Avian Origins

Avian Origins
The class Aves, with its more than 10,000 living species inhabiting the earth, is the most diverse of land vertebrates. Its members are examples of one of the richest and most controversial passages in animal evolution, and the descendants of an ancient radiation that finds its origins well into the Cretaceous period (Chiappe & Dyke, 2002). Even though birds represent one of most common and easily recognizable class within the animal kingdom, their origins remain the center of a debate that has for over a century filled the pages of scientific journals and produced spirited arguments among and between paleontologists and ornithologists. Despite such controversies, scientists from both disciplines agree that birds evolved from reptiles. While some paleontologists place avian origins some 230 million years ago among the archosaurs of the Triassic and long before the split between Saurischia and Ornithiscia (Feduccia, 1999), others sustain that modern birds rise from the mighty Theropoda (Chiappe & Dyke, 2002). Nevertheless, the research conducted for the purposes of this paper on the origin of birds yielded an overwhelming support for the latter hypothesis. Although the major focus of this paper will be to present part of the evidence that upholds that “birds are theropod dinosaurs” (Chiappe & Dyke, 2002, p. 92), the counterargument will be briefly discussed so as to offer an alternative to the study of avian origins.
The discovery of Archaeopteryx: A brief historical review on the establishment of avian ancestry
In 1863, Sir Richard Owen described in detail the morphology of a fossil that had been found in the Late Jurassic Solnhofen limestones of Germany two years earlier. After a thorough examination, Owen decided that Archaeopteryx lithographica belonged within the Aves, making it the oldest and only Jurassic bird to date (Chiappe & Dyke, 2002; Cracraft, 1986). A few years later, Thomas Henry Huxley also undertook an examination of the specimen and found several similarities between Archaeopteryx and reptiles, which prompted him to hypothesize the origin of birds within them, “with dinosaurs held to be highly probable sources” (Dodson, 2000, p. 505). These newly found phyletic connections of birds to reptiles were later challenged by Gerhard Heilmann, who proposed Thecodontia (an “incongruous assemblage…not easily definable by modern standards” [Feduccia, 1999, p.47], that is less anatomically specialized than Theropoda and includes most Triassic archosaurs such as pterosaurs, crocodiles, and dinosaurs [Prum, 2003]) as the ancestors of birds. Even though Heilmann had seen the connection between birds and dinosaurs, he considered the dinosaurs’ apparent lack of clavicles (the structure that preceded the avian furcula [wishbone] and which was later recognized among major Theropoda clades including dromaeosaurs, oviraptors, tyrannosaurs, allosaurs, and coelophysids [Prum, 2003]) as a drawback for the dinosaurian ancestry of birds.
Heilmann’s hypothesis reigned as the established answer to the origin of birds for forty years, until the early 1970s when John Ostrom—briefly after having concluded his study on Deinonychus antirrhopus and his examination on a newly discovered specimen of Archaeopteryx—published a series of papers in which he provided the “first detailed evidence” (Cracraft, 1986, p. 384) and proposed a close relationship between theropod dinosaurs—more specifically coelurosaurus, and within these maniraptorans—and birds. Ostrom had noticed several synapomorphies between higher coelurosaurs and Archaeopteryx; however the one trait that led him to question the established hypothesis and consequently support the theropod-avian relationship was the fused distal carpals 1 and 2 which allowed the wrists to swivel sideways (Prum, 2002). In 1986, Jacques Gautier proposed an initial cladistic interpretation of the theropod origin of birds which has been, and continues to be, modified and amended with new discoveries (see Appendix A).
Although widely accepted, the dinosaur-avian relationship remains much debated, not only among those who oppose it—who firmly believe that the phyletic connections still do not offer solid evidence for the case—but also among those who support it in their enthusiasm for determining the exact sister-taxon of Aves among coelurosaurians; dromaeosaurids (e.g. Deinonychus, Velociraptor), troodontis (e.g. Troodon), oviraptorids (e.g. Oviraptor), and alvavezsaurids (e.g. Shuvuuia deserti) are the most commonly cited taxa (Chiappe & Dyke, 2002).
Birds and dinosaurs: Getting closer
Much evidence has been accumulated since Ostrom’s work in support of the theropoda origin of birds. Although there are many traits shared by the two groups, there are a few that could be considered of particular interest as they better illustrate the relationship between birds and coelurosaurian theropods.
Feathers
Numerous wonderfully preserved specimens of coelurosauria from the Early Cretaceous of China have provided strong evidence that feathers arose within theropods and that they preceded the origin of flight and, by extension, the origin of birds as we know them (Wong, 2001). The specimens recovered from northeastern China’s Liaoning Province—which have preserved integumentary structures interpreted as feathers—include at least six taxa of non-avian theropods (Chiappe & Dyke, 2002). These structures vary in size and shape from the simpler filament-like to tufts attached at their bases, to more complex ones with vanes. Although their elaborated arrangements resemble those of avian feathers and are located externally, supporters of the opposing views of the origin of birds have claimed that these structures are only “frayed internal composite fibers of the structural protein collagen” (Chiappe & Dyke, 2002, p. 107). The discovery of structures surrounding a specimen of the alvarezsaurid Shuvuuia deserti provided further support for the avian hypothesis when they were shown to be composed of only β-keratin, the same component of avian feathers.
