A model of Archaeopteryx lithographica at the Oxford University Museum of Natural History.
Archaeopteryx lithographica in Greek means ancient wing from the printing stone. ἀρχαῖος archaios from the greek, 'ancient' and πτέρυξ pteryx which means, 'feather' or ‘wing’ (Nedin, 1999). It is named after the limestone in which it was discovered, in the Solnhofen Limestone deposits in southern Germany (George Washington University, 2002). Therefore, it is also sometimes called by its German name Urvogel, which means ‘first bird’ or ‘original bird’.
Archaeopteryx is an early prehistoric bird, dating from about 150 million years ago during the Jurassic period. Consequently, it is considered by many to be the oldest known bird. Archaeopteryx shares both the features of theropod dinosaurs as well as modern birds. It is thus widely considered a transitional fossil between the birds and reptiles. In many ways, Archaeopteryx is more similar to small theropod dinosaurs than it is to modern birds. However, the evolutionary history of Archaeopteryx has never been that simple. It has always been hugely controversial in the past and continues to be an integral part of many scientific debates about the origin and evolution of birds.
The Origin of Birds: Evolutionary History
Archaeopteryx is the earliest, most primitive bird known. The skeletal anatomy of Archaeopteryx is very similar to that of contemporaneous coelurosaurian dinosaurs (Ostrom, 2008). Every skeletal feature of Archaeopteryx closely relates to contemporaneous coelurosaurian dinosaurs, with the only exception of fused clavicles and unique ischial morphology (Ostrom, 2008). This has been shown by analysing five known skeletal Archaeopteryx specimens and comparing them with the skeletal anatomy of several reptilian groups such as Ornithopoda, Theropoda, Pseudosuchia and Sphenosuchia, which were proposed as possible ancestors of birds (Ostrom, 2008). The presence of many commonly derived characters establishes that coelurosaurian theropods as the closest ancestral relatives of Archaeopteryx (Ostrom, 2008).
It has been argued that feathers are unique and only evolved in birds; hence making modern birds the closest relatives of Archaeopteryx. This theory is weakening with the emergence of newer data. Recently, Dr. Kundrat provided evidence representing the oldest feathered dinosaurs. The new fossil data from China and Mongolia suggest that feathers are not uniquely an avian feature (Kundrat, 2004). Kundrat (2004) pointed to the appearance of feathery appendages in theropods and showed their evolutionary importance in achieving higher metabolic rates, improved locomotion abilities associated with distinct behaviours and visual communication. This important evidence for pre-Archaeopteryx feathery appendages dispels the theory about uniqueness of feathers in Aves.
History of Discovery
Archaeopteryx is widely considered a transitional fossil between the birds and reptiles (Bakalar, 2005). It was one of the most important discoveries in the Jurassic Solnhofen limestone deposits of southern Germany (Bakalar, 2005). In 1860, a single feather was unearthed thus marking the start of the study of Archaeopteryx. The discovery was described by Christian Erich Hermann von Meyer, a German palaeontologist; the Archaeopteryx fossil was found in a limestone slab as a feathered impression. It was about 6 cm long and 1.1 cm wide. Apparently, at first glimpse it looked like a feather impression of a living bird. Except that the feather was 150 million years old! (Herreid, 1999). Within a month, Hermann von Meyer found a fossil skeleton, which looked like a bird with a lot of reptilian characteristics. It was a remarkable discovery and despite of its distinct reptilian features, von Meyer called it a bird and hence the name Archaeopteryx, ‘ancient wing’ (Herreid, 1999). The fossil skeleton was then sold to the British Museum of Natural History, while the slab and counterslab of the feather were sold to museums in Munich and Berlin (Herreid, 1999). Soon after, several different specimens were discovered and to this day, ten skeletal fossil specimens and an Archaeopteryx feather have been discovered (Bakalar, 2005).
