You can’t go for a month without seeing a claim that some new discovery has rewritten evolutionary history. If headlines are to be believed, phylogeny – the business of drawing family trees between different species – is an etch-a-sketch science. No sooner are family trees drawn before they’re rearranged. It’s easy to rile against these seemingly sensationalist claims, but James Tarver from the University of Bristol has found that the reality is more complex.
Tarver focused on two popular groups of animals – dinosaurs and catarrhines, a group of primates that includes humans, apes and all monkeys from Asia and Africa. Together with Phil Donoghue and Mike Benton, Tarver looked at how the evolutionary trees for these two groups have changed over the last 200 years. They found that the catarrhine tree is far more stable than that of the dinosaurs. For the latter group, claims about new fossils that rewrite evolutionary history (while still arguably hyperbolic) have the ring of truth about them.
National Geographic should have a 3-D animation up soon
The pursuit of accurate dinosaur colours just turned into a race, and a heated one at that. Just last week, I wrote about a group of scientists who claimed to have accurately identified the colours of some feathered dinosaurs by microscopically analysing three fossils. According to that study, Sinosauropteryx had a tail covered in ginger stripes. Now, another group have revealed the palette of an entire dinosaur, Anchiornis. This tiny predator had a dark grey body and the limbs bore long, white feathers tipped with black spangles. Its head was mostly grey with reddish-orange and black specks, and an extravagant reddish-orange crown.
Both reconstructions are based on microscopic structures called melanosomes. They’re partly responsible for the brilliant colours of modern bird feathers, they’re packed with pigments, and they happen to fossilise well. There are two major types. Spherical ‘phaemelanosomes’ contain a reddish-brown or yellow pigment while the rod-like ‘eumelanosomes’ have black-grey tints.
The technique of inferring colours from fossil melanosomes was pioneered by Jakob Vinther at Yale University. He used it to show that a Cretaceous bird feather probably had black and white stripes and, later, that another fossil feather had an iridescent starling-like sheen. But these were analyses of single papers and even last week’s paper coloured Sinosauropteryx by looking at just one part of a single individual.
Vinther isn’t impressed with his rivals. “They are in the Stone Age when it comes to understanding melanosome fossilization and interpretation of original colors,” he says. To him, it’s simply not enough predict colours based on the presence of one type of melanosome. Even the hues of single feathers can depend on a mix of the two melanosome types with different concentrations of pigments. So you need to know the distribution of melanosomes across an animal and even then, you still need to work out how that translates to different colours.
And that’s exactly what he’s done. When I spoke to Vinther last week, he said, “We are still far from putting colours on dinosaurs [but] the future is promising. Eventually we will have dinosaurs in technicolour. We are working seriously on that currently.” He wasn’t kidding!
He had been working on a new specimen of Anchiornis with the catchy name of BMNHC PH828. The tail is missing but the rest of the skeleton is beautifully preserved, including the skull and both sets of limbs with their elegant plumes. Rather than looking at individual body parts, Vinther took 29 samples from the specimen, representing every type of feather types across different body parts. In each one, he thoroughly analysed the size, shape, density and distribution of melanosomes.
To interpret this goldmine of data, he worked with his colleague Matt Shawkey to catalogue the melanosomes from a wide variety of living birds, from ravens to finches to mallards. This modern data set was a cross between a paint catalogue and a Rosetta stone. It told Vinther how different combinations of melanosomes led to different colours and allowed him to correctly paint his Anchiornis.
The question of whether dinosaurs were warm-blooded or cold-blooded is one of the most enduring in palaeontology. Did they generate their own body heat like today’s mammals; was their temperature more influenced by their environment like today’s reptiles; or did they use a mixture of both strategies? Scientists have put forward a slew of arguments for all of these alternatives, but Herman Pontzer from Washington University has a new take on things which suggests that many dinosaurs were indeed warm-blooded.
Based on our knowledge of living animals, Pontzer worked out the energy that 14 dinosaur species would have used while walking or running. His model reveals that these ancient reptiles would have needed more energy than a cold-blooded physiology could supply. Their metabolic demands were within the range of modern warm-blooded animals like mammals and birds, which can keep up their physical activity for far more time than their cold-blooded peers.
While both warm-blooded and cold-blooded animals can be equally active over short bursts, warm-blooded ones have the advantage in the long run with their higher capacity for aerobic exercise. This aerobic capacity is signified by a measurement called VO2max, which is often measured by getting animals to run on a treadmill. Obviously, that’s not feasible if the animal in question has been dead for 65 million years before the invention of the treadmill, but Pontzer had a solution. In earlier work, he showed that you can predict with 98% accuracy how much energy an animal needs to run or walk by looking at how high their hips were from the ground.
