Researchers discover overlooked Jurassic Park of lizards
New research published today in eLife by researchers from the Institut Català de Paleontologia Miquel Crusafont (ICP) and the University of Bristol (UB) moves back the moment of the radiation of squamates — the group of reptiles that includes lizards, snakes and worm lizards — to the Jurassic, a long time before current estimates.
The Squamata is the largest order of reptiles, including lizards, snakes and worm lizards. Squamates are all cold-blooded, and their skins are covered by horny scales. They are key parts of modern terrestrial faunas, especially in warmer climates, with an astonishing diversity of more than 10,000 species. However, understanding the evolutionary paths that forged their success are still poorly understood.
There is consensus that all the main squamate groups had arisen before the event that wiped out dinosaurs and other groups of reptiles at the end of the Mesozoic era. Before that global catastrophic event, through the Cretaceous, many terrestrial tetrapod groups like mammals, lizards and birds, apparently underwent a great diversification during the so-called Cretaceous Terrestrial Revolution, triggered by the rise of flowering plants. The scarcity of fossil remains of squamates through the Jurassic suggested that the main burst of squamate evolution happened in the Cretaceous (between 145 and 66 Myr.), when their fossil record dramatically improves.
Now, a new paper published in eLife, led by Arnau Bolet, paleontologist at the Institut Català de Paleontologia Miquel Crusafont and the University of Bristol, however, challenges this view by suggesting a much earlier radiation of squamates. Along with colleagues from the University of Bristol Michael Benton, Tom Stubbs and Jorge Herrera-Flores, their research concludes that this group of reptiles probably achieved a diverse array of adaptations in the Jurassic (between 201 and 145 Myr.), long before previously thought. “Even though Jurassic squamates are rare, reconstructed evolutionary trees show that all the main specializations of squamates evolved then, and it’s possible to distinguish adaptations of geckoes, iguanas, skinks, worm lizards, and snakes some 50 million years earlier than had been thought,” explains Michael Benton, co-author of the research.
But how could the scarce Jurassic fossils suggest an early burst in evolution? The key is in their anatomy. The few Jurassic squamates do not show primitive morphologies as would be expected, but they relate directly to the diverse modern groups. “Instead of finding a suite of generalized lizards on the stem of the squamate tree, what we found in the Jurassic were the first representatives of many modern groups, showing advanced morphological features,” says Arnau Bolet, lead author of the article.
The observed times of divergence, morphospace plots and evolutionary rates, all suggest that the Jurassic was a time of innovation in squamate evolution, during which the bases of the success of the group were established. According to these results, the apparent sudden increase in diversity observed in the Cretaceous could be related to an improved fossil record, capable of recording a larger number of species, or to a burst of origins of new species related to the new kinds of forests and insects.
Establishing the timing and mode of radiation of squamates is key for not only understanding the dynamics of terrestrial ecosystems in the Mesozoic, but also for deciphering how the group achieved an astonishing diversity of more than 10,000 species, only rivalled by birds among tetrapods.
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Pterosaur discovery solves ancient feather mystery
An international team of palaeontologists has discovered remarkable new evidence that pterosaurs, the flying relatives of dinosaurs, were able to control the colour of their feathers using melanin pigments.
The study, published in the journal Nature, was led by University College Cork (UCC) palaeontologists Dr Aude Cincotta and Prof. Maria McNamara and Dr Pascal Godefroit from the Royal Belgian Institute of Natural Sciences, with an international team of scientists from Brazil and Belgium.
The new study is based on analyses of a new 115 million year old fossilized headcrest of the pterosaur Tupandactylus imperator from north-eastern Brazil. Pterosaurs lived side by side with dinosaurs, 230 to 66 million years ago.
This species of pterosaur is famous for its bizarre huge headcrest. The team discovered that the bottom of the crest had a fuzzy rim of feathers, with short wiry hair-like feathers and fluffy branched feathers.
“We didn’t expect to see this at all,” said Dr Cincotta. “For decades palaeontologists have argued about whether pterosaurs had feathers. The feathers in our specimen close off that debate for good as they are very clearly branched all the way along their length, just like birds today.”
