Archaeopteryx

This article is about the dinosaur. For the ancient plant, see Archaeopteris. For other uses, see Archaeopteryx (disambiguation).

Archaeopteryx, sometimes called by its German name, "Urvogel" (meaning 'Primeval Bird'), is an extinct genus of bird-like dinosaurs. Its name comes from the Ancient Greek words for 'ancient' and 'feather, wing'. From the late 19th century to the early 21st century, Archaeopteryx was generally accepted by palaeontologists and popular reference books as the oldest known bird (a member of the group Avialae). Older possible avialans have since been identified, including Anchiornis, Xiaotingia, Aurornis, and Baminornis.

Archaeopteryx lived in the Late Jurassic, around 150 million years ago, in what is now southern Germany. At that time, Europe was an archipelago of islands in a shallow, warm, tropical sea, much closer to the equator than it is today. It was similar in size to a Eurasian magpie, and the largest kind could grow to about 50 cm (20 in) long, possibly reaching the size of a raven. Despite its small size and broad wings, Archaeopteryx had more in common with other small Mesozoic dinosaurs than with modern birds. It shared many features with the dromaeosaurids and troodontids, such as jaws with sharp teeth, three fingers with claws, a long bony tail, and feathers, which also suggest warm-bloodedness.

Because of these features, Archaeopteryx is a strong candidate for a transitional fossil between non-bird dinosaurs and birds. This makes it important not only for the study of the origin of birds, but also for the study of dinosaurs. It was named from a single feather in 1861, and the identity of that feather has been debated. That same year, the first complete specimen was announced. Over the years, twelve more fossils have been found. Although these fossils vary, most experts think they all belong to a single species, or at least a single genus, though this is still debated.

Most of these 14 fossils show impressions of feathers. Because these feathers are an advanced form (flight feathers), the fossils suggest that feathers began to evolve before the Late Jurassic. The type specimen was discovered just two years after Charles Darwin published On the Origin of Species. Archaeopteryx seemed to support Darwin's ideas and has since become a key piece of evidence for the origin of birds, the debate about transitional fossils, and the confirmation of evolution. For a long time, it was considered the start of the evolutionary tree of birds. In recent years, however, the discovery of several small, feathered dinosaurs has raised new questions for palaeontologists about which animals are the ancestors of modern birds and which are their relatives.

History of discovery

Main article: Specimens of Archaeopteryx

Fourteen body fossils of Archaeopteryx have been found so far. All of them come from limestone deposits near Solnhofen, Germany, which people have quarried for centuries. The first discovery was a single feather, found in 1860 or 1861 and described in 1861 by Hermann von Meyer. It is now in the Natural History Museum of Berlin. The first skeleton, the London Specimen, was described in 1863 by Richard Owen. The genus name comes from Ancient Greek words meaning 'ancient' and 'feather, wing'. Since then, other specimens have been found, including the Berlin Specimen, which is the most complete and the first with a complete head. In 2024, the Field Museum in Chicago announced a fourteenth specimen, and a 2025 study described it.

Starting in 1985, a small group including Fred Hoyle and Lee Spetner claimed that the feathers on the Berlin and London specimens were forged. Scientists at the Natural History Museum in London, including Alan J. Charig, rejected these claims. The researchers who made them had misunderstood how fossils form, and they did not explain why the other feathered specimens were also real. Charig and his colleagues pointed to hairline cracks and mineral growth in the rock as signs that the feathers were original. Most palaeontologists did not accept the forgery idea.

Description

Most Archaeopteryx fossils come from the Solnhofen limestone in Bavaria, southern Germany, a rare Lagerstätte, a rock layer known for very detailed fossils from about 150 million years ago. The animal was roughly the size of a raven, with broad, rounded wings, a long tail, and small teeth. It could reach about 50 centimetres in body length and 70 centimetres in wingspan. Its feathers were very similar to those of modern birds. It had many bird features, but also many theropod dinosaur features, such as small teeth and a long bony tail. Because it mixed both, it has often been seen as a link between birds and dinosaurs. In the 1970s, John Ostrom followed Thomas Henry Huxley's idea from 1868 and argued that birds evolved from theropod dinosaurs, using Archaeopteryx as key evidence. He found it remarkably similar to the Dromaeosauridae family. It had three separate digits on each fore-leg, each ending in a claw, a feature few birds have, though ducks, swans, Jacanas, and the hoatzin have them hidden beneath their leg-feathers.

Its well-developed flight feathers had vanes held firm by a barbule and barbicel arrangement, like those of modern birds. The thumb did not yet have a movable tuft of stiff feathers. The body feathers are less well known, mostly from the Berlin specimen, which had "trousers" of feathers on its legs, some lacking barbicels as in ratites. A patch of pennaceous feathers ran along its back, and fluffy "proto-down" similar to that of Sinosauropteryx covered much of the rest of the body. Most specimens were buried in anoxic sediment after drifting in the sea, and their bent necks and tails give them the death pose seen in the fossils. In 2011, Ryan Carney and colleagues used scanning electron microscopy and energy-dispersive X-ray analysis to find melanosomes in one feather, probably a dorsal covert, and found it was black. A 2020 study confirmed it was matte black, but the rest of the plumage pattern remains unknown.

Classification

Today, fossils of the genus Archaeopteryx are usually sorted into one or two species, A. lithographica and A. siemensii, but its naming history is complicated. As understood today, the name A. lithographica first referred only to a single flight feather described by Meyer. That feather may belong to a different, smaller feathered theropod. In 1954, Gavin de Beer concluded that the London specimen was the holotype. Early names were often part of an argument between Meyer and Johann Andreas Wagner, and the ICZN later removed many of them. Because the feather was the type specimen, applying the name to the skeletons caused nomenclatorial confusion, so the London specimen was made the neotype on 3 October 2011.

