are dinosaurs reptiles or birds

Roger Benson’s latest paper features a feathered, chicken-sized, bird-like dinosaur revealed to have the hearing ability to rival a barn owl – a specialised nocturnal predator. Combined with short, muscular arms ending in a single giant claw for digging, Shuvuuia deserti (meaning ‘desert bird’) is not what you might classically expect from a dinosaur. Such are the revelations from fossil discoveries in recent decades that are changing how we see birds today. Professor of Palaeobiology at the University of Oxford, Roger researches the evolution of dinosaurs – including bird origins – and large-scale evolutionary patterns. He explains why ‘dinosaur’ is more ‘incredible bird’ than ‘terrible lizard’…

There’s no longer really any doubt that birds are a type of dinosaur. These days, the debate is about details. The strong evidence doesn’t just come from fossilised bones and similarities found across the skeleton, but from fossilised soft tissue – especially feathers. Many dinosaurs had not just some kind of body covering, but distinctive bird-like feathers. Rare fossils also give us glimpses of the behaviour of bird-like dinosaurs, such as Mei long, a small, duck-sized bipedal dinosaur from the Cretaceous era. It was found preserved in volcanic ash falls – a bit like Pompeii – captured curled up in a sleeping position very similar to how a lot of birds roost today.

Birds belong to the theropod group of dinosaurs that included T. rex. Theropods are all bipedal and some of them share more bird-like features than others. Archaeopteryx, discovered in 1861, was for a long time the only truly bird-like dinosaur – it’s from the Late Jurassic era (150 million years ago). Others closely related to birds, like Velociraptor, can be from the Late Cretaceous (100-66 million years ago), so they’d also had a lot of time to evolve independently. It’s the Late Jurassic where we start finding really interesting, distinctive, bird-like dinosaurs – especially with recent fossils from China preserved in fine-grain sediments from lake beds.

Anchiornis is a Late Jurassic winged dinosaur, with large feather arrays on its legs. Fossils like this suggest the intriguing possibility that birds evolved from a gliding ancestor that had effectively four wings. That is cool. Also, Yi qi was discovered in the last couple of years. Its fossil has preserved soft tissue with a bat-like wing membrane.

Not all of the dinosaurian close relatives of birds could fly. But those that could, flew in a range of different ways – suggesting early evolutionary experiments of flight, with birds being the most successful of those experiments, and persisting to the present.

We can confidently think of Velociraptor as having a bird-like feather covering, even though its fossils only preserve the bones. The skeleton has quill knobs on the ulna (wing bone – also found on today’s birds). Close relatives are better-preserved and show a complete body covering, ranging from down to quill feathers. They couldn’t fly, so the feathers could be to do with display. Reconstructions have moved on a bit since Jurassic Park… Artists have only recently let go of scaly, reptilian-like depictions. We can think of dinosaurs as more bird-like than reptile-like.

For dinosaurs closest to birds – or, in fact, dinosaurs in general – we have so much evidence that suggests they were warm-blooded, short of actually sticking a thermometer in one. Growth rates and insulation are the smoking gun. They grow fast – we know from cutting up bones – faster than reptiles (including those from the same period), but not quite as fast as modern birds or mammals. For theropods where we can see soft tissue, we can see insulating feathers. All dinosaurs were on the road to becoming warm-blooded, with steps towards faster metabolic rates and very high body temperatures somewhat after the origin of birds. There wouldn’t have been a sluggish T. rex waiting for prey to walk by; we should think of them as active, curious animals.

If birds aren’t dinosaurs, then we have no idea what they are. Birds share so many features with theropods and there are no other candidate fossil groups. When you understand that birds are a type of dinosaur, that the evidence has stacked up, everything starts to make more sense. Birds inherit their bipedalism from theropods, explaining why they evolved flight using just their forelimbs, unlike bats or pterosaurs. If that hypothesis was wrong, we’d expect to be just as uncertain about bird origins today as 30 years ago.