The team then studied the feathers with high-powered electron microscopes and found preserved melanosomes — granules of the pigment melanin. Unexpectedly, the new study shows that the melanosomes in different feather types have different shapes.
“In birds today, feather colour is strongly linked to melanosome shape.” said Prof. McNamara. “Since the pterosaur feather types had different melanosome shapes, these animals must have had the genetic machinery to control the colours of their feathers. This feature is essential for colour patterning and shows that coloration was a critical feature of even the very earliest feathers.”
Thanks to the collective efforts of the Belgian and Brazilian scientists and authorities working with a private donor, the remarkable specimen has been repatriated to Brazil. “It is so important that scientifically important fossils such as this are returned to their countries of origin and safely conserved for posterity” said Dr Godefroit. “These fossils can then be made available to scientists for further study and can inspire future generations of scientists through public exhibitions that celebrate our natural heritage.”
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T. rex’s short arms may have lowered risk of bites during feeding frenzies
Over the two decades paleontologist Kevin Padian taught a freshman seminar called The Age of Dinosaurs, one question asked frequently by undergraduates stuck with him: Why are the arms of Tyrannosaurus rex so ridiculously short?
He would usually list a range of paleontologists’ proposed hypotheses — for mating, for holding or stabbing prey, for tipping over a Triceratops — but his students, usually staring a lifesize replica in the face, remained dubious. Padian’s usual answer was, “No one knows.” But he also suspected that scholars who had proposed a solution to the conundrum came at it from the wrong perspective.
Rather than asking what the T. rex’s short arms evolved to do, Padian said, the question should be what benefit those arms were for the whole animal.
In a paper appearing in the current issue of the journal Acta Palaeontologia Polonica, Padian floats a new hypothesis: The T. rex’s arms shrank in length to prevent accidental or intentional amputation when a pack of T. rexes descended on a carcass with their massive heads and bone-crushing teeth. A 45-foot-long T. rex, for example, might have had a 5-foot-long skull, but arms only 3 feet long — the equivalent of a 6-foot human with 5-inch arms.
“What if several adult tyrannosaurs converged on a carcass? You have a bunch of massive skulls, with incredibly powerful jaws and teeth, ripping and chomping down flesh and bone right next to you. What if your friend there thinks you’re getting a little too close? They might warn you away by severing your arm,” said Padian, distinguished emeritus professor of integrative biology at the University of California, Berkeley, and a curator at the UC Museum of Paleontology (UCMP). “So, it could be a benefit to reduce the forelimbs, since you’re not using them in predation anyway.”
Severe bite wounds can cause infection, hemorrhaging, shock and eventual death, he said.
Padian noted that the predecessors of tyrannosaurids had longer arms, so there must have been a reason that they became reduced in both size and joint mobility. This would have affected not only T. rex, which lived in North America at the end of the Cretaceous period, he said, but the African and South American abelisaurids from the mid-Cretaceous and the carcharodontosaurids, which ranged across Europe and Asia in the Early and Mid-Cretaceous periods and were even bigger than T. rex.
“All of the ideas that have been put forward about this are either untested or impossible because they can’t work,” Padian said. “And none of the hypotheses explain why the arms would get smaller — the best they could do is explain why they would maintain the small size. And in every case, all of the proposed functions would have been much more effective if the arms had not been reduced.”
He admitted that any hypothesis, including his, will be hard to substantiate 66 million years after the last T. rex became extinct.
Arms and the T. rex
When the great dinosaur hunter Barnum Brown discovered the first T. rex fossils in 1900, he thought the arms were too small to be part of the skeleton. His colleague, Henry Fairfield Osborn, who described and named T. rex, hypothesized that the short arms might have been “pectoral claspers” — limbs that hold the female in place during copulation. This is analogous to some sharks and rays’ pelvic claspers, which are modified fins. But Osborn provided no evidence, and Padian noted that the T. rex’s arms are too short to go around another T. rex and certainly too weak to exert any control over a mate.
