Your Holiday Dinosaur
December 2, 2014
Most vertebrate paleontologists agree that modern birds evolved from dinosaurs. To paleontologists, there is no simple bird-dinosaur dichotomy. Rather there is a continuum of animals that are at first, clearly dinosaurs (like Tyrannosaurus and Velociraptor), then at the end are clearly birds, like all the modern birds that we see. We recognize lots of ‘in between’ animals, like Archaeopteryx that appear to be at once both bird and dinosaur.
To simplify, many, including me, refer to birds as dinosaurs. Sometimes, we add the term ‘avian’ or ‘non-avian’ to the front of dinosaur, to distinguish between modern, flying birds and their relatives, and the big scary ones that went extinct 65 million years ago.
The interesting outcome is that what this means is that, at least in the United States, we traditionally have a huge family meal on the fourth Thursday of November, in which we consume vast quantities of roasted dinosaur meat.
All right, but how do we know that?
If you’re like me, you need only go outside and hang out with the chickens to realize that these are just feathered versions of the velociraptors from the original Jurassic Park movie.
Yeah, but maybe they based those off of chickens.
I can do better than that, I promise. What is presented here is a re-hashing of what is already available here, at the University of California Museum of Paleontology’s website.
They start with a simple turkey skeleton and explain some of its dinosaurian features:
I’ll be expanding on this using this nice summary of the similarities between birds and dinosaurs prepared by Thomas Holtz, based on the skeletal drawing above.
There are a lot of terms here that might be a little overwhelming, so I’d like to summarize them first. We’ll go step by step, highlighting the evolution and relationships between birds and dinosaurs.
First, let’s consider the taxonomy – the scientific names. These are listed from most inclusive (the biggest groups) to the most specific (smaller groups – subsets of the next group up).
- Archosauriformes – A subset of reptiles including crocodylians, dinosaurs, and birds.
- Ornithodira – A subset of archosauriforms that are more closely related to dinosaurs and birds than to crocodylians.
- Dinosauromorpha – Includes only dinosaurs and birds, and some other closely related animals.
- Dinosauria – The dinosaurs (which includes birds).
- Theropoda – Mostly carnivorous dinosaurs, characterized by being bipedal and having three functional digits on the feet, and a fourth digit separated from the rest of the toes
- Neotheropoda – A broad group that includes all but the most primitive of theropods
- Tetanuridae – A subgroup of neotheropods that are characterized by having a stiff tail.
- Avetheropoda – These are more advanced theropod dinosaurs and birds. These are more bird-like.
- Maniraptoraformes – Avetheropods that possess wings and stiff feathers.
- Maniraptora – Includes long arms and three-fingered hands with some special adaptations to the wrist.
- Eumaniraptora – specialized maniraptorans with an especially mobile tail. Archaeopteryx is a Eumaniraptoran.
- Pygostylia – possess the specialized tail of modern birds called the pygostyle, which is short and stubby, but great for tail feathers to attach to.
- Ornithothoraces – show fusion of ankle and foot bones, as well as fusion of wrist and hand bones, characteristic of birds.
- Euornithes – Have a fully modern pygostyle, but still has primitive features like belly ribs (gastralia).
- Carinatae – possess a keeled sternum, used in powered flight
- Aves – lack teeth. These are modern birds.
We can illustrate these relationships in the following drawing. Bigger boxes are larger taxonomic divisions and include more animals. The divisions get smaller and smaller as we move toward modern birds, Aves (in darkest red).
Pygostylia – Changchengornis hengdaoziensis – Matt Martyniuk CC 3.0 NC-SA-By
Eumaniraptora – Archaeopteryx – T.M.Keesey Public Domain
Maniraptora – Jianchangosaurus yixianensis – Hanyong Pu, Yoshitsugu Kobayashi, Junchang Lü, Li Xu, Yanhua Wu, Huali Chang, Jiming Zhang, Songhai Jia & T. Michael Keesey CC 3.0 By
Neotetanurae – Sciurumimus albersdoeferi – Gareth Monger CC 3.0 By
Theropoda – Tyrannosaurus rex – Scott Hartman CC 3.0 NC-SA-By
Theropoda – Eodromaeus murphi – Conty (Modified) CC 3.0 By
Apatosaurus – Scott Hartman CC 3.0 NC-SA-By
Pterosauria – Preondactylus buffarinii – Mark Witton CC 3.0 NC-SA-By
Ornithodira – Dromomeron romeri – Nobu Tamura CC 3.0 By
Crocodylia – Crocodylus porosus – Steven Traver Public Domain
Archosauromorpha – Prolacerta broomi – T.M. Keesey Public Domain
Features of all of these groups are seen in the turkey. Let’s go through and explain them all.
- Antorbital fenestra – This refers to a particular opening in the skull that lies directly in front of the eye socket. This is why it’s hard sometimes to figure out which hole is the eye-hole in a T. rex skull. Antorbital literally means ‘in front of the eye.’ Fenestra means ‘window.’
