'Juvenile T. rex' fossils are a distinct species of small tyrannosaur

A new analysis of fossils believed to be juveniles of T. rex now shows they were adults of a small tyrannosaur, with narrower jaws, longer legs, and bigger arms than T. rex. The species, Nanotyrannus lancensis, was first named decades ago but later reinterpreted as a young T. rex.

The first skull of Nanotyrannus was found in Montana in 1942, but for decades, paleontologists have gone back and forth on whether it was a separate species, or simply a juvenile of the much larger T. rex.

Dr Nick Longrich, from the Milner Centre for Evolution at the University of Bath (UK), and Dr Evan Saitta, from the University of Chicago (USA) re-analysed the fossils, looking at growth rings, the anatomy of Nanotyrannus, and a previously unrecognized fossil of a young T. rex.

Measuring the growth rings in Nanotyrannus bones, they showed that they became more closely packed towards the outside of the bone -- its growth was slowing. It suggests these animals were nearly full size; not fast-growing juveniles.

Modelling the growth of the fossils showed the animals would have reached a maximum of around 900-1500 kilograms and five metres -- about 15 per cent of the size of the giant T. rex, which grew to 8,000 kilograms and nine metres or more.

The researchers have published their findings in Fossil Studies.

"When I saw these results I was pretty blown away," said Longrich. "I didn't expect it to be quite so conclusive.

"If they were young T. rex they should be growing like crazy, putting on hundreds of kilograms a year, but we're not seeing that.

"We tried modeling the data in a lot of different ways and we kept getting low growth rates. This is looking like the end for the hypothesis that these animals are young T. rex."

Supporting the existence of distinct species, the researchers found no evidence of fossils combining features of both the Nanotyrannus and T. rex - which would exist if the one turned into the other. Every fossil they examined could be confidently identified as one species or the other.

Neither did the patterns of growth in other tyrannosaurs fit with the hypothesis that these were young T. rex.

Dr Longrich said: "If you look at juveniles of other tyrannosaurs, they show many of the distinctive features of the adults. A very young Tarbosaurus - a close relative of T. rex - shows distinctive features of the adults.

"In the same way that kittens look like cats and puppies look like dogs, the juveniles of different tyrannosaurs are distinctive. And Nanotyrannus just doesn't look anything like a T. rex.

"It could be growing in a way that's completely unlike any other tyrannosaur, or any other dinosaur- but it's more likely it's just not a T. rex."

But that raises a mystery -- if Nanotyrannus isn't a juvenile Tyrannosaurus, then why hasn't anyone ever found a young T. rex?

"That's always been one of the big questions. Well, it turns out we actually had found one," said Longrich. "But the fossil was collected years ago, stuck in a box of unidentified bones in a museum drawer, and then forgotten."

The research led Longrich and co-author Evan Saitta to a previous fossil discovery, stored in a museum in San Francisco which they identified as a juvenile Tyrannosaurus.

That young T. rex is represented by a skull bone -- the frontal bone -- with distinctive features that ally it with Tyrannosaurus, but which aren't seen in Nanotyrannus. It comes from a small animal, one with a skull about 45 cm long and a body length of around 5 metres.

Dr Longrich said: "Yes, it's just one specimen, and just one bone, but it only takes one. T. rex skull bones are very distinctive, nothing else looks like it. Young T. rex exist, they're just incredibly rare, like juveniles of most dinosaurs."

The researchers argue these findings are strong evidence that Nanotyrannus is a separate species, one not closely related to Tyrannosaurus. It was more lightly-built and long-limbed than its thick-set relative. It also had larger arms, unlike the famously short-armed T. rex.

"The arms are actually longer than those of T. rex. Even the biggest T. rex, has shorter arms and smaller claws than in these little Nanotyrannus. This was an animal where the arms were actually pretty formidable weapons. It's really just a completely different animal -- small, fast, agile.

"T. rex relied on size and strength, but this animal relied on speed."

The long arms and other features suggest it was only distantly related to T. rex - and may have sat outside the family Tyrannosauridae, which T. rex is part of, in its own family of predatory dinosaurs.

The new study is the latest in a series of publications on the problem, going back decades.

Longrich said: "Nanotyrannus is highly controversial in paleontology. Not long ago, it seemed like we'd finally settled this problem, and it was a young T. rex.

