By Anthony King
"Dinosaurs as a group are not extinct."
This is how UCD's Dr Gareth Dyke began his talk at an Alchemist Café event earlier this year. "Maybe ninety-nine percent of the lineages went extinct when a meteorite hit Mexico about 65 million years ago, but not all." There hasn't been an astounding discovery of a Jurassic hideaway on a remote island, however. The living dinosaurs, he explained, are the birds, which palaeontologists now recognise as descendants of a group close to ferocious bipedal dinosaurs such as Velociraptor in Jurassic Park.
Fossil evidence uncovered in China in recent years revealed that birds weren't unique in possessing feathers. Dinosaurs weren't scaly reptiles, said Dyke. "They were fluffy, covered in feather and down." Animals like Tyrannosaurus rex and the big long-necked sauropod dinosaurs had downy feathers, he added.
"Most of the flesh eating dinosaurs had feathers," agreed Mike Benton, Professor of Vertebrate Palaeontology at Bristol University. "Some of the earliest dinosaurs probably had a fuzz of wispy feathers. Early on the tail and neck were covered, but more advanced ones had complete cover all over the body," he said.
Feather preservation in fossils is so rare that palaeontologists still aren't sure about the extent of feathers in many groups. No feathers have been found on T. rex, but other tyrannosaurids have been found with feather-like covering. According to Dr Matthew Wilkinson of the University of Cambridge, T. rex would therefore be expected to have some kind of feathers, but this is complicated by its large size. If it was covered in down and warm-blooded in the modern sense, it may have suffered from overheating, he said.
What is certain is that feathers evolved for a function other than flight. "We don't know why feathers first evolved," explained Wilkinson. "Insulation is a strong contender, but display and defence are also possible explanations."
Two years after Darwin published The Origin of Species, a 150-million-year-old fossil marvel turned up in a limestone quarry in Germany. Archaeopteryx ("ancient wing"), as it was called, had feathers and was definitely capable of flight. But the fossilised skeleton looked like a dinosaur, and it had teeth, claws, and a long bony tail.
"Key anatomical features show these early birds were much more similar to dinosaurs, especially Velociraptor and other small predatory ground-running dinosaurs, than to living birds today," said Dyke, who is one of the world's leading experts on bird evolution. A nearly complete specimen of Archaeopteryx described in the journal Science in late 2005 offered yet more evidence, from the skull and feet, to link birds to dinosaurs.
It's now widely accepted that birds evolved from a small carnivorous dinosaur in the Jurassic, some 150 million years ago. The feathers, large sternum, hollow limb bones and possibly warm bloodedness of birds were already present in the dinosaur ancestor, so flight was a story of co-opting existing structures to new purposes.
Kevin Padian writes in The Dinosauria that flight -- evolving in one lineage of dinosaurs (Maniraptorans) -- involved "a transition from the predatory forelimb slash-grab of a vicious predatory dinosaur to the graceful stroke of birds taking wing." Subsequent bird evolution has been all about reducing the amount of bone in the skeleton to make it lighter.
For some fossil animals, palaeontologists are uncertain whether to class them as birds or dinosaurs. One example is Mononykus (meaning "single claw"), a long-legged, lightly built animal with a well-developed breast bone, large eyes and tiny arms. Common sense dictates that birds and dinosaurs must be closely related if the experts have trouble telling them apart.
Images of the Cretaceous period (144 to 65 million years ago) depict dinosaurs with reptilian pterosaurs soaring above their heads. Missing from these pictures are flocks of colourful birds. "I would say that birds during the late Cretaceous, say 80 to 65 million years ago, were just as common as they are today," said Dyke. "The birds at the end of the Cretaceous were very similar to modern birds. They differed in some anatomical details, such as the shape of some bones, but the differences were very small," he said.
The "opposite birds" (Enantiornithes), named for their foot bones, were a common group in the earlier Cretaceous and have been compared to modern songbirds in terms of their diversity. The first "opposite bird" was identified in the 1980s, but many more fossils were discovered in the 1990s. They could perch on branches, were good fliers, and had the extra winglet (the alula) seen in modern birds, which is used to prevent stalling at slow speed. Many also had primitive features like teeth and clawed hands.
Dyke has been carrying out field work in Kazakhstan for the last number of years and will shortly publish a description of a new "opposite bird" from the Gobi Desert. He also recently reported a marine bird from Kazakhstan that is a Hesperornis ("western bird"). These were large, flightless diver birds that could be over a metre tall. They too had teeth, but were more closely related to modern birds than the "opposite birds." Until the recent spate of discoveries, Hesperornis and one other Cretaceous seabird formed the basis of studies into the origin of modern birds.