Below is a cladogram published in 2013 by Godefroit et al.

Some researchers think all the specimens belong to one species, while others see two. The specimens differ in size, finger proportions, snout shape, and possibly the presence of a sternum. Most specimens have been given their own names at some point. In 2007, a review concluded that two species could be supported: A. lithographica and A. siemensii. They are mainly told apart by large flexor tubercles on the claws of A. lithographica. A supposed extra species, Wellnhoferia grandis, seems to differ from A. lithographica only in size. Names are listed in the table below, following zoological nomenclature, where a name in parentheses means the taxon was first described in a different genus.

"Archaeopteryx" vicensensis is a nomen nudum for what appears to be an undescribed pterosaur.

Modern scientists have often called Archaeopteryx the most primitive bird, but it is thought to be a close relative of the true ancestor of modern birds rather than that ancestor itself. Some researchers have questioned whether it was the first bird, and suggested that Avimimus may have been a more likely ancestor. Other studies have noted that many maniraptoran groups were very birdlike. After the discovery of the closely related Xiaotingia in 2011, some analyses placed Archaeopteryx as a deinonychosaur rather than a true bird, but a later study returned it to its traditional place at the base of Avialae, and scientists say uncertainty remains. Studies including Eosinopteryx brevipenna and Jinfengopteryx elegans gave different results, and some work grouped Archaeopteryx with Wellnhoferia, Microraptoria, and Unenlagiinae. One analysis defined Avialae using Passer domesticus, excluding Dromaeosaurus albertensis and Troodon formosus, and placed Archaeopteryx in a grade with Aurornis. Other studies by Hu et al., Wang et al., and Hartman et al. considered it a deinonychosaur, and Hartman et al. suggested that it, like Yi, may have evolved flight independently from bird ancestors, possibly among the flightless "anchiornithids."

Palaeobiology

Like modern birds, the flight feathers of Archaeopteryx were slightly uneven, and its tail feathers were fairly broad. This suggests its wings and tail helped it get lift, but scientists are not sure whether it could flap or only glide. It had no strong bony breastbone, so it was probably not a very strong flier. Its coracoids, a cartilaginous sternum, and the joint between its scapula and humerus may show that it could not lift its wings above its back, which modern flapping flight needs. In 2006, Philip Senter suggested it may have used a downstroke-only flap to help it glide. Its large, short, rounded wings would have allowed a low turning radius and helped it move through cluttered places like trees, similar to crows and pheasants. Its "hind wings," similar to those of Microraptor, may also have helped it move in the air. Some scientists in 2010 questioned whether its thin rachises could support powered flight, as shown in a study of Confuciusornis and Archaeopteryx. Other scientists, including Philip J. Currie and Gregory Paul, disagreed with that study, and some suggested it may have run over water like the basilisk lizard (see Origin of avian flight).

In 2004, scientists studied a detailed CT scan of the braincase of the London Archaeopteryx. They found its brain was much larger than that of most dinosaurs, with areas for vision, hearing, and muscle control well developed, which suggests it had the senses needed to fly. Its brain was more advanced than that of non-coelurosaurian dinosaurs such as Carcharodontosaurus or Allosaurus. Newer research shows its brain was not especially large among Maniraptora dinosaurs, which suggests flight-related brain development was already common in that group. Scientists continue to debate how Archaeopteryx lived, with ideas from O. C. Marsh and Samuel Wendell Williston about whether it climbed, ran, or did both. A 2002 idea by Elżanowski suggested it used its wings mainly to escape predators. In 2018, scientists reported it may have flapped in a way closer to maniraptorans than to modern birds.

A histological study in 2009 suggested Archaeopteryx grew relatively slowly, similar to some kiwi birds, perhaps with a similar basal metabolic rate. Its growth was estimated to have taken at least 970 days to reach adult size. Comparisons of its scleral rings with those of modern birds and reptiles suggest it was diurnal, like most modern birds.

Palaeoecology

The rocks at the Solnhofen limestones, where every Archaeopteryx fossil has been found, show a Jurassic Bavaria that was very different from today. The area was at a similar latitude to Florida, but it was probably drier. Plants such as cycads and conifers were found, along with insects, small lizards, pterosaurs, and Compsognathus. The fossils are so well preserved that the animals probably did not travel far, so Archaeopteryx likely lived on the low islands around the lagoon. Pterosaurs were more common there than Archaeopteryx. They included Rhamphorhynchus from the Rhamphorhynchidae group, which dominated the ecological niche now held by seabirds and died out at the end of the Jurassic. Pterodactylus was also found, and the pterosaurs were common enough that the specimens were unlikely to be vagrants from larger islands 50 km (31 mi) to the north.

The islands were low, semi-arid, and sub-tropical, with a long dry season and little rain. The closest modern match is said to be Orca Basin in the northern Gulf of Mexico, though it is much deeper. The flora on the islands was adapted to dry conditions and was mostly low shrubs about 3 m (10 ft) tall. Few tree trunks have been found, and fossilized tree pollen is absent. Researchers disagree about how Archaeopteryx lived. Some think it mostly lived on the ground, while others think it mostly lived in trees, based on claw curves that have since been questioned. Some think it was a generalist that could feed in shrubs, open ground, and along the lagoon shore. It most likely hunted small prey, grabbing it with its jaws or claws.