly, the earliest and latest discovered fossils of each species indicate the period during which a species survived, but the boundary at which a species becomes distinctly different from an ancestral species is not always clear (10). Here are some terror birds whose habitats are better understood:

• Phorusrhacos longissimus: an environment with water bodies and a mix of open and enclosed areas. For instance, the first discovered terror bird fossils originated from longissimus individuals living in what was later reconstructed to be temperate forests and bushlands. This bird survived during parts of the Miocene period (23 mya to 5 mya) (8,10)

• Titanis Walleri: Tropical grasslands with springs, similar to today’s Florida. This species lived in a more unique environment than other terror birds, from 5 mya to 1.8 mya (5,6)

But what did all the terror birds, large and small, have in common regarding how they hunted? 

From the structure of the terror birds’ legs, feet and hips, a paleontologist can infer features that suggest some terror birds were fast runners (11), or otherwise had limbs adapted for running. Despite natural uncertainties associated with paleontology, there is some headway into the running speeds of some terror bird species. For instance, the running speed of the 1m tall, 45kg Patagornis marshi was estimated to be 50 km/h (12,13), more than enough to chase down their prey. Once the prey was chased down, some terror birds would use their powerful legs to kick and incapacitate it, as suggested by features indicating strength in the bones of some species (14). Furthermore, some terror bird species possessed sharp claws, which are thought to have been used to stab prey (14). 

Though not all terror birds – especially the smaller species – were fast runners, all terror birds used their beaks when hunting, relying on beak strikes rather than the biting force used by many other birds. Their long necks were able to be flexed far backwards and forwards, allowing them to frontally strike prey repeatedly and powerfully with their beaks. Unlike that of many other birds, their ancestors and even their closest living relatives (the seriemas), the skull structure of most terror bird species is such that there is no moveable hinge between the upper beak and the skull due to the fusion of some bones in that region. This adaptation allows the skulls of Phorusrhacids to specifically resist loads from striking prey without suffering damage – though only if the strikes are precise (15). 

Other interesting features of the terror birds include gaze stabilisation and their hearing capacity. Based on their inner ear anatomies, the terror birds had the capacity for fast head movements while maintaining sight on their prey, evidencing their agility. Further evidence from the inner ear anatomies indicate the enhanced ability of the terror birds to hear low frequency sounds. Low frequency sound waves can travel a longer distance and are less affected by obstacles that absorb and scatter sound, allowing the terror birds to hear prey far out of sight. If terror birds were capable of producing low frequency sound as well, this would have enabled them to communicate from long distances apart (11). 

If one were to picture the heterogeneity of the terror bird species, they would probably imagine a predator in the act of hunting, or doing something else. In periods of geologic time with the greatest terror bird diversity, you may even be able to picture individuals of two different terror bird species, though you wouldn’t see two species of apex predator terror birds together (10). However, if you were to imagine beyond the bird, you would wonder how the flora, the other animals present, the climate, and many more all played a role in the story of the terror birds.

Tracing the lineages of the Phorusrhacids backwards, one would reach a bird capable of flight. The characteristic of complete flightlessness arose specifically in large Phorusrhacids species, which were apex predators that consumed large mammals (10). The extinction of dinosaurs, and the absence of large placental carnivores in South America from 65 mya to 3 mya, made the apex predator niche unfilled (16). Subsequently, they started to be filled by the ancestors of large Phorusrhacids. But with diverse fauna, why did terror birds become one of the apex predators, and not many other animal groups, for instance the South American marsupial mammals? It is a persistent evolutionary mystery in perhaps all of paleontology, with many possible explanations but few, if any, ways to test them (17). 

Two hypotheses have been proposed to explain the demise of the terror birds: the encroachment of North American fauna into South America beginning 9 mya; and the episode of global cooling that occurred 3 mya. Due to continental drift, the North and South American continents were drifting towards each other, with a land bridge formed by 3 mya, though the movement of some groups of animals across the gap began much earlier. Known as the Great American biotic exchange, North American placental carnivores, some of them large predators, moved into South America and rapidly diversified (9). The former hypothesis suggests that competition with these predators drove the terror birds to extinction. In the latter hypothesis, rapid cooling not only affected the terror birds, but also affected the ecosystems where the terror birds lived (9). Despite the lack of direct evidence that is able to resolve this uncertainty, the contingent belief is that the latter hypothesis is more likely to be true and that the encroachment of North American fauna in the former hypothesis had a small to none effect on the extinction of the terror birds (9,12). 

