From dinosaur to bird
From metabolism and genetics to diseases and behaviour, many of the things we associate with birds first evolved in the mighty dinosaurs.
Sometime in the early Cretaceous, a duck-sized dinosaur scratches, stretches and curls up among the fern fronds near the forest floor to sleep. She folds her hind limbs up beneath her body, and her feathered forearms in against her sides, before turning her long neck back and tucking it behind her elbow in much the same way birds roost, with their heads under their wings. The position is one this little meat eater adopts each day come nightfall, and it helps her keep warm in the chilly air. During her peaceful slumber a blanket of noxious gas from a volcanic eruption suffocates her, while ash rains down upon her, leaving her frozen in this sleeping posture and buried underground near Beipiao, Liaoning, for 130 million years.
That was until she was excavated and described in 2004 by Mark Norell, of the AMNH in New York, and Xu Xing, of the IVPP in Beijing. They called her Mei long – Chinese for ‘soundly sleeping dragon’. Finding clues to behaviour in fossils is rare, but when that behaviour is sleep it represents an extraordinary discovery. Not only did Mei hint that many common behaviours of modern birds evolved in their dinosaur ancestors, but she also added to the evidence that dinosaurs were active, warm-blooded creatures, similar to birds and mammals.
‘This specimen displays the earliest recorded occurrence of the stereotypical sleeping or resting behaviour found in living birds’, Xu and Norell wrote in their Nature paper on the find. ‘In birds, the tuck-in posture reduces surface area and conserves heat in the head, a major region of heat loss in these animals. It is therefore usually associated with heat conservation.’ The fact the animal was a primitive member of the troodontid group of small carnivores also suggested that the tuck-in sleeping posture was adopted in dinosaurs long before they evolved into birds.
In January 2012 the discovery of a second specimen of Mei long was reported, and it was in precisely the same bird-like roosting position, reinforcing the idea that the fossils were capturing a typical behaviour rather than a freak incident. As British author Colin Tudge writes, ‘It is pleasing to think that behaviour we assume is exclusively avian in fact was shared by some long-gone creature an almost incomprehensibly long time ago – not a monster at all, but rather sweet’.
Ancient heritage
The roosting behaviour of Mei is just one example of many, many traits and behaviours that were once thought to be the exclusive province of birds, but which we now know were shared with the dinosaurs. Of course these include such things as skeletal features, feathers, flight, beaks and nesting behaviour, but also more subtle similarities such as diseases and aspects of metabolism. Clues to many of these were found with new fossils; others came from existing dinosaur fossils in museum collections when experts re-examined them in light of the fact they were closely related to birds.
‘If you took an evolutionary tree [of dinosaurs and birds], and marked off all the features that we used to think of as unique to birds, you’d see loads of them go all the way back down that family tree now’, says Paul Barrett. ‘They are spread out along that branch rather than all being clustered where birds appear. Lots of things we thought were unique to birds go way back down the theropod family tree.’
Breathing system
One of these traits we now know evolved long ago is the unusual and specialised breathing apparatus birds have. Unlike mammals (which breathe by contracting and expanding their lungs to take air in and out via the same route) birds have a one-way system to pump air right through their bodies. This means they are much more efficient at getting oxygen in and out of their blood than mammals, and – combined with their light bones – it’s likely to have been a major factor in helping them take to the air, given flying requires great power and energy.
Birds have a small, rigid lung fixed to their backbone and attached to a massive system of flexible air sacs and capillaries. These work their way between the organs and tissues, and even invade the hollow bones, helping to create the honeycomb structure that makes them a fraction of the weight of mammal bones. This feature of bird bones is called pneumatisation.
‘A bird lung is just a stop along the way for air that’s flowing around this big system’, Barrett says. ‘The air goes in through the lungs, through this very complicated set of outpocketings of the lungs, and then out again. It’s a one-way system and it occurs in the opposite direction to blood flow, so it’s amazingly efficient.’ The oxygen content of the air in the lungs is at its highest when it comes in, and this is where the oxygen content of the blood is at its lowest, which results in excellent oxygen exchange between the two systems. This means birds can sustain the very high metabolic rates essential for flight.
