Archaeopteryx

Kingdom: Animalia

Phylum: Chordata

Class: Reptilia

Order: Saurischia

Family: Archaeopterygidae

Genus:Archaeopteryx

Species:lithographica

Introduction

When a fossilized feather was found in a limestone quarry in southern Germany in 1861, it caused a sensation in the scientific community. The feather, which was identical in structure to the feathers of contemporary birds, represented the first concrete evidence paleontologists had ever seen of the existence of a prehistoric bird-like animal. Several months afterward, a complete feathered skeleton was found and given the name Archaeopteryx lithographica by German scientist Hermann von Meyer; later, nine other Archaeopteryx specimens were positively identified. The animal's appearance—a blend of reptile and bird—helped to bolster the theory that birds had evolved from dinosaurs.

Classification

There has been considerable scientific debate over the proper classification of Archaeopteryx, largely due to the disagreement over whether it should be considered a true bird—and therefore placed in the class Aves (the group of vertebrates comprising birds)—or whether its more typically dinosaurian characteristics make it more suited to the class Sauropsida. One method scientists have of classifying species is known as cladistics. This is an approach that analyzes the number of measurable evolutionary novelties creatures have in common. The more shared novelties two creatures have, the more likely it is that they also shared an ancestor. A cladistics-based approach to classifying Archaeopteryx places it in the clade (or group) Paraves, which includes both birds and deinonychosaurs (a group of theropod dinosaurs). Other creatures in the same clade include the winged Jeholornis, which shared Archaeopteryx's long tail, and Sapeornis, which was larger than Archaeopteryx and had more defined digits on its hands.

Anatomy

The specific details of Archaeopteryx's anatomy have been of special interest to scientists since its discovery because of the early, but now less-widely accepted, contention that it represented a key transitional species, or “missing link,” between dinosaurs and birds. Archaeopteryx was an animal of about the size of a modern crow or raven, and possessed many of the classic reptilian features seen on some dinosaurs. These included teeth, a long, stiff, and bony tail, large, well-developed hind limbs (well-suited to walking), three-clawed fingers on each hand, and hyperextensible second toes. In particular, the anatomical features of Archaeopteryx are very similar to those of Compsognathus, a carnivorous, bipedal dinosaur from the theropod group. As a result, at least one Archaeopteryx specimen was misidentified as a young Compsognathus because its feathers were not well preserved and had been overlooked.

It is these feathers that are Archaeopteryx's most striking and scientifically provocative anatomical characteristic—and the feature that clearly sets it apart from similar species. Fossil evidence shows that the creature had a full set of plumage on its forelimbs, the bone structure of which resembled that of wings. Each feather sported asymmetrical vanes, like modern flying birds (flightless birds have symmetrical feathers). In addition, its flight feathers were arranged within the wing in the same way as modern birds. It also had long feathers covering its tail and hind limbs. Other characteristics of Archaeopteryx that are present in avian anatomy include a furcula, or wishbone, as well as generally light, hollow bones and primitive air sacs within the bones.

Intelligence

Over the years, scientists’ ideas about Archaeopteryx's EQ have changed. Based on an estimated brain size of about 0.92 milliliters, collected in the 1970s, the animal's EQ can be calculated at a relatively low 0.198 to 0.260. However, new evidence suggests the size of Archaeopteryx's brain was more like 1.6 ml in volume, raising its EQ significantly.

Using computer software to create a 3D model of the Archaeopteryx braincase from fossils, scientists recently concluded that the regions of the brain responsible for controlling sight and movement in animals were relatively large, much as they are in birds today. Its overall brain size compared to its body was also similar to that of modern birds—and approximately three times higher than the brains belonging to similarly sized reptiles that do not fly. This new evidence supports the idea that Archaeopteryx did, in fact, have the “brain power” needed to allow it to take flight. Whether its muscles and bones allowed for flight remains something of an open question.

Reproduction and Population

Scientists have not found very much evidence in the fossil record that sheds light on how large the population of Archaeopteryx was, how it mated and reproduced, and whether it was marked by significant sexual dimorphism (when males and females show physical differences). However, while no fossilized eggs or nests belonging to Archaeopteryx itself have been found, like most dinosaurs, it was most likely an oviparous, or egg-laying, species, and it probably constructed nests to protect its eggs and young.

Differing views of the origin of feathers provide various insights into the potential breeding habits of Archaeopteryx. Some researchers have speculated that the animal first evolved feathers largely as an insulating feature, designed to keep its body temperature constant as it nested and incubated eggs in relatively cool treetops. Others point to the fact that in many living birds, feathers serve another purpose, that of showing off the health and fitness of male individuals to potential female mates. If Archaeopteryx's feathers evolved for this reason, it is possible that the animal may have engaged in elaborate courtship displays involving posture and motion.

Diet

Although scientists agree that Archaeopteryx was not primarily or even partially a plant-eater, there is some uncertainty as to the exact nature of its carnivorous diet. Because of its slim beak, lined with teeth (more like a lizard's mouth than a bird's), the animal was initially believed to have subsisted on insects. However, given the fact that Archaeopteryx fossils have commonly been unearthed in what were once shallow, salty lagoons, it is likely that it fed at least in part on aquatic prey such as fish and marine reptiles. If so, it probably caught them with the large, hook-shaped claws at the ends of the digits on its arms (similar in shape to those found on modern bats that feed on fish), then tore them apart using its small, sharp, conical teeth (similar in shape to those found on modern marine birds). Archaeopteryx may also have used its claws to catch terrestrial prey such as small invertebrates.

