Were you to visit a wealthy philanthropist’s dinosaur island, you’d no doubt feel cheated to find yourself faced with an ostrich. It might not be as fearsome as Tyrannosaurus rex, but John Hammond wouldn’t technically be conning you out of your price of entry. After all, birds are dinosaurs.
The “wonderchicken” was among the first crown birds to emerge from the dinosaur lineage, living just before all the other dinosaurs were wiped out 66 million years ago. What defines it as one of the oldest known birds is that it shared a common ancestor with all birds alive today, touching a group that’s home to more species than mammals. If that’s the case – did the “age of dinosaurs” ever really end?
All birds are dinosaurs, but not all dinosaurs became birds
If you think of a pigeon whilst holding a figurine of Brachiosaurus, the concept that birds are dinosaurs seems laughable – and with good reason. Those long-necked vegetarian giants hail from a group of dinosaurs known as Sauropodomorpha, one of the three main clades of dinosaurs. You also have the Ornithischia, whose name literally means “bird-hipped”. Somewhat ironic, because this group doesn’t contain birds either.
Take a look at the third main clade, Theropoda, and things start to get interesting when it comes to the “birds are dinosaurs” idea. Among that diverse group of bipedal animals – which means walking on two legs, just like a pigeon – we have the Tyrannoraptorans, encompassing T. rex among many other Theropod groups and species.
One of those is the Aves, a crown group of warm-blooded vertebrates that includes birds and their closest relatives. One of its oldest members is Archaeopteryx – a small flying dinosaur that also qualifies as a bird. It’s considered a link between reptiles and birds, as well as being one of the most important fossil finds in palaeontological history.
If you want to sound really smart you can decry your humble pigeon a Pennaraptoran Tyrannoraptoran Theropod Saurischian dinosaur.
As we continue along the Aves family tree, we encounter Anseriformes, better known as waterfowl, and Galliformes, which includes gamebirds like pheasants and chickens. Eventually we come to Neoaves – an incredibly diverse group that contains almost every other living bird you can think of (bar ratites – those flightless, long-necked big boys).
It’s not the easiest family tree to visualize, but handily, it is one that Field Museum palaeontologist Dr Jingmai O’Connor went to the trouble of mapping out on a chalkboard for WTTW. Follow that chalk demonstration and you’ll see clear as day that yes, birds are dinosaurs, and if you want to sound really smart you can decry your humble pigeon a Pennaraptoran Tyrannoraptoran Theropod Saurischian dinosaur. Good luck down at the pub with that one.
Animals of a feather, flock together
That groundbreaking discovery of Archaeopteryx marked the first time we’d found an avian dinosaur with feathers back in 1861, however, we now know that feathers weren’t unique to this group. The first non-avian feathered dinosaur cropped up in 1996 at a time when we were still arguing whether birds were dinosaurs or not. It was called Sinosauropteryx, a compsognathid dinosaur (think the little green dinos in Jurassic Park) that didn’t sit within the Aves, and it blew that little “birds aren’t dinosaurs” argument right out of the water.
Finally, the world had the tangible connection it needed to point at and go, “Look! Feathers on a dinosaur!” In the modern world, feathers are unique to birds – we don’t find them anywhere else in the animal kingdom – but we can now see that they inherited their plumage from dinosaurs.
That first fossil had proto feathers that looked more like a hair. Modern feathers have a complex structure of barbs, barbules, and hooks that enable feathers to be pulled apart and zipped back up, but proto feathers weren’t quite so fancy. Just a single barb of a feather, they don’t look much like what we know to be feathers today, so people debated at the time of its discovery if they counted. Two years later, the fossil of the theropod dinosaur Caudipteryx was described and – being more closely related to birds than Sinosauropteryx – it had modern feathers.
What makes a bird a bird?
So, if all of these “bird-like traits” only emerged among the theropods, where do you draw the line? Where is it in the Tree Of Life that bird-like theropods become actual birds?
It pays to remember that the idea of species is – to some extent – a human construct. We can only build these groups based on the information available to us, and that becomes much harder when dealing with extinct animals. It comes down to looking at shared traits, but that gets more confusing the closer to that line you look.
Take Caudipteryx, for example. It was an Oviraptorosaur that was one of the first-known feathered dinosaurs that’s also said to have incubated its eggs, which were colorful – two very birdy characteristics. Then again, they also had long tails, teeth, and couldn’t fly. Not so birdy after all.
The fact is the differentiations we make don’t match up to what happens in evolution, and in nature species don’t suddenly morph from one thing to another. There was a time when flight was used as a deciding factor in the switch from dinosaurs to birds, but we now know that flight cropped up several times among dinosaurs.
O’Connor describes “What makes a bird a bird?” as one of the most complex palaeontological questions you can ask, but we did it anyway.
“When you look at the fossil record, essentially you have a problem where you can’t really say what makes a bird a bird when you’re looking at non-avian dinosaurs, and also early birds, because you can see that most bird features like bird features today, were actually features that were present in non-avian dinosaurs very closely related to birds and birds inherited these features, so it’s not unique to them,” she told IFLScience.
“A lot of avian evolution occurred within theropods, so these are features that were not present in all dinosaurs, but they evolved in theropods, and they become concentrated around the theropods most closely related to birds. You’re not going to see colored eggs, or you’re not going to see feathers forming a wing-like arrangement in Tyrannosaurus, which are a little bit further back. But if you’re looking at the dinosaurs really close to birds, they’re going to have colored eggs, they’re going to have feathers, like modern birds, forming little wing-like arrangements that we don’t know what they were using them for.
“So, it’s like the evolution of birds is something that occurred both in dinosaurs, and then also during the unique evolution of birds in the Cretaceous, which occurred for 90 million years.”
Do we still live in an age of dinosaurs?
If you consider diversity to be a major marker of success, then technically dinosaurs are still the most successful, diverse group of amniotes on our planet.
Dr Jingmai O’Connor
It’s certainly empowering to think your pet budgie could make such a mighty claim, but the answer hinges on whether you measure dominance by diversity or impact. Given who’s to blame in the latter scenario, we know which answer we prefer.
“Mammals were much less diverse when they first appeared in the Jurassic, and became more successful into the Cretaceous, but at the same time birds outnumber dinosaurs at least two to one, if not much higher than that,” said O’Connor. “We roughly estimate there’s about 4,000 species of mammals. A conservative estimate for bird avian diversity is 11,000, but there are estimates that are even more than double that.”
“If you consider diversity to be a major marker of success, then technically dinosaurs are still the most successful, diverse group of amniotes on our planet,” O’Connor said. ”In terms of impact, I guess you could argue that humans are making a greater impact – not necessarily a good impact – but in terms of what is really making up our ecosystems, the species diversity, dinosaurs are still dominant.”
This article first appeared in Issue 24 of our digital magazine CURIOUS. Subscribe and never miss an issue.
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