Confuciusornis

Confuciusornis is a kind of small dinosaur from long ago, living during the Early Cretaceous Period, about 125 to 120 million years ago, in what is now China. It was about the size of a crow and had a beak without teeth, which was different from some of its relatives that still had teeth. Scientists thought it was the oldest known bird with a beak, but now we know an even older relative called Eoconfuciusornis had a beak too.

Confuciusornis was named after the famous Chinese philosopher Confucius, who lived from 551 to 479 BC. Many fossils of Confuciusornis have been found, making it one of the most well-known animals from that time. These fossils help scientists learn more about how birds evolved from their dinosaur ancestors.

History of discovery

In November 1993, scientists visited a fossil collector who showed them a fossil bird specimen. Soon after, they found another specimen. In 1995, these specimens were described as a new type of bird, Confuciusornis sanctus. The name combines the philosopher Confucius with the Greek word for bird, "ornis".

These birds lived during the Early Cretaceous Period, around 125 to 120 million years ago, in what is now China. Many well-preserved fossils of Confuciusornis have been found, helping scientists learn more about these ancient birds. At one site, many fossils were found close together, suggesting that a whole group of birds may have been affected by a volcanic eruption.

Description

Confuciusornis was about the size of a modern crow, with a total length of 50 centimetres (1.6 feet) and a wingspan of up to 70 cm (2.3 ft). Its body weight has been estimated to have been as much as 0.5 kilograms (1.1 lb), or as little as 0.2 kg (0.44 lb).

Confuciusornis shows a mix of simpler and more advanced traits. It was more "advanced" than Archaeopteryx in possessing a short tail with a pygostyle (a bone formed from a series of short, fused tail vertebrae) and a bony sternum (breastbone), but more basic than modern birds in retaining large claws on the forelimbs, having a simpler skull with a closed eye-socket, and a relatively small breastbone.

The wing feathers of Confuciusornis were long and modern in appearance. The primary wing feathers of a 0.5-kilogram individual reached 20.7 centimetres in length. Many specimens preserve a pair of long, narrow tail feathers, which grew longer than the entire length of the rest of the body.

Classification

Scientists first placed Confuciusornis in a group called Confuciusornithidae in 1995. They thought it was closely related to another group called Enantiornithes, but later realized it was more closely linked to them in a larger group called Sauriurae. Some scientists disagreed with this idea.

In 2001, a new idea suggested Confuciusornis might be closely related to a group called Ornithothoraces. By 2002, further studies supported this, placing Confuciusornis in a new group called Pygostylia, making it one of the earliest members. Confuciusornis had some features that show its place in bird evolution. It had a simpler skull than Archaeopteryx, but it was the first known bird to lose its long tail and develop fused tail bones, called a pygostyle.

One of its close relatives, Changchengornis, lived in the same area and also had long tail feathers, like some more advanced enantiornith birds. True tail fans appeared later in ornithuromorph birds.

Paleobiology

The large, fleshy foot pads, small foot pads, and curved foot claws of Confuciusornis are traits shared with modern tree-dwelling, perching birds, suggesting it may have had a similar lifestyle.

Comparisons between the rings supporting the eyes of Confuciusornis and modern birds and other reptiles indicate that it may have been active during the day, similar to most modern birds.

Flight

Confuciusornis has traditionally been assumed to have been a competent flier based on its extremely long wings with strongly asymmetrical feathers. Other adaptations for improved flight capabilities include: a fused wrist, a short tail, an ossified sternum with a central keel, a strut-like coracoid, a large deltopectoral crest, a strong ulna (forearm bone) and an enlarged second metacarpal. However, it may have had a different flight stroke due to being incapable of rotating its arm behind the body, and its relatively smaller sternal keel indicates that it likely was not capable of flight for extended periods of time.

Several contrary claims have been made against that the flight capabilities of Confuciusornis. Some authors proposed that Confuciusornis used its large thumb claws to climb tree trunks. Others considered it very unlikely that Confuciusornis climbed trunks.

Also a controversy is the strength of the feathers. Some researchers argued that the central shafts of the primary feathers were too thin and weak to have remained rigid during the power stroke required for true flight, suggesting that Confuciusornis would at most have employed gliding flight. Others disagreed with their study, arguing that more accurate weight estimates allowed powered flight even with relatively narrow shafts.

In 2016, researchers argued in favor of flight capabilities for Confuciusornis using evidence from laser fluorescence of two soft tissue-preserving specimens. They found that the central shafts of Confuciusornis were relatively robust, and the wing shape is consistent with either birds that live in dense forests or gliding birds. The substantial layer of tissue would have produced a generous amount of lift, while the large layer of tissue would have provided a large attachment area for the feathers, which would have kept them as a straight surface. This collectively is strongly indicative that Confuciusornis was capable of powered flight, if not only for short periods of time.

