Chris Hellier/SPL
Fossil remains of Archaeopteryx established it as the first bird — but that title is now under threat.
The iconic status of
Archaeopteryx,
the first animal discovered with both bird and dinosaur features, is
under attack. More-recently discovered rival species show a similar mix
of traits. But
Archaeopteryx still hogged the opening symposium
at the 2014
Society of Vertebrate Paleontology meeting in Berlin last
month, and even festooned the official conference beer glasses.
As the simplistic idea of a ‘first’ bird
gives way to a messy evolutionary transition, newly discovered fossils
and improved analysis techniques put
Archaeopteryx in prime position to unravel the details. “Research on
Archaeopteryx
is really catching a new breath,” says palaeobiologist Martin Kundrat
of Uppsala University in Sweden, who co-organized the symposium at the
November meeting.
The first
Archaeopteryx
fossil specimens turned up in limestone quarries in Bavaria, southern
Germany, in the early 1860s. Until recently, they were the only fossil
specimens found to mix bird- and dinosaur-like features. On the one
hand, they are small — the fossils show juvenile creatures about the
size of a magpie, which as adults may have been raven-sized — and have
broad feathered wings that look good for gliding; on the other, they
have a jaw with sharp teeth, dinosaur-like claws and a bony tail. These
features led to the idea of the first bird, and generations of
scientists have treated the 145-million-year-old animal as a
‘transitional species’ — the key piece of evidence linking birds and
dinosaurs (
Archaeopteryx is Greek for ‘ancient feather’, whereas its German name,
Urvogel, means ‘first bird’).
Illustration
by Emily Willoughby; Graphics: Jasiek Krzysztofiak/Nature; Sources:
Brains, Ref. 3/Tree, J. Clarke Science 340, 690–692 (2013)/R. B. J.
Benson et al. PLoS Biol. 12, e1001896 (2014)/Archaeopteryx silhouette:
Vladimir Nikolov.
But starting in the 1990s, the unique status of Archaeopteryx faced a challenge from the discovery in China of other potential transitional species. Fossils of Anchiornis huxleyi and Microraptor gui reveal small-bodied creatures like Archaeopteryx, and they may have used their four wings to glide. Another, Aurornis xui, has legs, claws and a tail similar to those of Archaeopteryx, yet lived about 10 million years earlier, leading some to propose it as a better candidate for first bird (see
‘The fight for first bird’).
Many scientists now believe that
Archaeopteryx is just another dinosaur. Others find this hard to swallow. “To some ornithologists this is a really big deal —
Archaeopteryx
is the first bird,” says Gareth Dyke, a vertebrate palaeontologist at
the University of Southampton, UK. “They’d rather cut off one of their
legs than admit it has nothing to do with bird origins.”
New specimens of
Archaeopteryx are
also reinvigorating research. There are now 12 known fossils of the
creature, and at the symposium Kundrat unveiled the first scientific
description of the eighth to be discovered. Researchers lost track of
the ‘phantom’ after it was found in Germany in the early 1990s, until a
collector purchased it in 2009. It is now on loan to a Bavarian museum
with an agreement that it will always be available for research.
Kundrat reported that the phantom lived more recently than other
Archaeopteryx fossils, bolstering suggestions that there may been more than one
Archaeopteryx species. And in July, an analysis
1
of the eleventh fossil showed that the animals boasted feather-covered
legs, suggesting that the feathers evolved for purposes other than
flight, such as display or insulation. Kundrat says that the twelfth
specimen might soon emerge from a private collection.
Scientists are also developing techniques to analyse
Archaeopteryx.
Kundrat presented a three-dimensional (3D) scan of the rock-embedded
bones of the phantom, which he made with help from scientists at the
European Synchrotron Radiation Facility in Grenoble, France. “Now we
have the first very detailed picture of the
Archaeopteryx skull,” he says.
Palaeontologist Ryan Carney of Brown University in Providence, Rhode Island, presented a 3D X-ray-tomography body scan of an
Archaeopteryx fossil
known as Thermopolis and a surface scan of another fossil. (Carney
sports a black feather tattoo as a nod to his part in the discovery
2 of pigment structures in
Archaeopteryx’s feathers that suggest the bird was black.) Both analyses will help to model whether and how
Archaeopteryxflew and to determine its place in the transition from dinosaurs to birds.
Revelations about
Archaeopteryx’s
brain are already informing its place in the evolutionary tree. Last
year, evolutionary biologist Amy Balanoff of Stony Brook University in
New York and her colleagues reported
3 in
Naturethat the brains of
Archaeopteryx
and a variety of dinosaurs related to it were, like modern birds, all
relatively large for their body sizes. Because earlier dinosaurs did not
fly as well as modern birds, if they flew at all, the finding suggests
that the brain expansion occurred well before flight emerged. It is yet
another clue that
Archaeopteryx may not have been any more
bird-like than other suggested transitional species. “It’s another piece
of data that has taken away from the specialness of
Archaeopteryx,” says Balanoff.
“Archaeopteryx is becoming one more fossil in this really diverse family tree.”
Balanoff is now investigating whether the
Archaeopteryx
brain shares finer anatomical details — inferred from skull shape and
small impressions on it — with theropod dinosaurs, the lineage that gave
rise to birds. Her team also reported
3 the discovery of a structure called a wulst in the
Archaeopteryx
brain, which was previously found only in living birds. The wulst,
involved in perception, might have helped the creature to process visual
information during flight. But the question is whether it, too, is
present in
Archaeopteryx’s dinosaur kin.
Taken
together, the findings suggest that there was no neat transition from
dinosaur to bird, making the idea of a first bird seem increasingly
arbitrary and prone to the caprices of an incomplete fossil record. “I
love
Archaeopteryx. I did a thesis on the animal,” says Nicholas
Longrich, a palaeontologist at the University of Bath, UK. “But in some
ways, we’re moving beyond
Archaeopteryx. It’s becoming one more fossil in this really diverse family tree.”
Some
think the debate over the transition will one day be moot. “The
boundary between dinosaurs and birds in the future will completely
disappear,” says Ulysse Lefevre, a palaeontologist at the University of
Liège in Belgium, who discussed new fossils from China at the Berlin
meeting that might further blur the line.
What is clear is that
Archaeopteryx research is still key to understanding the dinosaur–bird family tree. “There are some controversies over whether
Archaeopteryx is the first bird or not. I don’t think it matters at the moment.”says Kundrat. “We are bringing
Archaeopteryx back to life through new quantifiable analysis.”