Attached to every bone and bone fragment is a history of discovery, of being dated, of measurement, of cataloging and sometimes, of reexamination. Every bone was once a part of the organism, each with the potential to yield valuable information. As a testament to how far science has come since the early days of fossil hunting, we now have a much larger cache of fossils to make comparisons to, we have the tools to model an organism’s mass and some of its biomechanics based on fossilised bones, and we even have the means to look at the bone structures under a light or electron microscope to infer some of an organism’s probable behavioural characteristics. 

The fact that we figured out this much about the birds is astounding. Fossils form only under specific conditions – an organism has to be buried before there is a chance of it being eaten and then covered with sediments in conditions where microorganisms that decompose the body cannot survive (such as anoxic environments). Scientists estimate that the fossil record contains less than 0.1% of all species that have ever lived (18)! Furthermore, it is common ecological knowledge that for every ecosystem, the population of apex predators is small and are less likely to be preserved in the fossil record. Many mysteries, ranging from their colours to their various behaviour, remain. Perhaps these mysteries are what deepen our curiosity and account for our fascination with these organisms. Still, we are truly fortunate to be able to infer so much from the terror birds’ unique morphology and get to know of them and their stories, beyond just what we imagine them to be. 

References

1. Buffetaut, E. Who discovered the Phorusrhacidae? An episode in the history of avian palaeontology. In: Göhlich UB, Kroh A, editors. Proceedings of the 8th International Meeting Society of Avian Paleontology and Evolution; 2013 Dec 10; Naturhistorisches Museum Wien. Vienna (AT): Naturhistorisches Museum Wien, 2013 [cited 2025 May 12.]. p.123-134. Available from: https://verlag.nhm-wien.ac.at/buecher/2013_SAPE_Proceedings/10_Buffetaut.pdf 

2. Jones W, Rinderknecht A, Alvarenga H, Montenegro F, Ubilla M. The last terror birds (Aves, Phorusrhacidae): new evidence from the late Pleistocene of Uruguay. PalZ [Internet]. 2018 Jun [cited 2025 May 12];92(2):365–72. Available from: http://link.springer.com/10.1007/s12542-017-0388-y 

3. Acosta Hospitaleche C, Jones W. Insights on the oldest terror bird (Aves, Phorusrhacidae) from the Eocene of Argentina. Historical Biology [Internet]. 2025 Feb [cited 2025 May 12];37(2):391–9. Available from: https://www.tandfonline.com/doi/full/10.1080/08912963.2024.2304592

4. Degrange FJ, Cooke SB, Ortiz‐Pabon LG, Pelegrin JS, Perdomo CA, Salas‐Gismondi R, et al. A gigantic new terror bird (Cariamiformes, Phorusrhacidae) from middle Miocene tropical environments of La Venta in northern South America. Papers in Palaeontology [Internet]. 2024 Nov [cited 2025 May 12];10(6):e1601. Available from: https://onlinelibrary.wiley.com/doi/10.1002/spp2.1601

5. Gould GC, Quitmyer IR. Titanis walleri: bones of contention. Bull Fla Mus Nat Hist. 2005 [cited 2025 May 12]. 45(4):201-229. Available from https://flmnhbulletin.com/index.php/flmnh/article/download/flmnh-vol45-no4-pp201-230/vol45-no4/1140 

6. Baskin JA. The giant flightless bird Titanis walleri (Aves: Phorusrhacidae) from the Pleistocene coastal plain of south Texas. Journal of Vertebrate Paleontology [Internet]. 1995 Dec 27 [cited 2025 May 12];15(4):842–4. Available from: http://www.tandfonline.com/doi/abs/10.1080/02724634.1995.10011266 