"I was very skeptical about Nanotyrannus myself until about six years ago when I took a close look at the fossils and was surprised to realise we'd gotten it wrong all these years."

The authors suggest that, given how difficult it is to tell dinosaurs apart based on their often-incomplete skeletons, we may be underestimating the diversity of dinosaurs, and other fossil species.

Read more at Science Daily

The other side of the story: How evolution impacts the environment

The story of the peppered moths is a textbook evolutionary tale. As coal smoke darkened tree bark near England's cities during the Industrial Revolution, white-bodied peppered moths became conspicuous targets for predators and their numbers quickly dwindled. Meanwhile, black-bodied moths, which had been rare, thrived and became dominant in their newly darkened environment.

The peppered moths became a classic example of how environmental change drives species evolution. But in recent years, scientists have begun thinking about the inverse process. Might there be a feedback loop in which species evolution drives ecological change? Now, a new study by researchers at the University of Rhode Island shows some of the best evidence yet for that very phenomenon.

In research published in the Proceedings of the National Academy of Sciences, the researchers show that an evolutionary change in the length of lizards' legs can have a significant impact on vegetation growth and spider populations on small islands in the Bahamas. This is one of the first times, the researchers say, that such dramatic evolution-to-environment effects have been documented in a natural setting.

"The idea here is that, in addition to the environment shaping the traits of organisms through evolution, those trait changes should feed back and drive changes in predator-prey relationships and other ecological interactions between species," said Jason Kolbe, a professor of biological sciences at the University of Rhode Island and one of the study's senior authors. "And we really need to understand how those dynamics work so we can make predictions about how populations are going to persist, and what sort of ecological changes might result."

For the last 20 years, Kolbe and his colleagues have been observing the evolutionary dynamics of anole lizard populations on a chain of tiny islands in the Bahamas. The chain is made up of around 40 islands ranging from a few dozen to a few hundred meters in area -- small enough that the researchers can keep close tabs on the lizards living there. And the islands are far enough apart that lizards can't easily hop from one island to another, so distinct populations can be isolated from each other.

Previous research had shown that brown anoles adapt quickly to the characteristics of surrounding vegetation. In habitats where the diameter of brush and tree limbs is smaller, natural selection favors lizards with shorter legs, which enable individuals to move more quickly when escaping predators or chasing a snack. In contrast, lankier lizards tend to fare better where the tree and plant limbs are thicker. Researchers have shown that this limb length trait can evolve quickly in brown anoles -- in just a few generations.

For this new study, Kolbe and his team wanted to see how this evolved limb-length trait might affect the ecosystems on the tiny Bahamian islands. The idea was to separate short- and long-legged lizards on islands of their own, then look for differences in how the lizard populations affect the ecology of their island homes.

Armed with specialized lizard wrangling gear -- poles with tiny lassos made of dental floss at the end -- the team captured hundreds of brown anoles. They then measured the leg length of each lizard, keeping the ones whose limbs were either especially long or especially short and returning the rest to the wild. Once they had distinct populations of short- and long-limbed lizards, they set each population free on islands that previously had no lizards living on them.

Since the experimental islands were mostly covered by smaller diameter vegetation, the researchers expected that the short-legged lizards would be better adapted to that environment, that is, more maneuverable and better able to catch prey in the trees and brush. The question the researchers wanted to answer was whether the ecological effects of those highly effective hunters could be detected.

After eight months, the researchers checked back on the islands to look for ecological differences between islands stocked with the short- and long-legged groups. The differences, it turned out, were substantial. On islands with shorter-legged lizards, populations of web spiders -- a key prey item for brown anoles -- were reduced by 41% compared to islands with lanky lizards. There were significant differences in plant growth as well. Because the short-legged lizards were better at preying on insect herbivores, plants flourished. On islands with short-legged lizards, buttonwood trees had twice as much shoot growth compared to trees on islands with long-legged lizards, the researchers found.

The results, Kolbe says, help to bring the interaction between ecology and evolution full circle.

"These findings help us to close that feedback loop," Kolbe said. "We knew from previous research that ecological factors shape limb length, and now we show the reciprocal relationship of that evolutionary change on the environment."

Read more at Science Daily