Nowadays almost everyone has heard that ‘birds are dinosaurs’. What does this mean? And what is a dinosaur anyway? This is going to be a long diary but I will break it into parts. In it I will briefly survey the evolutionary history leading up to dinosaurs, talk a bit about dinosaur diversity, and then delve into the history of the group of dinosaurs that includes birds. Along the way I’ll point out when characteristics of modern birds first appeared. I’m going to do my best to minimize obscure terminology while still including a significant amount of detail.
Birds are Amniotes. An amniote is a vertebrate that has an amniotic egg. The amniotic egg has membranes that allow the egg to exchange gases with the environment (or in the case of amniotes that are live bearing (e.g. mammals) to exchange gases and nutrients with mom). The shelled eggs of birds allow the embryo to develop in a dry environment by providing food and moisture but still allowing for the exchange of gases. There are three major, ecologically diverse and dominant groups of living amniotes: birds (about 10,000 species), snakes and lizard (also about 10,000 species) and mammals (about 5,000 species). There are also three smaller groups of living amniotes: turtles (350ish species), crocodilians (just over 20 species), and tuataras (2 species).
Very Brief Overview of Amniotes. The Amniotes appear to have originated in the Carboniferous which is the second to last period (i.e. subdivision) of the Paleozoic era. As mentioned above the amniotic egg was a major evolutionary innovation which allowed vertebrates to live permanently out the water. By the Permian period, which was drier than the Carboniferous, amniotes had become the dominant group of land vertebrates. In particular one group of amniotes, the synapsids, was the dominant group of large land animals.
All this changed 250 million years ago when the earth underwent the largest mass extinction in its history, the so-called ‘great dying’. Most of the synapsids went extinct. Some of the ones that survived were the ancestors of mammals. The great dying forms the boundary between the Permian (last period of the Paleozoic era) and the Triassic (first period of the Mesozoic era). During the Triassic a different group of amniotes, the archosaurs, became ecologically dominant. And this is important to our story, because birds are archosaurs. There was another, smaller, mass extinction at the end of the Triassic in which basically all large land vertebrates that weren’t archosaurs went extinct. All the synapsids except for the mammalian lineage went extinct. Mammals persisted through the rest of the Mesozoic as small carnivores (something like modern day shrews).
Diapsids, Archosaurs, and Dinosaurs. Very early in the history of the amniotes they split into two groups: the synapsids (described above) and the diapsids. The names refer to openings in the skull that don’t really concern us here. Mammals are the only living synapsids, all other living amniotes are diapsids. A number of different groups of diapsids first appeared in the Triassic: turtles*, ichthyosaurs*, plesiosaurs*, lepidosaurs (the group that contains snakes, lizards, and tuataras), and the archosaurs. The archosaurs were very successful in the Triassic and, after the mass extinction, dominated terrestrial ecosystems in the Jurassic and Cretaceous periods. There are two major evolutionary groups of archosaurs: the crocodilians and their extinct relatives and the dinosaurs and their close relatives. Pterosaurs (the flying ‘reptiles’) are close relatives to the dinosaurs. Among other characteristics, archosaurs are distinguished from other diapsids in that their teeth are set in sockets (birds of course have lost their teeth). Dinosaurs are distinguished from other archosaurs in the orientation of their legs. Dinosaurs have hind limbs that stick directly down from their torso, similar to mammals (for dinosaurs that walked on all four limbs, the same is true of their front limbs as well). In contrast in other diapsids (think of a lizard or crocodile) the legs stick out to the sides in what is often described as a sprawling stance. Modern birds also have their hind legs oriented directly under them
*It now seems fairly certain that the ichthyosaurs, plesiosaurs, and turtles are all diapsids but because each group is so specialized it isn’t really very clear how they are related to other diapsids. The ichthyosaurs and plesiosaurs were Mesozoic aquatic diapsids. Ichthyosaurs were highly convergent in form to modern dolphins.
How Birds Fit Within Dinosaurs. The classic version of dinosaur classification was recently challenged (Baron et al. 2017). This doesn’t really change how birds fit into dinosaurs but it does provide some interesting context. What is not in dispute is the existence of three main groups of dinosaurs. The first of these is the Ornithischia (bird-hipped dinosaurs). This name is unfortunate because these dinosaurs are not closely related to birds. The Ornithischia were all herbivores and contained heavily armored types such as Stegosaurus, frilled dinosaurs such as Triceratops, and a lot of more generic appearing bipedal and quadrupedal types such as Iguanodon. The second group is the Sauropoda which were the famous gigantic long-necked and long-tailed herbivores such as Brontosaurus/Apatosaurus. The third group is the Theropoda which includes the birds. The non-avian theropods were bipedal and mostly carnivorous. They ranged in size from giants like Tyrannosaurus to things about the size of a pigeon.