Pontzer looked at the hip heights of 13 species of dinosaur including Tyrannosaurus, Velociraptor and Archaeopteryx, as well as a closely related non-dinosaur called Marasuchus, and used these values to calculate crude estimates of their aerobic capacity. He focused on species that walked on two legs, since the way they distributed their weight is clearer-cut than four-legged relatives like Diplodocus or Triceratops.
His figures showed that the aerobic capacity of his dinosaurs, especially the larger ones, were consistently over the maximum values for living reptiles, from alligators to iguanas. Even while walking, the energy demands of the largest dinosaurs, particularly the large meat-eaters, would have far exceeded anything that cold-blooded animals could have coped with.
For more accurate estimates, Pontzer also used a mathematical model to calculate the size of the dinosaurs’ walking muscles and from these, their aerobic capacity. Again, he came to the same conclusion. The largest species simply wouldn’t have been able to function with cold-blooded metabolisms, and the smaller ones like Velociraptor could have walked but not run. Only Archaeopteryx, the smallest of the baker’s dozen, had VO2max values that approached the range of living cold-blooded animals.
There are a couple of alternative interpretations. It’s possible the larger dinosaurs were cold-blooded but had adaptations that granted them greater aerobic capacities than modern reptiles can achieve, although Pontzer thinks this unlikely. It’s also possible that the dinosaurs didn’t go in for sustained bursts of speed and, instead, relied on sprints, as many monitor lizards use to run down prey. But that would saddle the largest species with unfeasibly long recovery periods, when they could barely function.
Pontzer says that the relationships between his 13 species support the idea that all dinosaurs powered their runs with a warm-blooded metabolism. If he takes a conservative view of his estimates, the alternative explanation is that warm-bloodedness evolved at least three times – in the early sauropods, in the tetanurans (including birds and most of the carnivores) and in modern birds – and was lost once in between among the small predatory coelurosaurs like Velociraptor. That reconstruction is not only messy, but it contradicts evidence from bones and primitive feathers suggesting that the coelurosaurs were warm-blooded.
Nonetheless, there’s a risk that by looking exclusively at two-legged dinosaurs, Pontzer has biased his dataset to species that were perhaps most likely to be warm-blooded anyway. Groups like the massive sauropods and the diverse ornithischians are represented only by three of their earliest members – Plateosaurus, Heterodontosaurus and Lesothosaurus.
It’s still possible that the largest of the plant-eaters had a different physiology altogether including “inertial homeothermy”, where they maintain a constant temperature simply because their gargantuan bulks lose heat very slowly.
Pontzer’s new study far from settles the debate about dinosaur physiology, but it adds another piece of evidence into the mix. This debate isn’t just an academic fancy – it’s critical for understanding how the dinosaurs lived, evolved, and ultimately died.
Reference: Pontzer, H., Allen, V., & Hutchinson, J. (2009). Biomechanics of Running Indicates Endothermy in Bipedal Dinosaurs PLoS ONE, 4 (11) DOI: 10.1371/journal.pone.0007783
In 1995, a palaeontologist called Mark Norrell reported an amazing discovery – the fossilised remains of a dinosaur called Troodon, sitting on top of a large clutch of eggs. The fossil was so well-preserved and its posture so unmistakeable that it provided strong proof that some dinosaurs incubated their eggs just as modern birds do. And since then, two other small predatory species – Oviraptor and Citipati – have been found in brooding positions on top of egg clutches.
But a subtler look at these fossils reveal much more about dinosaur parenting than the simple fact that it existed. To David Varricchio from Montana State University, they also tell us which parent took more responsibility for the young. Based on the size of the egg clutches and the bones of the parent, Varricchio thinks that it was the males that cared for the babies. And given that small, predatory dinosaurs were the ancestors of modern birds, fatherly care was probably also the norm for the earliest members of our feathered friends.
Of all the back-boned animal groups, none show a greater equality of parental care that the birds. Among mammals, the next generation is mainly the mother’s responsibility and fathers help out in less than 5% of species. By comparison, male birds help to care for eggs and chicks in over 90% of living species. But Varricchio (together with Norrell and others) argues that this joint parenting is not how the dynasty started off.
The team noted that the clutches so delicately incubated by Troodon, Oviraptor and Citipati contained a substantial number of eggs, about 22 to 30 eggs apiece. Compared to most of the 433 living birds and crocodilians whose clutch sizes have been studied, the dinosaurs were sitting on far more eggs than animals of their size normally do. The team found that species where both parents chip in, or where mum takes the lead, usually settle for smaller clutches. Only those where dad does almost all of the work tend to rear such large broods.