Behavior

Since relatively few Archaeopteryx specimens have been found, and each was found in isolation, it is unlikely that this was a species that lived in a herd or flock. Instead, like many modern birds, it was probably a somewhat solitary creature except during the season of mating, breeding, and nesting. Archaeopteryx's metabolism has been found to be similar to that of Velociraptor, and the two species also share an elongated clawed toe on their feet often known as a “killing claw.” These clues suggest that, like Velociraptor, the carnivorous Archaeopteryx was probably a swiftly moving, aggressive predator.

Habitat and Other Life Forms

Approximately 150 million years ago, toward the end of the Jurassic period and before the Cretaceous period, the area now known as southern Germany was covered with shallow seas filled with warm water. At the time, the region had a climate that was warm, humid, and tropical. The formation of reefs divided the seas into small, salty, stagnant lagoons divided by scattered islands. Short shrubs, bushes, and sandy plains covered the islands, while to the north of the lagoon area there was a larger landmass where taller trees grew.

It was in these surroundings that Archaeopteryx lived. There is some debate over which part of this environment the creature actually inhabited, and this question is especially crucial to establishing whether, how, and how far Archaeopteryx actually flew. For many years, researchers have been divided over whether the creature took to the air by getting a running start from the ground, leaped from trees and glided, or did not fly at all, with its feathers being solely insulating material. The current consensus is that Archaeopteryx could probably fly, but perhaps not very well or for long distances.

Plants, including conifers and cycads, blossomed during the Late Jurassic, as did large plant-eating dinosaurs known as sauropods. Other life forms that coexisted with Archaeopteryx included insects such as flies, beetles, cockroaches, and earwigs, as well as pterosaurs (flying reptiles), ichthyosaurs, and plesiosaurs (aquatic reptiles), and marine creatures like starfish and sponges.

Research

Because of its mixture of reptilian and avian characteristics (it had teeth and a stiff tail, but also possessed feathers and a wishbone), Archaeopteryx was long considered by researchers to be the world's first bird, solid proof that today's birds had in fact evolved from dinosaurs long ago. In fact, the discovery of Archaeopteryx represented one of the most convincing nineteenth-century arguments in favor of the theory of evolution.

Some recent scientific evidence suggests that Archaeopteryx itself may not have been the “missing link” between dinosaurs and birds. For example, some dinosaurs millions of years older than Archaeopteryx also had feathers, such as Anchiornis huxleyi. As a result, there is no solid consensus on where and when Archaeopteryx diverged from the dinosaur line. In addition, Archaeopteryx is now known to have taken years to develop from juvenile to adult form, not weeks or months as modern birds do. Although it seems indisputable that birds did evolve from dinosaurs, scientists now think that certain avian features, like feathers, may have evolved independently, at different times and in different species. One current hypothesis is that Archaeopteryx may have been an evolutionary “dead-end,” and that a different bird-like dinosaur, probably a theropod, evolved into the modern flying bird.

In 2015, M. Wang and other researchers published a study in which they argued, among other things, that Archeopteryx belongs to the Linnean class Aves rather than Reptilia. Another team of researchers published a study in Nature Communications in 2018, which found that Archeopteryx had a wing-bone structure similar to living quails and pheasants, avian species that can fly for short periods of time.

Bibliography

Alonso, Patricio Dominguez, et al. “The Avian Nature of the Brain and Inner Ear of Archaeopteryx.” Nature vol. 430, no. 5 (2004), pp. 666–69.

"Department of Paleobiology." National Museum of Natural History, Smithsonian, naturalhistory.si.edu/research/paleobiology. Accessed 21 May 2020.

Farlow, James Orville. Complete Dinosaur. Indiana UP, 2000.

Fastovsky, David E., and David B Weishampel. Dinosaurs: A Concise Natural History. Cambridge UP, 2009.

Long, John A., and Peter Schouten. Feathered Dinosaurs: The Origin of Birds. New York: Oxford University Press, 2008.

Mayr, Gerald, Burkhard Pohl, and D. Stefan Peters. “A Well-Preserved Archaeopteryx Specimen with Theropod Features.” Science vol. 310, 2005, pp. 1483–86.

Paul, Gregory S. Scientific American Book of Dinosaurs. St. Martin's Griffin, 2003.

Schomp, Virginia. Archaeopteryx and Other Flying Dinosaurs. Benchmark Books, 2004.

UC Museum of Paleontology, UC Berkeley, 2020, ucmp.berkeley.edu/. Accessed 21 May 2020.

Voeten, D.F.A.E., J. Cubo, E. de Margerie, et al. “Wing Bone Geometry Reveals Active Flight in Archaeopteryx.” Nature Communications, vol. 9, no. 923, 2018, doi:10.1038/s41467-018-03296-8. Accessed 26 May 2020.

Wang, M., et al. “The Oldest Record of Ornithuromorpha from the Early Cretaceous of China.” Nature Communications vol. 6, no. 6987, 2015, doi:10.1038/ncomms7987. Accessed 26 May 2020