Feathers appear to have had purposes other than flight. It is important to clarify at this point that while modern birds (or neornothines) possess feathers that are adapted for flight, the primordial structures may have gone through several stages of development, independent of their functionality, until becoming feathers that could have had an aerodynamic function. Prum (2003) has hypothesized that the initial stage was represented by “hollow tubes”, followed by a “tuft of barbs”, then by “doubly branched feathers”, and finally by “feathers with a closed pennaceous vane” (p. 556). Such gradual development spanning millions of years, hints that feathers did not originally evolve for flight, but perhaps for insulation (Wong, 2001) or even as a result of sexual dimorphism as it had been originally hypothesized for Archaeopteryx (Owen, 2006).
Shared traits
Further support of a coelusaurian origin of birds has been provided by the Malagasy Rahonavis ostromi or “Ostrom’s menace from the clouds” (Leutwyler, 1998, para. 1). Found and described in 1998, R. ostromi embodies a complex combination of characters that place it in a basal position within Aves (Chiappe & Dyke, 2002). It has very defined dinosaurian features such as a thicker toe bone structure and sickle claw on the second toe of its pes (typical of maniraptorans), and a long tail (just like Archaeopteryx’s), but also very bird-like characters including long hollowed and light bones, a reversed first toe, hip and leg structure similar to that of modern birds, and quill knobs on its forearms represented by six “bumps” (Leutwyler, 1998). This extraordinary array of features, though it provides invaluable and strong evidence, has not gone unchallenged. Several dissenters to the theropod hypothesis have claimed the specimen to be a sham composed of avian (forelimb and shoulder girdle), and non-avian theropod parts (hindlimb, pelvis, and tail); nevertheless, the fact that the specimen was found on a surface of less than 0.14 m² suggests that the bones belong to only one individual (Chiappe & Dyke, 2002).
Flight
One of the unique characteristics of birds that accounts for their aerodynamic features (elongated forelimb with flexible wrists, arrangement of vaned feathers in remiges and retrices, and a lateral orientation of the shoulder socket) is their ability to fly or glide (Chiappe & Dyke, 2002). Nevertheless, all of these features evolved before the Aves became the vertebrates we know today. The development of such complex locomotor system led not only to physiological and muscular transformations, but also to behavioral and neurological changes. “Each major lineage of Mesozoic birds provides direct evidence of the changes in the skeleton and plumage” that occurred before these creatures were able to take off (Chiappe & Dyke, 2002, p. 110). Although Archaeopteryx was originally interpreted as a bird capable of flight, it is probable that its lack of well-developed flight musculature and several aerodynamic features may have made it less of a flying specimen and more of a glider. A younger species (25 million years younger to be exact), Microraptor gui from the fossil beds of Liaoning Province in China, has shed some light on the mode of life of these ancestors. This four-winged bird has led scientists to speculate that the evolution from dinosaurs to birds included a phase in which birds had flight feathers on all their limbs (Mayell, 2003). Whether they were capable of flying (or gliding) remains another much debated question, as it opens the arena for yet two more hypotheses: Were these ancestors gliders (they climbed trees [or were tree-dwellers] and learned to glide before flapping their wings), or were they flappers (they developed legs that gave them speed and lifted them off the ground while energetically flapping their wings)? The team that recovered Microraptor is convinced that the feathers on the hind limbs would have represented a burden for the animal if it attempted to run (Mayell, 2003). However, whether these birds took advantage of gravity or launched themselves into the air is still a matter of interpretation with no conclusive answers.
Similar Habitats
Numerous paleontologists have argued on the subdivisions of birds of the basal lineages. Although some of these interpretations have not been evaluated within a cladistic framework, several analyses have shown a strong and closer association between confuciusornithids and enantiornithines, and extant birds (Chiappe & Dyke, 2002). Of all the Cretaceous groups, Enantiornithes, with about 20 species reported to date, are by far the most diverse. The early members of this group were toothed and small, and shared many anatomical features with their living relatives including pedal morphology that evidenced perching. Most enantiornithines occupied the niche of current land birds; however, some specimens have also been found in littoral and marine environments. The discovery of the ornithurine Apsaravis ukhaana in 1998 in the Gobi desert of southern Mongolia contradicts a previous hypothesis which suggested that the enantiornithines had pushed the ornithurines, the closest relative to living birds, to coastal areas and marine habitats. The recovery of an ornithurine in a sand dune environment prompted scientists to suspect that these early birds occupied the same wide variance of niches modern birds fill today (Wong, 2001).
Brooding
Finally, the presence of brooding behavior similar to that of the Aves has been suggested after the discovery of theropoda remains associated with their clutches (Chiappe & Dyke, 2002). Furthermore, embryonic remains of coelurosaurians have revealed that these non-avian dinosaurs and birds shared the features of their egg shells microstructure.
Conclusion
There is no doubt that modern birds have evolved from reptiles; furthermore, the fossil record seems to provide the strongest evidence in support of the coelurosaurian hypothesis of the origin of birds. Despite the evident gaps from one taxon to the next and continuous disagreements among the experts—which in some instances appear to be driven by zeal and rhetoric rather than science—the level of understanding on the evolution of birds achieved in the past fifty years has swept old conceptions and opened the debate to the possibility of new (and why not, even radical) interpretations. Even though the discipline has made tremendous advancements regarding the way we look at dinosaurs and birds, everything seems to indicate that we should continue to stay tuned for the changes future discoveries may bring along.