Tail feathers

Many specimens of Confuciusornis preserve a single pair of long, streamer-like tail feathers, similar to those present in some modern birds. Specimens lacking these feathers include ones that otherwise have exquisitely preserved feathers on the rest of the body, indicating that their absence is not simply due to poor preservation. Long tail feathers are present in about 5 to 10% of the specimens known.

Controversy arose from the observation that the known specimens of Confuciusornis can be divided into a small-sized and a large-sized group, but that this size distribution is unrelated to the possession of long tail feathers. Some researchers suggested that the pattern might reflect differences between males and females, with the streamer-like feathers only present in one sex which used them in courtship displays. Others argued that sexual differences is not the only but the most reasonable explanation, noting that in modern birds the length of ornamental feathers often varies between the sexes.

Several alternative hypotheses explaining the frequent absence of long tail feathers have been proposed. Some researchers discussed the possibility that individuals might lack tail feathers because they died during a certain period. Others suggested that Confuciusornis may have shed the feathers as a defense mechanism. In a study, researchers stated that even the presence of two separate groups, one with and one without long tail feathers, needs to be considered.

Reproduction

In 2007, a Confuciusornis skeleton preserving an egg near its right foot was mentioned — the first possible egg referable to the genus. The egg is roundish in shape and measures 17 mm in diameter, slightly smaller than the head of the animal. In modern birds, proportionally large eggs are commonly found in species whose hatchlings do fully depend on their parents, while smaller eggs are often found in species whose hatchlings are more developed and independent. As the estimated egg of the specimen would have been around 30% smaller than expected for a modern bird, it is likely that Confuciusornis was independent at hatching.

Growth

Growth can be reconstructed based on the inner bone structure. The first such study on Confuciusornis used scanning electron microscopy to analyze a femur in cross section. Because the bone was well vascularized and showed only a single line of arrested growth, these authors determined that growth must have been fast and continuous as in modern birds.

In 2008 researchers conducted a statistical analysis based on specimens to explore the relationship between body size and the possession of long tail feathers. The population showed a clear size distribution of the animals with two distinct weight classes. From this it was concluded that either the sexes did not differ in size or both sexes had the long feathers.

The idea of a growth was criticized, who argued that the body size of the smaller size class was too large to possibly have represented the youngest growth state.

Chiappe and colleagues defended their findings, arguing that the assumed short, growth period is unlikely. The calculation presented by a researcher of a growth phase of less than 20 weeks was based on the assumption that bone diameters grew by 10 micrometres per day, which is subjective. Rather, histology reveals the presence of different tissue types in the bone that grew at different rates, as well as pauses in growth as indicated by the lines of arrested growth. Thus, growth periods must have been longer than in modern birds and likely took several years.

Possible medullary bone

A histological study found medullary bone within the long bones of a short tailed specimen, while three long-tailed specimens lacked medullary bone. In modern birds, medullary bone only forms temporarily in females, where it functions as a calcium reservoir for eggshell production. Therefore, these authors suggested short-tailed specimens to be females, and long-tailed specimens to be males. The female specimen had already passed its rapid growth phase, although it was still significantly smaller than the maximum size reached by Confuciusornis exemplars. At least two lines of arrested growth could be identified, demonstrating an extended growth over several years. Long tail feathers were confirmed to occur in small individuals. Assuming that the occurrence of tail feathers indicates maturity, the authors concluded that the latter must have occurred well before the animals reached their final size, unlike in birds but similar to non-avian dinosaurs.

Diet

In 1999, a paleontologist suggested that Confuciusornis was likely herbivorous, though no stomach contents were yet known, pointing out that the beak curved upwards and was not raptorial.

In 2006, researchers described a specimen which preserves several ribs of a small fish, probably Jinanichthys. These fish bones are formed into a tight cluster about 6 mm across, and the cluster is in contact with the seventh and eighth cervical vertebrae of the bird. Though it is unknown how common fish were in the diet of Confuciusornis, the finding did not support a herbivorous diet, and the researchers pointed out that no specimens have been found with stomach stones, which are swallowed by birds to help digest plant fibers. Instead, they suggested it would have been omnivorous, similar to for example crows.

Paleoenvironment and paleoecology

Confuciusornis was found in the Yixian and Jiufotang Formations, which are part of the Jehol Biota. These rock layers contain lots of tuff from volcanic eruptions, along with shale and mudstone. Scientists can figure out the age of these formations, which is about 125 to 120 million years old, by studying the tuff.

The fossils of Confuciusornis and many other creatures were preserved because of flooding and volcanic ash. This created very flat fossils, and even some soft parts like feathers were saved. The climate back then was cool and temperate, with an average temperature around 10 °C. Many different plants and animals lived here, including many types of insects, fish, salamanders, mammals, lizards, pterosaurs, and dinosaurs. Some of these dinosaurs had feathers, showing how birds might have evolved from them. There were also many early birds in this area.