Many groups have lost teeth and evolved a beak. Triceratops had a beak at the front of the mouth. Sheep also lack teeth at the front in their upper jaw. That’s for grabbing vegetation, and evolves frequently.

Palaeontologists ask ‘what makes Archaeopteryx a fossil bird rather than another bird-like dinosaur?’, and it’s the capability of powered flight. The wing feather arrangement is much more similar to modern birds. But the more we know about bird-like dinosaurs, the more we find that specific features of birds have an older origin. Walking on two legs, having feathers, laying eggs, warm bloodedness – they’re just inherited features from dinosaurs.

Not everyone agrees, but many think they were tree-climbing animals that glided. I think they evolved flight from the trees down. From an aerodynamic perspective it’s easier to see how that would work. To evolve flight from the ground up, evolution would need to master a number of different things about flight control and power quickly. That’s more difficult. When did they learn to climb trees? We would like to know!

Why were birds the only dinosaurs that survived the mass extinction at the end of Cretaceous?

We know from the fossil record that large-bodied land animals were hit hard. Only the tiny survived. The smallest dinosaurs weighed about 500 g, but to survive as a land mammal you needed to weigh less than 50 g, and even then the chances were very slim. Lots of bird groups also went extinct. All sorts of reasons have been suggested – such as being a seed-eater or fish-eater (after the 10-km diameter meteor struck, there was a lack of sunlight due to dust, and freshwater ecosystems were a refuge). Some places would have been less terrible than others – clearly proximity to the impact zone in Mexico would have been terrible – but there were global effects.

Will only small animals survive the current sixth mass extinction and the bird declines BirdLife scientists are monitoring today?

We know from the fossil record that mass extinctions happen, but each one has a different cause and pattern. So we can’t predict what will happen next based on past mass extinctions. It’s much better to look at what’s happening to birds and other animals right now. The extinction of the dinosaurs is the most abrupt – it could have happened in a single year. The current mass extinction seems a lot faster than some of the other mass extinction events, though, in terms of rate of decline of abundance and rate of species loss. It’s terrifying, but only if it continues. And we have some control of this if people take action.

The other lesson from mass extinctions is that the biosphere will recover. But not in our lifetimes – only on timescales that aren’t useful for human society. Some of our best fossil record studies monitor extinctions with data points every 100,000 years … So today, monitoring birds is one of the most important things we can do to catch species before they’re unknowingly lost.

I really like the Inaccessible Island Rail, the smallest flightless bird, living precariously on a tiny island. How can it be so small? It’s got something to tell us about evolution. No dinosaur had ever been that small. Islands are fascinating for an evolutionary biologist and we can’t risk losing this information from science, the sum of human knowledge. That’s just one reason why BirdLife’s work to protect island birds from introduced species is so important.

I like watching them because I like animals and birds are some of the most visible. You can say that a pigeon’s foot is similar to a dinosaur’s – birds have inherited so much from dinosaurs but are also so distinctive in their own right. I respect that and see them doing something fundamentally different to what most dinosaurs would have done. They use resources in a way that ground-walking animals can’t. At any one time there were probably only about 1,000 species of dinosaur on earth, whereas birds have taken what they’ve inherited from dinosaurs and done a lot more with it, giving rise to an enormous diversity of 11,000 species. People love dinosaurs and people love birds. What could be more interesting?

Though not everyone concurs, many believe they were animals that could glide up trees. I think they evolved flight from the trees down. Aerodynamically speaking, it is simpler to understand how that would function. Evolution would need to swiftly acquire a variety of skills related to power and flight control in order to develop flight from the ground up. That’s more difficult. We are curious as to when they learned how to climb trees!

Not every close relative of birds could fly during the dinosaur era. However, those that were able to fly did so in a variety of ways, indicating the earliest successful evolutionary flight experiments, of which birds are still extant.