There was already evidence that dinosaurs had the same kind of respiratory set-up, but a fossil of an Allosaurus relative revealed in 2008 clinched it. The wishbone, hipbone and ribs of the 10-metre carnivore discovered on the banks of Argentina’s Rio Colorado were filled with spaces and had the spongy look characteristic of pneumatisation. The experts behind the discovery – including David Varricchio of the University of Montana and Paul Sereno of Chicago’s Field Museum – named the species Aerosteon riocoloradensis.
More than any other dinosaur discovered so far, this fossil provided telltale signs that dinosaurs shared the bellows-like breathing system of birds. There was also a tantalising clue that Aerosteon – as heavy as an elephant – might have used the air sacs as a cooling mechanism as well. Cooling can be a significant problem in large animals, which lose heat more slowly to the environment than small animals (particularly dinosaurs, which didn’t have sweat glands as we and other mammals do). According to Sereno, Aerosteon had air sacs in unusual places, which came around the outside of the body and entered ribs in the belly region. It also appeared to have a network of tubes under its skin. Could it be that it was using them to dissipate body heat to the environment? Only more fossil evidence will tell, but it’s an interesting idea.
Many other fossilised dinosaur bones have cavities in them suggestive of air sacs, which leave characteristic marks on bones. Most theropods and some sauropods have these holes, strongly hinting that they had bird-like one-way lungs. The fact that the feature is widespread across terrestrial dinosaurs – and their close relatives the flying pterosaurs – suggests it appeared before either group evolved.
But if it wasn’t for flight, it’s not completely clear what the purpose might have been. One idea is that it helped dinosaurs attain massive size by allowing them to get oxygen into body tissues that would otherwise have been too far away from their lungs. Another suggestion is just that dinosaurs and pterosaurs were very active animals with high metabolisms, and the efficient breathing system allowed this. ‘People used to think the air sacs were simply associated with saving weight – and that makes a lot of sense with the giant size of dinosaurs’, says Mike Benton of the University of Bristol. ‘But many are now convinced that if they did have a sort of avian respiration with one-way flow of oxygen, that would have enhanced their efficiency.’
Other studies have shown that a number of dinosaurs, including Velociraptor and the ‘first bird’ Archaeopteryx, may have had bony levers (called uncinate processes) on their ribs. These work in birds to help move the ribs and sternum efficiently during breathing. The diaphragm of mammals allows them to force air into and out of the chest cavity. Instead, birds use muscles attached to both these rib-mover bones and their sternum to pump air through the sacs in their chest cavity effectively.
Modern birds occasionally use the air sac system for display purposes or to produce sounds (such as the male great frigatebird, found on tropical islands across the world, which inflates a large, red, balloon-like sac on its throat to woo females). Since dinosaurs had the same system of air sacs, it seems reasonable to assume they may have occasionally employed it for display purposes. While the thought of a dinosaur inflating a throat sac the size of a small car in an attempt to woo a mate is an intriguing one, of this the fossil record has sadly left no trace.
Growth and development
Development and growth is another area where dinosaurs are thought to have been surprisingly bird-like. Before the 1990s the prevailing wisdom was that dinosaurs grew in a similar way to living reptiles, such as crocodiles, which slowly increase in size throughout their lives, only attaining their greatest size in old age. But many studies of fossil dinosaur bones now hint that they, in a similar way to birds, underwent much more rapid bursts of growth in early life to reach a large size relatively swiftly.
‘Whereas dinosaurs may not have grown at rates exactly like those of extant birds and mammals, they seem generally to have been more like them than like other extant reptiles’, wrote Kevin Padian and other experts in Nature in 2001. When bones grow they create microscopic structural features that – somewhat similar to tree rings – can tell us about their growth rates. These features in the dinosaur bones showed a clear similarity to those in birds. The discovery was significant, because it offered clues as to how some dinosaurs, particularly the long-necked herbivorous sauropods, could grow larger than any other land creatures in the history of life on earth. The largest sauropods are now known to have weighed in at more than 100 000 kilograms, equal to about 20 African elephants.
In 2013 a remarkable cache of baby sauropod bones from Lufeng in China provided the best glimpse yet of how embryonic dinosaurs grew and developed. This collection of hundreds of minute fossil bones – many no wider than the lead in a pencil – and pieces of eggshell dates to more than 190 million years ago, in the Jurassic. As most juvenile dinosaur fossils are from the Late Cretaceous, this pushed the date of the oldest fossils of baby dinosaurs back by more than 100 million years. The bones are those of around 20 young Lufengosaurus (the most common dinosaur in this region in the Early Jurassic), and came from a number of different egg clutches at a range of stages of development. By comparing the length of the thighbones of young at different stages of development, experts led by Robert Reisz of the University of Toronto in Canada, showed they were growing at a faster rate than ever recorded in any other species of bird, mammal or dinosaur.
