Repost: Terror Birds Ain’t What They Used to Be – A Titanis Takedown

[This essay was originally posted on February 12, 2011.]

You know a novel is going to be bad when the main endorsement on the jacket comes from the movie producer who is trying to turn the pulp horror story into a film. It’s the literary equivalent of saying “Well, my mom thinks I’m handsome.” All the same, I just couldn’t resist picking up James Robert Smith’s The Flock.

What drew me to Smith’s debut novel was his choice of antagonist. Instead of yet another bloodthirsty fish from depths unknown or a genetic experiment gone awry, he settled on Titanis walleri, one of the great ‘terror birds’ from the recent fossil record of Florida. (Just how distant has been a matter of dispute, but we’ll get to that in a moment.) Given a reprieve from the Pleistocene mass extinction, the birds raise hell for a Disney-esque suburban development, the local Fish and Wildlife authority, and an unhinged former Marine Colonel.

Unfortunately, the birds fail to live up to their fearsome reputation. Even though Smith gave them an overhaul – adding claws, a long tail, the ability to instantly camouflage themselves, and a knack for perfectly imitating human speech – the birds do virtually nothing but dither for about 300 pages, and their fate is ultimately left unresolved in a ham-fisted attempt to leave the story open for a sequel. Somehow I get the feeling that I am eventually going to see The Flock as one of the low-budget SyFy original movies, but I do have to thank Smith for piquing my interest in Titanis. Just how badass was Florida’s own terror bird, exactly?

That Titanis lived in what would eventually become the sunshine state is remarkable by itself. This bird was an immigrant to North America. Every other known terror bird – technically called phorusrhacids – lived in South America between 62 to 2 million years ago. Built like stout ostriches with large, hatchet-shaped heads, the terror birds were among the major predators of their day; a lineage of distant dinosaur descendants that lost the ability to fly and became adapted to hunting on the ground. This was not a unique occurrence. Large, ground-dwelling, carnivorous birds evolved multiple times over the past sixty five million years, including Gastornis from North America and Europe, some of the demon ducks of Australia, and the relatively recent giant stork of Flores Island. Big carnivorous birds were a recurring theme in prehistory, and the terror birds were arguably the most spectacular of all.

What brought the terror birds to North America was one of the greatest exchanges of animals to have ever occurred on the planet. Around three million years ago, the isthmus of Panama formed a solid connection between North and South America. What George Gaylord Simpson called the “splendid isolation” of South America was finally broken. Small animals had already been dispersing between the continents for millions of years by this point, but this final connection allowed many of the larger species – those incapable of island-hopping, flying, or rafting – to move between continents. Prehistoric elephants and saber-toothed cats moved south, giant sloths and armored glyptodonts shuffled into North America, and among the last of the terror birds also made the trek northwards.

Scientists first learned of the existence of Titanis in 1963. In a short report printed in The Auk, ornithologist Pierce Brodkorb described the massive bird on the basis of the lower part of the avian’s ankle – the tarsometatarsus – and an associated toe bone. It was not very much to go on, but the size and distribution of subtle anatomical landmarks identified the scraps as belonging to a terror bird, which Brodkorb thought was over six feet tall and comparable in size to its famous cousin Phorusrhacos from South America. The newly-discovered terror bird was closer to us in time, though, and Brodkorb took the fact these bones were found alongside still-living species of cormorant, scaup, and merganser to indicate that Titanis had lived during the Late Pleistocene, a little more than 12,000 years ago.

Titanis turned up in Texas, too. A 1995 note by Jon Baskin in the Journal of Vertebrate Paleontology announced the discovery of a toe bone from the big bird. Serendipitously, the bone came from the same position in the foot as the toe bone Brodkorb had described three decades before, and so the terror bird’s existence in Texas could be directly confirmed. Just how long ago Titanis lived in Texas was unclear – the fossil was dredged from a gravel pit where the age of the fossil layers was a bit fuzzy – but, on the basis of the fossils found at the same site, Baskin also proposed a Pleistocene age for the bird. Studies made after Brodkorb’s announcement had revised the Florida fossils to about 2.5 million years ago, but the Texas find brought Titanis back into the Pleistocene. Perhaps humans who wandered along the Gulf coast encountered these imposing birds, among the last of a long line of speedy, sharp-beaked killers.

