The Urvogel’s Old, New Clothes

On May 5th, 1877, the German paleontologist Karl Zittel first laid eyes on one of the most stunning fossils ever discovered.

The 150 million year old specimen belonged to Ernst Häberlein, which he showed off at a meeting of the Bavarian Academy of Sciences, and only little of the animal could yet been seen. “Mr. Häberlein was able to expose an area of a hand’s breadth with the posterior portion of the tail,” Zittel wrote, “And there one can see the feathers at both sides of the elongated tail vertebrae in faultless beauty.” There was no question to what this animal was – an Archaeopteryx, an animal that combined reptilian and avian traits to mark the deep origin of birds.

This was only the third Archaeopteryx fossil to be found – an isolated feather and a slightly-jumbled skeleton surrounded by feather impressions were the first two – and even the small section Häberlein had prepared promised that this particular early avian was even more complete and beautifully preserved. That’s why Häberlein set the price high when he shopped the fossil to potential buyers.

Häberlein’s initial asking price was just under 26,000 marks. Museum after museum failed to come up with the cash. Ultimately the wealthy industrialist Werner von Siemens bought the fossil for himself for 20,000 marks with an offer to let Berlin’s Mineralogical Museum purchase it from him within the year, which the institution did in 1880. Hence this second Archaeopteryx became known as the “Berlin specimen”, and the fossil has become one of the most cherished representations of grand evolutionary change.

By the time of the sale Häberlein had prepped away much of the limestone surrounding the bird. The first photograph of the fossil, taken by Häberlein in 1879, shows the bird’s strangely beautiful bony contortion, surrounded by feather traces. But that photo doesn’t quite match the fossil as you see it today. Some parts, like the skull, have been more delicately prepped, but other portions have been lost or obscured. Most prominent among those altered parts are the bird’s “breeches”, long feathers jutting from either side of the right leg.

You can see remnants of these feathers on the fossil today, but they’re not as prominent as they used to be. Over a century of preservation and prep work all but denuded the urvogel’s legs, and, as paleontologist Kálmán Lambrecht wrote in 1933, “Today, the main slab of the [Archaeopteryx] is a real chemical laboratory” created by the various treatments used to keep the fossil together for study and casting. That means paleontologists often  turn to the little-seen, incomplete counterslab to get a better look at the dinobird’s feathers, and that piece offers little detail of the fossil’s feathery legs.

Other Archaeopteryx have done little to fill in these human-made gaps. While paleontologists eventually found and identified a total of 10 Archaeopteryx, most of those did not preserve feather traces in anywhere near the same detail as the Berlin specimen. Now that has changed.

In 2011, the 150th anniversary of when Archaeopteryx was named, paleontologists announced the acquisition of an 11th specimen with feathers to rival those of the Berlin fossil. (The 11th specimen is in the private collection of Burkhard Pohl, who brokered the controversial sale of the “Thermopolis specimen” of Archaeopteryx into private hands.) While the contorted fossil isn’t quite as skeletally intact as its Berlin relative, the fossil’s feather preservation is so outstanding that it’s the new focus of a short Nature letter by Christian Foth, Helmut Tischlinger, and Oliver Rauhut.

The news hook for the paper, taken directly from Nature‘s press materials, is that Archaeopteryx had “trousers” or “hipster feather pants.” This isn’t really news, given that paleontologists have known about the bird’s breeches for decades, although it’s still fortunate that there’s a new Archaeopteryx which preserves what was lost on the Berlin specimen. More significant is that this new specimen adds a little more perspective on what kinds of feathers Archaeopteryx had and the fact that plumage for display and insulation evolved long before feathers for flight.

The feathers on the neck and body of the 11th Archaeopteryx are pennaceous, meaning that they have a central shaft or quill. The bird’s leg feathers have central vanes, too, and while they’re consistently symmetrical in shape they decrease in size from the top of the leg towards the ankle. And, for the first time, paleontologists can see the very tip of the Archaeopteryx tail, which, in life, bore elongated feathers that might have played some aerodynamic role (that is, if Archaeopteryx actually flew, which paleontologists continue to debate).

Archaeopteryx is now just one of many, many feathery dinosaurs, of course. And through a new evolutionary tree of early birds and their fluffy, non-avian dinosaur relatives, Foth and coauthors suggest that pennaceous feathers had a complex evolutionary history. They hypothesize a single origin of pennaceous feathers among theropod dinosaurs – a point on the tree they’ve named Pennaraptora – but from that point those barbed feathers diversified on different body parts among the diverging lineages. While pennaceous body feathers were a common feature, for example, wing feathers – called remiges – evolved to be asymmetrically-shaped flight feathers in dinosaurs like Microraptor while they offered no flying function to the earthbound, parrot-beaked Caudipteryx.

The upshot is that pennaceous feathers did not initially evolve as an adaptation for flight. Plenty of ground-dwelling dinosaurs had them, Foth and coauthors point out, and so they fit into the bigger story of feather evolution that has been apparent for some time. Pennaceous feathers evolved according to evolutionary pressures related to insulation, display, camouflage, and balance first, and were later co-opted in some lineages to allow dinosaurs like Microraptor and (possibly) Archaeopteryx to fly. In the case of dinosaur breeches, for example, they could have been flashy signals to non-flyers such as Anchiornis but evolutionarily altered for flight in the quad-winged Microraptor. In evolution, as well as science news, what was old can become new again.

References:

Foth, C., Tischlinger, H., Rauhut, O. 2014. New specimen of Archaeopteryx provides insights into the evolution of pennaceous feathersNew specimen of Archaeopteryx provides insights into the evolution of pennaceous feathersNew specimen of Archaeopteryx provides insights into the evolution of pennaceous feathers. Nature. DOI: 10.1038/nature13467

Wellnhofer, P. 2009. Archaeopteryx: The Icon of Evolution. München: Verlag Dr. Friedrich Pfeil. pp. 60-76