Long neck helped reptile hunt underwater

UNIVERSITY OF ZURICH

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IMAGE: NOSTRILS LOCATED ON THE TOP OF THE SNOUT AND CURVED TEETH, PERFECTLY ADAPTED FOR CATCHING SLIPPERY PREY: THE SKULL OF TANYSTROPHEUS HAS SEVERAL CLEAR ADAPTATIONS FOR LIFE IN WATER. view more 

CREDIT: EMMA FINLEY-JACOB

Its neck was three times as long as its torso, but had only 13 extremely elongated vertebrae: Tanystropheus, a bizarre giraffe-necked reptile which lived 242 million years ago, is a paleontological absurdity. A new study led by the University of Zurich has now shown that the creature lived in water and was surprisingly adaptable.

For over 150 years, paleontologists have puzzled over Tanystropheus, its strangely long neck and whether it lived mostly underwater or on land. An international team led by the University of Zurich has now reconstructed its skull in unprecedented detail using synchrotron radiation micro-computed tomography (SRμCT), an extremely powerful form of CT scanning. In addition to revealing crucial aspects of its lifestyle, this also shows that Tanystropheus had evolved into two different species.

Underwater ambush predator

The researchers were able to reconstruct an almost complete 3D skull from a severely crushed fossil. The reconstruction reveals that the skull of Tanystropheus has several very clear adaptations for life in water. The nostrils are located on the top of the snout, much like in modern crocodilians, and the teeth are long and curved, perfectly adapted for catching slippery prey like fish and squid. However, the lack of visible adaptations for swimming in the limbs and tail also means that Tanystropheus was not a particularly efficient swimmer. "It likely hunted by stealthily approaching its prey in murky water using its small head and very long neck to remain hidden," says lead author and UZH paleontologist Stephan Spiekman.

Two species living together

Tanystropheus remains have mainly been found at Monte San Giorgio on the border between Switzerland and Italy, a place so unique for its Triassic fossils that it has been declared a UNESCO World Heritage Site. Two types of Tanystropheus fossils are known from this location, one small and one large. Until now, these were believed to be the juveniles and adults of the same species.

However, the current study disproves this assumption. The reconstructed skull, belonging to a large specimen, is very different from the already known smaller skulls, particularly when it comes to its dentition. In order to see whether the small fossils actually belonged to young animals, the researchers looked at cross sections of limb bones from the smaller type of Tanystropheus. They found many growth rings which form when bone growth is drastically slowed down. "The number and distribution of the growth rings tells us that these smaller types were not young animals, as previously considered, but mature ones," says last author Torsten Scheyer. "This means that the small fossils belonged to a separate, smaller species of Tanystropheus."

Specialists in different food sources

According to Spiekman, these two closely related species had evolved to use different food sources in the same environment: "The small species likely fed on small shelled animals, like shrimp, in contrast to the large species which ate fish and squid." For the researchers, this is a really remarkable finding: "We expected the bizarre neck of Tanystropheus to be specialized for a single task, like the neck of a giraffe. But actually, it allowed for several lifestyles."

Reconstructions of the skeletons of Tanystropheus hydroides (large species, newly named) and Tanystropheus longobardicus (small species). The outline of a 170 cm tall diver serves as the scale.###

Fossil mystery solved: Super-long-necked reptiles lived in the ocean, not on land

Twenty-foot-long specimens described as separate species from their cousins, named after mythology's Hydra

FIELD MUSEUM

IMAGE: AN ILLUSTRATION SHOWING TANYSTROPHEUS HYDROIDES HUNTING. view more 

CREDIT: EMMA FINLEY-JACOB

A fossil called Tanystropheus was first described in 1852, and it's been puzzling scientists ever since. At one point, paleontologists thought it was a flying pterosaur, like a pterodactyl, and that its long, hollow bones were phalanges in the finger that supported the wing. Later on, they figured out that those were elongated neck bones, and that it was a twenty-foot-long reptile with a ten-foot neck: three times as long as its torso. Scientists still weren't sure if it lived on land or in the water, and they didn't know if smaller specimens were juveniles or a completely different species--until now. By CT-scanning the fossils' crushed skulls and digitally reassembling them, researchers found evidence that the animals were water-dwelling, and by examining the growth rings in bones, determined that the big and little Tanystropheus were separate species that could live alongside each other without competing because they hunted different prey.

