Following the successful tour of the Natural History Museum on Saturday the 24th, I’m launching a series of blog posts entitled “Evidence for Common Descent”, which will help many newly freed ex-Jehovah’s Witnesses and others to understand various aspects of the compelling evidence for common descent and by extension evidence for evolution by natural selection.
The question to start off with is, have dinosaurs gone extinct?
This may seem like a foolish question as we ALL know from childhood that 65 million years ago, an asteroid wiped out most life on the planet, including dinosaurs, leaving only a few small reptiles and mammalso on land.
Well, the answer is no, the dinosaurs didn’t go extinct.
On Saturday we talked about birds and why biologists believe they are related to dinosaurs (or more accurately, why birds ARE dinosaurs). This Thanksgiving and Christmas season, many people will be sitting down with family and friends to eat Turkey. Little do you know, you’ll be eating a dinosaur.
The way that we can tell that one organism is related to something else is by looking at its anatomy and what kind of characteristics that it shares with the other organism. A simple example of this would be how you look more like your parents than anyone else. but another example would be how both humans and chimpanzees have opposable thumbs.
These are called synapomorphies, traits shared by two or more groups of organisms, and derived from a common ancestor.
Over the last few hundred years, we’ve found many varieties of dinosaur bones in the ground and formed family trees on dinosaurs similar to the ones of living organisms. Just like we can make an assumption (educated guess) about the ethnic origin of a human by looking at their skin colour, hair and eye colour, nose and eye shape and hair type, we can look at the characteristics of birds to see which groups and sub-groups birds belong to.
There’s actually lots of clues preserved in bird bones that show that they are very closely related, and that birds are actually dinosaurs. So if you have a turkey for a Thanksgiving or Christmas, or a chicken for Sunday lunch you can see the evidence first hand. If you are a vegetarian or a vegan, you’ll have to rely on second-hand evidence.
In 1861, a fossil of an ancient feather was unearthed and then described by Hermann von Meyer, a German palaeontologist. Although they only had a single feather they named the creature Archaeopteryx meaning “ancient feather”. The feather was asymmetrical in shape, which indicates it had a higher aerodynamic performance. This indicated that the feather was already in a late stage of development and had come from earlier simpler feathers.
Within a year, they had found an almost complete skeleton of Archaeopteryx. It had a long lizard-like tail rather than the stumpy pygostyle of birds and seemed in every way to be intermediate between dinosaurs and modern birds. It became clear, to sceintists like Thomas Henry Huxley that these extinct animals were not birds but reptiles… but equally that they were the ancestors of birds. The similarities were just too much to deny.
Charles Darwin, in his On the Origin of Species, commented that some people thought “that the whole class of birds came suddenly into existence during the eocene period; but now we know, on the authority of professor Owen, that a bird certainly lived during the deposition of the upper greensand; and still more recently, that strange bird, the Archaeopteryx, with a long lizard-like tail, bearing a pair of feathers on each joint, and with its wings furnished with two free claws, has been discovered in the oolitic slates of Solnhofen. Hardly any recent discovery shows more forcibly than this how little we as yet know of the former inhabitants of the world.”
1. The Archosaur Split
Birds, crocodiles and dinosaurs are Archosaurs. The dinosaurs split from the crocodilian reptiles (Pseudosuchia) about 250 million years ago, but where do birds come in?
We know birds are Archosaurs because of a number of shared characteristics, including a distinct opening in front of the eyes called an antorbital fenestra, which is still present in the skull of modern chickens.
So now that we know birds are Archosaurs, which side of this split do birds fall on, dinosaur or crocodile? Well, a big clue is the way both these creatures walked. The crocodile has a “sprawled stance”, meaning it walks more like a lizard with its legs coming out of the side of its body. On the other hand, the dinosaurs walked upright on legs that went straight down. This is often called the “improved stance” and gave dinosaurs a big advantage over other groups. The sprawled stance caused crocodilians to live in niche environments, whereas the improved stance allowed them to adopt a wide range of lifestyles, and allowed them to rule prehistoric Earth, seas and skies.