Almost all bird groups died out during the famous K-T (Cretaceous-Tertiary) extinction event 65 million years ago that wiped out the dinosaurs and pterosaurs. The modern birds (Neornithes) were the only group to survive. "This one lineage radiated rapidly after the extinction and gave rise to the 10,000 species we see today," explained Dyke. Modern birds appear to have been rare during the Cretaceous, and few of their fossils have been found. Why this group of birds survived the extinction but all others died out is a mystery.
Stig Walsh, a palaeontologist at the Natural History Museum in London, has proposed a solution. Walsh has a theory that "modern birds survived the Cretaceous-Tertiary extinction event due to enlargement of certain parts of their brains." This would have set them apart from the more archaic lineages of birds. "We're trying to test the idea," he explained, "using high resolution computed tomography of fossils, but we don't have anything conclusive as yet."
Dyke outlined another theory: "Two of the three lineages of birds went extinct at the K-T boundary. The fact that one survived may have been for ecological reasons." Perhaps the fact that they lived in specific habitats, such as shorelines, allowed them survive in small numbers, he said. Research into this area continues and may well depend on new discoveries of fossils in the field.
The thin-walled bones of birds don't fossilise easily. As Dyke explained, the entire fossil record of birds from the Cretaceous could fit on a large dining table. "Even T. rex is known from just twelve specimens, only two of which are complete," he said. Yet this large dinosaur was on Earth for 10 million years. Limited data is a fact of life in vertebrate fossils, and many records of modern birds from the Cretaceous are based on a single bone.
Molecular clock studies have indicated that the modern bird lineage originated over 100 million years ago, but there are no anatomically modern birds found before the terminal stages of the Cretaceous. Even those fossils are problematic. "Most of the specimens are fragmentary and we do not understand the characters well enough to place most of them," said Joel Cracraft, ornithologist at the American Museum of Natural History.
The number of archaic and modern bird species discovered from the Cretaceous has more than tripled since the 1990s. The net is also widening, taking in new areas of the world. This summer, a team including UCD's Gareth Dyke scoured remote areas of Kazakhstan for fossils. Other areas of the world await exploration. The fossil record in the Southern Hemisphere has received less attention and it could be that evolution of modern birds during the Cretaceous took place there.
Fossilisation requires particular conditions. For a feather to be preserved, it must be buried quickly, gradual sedimentation must occur, and the carcass must remain undisturbed. Consequently, the fossil fauna is predominantly from marine and lake environments.
Another problem with the fossil record is that fossils often say little about the behaviour of an animal or the ecology of the time. Strikingly visual features in modern birds, such as the brilliant plumage of birds of paradise, may have been present in Cretaceous birds, but such delicate traits do not fossilise easily.
Field work, such as Dyke's surveys in Kazakhstan, may yield important new finds of dinosaurs and birds. And ongoing research on the feathered dinosaurs of China and Mongolia is critically important to our understanding of the origin of birds. But, it is clear that birds have a longer history than many of us would have imagined. Equally, the image of dinosaurs as scaly reptiles was all wrong.
There has been a renaissance in dinosaur research in the last three decades and bird evolution is now very much part of this story. "At least anatomically, birds are one kind of dinosaur," Dyke said, "So you can go look in the garden and see dinosaurs flapping around." Next time you see a bounding magpie, just picture a scaled-up toothed version with claws.
After the K-T extinction, some birds appear to have taken over the role of some of their dinosaur ancestors. The radiation of mammals at this time did not include large carnivores, and birds seem to have become top predators in some parts of the world. The Phorusrhacids, possible relatives of cranes, reached heights of three metres or more and have been pictured seizing horse-like mammals. These giant flesh-eating terror birds lived on in South America longer than anywhere else and were eventually replaced by predatory cats and dogs in the Pleistocene, less than two million years ago.
The group that most palaeontologists believe gave rise to birds is the Maniraptorans ("seizing hands").Exact membership of this dinosaur group is debated. Maniraptorans had a specialised wrist and forelimb that was later used for the flight stroke in birds. In the 1920s, Oviraptor ("egg thief") got a bad rap when it was discovered lying above a nest of eggs. Further finds of Oviraptors in the 1990s, including an unhatched embryo, showed these dinosaurs were protecting their nests in an avian brooding position. Some experts place Oviraptor with the birds.
Microraptor is a small feathered dinosaur that recently featured in the TV series Prehistoric Park. The feathers of this dromaesaurid were asymmetrical -- important since asymmetry generates lift. The long feathers in the forelimb of Microraptor form a wing of essentially modern design which suggests that the animal was capable of flight. Whether it glided or actively flapped is a source of controversy. Writing recently on the subject, Dyke and Chiappe noted that such new discoveries not only document the presence of feathers outside birds, "but they also suggest that some non-avian theropod dinosaurs may, to some extent, have been able to fly."