"I've been studying Tanystropheus for over thirty years, so it's extremely satisfying to see these creatures demystified," says Olivier Rieppel, a paleontologist at the Field Museum in Chicago and one of the authors of a new paper in Current Biology detailing the discovery.

Tanystropheus lived 242 million years ago, during the middle Triassic. On land, dinosaurs were just starting to emerge, and the sea was ruled by giant reptiles. For a long time, though, scientists weren't sure whether Tanystropheus lived on land or in the water. Its bizarre body didn't make things clear one way or the other.

"Tanystropheus looked like a stubby crocodile with a very, very long neck," says Rieppel. The larger specimens were twenty feet long, with their necks making up ten feet of that length. Oddly for animals with such long necks, they only had thirteen neck vertebrae, just really elongated. (We see the same thing with giraffes, which have only seven neck bones, just like humans.) And their necks were rather inflexible, reinforced with extra bones called cervical ribs.

In the same region where many of the big Tanystropheus fossils were found, in what's now Switzerland, there were also fossils from similar-looking animals that were only about four feet long. So not only were scientists unsure if these were land-dwellers or marine animals, but they also didn't know if the smaller specimens were juveniles, or a separate species from the twenty-footers.

To solve these two long-standing mysteries, the researchers used newer technologies to see details of the animals' bones. The large Tanystropheus fossils' skulls had been crushed, but Stephan Spiekman, the paper's lead author and a researcher at the University of Zurich, was able to take CT scans of the fossil slabs and generate 3D images of the bone fragments inside.

"The power of CT scanning allows us to see details that are otherwise impossible to observe in fossils," says Spiekman. "From a strongly crushed skull we have been able to reconstruct an almost complete 3D skull, revealing crucial morphological details."

The skulls had key features, including nostrils on top of the snout like a crocodile's, that suggested Tanystropheus lived in the water. It probably lay in wait, waiting for fish and squid-like animals to swim by, and then snagged them with its long, curved teeth. It may have come to land to lay eggs, but overall, it stayed in the ocean.

Rieppel wasn't surprised that evidence pointed to a water-dwelling Tanystropheus. "That neck doesn't make sense in a terrestrial environment," he says. "It's just an awkward structure to carry around.

The digitially reconstructed skull of Tanystropheus, using CT scans of the crushed skull pieces.

So that answered one question, about where Tanystropheus lived. To learn whether the small specimens were juveniles or a separate species, the researchers examined the bones for signs of growth and aging.

"We looked at cross sections of bones from the small type and were very excited to find many growth rings. This tells us that these animals were mature," says Torsten Scheyer, the study's senior author and a researcher at University of Zurich.

"The small form is an adult, which basically sealed the case," says Rieppel. "It's proven now that these are two species." The researchers named the larger one Tanystropheus hydroides, after the long-necked hydras in Greek mythology. The small form bears the original name Tanystropheus longobardicus.

"For many years now we have had our suspicions that there were two species of Tanystropheus, but until we were able to CT scan the larger specimens we had no definitive evidence. Now we do," says Nick Fraser, Keeper of Natural Sciences at National Museums Scotland and a co-author of the paper. "It is hugely significant to discover that there were two quite separate species of this bizarrely long-necked reptile who swam and lived alongside each other in the coastal waters of the great sea of Tethys approximately 240 million years ago."

The animals' different sizes, along with cone-shaped teeth in the big species and crown-shaped teeth in the little species, meant they probably weren't competing for the same prey.

"These two closely related species had evolved to use different food sources in the same environment," says Spiekman. "The small species likely fed on small shelled animals, like shrimp, in contrast to the fish and squid the large species ate. This is really remarkable, because we expected the bizarre neck of Tanystropheus to be specialized for a single task, like the neck of a giraffe. But actually, it allowed for several lifestyles. This completely changes the way we look at this animal."

This "splitting up" of a habitat to accommodate two similar species is called niche partitioning. "Darwin focused a lot on competition between species, and how competing over resources can even result in one of the species going extinct," says Rieppel. "But this kind of radical competition happens in restricted environments like islands. The marine basins that Tanystropheus lived in could apparently support niche partitioning. It's an important ecological phenomenon."

"Tanystropheus is an iconic fossil and has always been," adds Rieppel. "To clarify its taxonomy is an important first step to understanding that group and its evolution."