Additional fragments of Titanis seemed to justify its fearsome reputation. Even though isolated fossils from the Florida sites were not enough to reconstruct the entire creature, one unusual bone was taken as a sign that this terror bird had re-evolved large claws for grappling its prey into submission. Described by Robert Chandler in 1994, the fused bones of the bird’s hand – the carpometacarpus – appeared to have a large, round attachment for a flexible thumb. Along with a thick portion of the bird’s humerus, Chandler used this strange osteological landmark to propose that the“hand of Titanis could not be folded underneath the rest of the wing as in other birds, and that the bird may have been equipped with a large thumb claw. Carl Zimmer, in a 1997 story about Chandler’s work, wrote that Titanis

… held [it’s wings] out in front of its body, palms facing inward, and on each hand was a giant movable claw and two smaller fixed claws. Titanis would stalk mammals in the tall grass, then attack at high speed, and strike with its giant beak, possibly taking a quick swipe at a victim’s spine to paralyze the prey as lions do. And they would use their arms, Chandler suggests, to keep the prey from goring them with their horns or kicking them with their feet. As much as an antelope might struggle, the terror bird’s sturdy arm bones could resist its force. They could manipulate prey with their hands and impale them with their claws. Chandler further speculates that Titanis’s arms were probably bare–feathers would get matted down with blood and be a likely haven for infection.

Over 60 million years after Tyrannosaurus and other Cretaceous monstrosities disappeared, Zimmer suggested, their avian cousins carried on their rapacious legacy. And it is little wonder that Smith used a bit of artistic license to add long tails to The Flock’s terror birds – with just that little extra touch, he had brought dinosaurs back to life without the need for complicated genetics laboratories or a lost world isolated on a rainforest pleateau. After all, twelve thousand years is a much more reasonable gap to overlook than sixty five million or more, making Titanis a convenient monster to bring up to the present day.

But, just as biologists have tossed out antiquated restorations of the dodo, scientists have recently given Titanis a makeover. For one thing, there is no sign that Titanis had clawed hands for clutching their hapless victims. In a 2005, state-of-the-fossil review, Gina Gould and Irvy Quitmyer noted that the closest living relatives of the terror birds – the seriemas of South America – also have rounded ball joints on their wings, yet they lack claws. If living seriemas don’t have claws on their wings, Titanis and other terror birds probably did not have them. Nor is there evidence that Titanis held its wings out in front or that its wings were especially robust. Of all the terror birds, Gould and Quitmyer calculated, Titanis had some of the smallest wings relative to its body size.

Titanis didn’t hunt humans, either. Confirmed in a 2007 Geology paper, this terror bird lived and died before people arrived at its coastal haunts. Previously the age of Titanis was based upon the estimated age of the other animals it lived alongside, but fossils from different layers appeared to be mixed together and confounded efforts to determine definite dates. To solve this problem, paleontologists Bruce MacFadden, Joann Labs-Hochstein, Richard Hulbert, and Jon Baskin looked at the signature of rare earth elements in the Titanis bones. Since bones take in these tell-tale markers during the fossilization process, bones from animals that lived about the same time should have more similar chemical signatures than those of animals that lived at more distant times.

