A few days ago I started working on a diary about bird evolution which has rapidly grown to such an enormous size that I am going to split it in two. I'm publishing the first part, on the fossil history of birds, during the old Dawn Chorus time slot (I obviously missed that - took longer than I thought). Part II on evolutionary relationships among modern birds will be published Sunday evening at approximately 8 PM EST.
So follow me below the fold to learn about the evolutionary history of birds. You may never look at a goldfinch on your feeder the same way again.
Model of Iberomesornis romerali a finch sized bird from the early Cretaceous, one of the first birds with a tail structure typical of modern birds.
Birds are among the most thoroughly studied organisms on the planet from a taxonomic viewpoint. So we've probably got their evolutionary history down pat. Right? Wrong.
Studying the evolutionary history of birds has faced two distinct hurdles. The first is that birds have a poor fossil record. They have fragile bones and don't tend to live in places like the bottoms of shallow seas where fossilization is common. The second is that modern birds appear to have diverged rapidly into major groups over a short period of time. We'll discuss this in more detail in the second diary.
A major resource of studying the diversity of life on the web is the Tree of Life, a two decade old project with contributions from experts on all kinds of organisms. This web page is an attempt to build a phylogeny (evolutionary family tree) for all living things. A phylogeny is a diagram that shows the evolutionary relationships among different groups of organisms. The page that contains all the birds including now extinct groups is found here - note the absence of a tree indicating uncertainty about evolutionary relationships.
Famously the earliest fossil bird is Archaeopteryx. The outstanding fossils of these birds clearly demonstrate the ancestry of birds within the Theropod dinosaurs. Theropods are the bipedal carnivorous dinosaurs such as Allosaurus, Tyrannosaurus, and Velociraptor. There were many smaller theropods that would not be useful fodder for a horror film. Birds are the only living theropods.
A fossil Archaeopteryx. The characteristic pose with the bent neck is thought to be the result of the body floating at sea and partially decomposing before settling into marine sediment.
Dinosaur-like features of Archaeopteryx. On the left, similarity of forelimb appendages with a non-avian theropod. On the right the long , 'reptile=like' tail of Archaeopteryx compared to a modern bird
Archaeopteryx was present in the Jurassic, the middle of the three Periods of the Mesozoic Era (the age of dinosaurs). Birds became more diversified during the Cretaceous, the last period of the Mesozoic. Interestingly a number of non-avian theropods from the Cretaceous are very bird-like. They occur in three families of theropods thought to be closely related to the birds, the Oviraptorosauria, Dromaeosauridae, and the Troodontidae. All three of these groups had feathers. The Troodontidae are particularly birdlike with large brains. Like most of the theropods they were primarily predators but some appear to have eaten seeds.
Anchiornis, a Troodontid.
The Dromaeosuaridae is the group that contains the famous Velociraptor and several larger predators. It also includes the intriguing Microraptor a small theropod with two pairs of wings! Both the front and rear legs had flight feathers. It is not believed to have been capable of powered flight but could definitely glide with its legs in a biplane configuration.
Microraptor
Increasing numbers of bird fossils have been found, establishing that birds with a 'modern' skeletal structure (as opposed to Archaeopteryx and similatr earlier birds that retained the long bony tail and other ancestral features) had appeared by the Cretaceous (the last period of the Mesozoic, the age of dinosaurs). Much of the early modern bird fossils are of aquatic birds, either from extinct groups or apparently similar enough to be assigned to some of the modern order. Aquatic birds are more likely to fossilize and may also, on average, have more robust skeletons so it is not necessarily the case that aquatic birds represent earlier lineages.
Among the earliest known birds to have a pygostyle (the shortened fused tail of modern birds) and lack teeth is Confuciusornis from the early Cretaceous and named after Confucius (fossils are from China). This was a crow-sized bird with elongated tail feathers much like those used in courtship displays by modern birds.
Confuciusornis
The most diverse group of Cretaceous birds was the Enantiornithes which is also the most diverse group of birds outside of the modern birds. These were mostly small to medium sized animals (one example is pictured above the fold). Their shoulder joints were different from those of modern birds but like them, they could raise their wings high above their backs, an important feature in powered flight.
Another well known group from the Cretaceous are the Hesperornithes which were large, flightless, loon-like toothed birds. It appears that the loss of teeth may have multiple times in the early evolutionary history of birds (either that or teeth re-evolved which seems less likely). They are one of several groups closely related to but outside of the modern, living birds.
All of the groups mentioned above went extinct in the last mass extinction at the end of the cretaceous. All existing birds represent one single lineage . The earliest representative of a specific modern group of birds is also known from the Cretaceous. It is Vegavis a member of the Anseriformes, the group containing ducks, geese, and swan. It is known only from a single fossil from Argentina. The significance of a duck-like 'early bird' will become more apparent in the second diary. There are a large number of other fossils (mostly fragmentary) of birds that belong to the 'modern birds' but they mostly belong to early branches within the group that are now extinct.
In the modern era, the Cenozoic, began sixty five million years ago and continues to the present. Most bird fossils date from this era with most that can be tied specifically to modern groups of birds dating back to about half way through the Cenozoic. Older ones, again, largely represent groups that are now extent that are closely related to modern groups. Our knowledge is somewhat biased towards large species with robust skeletons that fossilize well.
Here are a few examples.
The 'demon ducks' of Australia (they were related to ducks and geese). These were huge flightless birds up to 10 feet tall and weighing half a ton. They had massive beaks but did not appear to be fast runners or claws adapted for killing prey. It is not clear if they were herbivores or scavengers. At least some species were still present in Australia at the time of human arrival circa 50,000 years ago.
'Terror Birds' from South America, ranging north to the far south of the US. Mostly about 1 meter tall at least one species got to be 3 meters tall. They had massive skulls and huge predatory beaks. Believed to be very fast, agile runners (flightless) the larger species would have been top predators.
The Teratorns were birds closely related to New World Vultures and probably ecologically similar to modern Condors although perhaps somewhat more predatory. The well known Teratornis from western North America was somewhat larger than an Andean Condor. The South American Argentavis is the largest flying bird known with an estimated wingspan of over 20 feet and standing over six feet tall on the ground!