The fossil record demonstrates that large-bodied land animals suffered severe impacts. Only the tiny survived. The tiniest dinosaurs weighed roughly 500 g, but even then, your chances of surviving as a land mammal were extremely slim if you were heavier than 50 g. Lots of bird groups also went extinct. Various explanations have been put forth, including the fact that it is a fish- or seed-eating species (due to the dust caused by the 10-km-diameter meteor impact, freshwater ecosystems provided shade during this period). There were worldwide effects, though some locations would have been worse than others. For example, being close to the impact zone in Mexico would have been horrible.

Without actually inserting a thermometer, there is a wealth of evidence indicating that dinosaurs, particularly those closest to birds, were warm-blooded. Growth rates and insulation are the smoking gun. By dissecting bones, we can determine that they grow more quickly than reptiles, even those from the same era, but not quite as quickly as contemporary birds or mammals. In the case of theropods, insulating feathers are visible where soft tissue is visible. All dinosaurs were moving toward warm-bloodedness, with rapid metabolic rates and extremely high body temperatures, somewhat following the emergence of birds. There wouldn’t have been a sluggish T. Rex should be viewed as curious, energetic animals that are patiently waiting for prey to pass by.

Why did only birds escape the mass extinction that occurred at the end of the Cretaceous period?

You may wonder why biologists have two systems of classification. Of course, their respective histories are a contributing factor, but they are also both beneficial in different ways. While the Linnaean system is more helpful for understanding how animals live, the phylogenetic system is useful for understanding the relationships between animals. Its sort of like cooking. All products derived from peanuts would be arranged on the same shelf if your ingredients were arranged according to their evolutionary relationships. The relationship between peanut butter, peanut oil, and peanut brittle was then evident. However, if you truly wanted to cook, you would combine all of your dry goods, oils, and other ingredients using something like the Linnaean system. So both systems have their uses.

Dinosaurs and crocodiles descended from archosaurs, but modern snakes, lizards, and turtles—groups that broke off at different times—were only tangentially related to them. Then, a massive extinction event occurred 65 million years ago, wiping out all dinosaur species save for a small group of feathered dinosaurs. Over the following 65 million years, these gave rise to modern birds. Thus, birds are actually dinosaurs, not just closely related to them! The closest living relative of birds is a crocodile, which descended from archosaurs. Although technically, birds, reptiles, and mammals all share a reptile-like ancestor, this is what most people mean when they say that birds are reptiles.

When people refer to birds as reptiles, they usually mean that they are more closely related to reptiles than to anything else, which is somewhat accurate, but there are many different kinds of reptiles. Birds are most closely related to crocodiles. To understand this, we should look at some history. Around 320 million years ago, the first groups of animals that resembled reptiles began to evolve. A group known as therapsids split off approximately 40 million years later—quite quickly by geologic standards—and eventually gave rise to modern mammals. Over the course of the next 120 million years, other groups of reptiles split off, with the archosaurs being one of the most successful branches.

Yes, birds are reptiles, but let me explain a bit. Linnaean and phylogenetic classification systems are the two types used by biologists. Carolus Linnaeus created the Linnaean system in the 1730s. Regardless of their lineage, organisms are categorized using traits in the Linnaean system. Thus, an ectothermic animal with scales is called a reptile; birds are not considered to be reptiles. A biologist named Willi Hennig developed a different classification scheme in the 1940s and named it phylogenetics. Under this system, the only way to group organisms is based on their ancestry, and traits are only used to determine ancestry. Therefore, any animal descended from the original reptile group is considered a reptile. Although mammals and birds have common ancestors that are occasionally referred to as reptile-like animals (Reptiliomorpha), it is uncommon for people to refer to mammals as reptiles. The situation is different for birds. Together with all other living reptiles (tuataras, crocodilians, turtles, and squamates, which primarily consist of snakes and lizards), birds are members of the Diapsida group.

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