Fossil Friday #4 - Phorusracids (Terror Bird)

66 million years ago, the K-Pg extinction event saw the end of the (non-avian) dinosaurs. This marked the beginning of the reign of mammals on earth, as they were able to diversify and evolve to fill almost all niches that were previously unattainable during the Mesozoic era. As with any trend in evolutionary biology, there are always outliers. Up until ∼100,000 years ago, South America was ruled by a group of theropods, the Phorusrhacids, also known as the Terror Birds.

A diverse group of birds, 18 species have been discovered so far, mostly in South America, an area that produced some of the Cenozoic's most astonishing animals, due to its isolation from the rest of the world up until the land bridge between North and South America was formed ∼2.6ma (AZ Animals, 2020). The type specimen, belonging to Phorusracos longissimus, is of a fragmented mandible, equalling 60cm in length, and was discovered by Florentino Ameghino at the Santa Cruz formation, Patagonia in 1887. At first, it was questioned as to what type of animal the mandible belonged to, and it wasn't until 1891, after more discoveries, that it was thought to have belonged to a very large bird (Alvarenga and Höfling, 2003).

Mandible of the holotype specimen of Phorusracos longissimus.

The presence of Phorusrhacids in South America meant that mammals were unable to flourish as much as they had throughout the rest of the world since the start of the Cenozoic era. They remained top predators and were able to diversify and spread across the continent due to a lack of competition from other groups. This changed ∼2.6ma during the great American interchange, when a land bridge was formed between North and South America, due to lowering sea levels that exposed parts of Central America, including what is now Panama. This changed the fate of this successful group of birds, as dominant predators from the north, such as the infamous Smilodon populator, quickly out-competed them in their environments, as the more powerful predator in South America (Woodburne, 2010; Gelfo and Plata, 2016). There are some rare examples, however of Terror birds successfully migrating to North America, as fossils have been found in the southern states of the USA. 

Titanis walleri, a relative of Phorusracos longissimus is a great example of this. First discovered in Florida in 1961, by its namesake, Benjamin Waller, it was the first Phorusrhacid to be found in North America. This species has been found to have been smaller in size than its South American counterparts, perhaps due to the fact there were other large predators already existing on the North American plains, that did not allow Titanis to grow larger (Brodkorb, 1963; Baskin, 1995). Despite this, it still stood at an impressive 1.9 meters tall and would have preyed upon mammals, as their relatives did in South America. Its long slender legs, a signature trait of predatory birds, would have allowed it to run at great speeds, allowing it to catch difficult prey, meaning it could achieve the difficult task of migrating to the North American plains during the Pliocene epoch.

  
Titanis walleri skeleton (Florida Museum of              Artists reconstruction of Titanis walleri, based on        Natural History, 2020).                                                features from large modern birds.

References

Alvarenga, H.M.F. and Höfling, E. (2003) “Systematic revision of the Phorusrhacidae (aves: Ralliformes),” Papéis Avulsos de Zoologia (São Paulo), 43(4). Available at: https://doi.org/10.1590/s0031-10492003000400001.

Baskin, J.A. (1995) “The giant flightless bird Titanis walleri (aves: Phorusrhacidae) from the Pleistocene Coastal Plain of South Texas,” Journal of Vertebrate Paleontology, 15(4), pp. 842–844. Available at: https://doi.org/10.1080/02724634.1995.10011266.

Brodkorb, P. (1963) “A giant flightless bird from the pleistocene of Florida,” The Auk, 80(2), pp. 111–115. Available at: https://doi.org/10.2307/4082556.

Gelfo, J.N. and Plata, M. (2016). “Horned Armadillos and rafting monkeys. the fascinating fossil mammals of south America". Cloth 320 pages, 197 color illus. series: Life of the past, Indiana University Press. ISBN: 978-0-253-02084-0,” Ameghiniana, 53(6), pp. 705–706. Available at: https://doi.org/10.5710/amgh.v53i6.1.

Phorusrhacos (2022) AZ Animals. Available at: https://a-z-animals.com/animals/phorusrhacos/#single-animal-text (Accessed: January 19, 2023).

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. Available at: https://doi.org/10.1007/s10914-010-9144-8.