By comparing the patterns of rare earth elements in the Titanis bones to those of mammals known to come from older Pliocene layers or relatively younger Pleistocene layers, the researchers were able to determine that Titanis was older than previously thought. The Texas fossils dated to about 5 million years ago, while those from Florida are about 2.2 to 1.8 million years old. Not only was Titanis gone by the time humans showed up at the close of the Pleistocene, but it was actually a relatively early immigrant to North America. The fossils from Texas are about two million years older than the final closure of the land bridge between North and South America, meaning that Titanis must have island hopped or swum across shallow waterways to arrive in North America before the peak of the intercontinental exchange. A paper published last year provides evidence that terror birds may have survived to the Late Pleistocene in Uruguay, but there is no sign that Titanis held out as long.

But, regardless of when it lived, the disappearance of Titanis is disappointing. Today’s ostriches, rheas, and cassowaries are not substitutes for the terror birds, just as the fact that birds are living dinosaurs provides little solace for those who have the impossible wish of seeing a living Tyrannosaurus or Allosaurus. “Velociraptor in fact was terror incarnate, a prime example of the general dinosaur type whose disappearance we so mourn,” Zimmer wrote in his profile of Titanis, “A crow hardly makes up for its loss.” The same could be said of terror birds and their seriema relatives, and the fact that Titanis had tiny wings instead of dinosaur-like arms tipped with vicious claws adds insult to injury. But die-hard terror bird fans should not lose heart. Thanks to new scientific techniques, we are beginning to investigate how birds like Titanis subdued their prey.

Frustratingly, very little of Titanis is actually known. The material is so scrappy that we can’t even be sure of just how big it was, although Gould and Quitmyer estimate that it was a relatively modest five feet tall. Reconstructions of its skeleton, like the one on display at the Florida Museum of Natural History, are composites which rely on the anatomy of better-known terror birds like Phorusrhacos to fill in the gaps, and we also have to turn to the relatives of Titanis to understand its hunting technique.

For decades, terror birds have been reconstructed as predators that swiftly ran down their prey and used their heavy beaks to slice through skin and muscle. This is apparent on the basis of their anatomy alone, but how fast could they run? Ernesto Blanco and Washington Jones approached this question six years ago by estimating the strength of the tibiotarsus – the lower leg bone between the femur and ankle – in three terror birds of varying size. By determining how strong this bone was, the maximum running speed of the birds could be calculated. Both a large, unnamed bird and the mid-sized Patagornis were estimated to reach speeds up to 30 miles per hour, while the smaller Mesembriornis was projected to reach the astonishing speed of 60 miles per hour – as fast as a cheetah. Did these birds really run this fast, though? Perhaps not. These are estimates of maximum speed based upon the strength of the bones, and, Blanco and Jones argue, there might have been other reasons for terror birds to have strong legs. In the case of Mesembriornis, specifically, its legs seem to be overbuilt, and the scientists suggest that this bird likely had a powerful kick to kill prey and perhaps crack bones to get at the marrow inside.

Terror birds varied in size and ran at different speeds. Some of the biggest were likely on the slower end of the spectrum, and there was enough variety among the terror birds – a major 2003 review recognized 13 genera and 17 species – that we should be wary of making blanket statements. Nevertheless, their foot claws and large, hooked beaks indicate that they were carnivorous, and this raises the question of how they used their beaks.

Just like body size, beak shape varied among the terror birds. Paraphysornis had a relatively short and deep beak, Mesembrionis had a beak reminiscent of modern shoebills, and the enormous Kelenken had an elongated, shallow beak with a pronounced hook on the end. Since terror birds lived for tens of millions of years, came in a variety of sizes, and ranged over an entire continent, it is probable that the different beak shapes indicate differences in diet, but a study published last year at least gives us a start for determining what their beaks were capable of.

Published in the PLoS One, the study by Federico Degrange and co-authors looked at the properties of an Andalgalornis skull. While it belonged to a different subgroup of terror birds, Andalgalornis had the classic, deep beak shape of the better-known Phorusrhacos, which Titanis likely shared, too. Despite the popular reputation of these birds as sueprpredators, though, their jaws were not very well-suited to tackling large prey – their rigid skulls were weak against the side-to-side stresses which would have been made by struggling prey. Terror bird skulls were better suited to dealing with forces in a front-to-back plane, as well as stresses on the hook at the front of their beak which would have been incurred when they tore away flesh from carcasses.