‘We are opening a new window into the lives of dinosaurs’, Reisz told reporters. ‘This is the first time we’ve been able to track the growth of embryonic dinosaurs as they developed. Our findings will have a major impact on our understanding of the biology of these animals.’ He argued that rapid, sustained growth of embryos, and brief incubation times in the egg, were factors that allowed Lufengosaurus to swiftly outgrow the carnivores that preyed upon them, and to reach lengths of about 9 metres.
More clues from the bones showed that the thighbones were being reshaped during growth, which suggested muscles were contracting and pulling against them. This was a clue that dinosaurs moved around inside their eggs, much as birds do, and was the first time this kind of avian behaviour had been detected in a dinosaur.
Warm-bloodedness
For several decades now the debate has continued to rage among dinosaur scientists about whether these animals were warm- or cold-blooded. Warm-blooded animals – including humans, all other mammals and birds – tend to have high body temperatures and can tightly control this temperature within a narrow range, generating heat internally as required. Cold-blooded animals – including reptiles such as crocodiles, snakes and lizards, as well as fish and amphibians – have body temperatures that vary with the conditions of the environment they are in, and they tend to have slower metabolisms.
Birds are active, warm-blooded animals with the high metabolism that seems necessary for flight. If this ability evolved somewhere in the lineage running up to birds, it’s likely that at least the carnivorous theropods from which they are descended were warm blooded too. Some experts argue that the herbivorous long-necked sauropods weren’t warm-blooded, as their massive body size would have caused them to retain all the heat they required. But the pterosaurs, close relatives of the dinosaurs, must surely have had high metabolisms, as they were active, flying animals too, and if they were warm-blooded, then maybe all dinosaurs were as well.
‘Based on the evidence of posture and locomotion, we actually think that some of these more predatory dinosaurs might have been warm-blooded, whereas some of the larger vegetarian animals might have been cold-blooded’, says Michael Novacek a palaeontologist at the AMNH. ‘We don’t know that directly, but one thing is certain: birds are warm-blooded animals and birds are a kind of dinosaur. So somewhere in the evolution of dinosaurs warm-bloodedness evolved, maybe in those dinosaurs that aren’t strictly classified as birds. Maybe Tyrannosaurus was warm-blooded. We don’t know that for sure, but it’s a good supposition.’
A number of different methods have been used over the years to probe fossils for evidence that might answer the question – the microstructure of the bone, for example. In cold-blooded species bone grows in dense rings, but in warm-blooded animals tightly packed cavities called Haversian canals can permeate the structure. Tests of many dinosaur bones appear to show evidence of Haversian canals, perhaps suggesting they were warm-blooded.
Accelerated development
Though the rapid development of young was a method already employed by the non-avian dinosaurs, the ancestors of modern birds used another trick in the timing of their development, which may have been part of the key to the massive success of the group. It may also account for some of the difference in appearance between birds and dinosaurs.
Bhart-Anjan Bhullar at Harvard University in Cambridge, Massachusetts, studies the origins of major vertebrate groups such as birds and mammals, and the unique features that define them. One of his projects involved bird skulls. Bhullar noticed a puzzling similarity between the large eyes, flat faces and rounded shape of bird skulls and those of baby dinosaurs. Rather than having elongated heads with long snouts and jaws as adult dinosaurs do, baby dinosaurs have rounded heads that are relatively large compared to their body size.
‘No one had told the big story of the evolution of the bird head before’, Bhullar said. ‘There had been a number of smaller studies that focused on particular points of the anatomy, but no one had looked at the entire picture … The origins of the features that make the bird head special lie deep in the history of the evolution of archosaurs, a group of animals [including crocodiles and dinosaurs] that were the dominant, meat-eating animals for millions of years.’