The traditional view has been that the theropods and the sauropods together form a group called the Saurischia (lizard-hipped dinosaurs). However Baron et al, (2017) have challenged this view indicating that the theropods are more closely related to the ornithischians than they are to the Sauropods.
Theropoda (i.e. Birds and their Relatives). As stated above the birds are a group of theropod dinosaurs. This view has been widely held by paleontologists and other evolutionary biologists since the 1970s when skeletal similarities between Archaeopteryx (late Jurassic proto-bird) and theropods were noted. It’s not clear to me why this realization took so long as Archaeopteryx fossils had been discovered a century earlier. Various other hypotheses for the origin of birds have been proposed over the years. As more and better fossils have been discovered the evolutionary history of the theropods has been understood in more and more detail as has the position of the birds within the theropods.
Theropods are/were bipedal, with large hindlimbs and usually shorter front limbs (in some cases very short). They were mostly carnivores and many had feathers. A birdlike characteristic of the entire group is hollow bones. The most famous non-avian theropods were the giant forms such as Tyrannosaurus and Allosaurus but there were many smaller forms as well and, as we shall see, some really odd ones.
The group within the theropods containing the birds is known as the Coelurosuars. It is thought that all members of this group might have been feathered. This group has three major subgroups*. The tyrannosaurs are large carnivores including the infamous T. rex. These were originally placed in a group called the carnosaurs (which includes Allosaurus) but it is now thought that this giant carnivorous body form evolved twice independently.
A second, much more obscure but very interesting, group are the Ornithomimidae, or bird mimic dinosaurs. These were slender long-legged dinosaurs with small heads and a beaked mouth but a large brain. Based on their structure they may have been ecologically similar to the ratite birds (e.g. ostriches).
The third group is the maniraptors, the group than includes the birds.
*There are quite a few theropods that are known from a very small amount of fossil data and thus their relationships to other theropods are not will understood. For simplicity I’m leaving them out of this diary.
Maniraptors. This group includes the birds and several groups of close relatives, some of which are bird like and others not so much. The four major non-avian groups are the therizinosaurs, the oviraptors, the troodontids, and dromeosaurs. The latter two groups are thought to be particularly closely related to birds. A more technical discussion of the maniraptora can be found here.
The therizinosaurs are one of the bizarre groups I mentioned earlier. They have been classified as very different types of dinosaurs in the past and only recently recognized as maniraptorans. Unlike most other maniraptorans they were large and bulky. They were also herbivores which is unusual among theropods but also had enormous claws on their front limbs. They remain something of an enigma.
The oviraptors are also enigmatic. This is the first group to definitely have pennate feathers similar to those on the wings and tails of modern birds (although it is possible that therizinosaurs had them as well as indicated in the figure below).
The name oviraptor means egg thief which is based on a misconception due to an oviraptor fossil being found next to a fossil nest. It turns out that the nest was of its own species and multiple fossils of oviraptors on nests have been found indicating they had parental care. Their short beaks appear to be adaptations for eating hard food (mollusks?).
The remaining two groups are both closely related to birds. The exact relationships between the two groups and birds are not certain and, in some cases it is not clear whether a particular fossil is a troodontid or dromeosaur.
The troodontids were small theropods with large brains. There were relatively few fossils of these animals known until fairly recently. They had grasping front limbs, were fully feathered, and had very large brains and eyes.
The dromeosaurs are much more familiar to non-paleontologists as one species, Velociraptor, was prominently featured in Jurassic park films. The dromeosaurs were highly variable in size ranging from small species like Microraptor that was the size of a chicken to giants like Utahraptor which was 30 feet long and thus comparable in size to some of the huge carnosaurs. These were highly active predatory dinosaurs .
I’m going to stop here. The remaining dinosaurs are all in the group Aviales which includes modern birds and fossils that fall outside of the modern bird group but are similar enough in form and evolutionary origin that they are considered birds. I can discuss those in a future diary if there is interest.