At the British Museum of Natural History, the job to describe the skeletal Archaeopteryx went to Sir Richard Owen, a prominent anatomist of the British Museum. However, Thomas Huxley, an awe-inspiring anatomist, also known as ‘Darwin's Bulldog’ showed a lot of interest (Herreid, 1999). Huxley and Owen were old rivals; after Owen described the specimen, soon after Huxley pointed out grave inaccuracies in Owen’s analysis of Archaeopteryx. Huxley in his paper challenged Owen's many predictions including the presence of a toothless beak, just like other birds. Huxley further pointed out that the pelvis and feet of Archaeopteryx closely resembled those of several dinosaurs that walk on two feet, especially the small dinosaurs. He was critical of the fact that if Archaeopteryx did not have feathers, it would easily be mistaken for a reptile (Herreid, 1999). Thomas Huxley thus opened a new debate in favour of evolution and thus associated Archaeopteryx in close ancestral relationship with reptiles. His analysis was so fascinating, that on January 19th 1890, O. C. Marsh from the New York Herald responded, “He is certainly a wise man who to-day can tell a bird from a reptile, with only the fragments of an ancient form before him." (Majka, 1992).
Taxonomy
The "Berliner Exemplar" specimen of Archaeopteryx from Eichstätt, Germany.
A total of ten specimens of Archaeopteryx have been discovered till date. However, the matter is of dispute, whether all the specimens belong to the same species or not (Senter & Robins, 2003). Significant differences among the specimens have been noted which have resulted in an argument of taxonomic diversity within Archaeopteryx (Senter & Robins, 2003). Some of the noted disparity results from the following differences (Senter & Robins, 2003):
- differences in length of ulna relative to humerus
- differences in length of tibia relative to femur
- lack of pedal flexor tubercles
- lack of the iliofemoralis internus fossa
- lack of the anteroventral process on the anterior iliac blade in some specimens
- various dental differences
However, researchers, most notably Senter & Robins (2003), dispel those claims by arguing that iliac markings absent in the two smaller specimens can be attributed to the difference in age of the birds where tendency of bony markings increase in prominence with age in vertebrates. Dental differences between the specimens have been interpreted by researchers as a result of ontogeny (Senter & Robins, 2003). Because of varied opinion amongst researchers, the taxonomic history has become very complicated. However, most of the research data does point to a single species, Archaeopteryx lithographica. It is widely believed that Archaeopteryx forms the sister group to all other birds, and coelurosaurian theropods are its closest ancestral relatives.
Anatomy & Physiology
Archaeopteryx exhibits both reptilian and bird like characteristics. Similar to reptilians, Archaeopteryx had a complete set of teeth. Unlike all living birds, Archaeopteryx had a flat sternum, a long, bony tail, gastralia, and three claws on the wing, believed to be used in grasping its prey or maybe trees. However, it also had characteristics of a modern bird, which included feathers, wings, furcula and reduced fingers (UCMP, 2009).
Avian Features
Feathers: Feathers are generally a defining structure when looking at modern birds, although they cannot be relied upon as the sole identifying structures as this same feature has been found in fossils of the dinosaurs Protoarchaeopteryx robusta and Caudipteryx zoui (Ji et al. 1998). Archaeopteryx appears to have possessed well developed flight feathers, which are asymmetrical and contain a large amount of curvature as well as three distinct vanes. In some aspects however, the feathers of Archaeopteryx appear to be slightly more primitive than extant bird capable of flight with slightly less apparent asymmetry (Norberg 1995).
Limbs: The hallux, or first toe, in Archaeopteryx appears to be flexible, a trait not seen in remains of dinosaurs (Nedin 1999).
Reptilian Features
Limbs: Archaeopteryx possesses several characteristics present only in reptiles or bird embryos relating to the feet and claws, including the presence of claws on three toes (Metcalfe 1987). No extant bird species retains more than two claws on unfused digits entirely into adulthood. The “hand” bones of Archaeopteryx remain free and are not fused as seen in the carpo-metacarpus of modern birds. Also lacking in Archaeopteryx are fused metatarsals of the foot. Another feature present only in bird embryos, Archaeopteryx possessed foot bones free of the tarsometatarsus unlike modern adult birds (Nedin 1999).
