Terror birds weren't killed by humans — the Isthmus of Panama closed first
Terror birds were the apex avian predators that emerged on the island continent of South America after the dinosaurs died out, and they ruled for tens of millions of years; their eventual disappearance is tied to the great species interchange that followed the joining of the Isthmus of Panama, earlier than the arrival of humans in South America.
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The first fossil was filed as a relative of the anteater
In the 19th century the Ameghino brothers in Argentina were the first to find the animal, and all they had was a toothless piece of jawbone a few inches long. They took it for the lower jaw of a mammal whose teeth had dropped out during fossilization, and so placed it with the group containing anteaters, sloths and armadillos. Only later did it become clear that the jaw had no teeth because it was never a tooth-bearing jaw at all — it was part of a beak. That early confusion also explains why the nickname "terror bird" caught on more widely than the complicated scientific name.
— SteveTerror birds were already gone before the first humans reached South America
The oldest known terror bird fossils come from the Eocene, about 50 to 55 million years ago, but the fossil record is patchy and they may have appeared earlier — possibly getting under way not long after T. rex and Triceratops died out around 66 million years ago. Terror birds went on evolving and diversifying in South America, and the last group of them only vanishes from the fossil record a few tens of thousands of years ago — earlier than the arrival of the first humans in South America. That is why Brusatte is emphatic on the point: the extinction of the terror birds was not the work of humans. This time humans are not to blame.
— SteveOnly a continent cut off from the world could grow terror birds
For a long stretch after the dinosaurs died out, South America was an "island continent" like Australia today, connected to no other landmass — isolated for perhaps 60 to 90 million years. Gene flow for animals and plants was severed; in a biologist's phrase, everything evolved on its own in "splendid isolation," which is why the continent produced so many strange species found nowhere else. It was inside this ecological fortress that terror birds became the top predators; the cats, bears and dogs of North America had not yet entered South America, so birds held the large-predator niche.
— SteveBeing able to crack seeds was the get-out-of-extinction card after the impact
66 million years ago an asteroid about 10 kilometers wide struck near the Yucatán Peninsula in what is now the Gulf of Mexico, with more force than a billion nuclear bombs combined, plunging the world into years of darkness and cold. 75% of species went extinct. The large dinosaurs ate too much, grew too slowly and had a hard time hiding, so they fell first; the birds that survived were small-bodied, fast-growing, able to fly, and equipped with beaks that could eat seeds. With plants dying off en masse, seeds were just about the last food source left, and being able to crack them amounted to a get-out-of-extinction card.
— SteveBirds are dinosaurs, so the age of dinosaurs never ended
Once the nuclear winter lifted and sunlight came back through the clouds, the world was barren but full of opportunity. T. rex, Triceratops and the long-necked dinosaurs were gone, and evolution dislikes a vacuum: starting from a handful of survivors, birds and mammals diversified rapidly within a few million years, filling the herbivore, predator and every other niche. The host takes the moment to make the point: this period can still be called the age of dinosaurs, because birds are dinosaurs. Understand that sentence and you understand how terror birds could reach the top of the food chain in South America.
— SteveLosing flight wasn't decay; it was the trade that bought body size
The terror birds' closest living relatives are the seriemas; within the first few million years after the dinosaurs died out, birds of this group reached South America, and one lineage gave up flight. That was not degeneration but an evolutionary trade-off: a bird has to beat its wings with muscle, and once body size passes a certain threshold, flying becomes wildly uneconomical. Only by shedding the ability to fly could the body scale up to several hundred kilograms. Elephant birds and the "demon duck" were larger than terror birds, but they ate plants; the accurate title for a terror bird is therefore "largest avian predator," not "largest bird."
— SteveIt didn't bite and tear; it pinned prey down and jabbed repeatedly
A terror bird's skull was about the size of a horse's head. Birds have no teeth — in fact the embryo does start growing teeth inside the egg, but they are quickly replaced by the beak — and the terror bird's beak carried a large hook, like an eagle's beak enlarged and made sharper. Many of the bones in the skull were fused together, making it sturdier than in most living birds. Paleontologists built biomechanical models with engineering software and found that their bite force was strong, with the skull structure reinforced specifically to withstand that force. Brusatte's image: first pin the prey down with a clawed foot, then jab at it over and over in rhythm, like Muhammad Ali.
— SteveThe land bridge does not mean terror birds lost out to jaguars
About 2.5 million years ago North and South America were formally joined at the Isthmus of Panama, and the land corridor became a species highway: jaguars, bears and dogs moved south into South America, while opossums, armadillos and others spread north. This great species interchange was an enormous disturbance to South American ecosystems, and terror birds subsequently disappear from the fossil record. Brusatte does not simply put that down to losing a competition; the drastic ice-age climate shifts of the time may also have been at work, or both together. Some terror birds did reach Florida, Texas and California, but failed to establish themselves. The last terror birds went extinct tens of thousands of years before humans arrived in South America.
— SteveIn their own words · checked verbatim
Sometimes we find fossils of things that don't live anymore, we would have no idea they ever existed.
Steve2:02
These were birds that would have been terrified. The nickname suits them.
Steve3:03
It's really an evolutionary trade off because it's very hard to fly if you're big.
Steve21:06
Actually, birds start to grow teeth in the egg. but then very quickly, they're replaced by the beak.
Steve24:25
it's not clear if they were out competed necessarily by jaguars or bears they could have been on their way out otherwise there was climate change happening
Steve43:47
So we did not kill them. This one wasn't our fault.
Steve44:54
Figures
| Age of the oldest known terror bird fossils | About 50 to 55 million years ago | 6:17 |
| How long South America was an island continent | About 60 to 90 million years | 8:27 |
| Diameter of the impacting asteroid | About 10 kilometers (6 miles) | 10:38 |
| Force of the impact | Greater than a billion nuclear bombs detonating at once | 10:38 |
| Diameter of the crater | More than 100 miles | 10:38 |
| Share of species lost at the end of the Cretaceous | About 75% | 11:43 |
| When North and South America connected | About 2.5 million years ago | 41:33 |
| Last terror bird extinction relative to the first humans in South America | Tens of thousands of years ago (earlier than humans) | 44:54 |
Glossary
- Seriemas
- Ground-dwelling birds living in South America today, regarded as the terror birds' closest living relatives and far smaller than they were.
- Eocene
- The geological period after the dinosaurs died out, at the start of the age of mammals; the oldest terror bird fossils come from it.
- Isthmus of Panama
- The land corridor that connected North and South America about 2.5 million years ago, triggering the great species interchange.
How to listen
People gathering material for a podcast or for writing, and listeners who want a fast grip on how island evolution and a great species interchange reshape ecological niches.