Working with Timothy Rowe at the University of Texas, Bhullar and his team compared adult and baby skulls of birds, dinosaurs and crocodiles. Their results showed that though dinosaur skulls changed in shape dramatically as they matured, adult bird skulls remain remarkably similar to those of juveniles. The conclusion was that the ancestors to modern birds had undergone a dramatic change in their development. In a process known as paedomorphosis or neoteny, they evolved to reach sexual maturity at an earlier stage of development. This meant their adult body shape no longer differed significantly from their baby body shape, and it also meant they concluded their development much more rapidly – in as few as 12 weeks in some species of bird.
Mark Norell of the AMNH was also involved in the work. ‘It was actually Mark who brought to our attention the hypothesis of paedomorphosis in birds, pointing out the similarity between the skull of Archaeopteryx and the skull of some baby non-avian predatory dinosaurs he’d collected in Mongolia’, Bhullar says. ‘Our results had already started to converge on this result, and we quickly moved to test it by including some additional relevant specimens. The results were immediately striking.’
Domestic dogs, too, are thought to be paedomorphic versions of wolves, accounting for their low levels of aggression, playfulness, large eyes and puppyish ways. A similar developmental process may also have been at work in the history of our own species. Researchers believe that adult humans retain the juvenile features of apes, such as flat faces and large braincases. In fact, it may have been the benefits of having a relatively larger and more complex brain that made this a good strategy in both humans and birds. In birds, larger brains may have allowed for more sophisticated visual processing and coordination, both of which are essential for flight.
Bhullar argues that rolling back development to an earlier stage in adults can have some unusual benefits. Because adult paedomorphic animals are typically smaller than their ancestors, it means that, for their size range, they may have unusual traits and adaptations. ‘Suddenly a population of small animals will pop up with a set of adaptations totally unlike those of the pre-existing small animals’, he says. ‘These unique characters may allow the exploitation of radically different ecological niches from other similarly sized organisms.’
It could be that fiddling with their development enabled the massive evolution and radiation of the 10 000 living species of birds – from the diving penguins of the Antarctic to the running ostriches of Africa and minute hovering hummingbirds of the Americas. According to Bhullar, specific features of specialised animals – the kind of things that lock them in to using a particular habitat and prevent them from evolving new kinds of body plan – tend to appear late in embryonic development. This means that paedomorphosis ‘has the potential to roll back these features to a more generalised state’, unlocking a fresh pool of potential.
Hands to wings
Though the vast majority of evidence from embryonic development points to birds evolving from dinosaurs, until recently something remained that confounded the overall picture of a link between the two. A difference in the arrangement of the bones in dinosaur hands and bird wings had left palaeontologists scratching their heads. That was until Limusaurus, a vegetarian theropod with a bird-like beak, was discovered by a team including James Clark, from George Washington University in Washington, DC, and Xu Xing.
The standard ancestral model for a vertebrate hand, foot, paw or hoof has five digits, and most vertebrates still display five digits in the early stages of embryonic development. Any groups of vertebrate that have fewer than five digits have lost some through evolution (horses have taken things to the extreme and now make do with a single elongated toe). During embryonic development, the wings of birds appear to form from the fusion of the three middle fingers of the basic vertebrate hand (that’s fingers two, three and four, if you imagine the thumb is one and the little finger is five). Similarly, most theropod dinosaurs, such as Allosaurus, have three digits on their hands. This is all well and good – or at least it was until fossils of early theropods, such as Dilophosaurus, suggested that the digits of the dinosaur hand were numbers one, two and three. Fossils of Dilophosaurus seem to have much-reduced fourth digits and almost absent fifth digits.
Clark and Xu’s 2009 study revealed that Limusaurus had a reduced first digit as well as other features of its hand similar to those of Allosaurus. Taken together, these facts suggested that Allosaurus and other later theropods had hands that matched the bone configuration of bird wings after all. These carnivorous relatives of birds ‘had digits 2, 3 and 4, but … these have long been misidentified as digits 1, 2 and 3’, Xu told Nature.
This problem with digit development was one thing sceptics of the link between dinosaurs and birds had hung onto as evidence that birds were descended from another group. With it, according to Xu, has gone one of the final puzzles clouding the relationship between theropods and modern birds.
The shrinking genome
It’s all very well hypothesising that dinosaurs that are closely related to birds were animals with an active lifestyle and a high metabolic rate, but is there any way to test the idea?
The authors of one clever study found a way to infer metabolic rate in the ancestors of birds. Birds have unusually small genomes (the sum total of their DNA) compared to other vertebrates, a property they share with the mammalian fliers, the bats. In the 1970s, Polish researcher Henryk Szarski hypothesised that small genomes are useful precursors to flight because they allow for a smaller cell nucleus and smaller cells with a larger relative surface area, equalling greater gas exchange and overall increased efficiency.