Archosaurs include organisms like modern crocodilians, all dinosaurs, and birds. Modern lizards, snakes, and turtles are not archosaurs.
- Endothermy? – This is a fancy way to say ‘warm-blooded.’ It’s possible that warm-bloodedness – the ability to generate body heat – evolved in ornithodirans.
- Cervical vertebrae distinct from dorsal vertebrae – The bones of the back can be divided into several groups, the cervicals (neck), dorsals (where the ribs attach), lumbar (the lower back in humans – rib-free in other animals), sacral (where the pelvis attaches), and caudal (tail bones). Prior to this, the cervical vertebrae weren’t different than dorsal vertebrae. They all had ribs.
- Parasagittal stance – This is standing with the limbs directly under the body (like humans, horses, and birds), rather than sprawled out to the side (like crocodiles and salamanders).
- Hinge-like ankle joint – Because of the improved stance, the foot can now hinge nicely on the shinbone (tibia) in a forward and back motion. They were not able to rotate their ankles like we can.
- Obligate bipedality – This means they have no choice but to stand on two legs. Sure, they might be able to put their hands down, but they couldn’t walk on all fours any more than we can. Being on their feet is their preferred mode of transportation. Makes it easy to use the hands for other things…
- Endothermy? – It’s possible that warm-bloodedness evolved with the dinosaurs and not with the ornithodirans. We haven’t sorted this out yet.
- Feathers? -There’s lots of evidence for feathers on dinosaurs. We’re not sure yet if all dinosaurs had feathers, or only certain groups. At this point it looks like pretty-much all of them had feathers.
- Loss of digit V on hand – Digit V is equivalent to your pinky finger. Theropods only have four fingers.
- Furculum – This is the wishbone in your turkey. Yeah, Tyrannosaurus had one. It didn’t look quite like the one you’ll get from your turkey, but it was there. Here’s where the furculum sits in a pigeon.
The furculum of a dinosaur looks a little different than a turkey wishbone. But it’s there!
- Feathers? – Well, if feathers hadn’t evolved earlier, they almost certainly had evolved by the time the tetanurans came about.
- Loss of digit IV on hand – This is the same as your ring finger. Avetheropods only have three fingers.
- Pennaceous feathers – These are the stiff feathers that have a stalk or quill. Before this, it may have been mostly downy fuzz.
- Laterally-oriented shoulder joint – The shoulder joint aims to the side, like ours does, rather than pointing toward the ground like most other animals.
- Backwards-pointing pubis – The pelvis is composed of three parts, the ilium, ichium, and pubis. In theropods like T. rex, the pubis points forward, but in birds this points backwards.
- Enlarged sternum – This is where ribs connect in the front of the body. It is also where big muscles like the pectoralis connect. Enlarging the sternum allows for greater size and strength of the pectoralis muscles, which are important for flapping.
- Semilunate carpal – This is a specialized wrist bone which allows the wrist to bend to the side.
- Proximally-mobile caudal vertebrae – These are the tail bones. Much of the tail is stiff, but the bones closest to the body (proximal) are able to move quite a lot. In birds, this flexibility is what allows them the move their tails around and steer so well.
- Distally-placed metatarsal I – Metatarsal I is the foot bone connected to the big toe. To say ‘distally-placed’ means that it is placed far from the main part of the body, more close to the foot.
- Has a pygostyle – A pygostyle is specialized caudal vertebrae for the connection of tail feathers. It replaces the long, stiff tail of earlier dinosaurs.
- Carpometacarpus – This is a fusion of wrist and hand bones characteristic of birds.
- Tarsometatarsus – This is a fusion of ankle and foot bones also characteristic of birds.
- Additional cervical vertebrae – Unlike mammals, birds and their relatives aren’t limited to only seven neck vertebrae. They have many and kept gaining more allowing them to do crazy looking things like spin their heads around nearly 360 degrees.
- Backward pointing metatarsal I – One toe – the equivalent of our big toe – points backwards on the foot.
- No unguals on manus – The third bone of the fingers (the one that has a nail on it) is missing.
- Has a stubby pygostyle – Just like modern birds, the pygostyle is a little tiny thing.
- Keeled sternum – Just like a yacht, the sternum in most birds has a keel sticking out from it. This provides even more surface for the pectoral muscles to attach. (This is also that delicious breast meat that we like so much.)
- Tibiotarsus – Some bones of the shin and the ankle have fused together to make one bone.
- Toothless skull – No modern bird has teeth. The egg tooth they have when hatching is not a true tooth.
Your turkey shares all of these characteristics in common with various groups of dinosaurs. By extension, one could argue that all birds are dinosaurs.
A fun way to look at some of these relationships can be seen here, in this wonderful summary of the Origins of ‘Avian’ Characteristics (by Albertonykus on DeviantArt)
I invite you to discuss this over dinner this Thanksgiving or Christmas. I might do so myself. Maybe then the family will leave me alone.