7. Degrange FJ, Noriega JI, Areta JI. Diversity and paleobiology of the Santacrucian birds. In: Bargo MS, Kay RF, Vizcaíno SF, editors. Early Miocene Paleobiology in Patagonia: High-Latitude Paleocommunities of the Santa Cruz Formation [Internet]. Cambridge: Cambridge University Press; 2012 [cited 2025 May 13]. p. 138–55. Available from: https://doi.org/10.1017/CBO9780511667381.010 

8. Vizcaíno SF, Bargo MS, Kay RF, Fariña RA, Di Giacomo M, Perry JMG, et al. A baseline paleoecological study for the Santa Cruz formation (Late–early miocene) at the Atlantic coast of Patagonia, Argentina. Palaeogeography, Palaeoclimatology, Palaeoecology [Internet]. 2010 Jun [cited 2025 May 13];292(3–4):507–19. Available from: http://dx.doi.org/10.1016/j.palaeo.2010.04.022  

9. Prevosti FJ, Romano CO, Forasiepi AM, Hemming S, Bonini R, Candela AM, et al. New radiometric 40Ar–39Ar dates and faunistic analyses refine evolutionary dynamics of Neogene vertebrate assemblages in southern South America. Sci Rep [Internet]. 2021 May 10 [cited 2025 Jun 1];11(1):9830. Available from: https://doi.org/10.1038/s41598-021-89135-1  

10. LaBarge TW, Gardner JD, Organ CL. The evolution and ecology of gigantism in terror birds (Aves, Phorusrhacidae). Proc R Soc B [Internet]. 2024 Apr 30 [cited 2025 May 13];291(2021):20240235. Available from: https://royalsocietypublishing.org/doi/10.1098/rspb.2024.0235

11. Degrange FJ. Research: The “Terror Bird:” Paleobiology of a Fierce Bird. 2015. Accessed May 13, 2025. https://www.myfossil.org/research-the-terror-bird-paleobiology-of-a-fierce-bird/ 

12. Marsà JAG, Agnolín FL, Angst D, Buffetaut E. Paleohistological analysis of “terror birds” (Phorusrhacidae, Brontornithidae): Paleobiological Inferences. Diversity (14242818) [Internet]. 2025 Mar 1 [cited 2025 May 12];17(3):153. Available from: https://doi.org/10.3390/d17030153 

13. Blanco RE, Jones WW. Terror birds on the run: a mechanical model to estimate its maximum running speed. Proc R Soc B [Internet]. 2005 Sep 7 [cited 2025 May 13];272(1574):1769–73. Available from: https://royalsocietypublishing.org/doi/10.1098/rspb.2005.3133 

14. Melchor RN, Feola SF, Cardonatto MC, Espinoza N, Rojas-Manriquez MA, Herazo L. First terror bird footprints reveal functionally didactyl posture. Sci Rep [Internet]. 2023 Sep 30 [cited 2025 Jun 1];13(1):16474. Available from: https://doi.org/10.1038/s41598-023-43771-x 

15. Degrange FJ, Tambussi CP, Moreno K, Witmer LM, Wroe S. Mechanical analysis of feeding behavior in the extinct “terror bird” Andalgalornis steulleti(Gruiformes: Phorusrhacidae). Turvey ST, editor. PLoS ONE [Internet]. 2010 Aug 18 [cited 2025 Jun 1];5(8):e11856. Available from: https://doi.org/10.1371/journal.pone.0011856 

16. Marshall LG. Scientific American. 1994 [cited 2025 Jun 1]. The terror birds of south america. Available from: https://doi.org/10.1038/scientificamerican0294-90 

17. Olson ME, Arroyo-Santos A. How to study adaptation(And why to do it that way). The Quarterly Review of Biology [Internet]. 2015 Jun [cited 2025 Jun 1];90(2):167–91. Available from: https://www.journals.uchicago.edu/doi/10.1086/681438 

    
    

18. How can I become a fossil? [Internet]. 2018 [cited 2025 Jun 1]. Available from: https://www.bbc.com/future/article/20180215-how-does-fossilisation-happen