Most of the time, Andalgalornis probably killed and consumed small prey that could be swallowed whole, but, if it did try and tackle larger prey, the safest strategy would have been for it to swing its head downward to repeatedly strike the victim. If they tried to bite and hold onto large prey, such terror birds would have risked catastrophic injury to their skulls. Titanis, a terror bird that stood about 10 inches shorter than myself, was not the terror of the sloths, glyptodonts, and elephants it lived alongside. The creatures with the most to fear from Titanis were the lizards, snakes, rodents, and smaller birds that shared its habitat.

We are only just beginning to understand the natural history of the terror birds. Their fame is disproportional to what we really understand about their biology, and, despite its notoriety, Titanis is among the most poorly known. Titanis was not a monster or a Velociraptor returned from the dead, but a unique predator that gained a toehold on an unfamiliar continent among creatures its kind had never encountered before. That, alone, is an astonishing accomplishment, but the details of how this terror bird lived and why it disappeared so tantalizingly close to our own time remain mysterious.

Top Image: A reconstruction of Titanis on display at the Florida Museum of Natural History. Image from Wikipedia.

References:

Alvarenga, H., & Höfling, E. (2003). Systematic revision of the Phorusrhacidae (Aves: Ralliformes) Papéis Avulsos de Zoologia (São Paulo), 43 (4) DOI: 10.1590/S0031-10492003000400001

Alvarenga, H., Jones, W., & Rinderknecht, A. (2010). The youngest record of phorusrhacid birds (Aves, Phorusrhacidae) from the late Pleistocene of Uruguay Neues Jahrbuch für Geologie und Paläontologie – Abhandlungen, 256 (2), 229-234 DOI: 10.1127/0077-7749/2010/0052

Baskin, J. (1995). The giant flightless bird Titanis walleri (Aves: Phorusrhacidae) from the Pleistocene coastal plain of south Texas Journal of Vertebrate Paleontology, 15 (4), 842-844 DOI: 10.1080/02724634.1995.10011266

BERTELLI, S., CHIAPPE, L., & TAMBUSSI, C. (2007). A NEW PHORUSRHACID (AVES: CARIAMAE) FROM THE MIDDLE MIOCENE OF PATAGONIA, ARGENTINA Journal of Vertebrate Paleontology, 27 (2), 409-419 DOI: 10.1671/0272-4634(2007)27[409:ANPACF]2.0.CO;2

Blanco, R., & Jones, 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), 1769-1773 DOI: 10.1098/rspb.2005.3133

Brodkorb, P. (1963). A Giant Flightless Bird from the Pleistocene of Florida The Auk, 80 (2), 111-115

Chandler, R.M. (1994). The wing of Titanis walleri (Aves: Phorusrhacidae) from the Late Blancan of Florida. Bulletin of the Florida Museum of Natural History, Biological Sciences, 36, 175-180

Degrange, F., Tambussi, C., Moreno, K., Witmer, L., & Wroe, S. (2010). Mechanical Analysis of Feeding Behavior in the Extinct “Terror Bird” Andalgalornis steulleti (Gruiformes: Phorusrhacidae) PLoS ONE, 5 (8) DOI: 10.1371/journal.pone.0011856

Gould, G.C., and Quitmyer, I.R. (2005). TITANIS WALLERI: BONES OF CONTENTION Bulletin of the Florida Museum of Natural History, 45 (4), 201-229

MacFadden, B., Labs-Hochstein, J., Hulbert, R., & Baskin, J. (2007). Revised age of the late Neogene terror bird (Titanis) in North America during the Great American Interchange Geology, 35 (2) DOI: 10.1130/G23186A.1