Which side do birds fall on then? Evidently, the dinosaurian side, as birds typically have a fully erect posture, (as do mammals, but other evidence suggests each evolved it independently). The improved stance is often linked with the evolution of endothermy (warm-bloodedness) because it avoids Carrier’s constraint and thus allows prolonged periods of activity.
2. The Dinosaur Split
The next split took place within the dinosaurians. The Ornithischian and the Saurischian dinosaurs split about 240 million years ago.
We know that birds belong to the Saurischian group mainly because of their “hands”. Although birds don’t have hands in the sense that we hominids do, there is an analogue to hands which biologists usually call the “manus” (Latin for “hand”).
If you look at a bird’s manus, you can see that it has become extremely reduced. It is the relative length of the three bird fingers inside a wing which narrows down which group of dinosaurs birds descended from. In Ornithischian dinosaurs (eg. Stegosaurus and Triceratops), the third finger is longer, but in Saurischian dinosaurs, the second finger is longer. Birds evidently belong to the Saurischian dinosaurs.
In terms of number of fingers, you can see a big difference too, but we will come to that later.
n.b The Ornithischian dinosaurs also have a similar hip joint to birds, which is not shared with other Saurischian dinosaurs, but because of the number of other similarities, palaeontologists have determined that this is likely convergent evolution.
3. The Saurischian Split
The Saurischian dinosaurs further branched into two further groups known as Sauropods and Theropods. To know which one birds belong to, we need to look at the bird’s chest.
The biggest clue of which group birds belong to is their wishbone (known as a furcula). The furcula serves a purpose acting as a spring and supporting the flight muscles.
In the above picture, the top wishbone is that of a chicken and the bottom one that of an Allosaurus. A wishbone is actually a bird’s collarbones, but instead of two
separate collarbones like we have, theirs is fused into one.
Wishbones have only been found in birds and Theropod dinosaurs, like Allosaurus and Tyrannosaurus Rex, and they draw a clear line from those two extinct dinosaurs and modern birds.
We’ve found a furcula in most Theropod dinosaurs’ fossils that we’ve found around the world. Fossil wishbones are one of the most important pieces of evidence we have that birds descended from Theropod dinosaurs.
Another piece of evidence is their bones. Birds and Theropod dinosaurs are the only animals to have “hollow” bones. The walls of the bone is very thin and free of marrow.
4. The Theropod Split
Therapods themselves split into Ceratosauria and Tetanurae. Ceratosauria kept four fingers on their manus (and even 5 in some cases) and the Tetanurae lost one to give them a total of 3 fingers.
Again, as bird have only 3 fingers on the end of their manus, we can see that the are more closely related to the Tetanurae dinosaurs whose skeletons we have found.
One of our guests at the tour on Saturday noted that, the Tyrannosaurus rex skeleton in the Natural History Museum had only 2 fingers. It was a good observation because some Tetanuran dinosaurs, including Tyrannosaurus rex, lost a further finger, leading them to just 2 fingers on their manus.
We can get even more specific than Tetanuran. Another split happened among Tetanurans which can be seen in their hip-bone.
5. The Tetanurae Split
One of the branches, Tyrannosauridae (which includes Tyrannosaurus rex and other slightly smaller Tyrannosaurs such as Tarbosaurus, Albertosaurus, Daspletosaurus and Gorgosaurus), has a pubis which resembles all of the other dinosaurs in other clades, with that bone pointing forward, but the Maniraptora have a pubic bone which points backwards.
This helped the Maniraptors run at a much faster speed than the Tyrannosaurs. Films such as Jurassic Park show that Maniraptors such as Velociraptor were able to run as a great speed to catch prey and run from predators.
Another point of note is that many Maniraptors had feathers. They were probably used to keep the animal warm, in wing-assisted incline running and in courtship displays. It is highly controversial whether Tyrannosaurus rex has feathers, but due to recent discoveries, it is possible that they had feathers for at least a part of their life cycle.
Even Maniraptor feathers were whispy, more hair-like and similar to the downy feathers modern bird chicks. They had not developed barbs allowing the filaments to hook together, and they had not developed asymmetry allowing them to glide or fly.