Cranial Features: A feature common to extant birds is the connection of the skull to the spine at the base rather than the back as seen in reptiles and Archaeopteryx. Archaeopteryx lacks the keratin horn-cover on the maxilla and premaxilla, composing the bill in all extant bird species. These bones are also teeth bearing, a feature only seen in the development of bird embryos. Finally, the nasal openings of Archaeopteryx are positioned far forward on the skull, in front a large opening in the skull. This opening is generally absent or much reduced in modern birds (Metcalfe 1987, Nedin 1999).
Vertebrae, Tail: The tail of Archaeopteryx is unlike that of modern birds, in that it is long and possesses several individual vertebrae. Those of modern birds are generally much shorter and contain fused vertebrae (Nedin 1999).
Flight
The structure of feathers of the wings and tail appear to entail the ability of the animal to generate lift, although the extent to which it was able to do this is still a topic of debate. The flexibility in the wrist of modern birds is crucial for powered flight (Storer 1948). In Archaeopteryx the wrist joint possesses some of the traits necessary for flexibility, although it was clearly not as flexible as modern birds. Another important consideration is the structure of the breast and range of motion in the wings of Archaeopteryx. The shoulder joint present in the fossil record appears to be an intermediate form, somewhere between the dinosaurs and modern birds.
A crucial factor in the capability of powered flight is the ability of the wing to undergo both adduction and abduction. Archaeopteryx appeared to be capable of adduction, or the lowering of the arms below the shoulder, although Archaeopteryx was most likely incapable of abduction, or the raising of the arms above the back (Nedin 1999). This full range of motion is crucial for true powered flight (Storer 1948). The keel of the breastbone in modern birds is completely absent in Archaeopteryx suggesting that the pectoral muscles may have been anchored to an alternative location different from that in modern birds. This would have reduced the muscle power available for flight (Greenewalt 1975). It would appear that Archaeopteryx was not capable of the powered flight that is seen in modern birds, although that is not to say Archaeopteryx was incapable of flight in general. Given the multitude of features necessary for this type of flight that are missing in Archaeopteryx it is likely that it was not a strong flier, and was almost certainly incapable of true powered flight and a standing takeoff. Although Archaeopteryx may not have been capable of powered flight, it is likely that with the traits it did possess it was capable of some flapping and therefore flight after gaining a minimum speed, perhaps after a running start (Nedin 1999, Greenewalt 1975).
Pelvis
The pelvis of Archaeopteryx contains several features of both the modern birds and other saurischian dinosaurs. The pubis in Archaeopteryx faces backwards, parallel to the ischium, unlike that of saurischians. Unlike modern birds the pubis has retained a hooked end as seen in saurischians. The angle between the illium and the pubis in Archaeopteryx also appears to be an intermediate between the saurischians and the modern birds, but a look at the embryonic development of birds provides an interesting narrative. Early in the development of the chicken we see an angle of 45⁰, the same as Archaeopteryx (Nedin 1999). As the embryo develops the pubis rotates until it is almost parallel to the illium at the time of hatching (Metcalfe 1987).
Lifestyle
The behaviour and day to day life of Archaeopteryx has been a difficult topic to address. Until recently it was believed that it lived in a forested environment, but the fossil specimens were rarely if at all found in the presence of fossilized tree detritus suggesting a different lifestyle. It is entirely possible that Archaeopteryx thrived in smaller shrubs, feeding on smaller animals that would have lived within them (Chiappe 2007).
Controversy
Archaeopteryx has been at the center of several controversies, ranging from disagreements over its position in the phylogenetic tree to the validity of several fossil specimens. The arguments over the validity of the specimens originated over several perceived, but ultimately unsubstantiated discrepancies in the fossils. The hypothesis that Archaeopteryx is in fact more closely related to dinosaurs than birds has and continues to be a contentious area of debate. Part of this hypothesis states that Archaeopteryx is in fact not the earliest bird, which instead is Avimimus. A further assertion that Archaeopteryx is not an intermediate species at all and is in fact a true bird has also been claimed and is a cornerstone to the creationist argument.
Conclusion
While a general consensus of the position of Archaeopteryx in the evolution of modern birds is difficult to find, it is clear that it contained many transitory character states of both dinosaurs and modern birds. Although it was most likely not a transitional species itself, the character states that it did possess would suggest that the transition from dinosaur to modern bird did indeed occur.