It sounds almost implausible, as the total mass of DNA in cells is minuscule. Nevertheless, flying is one of the most energy-intensive things an animal can do, so even minor gains can confer an advantage. A 2009 study confirmed the relationship by showing that hummingbirds, which have the highest metabolic rate of all birds, also have the smallest genomes. But how does this relate to dinosaurs? Several years earlier, geneticist Chris Organ, then at Harvard University and now at the University of Utah, came up with an idea for estimating dinosaur genome size.
Organ started to wonder how the small genomes of birds evolved. ‘Previous research suggested it was related to flight, because smaller cells are thought to be more efficient’, he says. ‘But our work suggests the small genomes of birds first appeared long before birds, and therefore powered flight, evolved … the changes may have started to happen as early as the ancestors of dinosaurs and pterosaurs.’
‘My co-authors and I knew that a relationship exists between cell size and genome size, but that relationship hadn’t been established in bone tissue’, Organ says. ‘We thought that if we could establish a relationship between genome size and bone cell size, we could use lacunae – the pockets within bone where cells reside – from fossils to infer genome size.
Organ’s team used the relationship between genome size and bone cell size in living species to predict genome size in dinosaurs from the size of lacunae in fossil bones. They applied the technique to the bones of 31 species of dinosaur and found that genome reduction was probably underway 230–250 million years ago, in the ancestors of the saurischian dinosaur group, which includes both theropods and their long-necked sauropod cousins. In contrast, the less bird-like ornithischian group (which includes the duck-billed hadrosaurs and Triceratops) did not have such small bone cells and presumably branched off before the genome reduction occurred.
The scientists have found that birds, bats and pterosaurs – the three groups of flying vertebrates – all evolved smaller genomes compared with their close relatives, strongly suggesting it was related to flight.
If we could find dinosaur DNA in fossils we could compare it with the DNA of birds to glean much new information about many aspects of dinosaur physiology. Studies have shown, however, that DNA is very unlikely to last more than 1 million years (see chapter 10), so the chance of finding it in fossils more than 66 million years old is virtually non-existent. Nevertheless, more resilient proteins and even some tissue material have been found in a few fossils, so experts are nowadays paying more attention to the chemistry and other fine details of the fossils they find.
A common misconception about fossils is that they are pure rock, but in many cases fossils retain some original bone material. In 2004, a forensic analysis of bone controversially purported to have found the remains of soft tissue, including blood cells, blood vessels and protein, in a 68-million-year-old T. rex bone from Montana. Researchers, including John Asara of Harvard University and Mary Schweitzer of North Carolina State University, subsequently analysed collagen they believed they had found and showed that the sequence of amino acids in it was a close match to that in chicken collagen, providing the first genetic evidence for a link with birds (since the sequence of amino acids in proteins is directly related to the DNA sequence used to produce them). In an effort to silence their critics and prove this wasn’t a onetrick wonder, the same team has now extracted soft tissue from the bone of an 80-million-year-old hadrosaur, Brachylophosaurus canadensis.
Dinosaur diseases and parasites?
It now seems that dinosaurs were even suffering from some of the same diseases that afflict modern birds. A team including David Varricchio and Jack Horner of Montana State University, as well as Steve Salisbury at the University of Queensland in Brisbane, believes that holes in the jawbones of Tyrannosaurus specimens were caused by trichomonosis, a nasty parasitic disease that is endemic in pigeons and fatal to birds of prey. Eagles, hawks and related species can pick up the infection from eating pigeons and pass it on to their chicks. In severe infections, lesions develop throughout the lower jaw and throat and start to eat away at the bone.
Some of the most famous T. rex specimens – including the first skull from which the species was described and ‘Sue’, the world’s most complete specimen, housed at Chicago’s Field Museum – appear to have characteristic marks of the infection in their jaws. This could have led to swelling, problems eating and eventually death, concluded a 2009 study published in the journal PLoS One. ‘This finding represents the first evidence for the ancient evolutionary origin of an avian transmissible disease in non-avian theropod dinosaurs’, wrote the authors. ‘It also provides a valuable insight into the palaeobiology of these now extinct animals.’