6. The Maniraptora Split
Maniraptors further split into Dromaeosauridae and Avialae. The name of Avialae sort of gives this classification away as birds are classified as Aves, but apart from the name, there are features of the skeleton which can help us classify birds. Avialan dinosaurs were built for flight. Their feathers were fully developed asymmetrical which gave them better control in the air.
Dromaeosauridan Maniraptors like Velocirator had long tails, but Avialan Maniraptors like Confusiusornis had a reduced tail which in modern birds is called a pygostyle, (similar to our human coccyx which is also a reduced tail).
7. The Avialan Split
The final split was between modern birds and other Avialans. This saw the continued shortening of the pygostyle (tail), the specialisation of the fathers for flight, the loss of teeth (apart from the egg-tooth).
Due to the highly selective pressures of flights, various Avialans developed the features of modern birds in a non-linear manner, making it much more difficult to classify these more recent fossils than it was to place the fossils of their ancestors. However, what we do know is that a combination of those traits made it easier for modern birds to survive and that the Avialans with only one or two of these traits died out.
To conclude, birds are:
And these anatomical features are just a few of the similarities that scientists have used to place birds in the dinosaur family tree. There’s even more, from the shape of their necks, to their weird feet, to yes, even feathers.
If you click on the image below, you will see an interactive activity where you can take part in the classification of birds in the dinosaur family tree.
In addition, here is a video from HHMI summarising the information in this blog post:
- Epidexipteryx: bizarre little strap-feathered maniraptoran
- Long and Schouten’s Feathered Dinosaurs, a review
- Gary Kaiser’s The Inner Bird: Anatomy and Evolution
- Luis Chiappe’s Glorified Dinosaurs: The Origin and Early Evolution of Birds
- A truly tiny Cretaceous theropod… from England?
- Flight of the Microraptor
- Did Velociraptor and Archaeopteryx climb trees? Claws and climbing in birds and other dinosaurs
- Bird behaviour, the ‘deep time’ perspective
- Yi qi Is Neat But Might Not Have Been the Black Screaming Dino-Dragon of Death
- 50 Million Years of Incredible Shrinking Theropod Dinosaurs
- The Climbing, Flying Babies of Deinonychus
- The Romanian Dinosaur Balaur Seems to Be a Flightless Bird
- The Integrated Maniraptoran, Part 1
- The Integrated Maniraptoran, Part 2: Meet the Maniraptorans
- The Integrated Maniraptoran, Part 3 – Feathers Did Not Evolve in an Aerodynamic Context
Birn-Jeffery, A. V., Miller, C. E., Naish, D., Rayfield, E. J., Hone, D. W. E. 2012. Pedal claw curvature in birds, lizards and Mesozoic dinosaurs – complicated categories and compensating for mass-specific and phylogenetic control. PLoS ONE 7 (12): e50555. doi:10.1371/journal.pone.0050555
Hopson, J. A. 2001. Ecomorphology of avian and nonavian theropod phalangeal proportions: implications for the arboreal versus terrestrial origin of bird flight. In Gauthier, J. & Gall, L. F. (eds) New Perspectives on the Origin and Early Evolution of Birds: Proceedings of the International Symposium in Honor of John H. Ostrom. Peabody Museum of Natural History, Yale University (New Haven), pp. 211-235.
Lü, J. & Brusatte, S. L. 2015. A large, short-armed, winged dromaeosaurid (Dinosauria: Theropoda) from the Early Cretaceous of China and its implications for feather evolution.Scientific Reports 5: Article number 11775.
Yalden, D. 1985. Forelimb function in Archaeopteryx. In Hecht, M. K., Ostrom, J. H., Viohl, G. & Wellnhofer, P. (eds) The Beginnings of Birds – Proceedings of the International Archaeopteryx Conference, Eichstätt1984, pp. 91-97.
Zelenitsky, D. K., Therrien, F., Erickson, G. M., Debuhr, C. L., Kobayashi, Y., Eberth, D. A. & Hadfield, F. 2012. Feathered non-avian dinosaurs from North America provide insight into wing origins. Science 338, 510-514.