Over more than a century, other proposed explanations for the short arms included waving for mate attraction or social signaling, serving as an anchor to allow T. rex to get up from the ground, holding down prey, stabbing enemies, and even pushing over a sleeping Triceratops at night. Think cow-tipping, Padian said. And some paleontologists propose that the arms had no function at all, so we shouldn’t be concerned with them.
Padian approached the question from a different perspective, asking what benefit shorter arms might have for the animal’s survival. The answer came to him after other paleontologists unearthed evidence that some tyrannosaurids hunted in packs, not singly, as depicted in many paintings and dioramas.
“Several important quarry sites unearthed in the past 20 years preserve adult and juvenile tyrannosaurs together,” he said. “We can’t really assume that they lived together or even died together. We only know that they were buried together. But when you find several sites with the same animals, that’s a stronger signal. And the possibility, which other researchers have already raised, is that they were hunting in groups.”
Perhaps, he thought, the arms shrank to get out of the way during pack feeding. T. rex youngsters, in particular, would have been wise to wait until the larger adults were finished.
In his new paper, Padian examines speculations by other paleontologists, none of which appear to have been fully tested. The first thing he determined, by measuring the lifesize T. rex cast that dominates the atrium outside the doors of the UCMP, is that none of the hypotheses would actually work.
“The arms are simply too short,” he said. “They can’t touch each other, they can’t reach the mouth, and their mobility is so limited that they can’t stretch very far, either forward or upward. The enormous head and neck are way out in front of them and pretty much form the kind of death machine you saw in ‘Jurassic Park.'”
Twenty years ago, two paleontologists analyzed the arms and hypothesized that T. rex could have bench pressed about 400 pounds with its arms. “But the thing is, it can’t get close enough to anything to pick it up,” Padian said.
Beware of Komodo dragons
Padian’s hypothesis has analogies in some fearsome animals today. The giant Komodo Dragon lizard (Varanus komodoensis) of Indonesia hunts in groups, and when it kills prey, the larger dragons converge on the carcass and leave the remains for the smaller ones. Maulings can occur, as they do among crocodiles during feeding. The same could be true of T. rex and other tyrannosaurids, which first appeared in the Late Jurassic and reached their peak in the Late Cretaceous before becoming extinct.
Firmly establishing the hypothesis may never be possible, Padian said, but a correlation could be found if museum specimens around the world were checked for bite marks. That would be quite a feat of fossil crowdsourcing, he admitted.
“Bite wounds on the skull and other parts of the skeleton are well known in tyrannosaurs and other carnivorous dinosaurs,” he said. “If fewer bite marks were found on the reduced limbs, it could be a sign that reduction worked.”
But Padian has no illusion that his idea will be the end of the story.
“What I first wanted to do was to establish that the prevailing functional ideas simply don’t work,” he said. “That gets us back to square one. Then, we can take an integrative approach, thinking about social organization, feeding behavior and ecological factors apart from purely mechanical considerations.”
One problem in establishing the hypothesis is that there were several groups of large carnivorous dinosaurs that independently reduced their forelimbs, although in different ways.
“The sizes and proportions of the limb bones in these groups are different, but so are other aspects of their skeletons,” Padian said. “We shouldn’t expect them to be reduced in the same way. This is also true for the reduced wings of our large, living, flightless ratite birds, like the ostrich, the emu and the rhea. They evidently took different evolutionary paths for their own reasons.”
Padian sees a common thread in the history of explanations of short arms and other characteristics of T. rex.
“To me, this study of what the arms did is interesting because of how we tell stories in science and what qualifies as an explanation,” he said. “We tell a lot of stories like this about possible functions of T. rex because it’s an interesting problem. But are we really looking at the problem the right way?”
Padian’s paper is part of a Festschift honoring mammalian paleontologist Richard Cifelli, long-time head of the Oklahoma Museum of Natural History and Presidential Professor of Biology at the University of Oklahoma in Norman.
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Materials provided by University of California – Berkeley. Original written by Robert Sanders. Note: Content may be edited for style and length.