It’s plausible that some tyrannosaurs may have picked up the infection through cannibalism, but up to 60 per cent of specimens display evidence of face biting in battle, hinting at how the infection may have spread. ‘We can see similarities with what has been happening to Tasmanian devils recently, where a debilitating oral cancer is being spread by animals fighting and biting each other’s faces’, Salisbury says. ‘It’s ironic to think that an animal as mighty as Sue probably died as a result of a parasitic infection.’
The discovery and sale of ‘Sue’ is a whole fascinating story in itself. One of the largest, most complete and best preserved T. rex specimens ever discovered, she was found by chance in 1990 poking out of a cliff on a ranch near the Cheyenne River Sioux Indian Reservation in South Dakota. More than 90 per cent of the bones are preserved, and even the stapes, tiny bones that connect the eardrum to the inner ear, were found by preparators. At nearly 13 metres long, Sue is one of the largest of the 30 or so T. rex specimens discovered so far. The fact that the bones are so well preserved affords the opportunity to examine them for disease and other fine anatomical features, little trace of which are left in most other fossils.
Following FBI impoundment – and a protracted court battle between the commercial fossil hunters who discovered her and the rancher whose land they were on – Sue was eventually sold at auction to Chicago’s Field Museum for a figure of US$8.36 million in October 1997. Today this remains the most anyone has ever paid for a fossil, and it is thanks in part to sponsorship from Disney World and McDonald’s that Sue ended up in a scientific institution where she could be studied, rather than locked away by a wealthy collector.
Just a week before the auction, at Sotheby’s in Manhattan, one commentator showed a remarkable lack of foresight when he told the New York Times: ‘It’s going to cost Sotheby’s about a half million dollars to clean those bones, and the dinosaur bone market has been depressed by a lot of recent tyrannosaur discoveries … They’ll never get $1 million, and it’s really sad that Sue has come to this.’ How wrong he was. Today fossils are regularly auctioned for big money in the United States and there’s a thriving black market for specimens illegally shipped overseas from China and Mongolia (see chapter 5), but the auctioning of Sue was where it all began.
Trichomonosis is one disease thought to have been passed down from dinosaurs to birds, but another common affliction that followed the same route is lice. What may be the only genuine fossil louse ever discovered is a 44-million-year-old relative of the feather lice of modern birds, which was dug up from an ancient volcanic crater lake in Germany. Could an ancestor of this species have evolved as a parasite of feathered dinosaurs before the end-Cretaceous extinction event 66 million years ago? A 2011 study led by Vincent Smith of the Natural History Museum in London looked at the DNA of living lice and used it to create a family tree.
Smith’s team then used a clever method called the molecular clock. This estimates when species separated from one another based on differences in their DNA and on calculations of the rate at which changes tend to accrue naturally in genes as time goes by. Using what they knew about the 44-million-year-old louse species, they estimated that lice began separating out into different species on different host animals 115–130 million years ago.
But what were they living on? The great evolutionary radiations of feathery birds and hairy mammals that lice infect aren’t thought to have come until later. Perhaps these lice were infecting feathery dinosaurs instead. ‘Our analysis suggests that both bird and mammal lice began to diversify before the mass extinction of dinosaurs’, Smith told reporters. ‘And given how widespread lice are on birds, in particular, and also to some extent on mammals, they probably existed on a wide variety of hosts in the past, possibly including dinosaurs.’
Dinosaur diets
In 2012, experts including Phil Currie and Lida Xing announced they had found a specimen of a fuzzy 2.5-metre-long relative of Compsognathus, Sinocalliopteryx, with small birds and another feathered dinosaur species in its gut. They argued that the fact that Sinocalliopteryx was able to catch birds meant it was a highly capable stealth hunter that could strike them before they took flight.
Early birds such as Confuciusornis would not have been such proficient fliers as modern birds, however, and probably took longer to get airborne, so they may have been easier to catch.
That Sinocalliopteryx specimen had several other interesting characteristics. For a start, the smaller feathered dinosaur it had been eating along with the birds was Sinosauropteryx, a leg bone of which was found in its gut (so it had been chowing down on a Sinosauropteryx drumstick). It had gastroliths, too, small rounded stones in the gut used by modern birds in place of teeth to help grind down their food. In addition, one of the bones showed evidence of damage from a highly acidic foregut of the same kind used by modern crocodiles and vultures to break down and digest bones.
A specimen of four-winged flyer Microraptor found in the prehistoric Jehol ecosystem of north-eastern China had the remains of small Cretaceous birds in its stomach, and experts suggested it may have caught them while climbing and gliding between trees. A Velociraptor fossil with flying pterosaur bones in its gut is further adding to our picture of the diet and behaviour of these feathered predators.
More recent work on the crow-sized Microraptor show that in addition to the small birds and mammals it snacked upon in the forested, swampy environments of Cretaceous north-eastern China, it also ate fish, as seabirds do today. A 2013 analysis of the largest known fossil of Microraptor revealed a wad of partially digested fish bones inside it. The scientists behind that study also pointed to some other features suggestive of a piscivorous lifestyle that hadn’t been noticed before, such as forward-angled teeth for skewering slippery fish, and the fact that these teeth only have serrations on one side, perhaps an adaptation to prevent struggling prey from tearing itself apart. Only very few dinosaurs are known to have eaten fish, and these include the spinosaurs, such as Baryonyx, which had long crocodile-like snouts adapted for snatching fish out of the water. ‘Microraptor appears to have been an opportunistic and generalist feeder, able to exploit the most common prey in both the arboreal and aquatic microhabitats of the Early Cretaceous Jehol ecosystem’, wrote the authors of a study published in the journal Evolution.
What we know about diet and feeding behaviour even extends to the feeding style of some species of dinosaur, and here the evidence points to similarities with birds too. In 2013 a team of palaeontologists, medical imaging specialists and mechanical engineers used CT scans and sophisticated computer simulations to reconstruct the muscles, tendons and other soft tissues around the skull and neck of a 9-metre specimen of the predator Allosaurus. This enabled them to make the most detailed analysis yet of how Allosaurus could move its head and neck, and showed that it had great control, allowing it to tug at a carcass while holding it with its foot, ripping off chunks of flesh in the same style as a falcon or kestrel.
‘Allosaurus was uniquely equipped to drive its head down into prey, hold it there, and then pull the head straight up and back with the neck and body, tearing flesh from the carcass’, said lead author Eric Snively, a palaeontologist at Ohio University. ‘Many people think of Allosaurus as a smaller and earlier version of T. rex, but our engineering analyses show that they were very different predators.’ T. rex had a much heavier head, and research suggests it dismembered its prey by violently thrashing it from side to side, more in the style of a crocodile than a bird.
Attack of the venomous dinosaurs
There was even a tantalising hint in 2010 that another turkey- sized feathered theropod and close relative of Microraptor could have been venomous like a snake (or perhaps more accurately a Komodo dragon, which rather than injecting venom with fangs delivers it via saliva with a bite). Although the Jurassic Park movie – which depicted frill-necked, chirping Dilophosaurus spitting wads of poison into the eyes of its prey – implied we already knew about venomous dinosaurs, it was largely fanciful and not based on convincing fossil evidence. It was exciting news, then, in 2010, when Chinese palaeontologist Enpu Gong and colleagues of his at the University of Kansas suggested that feathered Sinornithosaurus may have been venomous. They reported evidence including long teeth with grooves ideal for delivering the poison, and a space in the skull that could have housed a venom gland.
‘Sinornithosaurus … has unusually long [upper] teeth that are morphologically similar to those of “rear-fanged” snakes specialised to carry poison’, they wrote:
This type of fang discharges venom along a groove on the outer surface of the tooth. The mechanism for dispensing the venom may be similar to the system used by openfanged snakes and lizards that discharge it under low pressure provided largely by force of the bite … We believe Sinornithosaurus was a venomous predator that fed on birds by using its long fangs to penetrate through the plumage and into the skin, and the toxins would induce shock and permit the victim to be subdued rapidly.
This was tantalising stuff, but six months later another team argued in Paläontologische Zeitschrift that the long teeth described by Gong’s group had just slipped out of their sockets in the specimen they’d looked at, and that the grooves for venom delivery were fairly common features of other related theropods that were not thought to be venomous. ‘We fail to recognise unambiguous evidence supporting the presence of a venom delivery system in Sinornithosaurus’, the authors of the rebuttal wrote. As is often the case in the world of science, it looks like it might be back to the drawing board on that one, but I haven’t given up hope that we’ll one day find evidence of exotic venomous feathered dinosaurs.