References
Chiappe, L. M., & Dyke, G. J. (2002). The Mesozoic Radiation of Birds. Annual Review of
Ecology and Systematics, 33(2002), 91-124. doi: 10.1146/annurev.ecolsys.33.010802.150517
Cracraft, J. (1986). The origin and early diversity of birds. Paleobiology, 12(4), 383-399.
Stable URL: http://www.jstor.org/stable/2400513
Dodson, P. (2000). Origin of birds: The final solution? American Zoologist, 40(4), 504–512.
Retrieved from http://icb.oxfordjournals.org/content/40/4/504.full.pdf on February 25, 2011.
Feduccia, A. (1999). The origin and evolution of birds. New Haven, CT: Yale University Press.
Leutwyler, K. (March 23, 1998). Flying dinosaurs. Scientific American. Retrieved from
http://www.scientificamerican.com/article.cfm?id=flyin-dinosaurs on February 25, 2011.
Mayell, H. (January 22, 2003). Four-winged dinosaurs found in China experts announce.
National Geographic. Retrieved from
http://news.nationalgeographic.com/news/2003/01/0121_030122_dromaeosaur.html on February 25, 2011.
Owen, J. (September 28, 2006). Dino-era bird flew with four wings, study says. National
Geographic. Retrieved from http://news.nationalgeographic.com/news/2006/09/060927-four-wings.html on February 25, 2011.
Prum, R. O. (2002). Why Ornithologists Should Care about the Theropod Origin of Birds. The
Auk, 119(1), 1-17. Stable URL: http://www.jstor.org/stable/4090007
Prum, R. O. (2003). Are current critiques of the theropod origin of birds science? Rebuttal
to Feduccia (2002). The Auk, 120(2), 550-561. Stable URL: http://www.jstor.org/stable/4090212
Wong, K. (January 11, 2001). Mongolian fossil sheds light on bird evolution. Scientific
American. Retrieved from http://www.scientificamerican.com/article.cfm?id=mongolian-fossil-sheds-li on February 25, 2011.
Wong, K. (April 26, 2001). Dinosaur discovery shows feathers came before flight. Scientific
American. Retrieved from http://www.scientificamerican.com/article.cfm?id=dinosaur-discovery-shows on February 25, 2011.

Appendix A
Figure extracted from Prum (2002).

 

Your Guide to Laos

Squeezed in a corner of Southeast Asia between Cambodia, Thailand, Burma, Vietnam, and China, Laos seems to rest quietly in an almost natural state. Twice the size of Pennsylvania, Laos has been defined as “the forgotten country” because is in fact one of the least visited and developed places in the area, yet the attributive that fits it best is “The Jewel of Southeast Asia”. A French colony for almost one-hundred years, Laos has remained isolated from foreign influences up until 1975, and shyly opened up to tourism in the last decade.

Heavily supported by NGOs (nongovernmental organizations), and one of the poorest countries in the world, Laos will offer you some exciting eco-touring options such as kayaking along the spine of the land that forms the Mekong river, trekking up in the mountains, or exploring into the Buddhist culture. Even the shortest visit to this enchanting, laid-back land will teach you to slow your pace to match its own. A trip to Laos is not about the destination, but the journey itself.

The country has a diverse geography that matches its population as every region conforms a piece of this rich tapestry of ethnic minorities. The best time to visit Laos is the cool season between November and February, as it rains least and isn’t too hot (temperatures oscillate between 71 and 75 degrees). Be advised that these are the peak tourist months since some major national and regional festivals (buns) take place during this period. During the rainy season -from May through October- most of the roads are washed out and the Mekong grows, offering a better chance to kayak its peaceful waters.

A visa is required to enter. Most border crossings will issue them upon arrival for 15 days. Make sure to have 2 passport size pictures and $30. You can obtain one for 30 days if issued at a Laos embassy. Once in Laos you can get an extension for an extra day at a travel agency for $3. Overstaying will risk you arrest, and a $5 fine for each day upon departure.

The cheapest way to get to Laos is flying to Bangkok, Thailand. A flight from Bangkok to the border city of Udon Thani is a third the cost of an equivalent one-way flight from Bangkok to Vientiane (the capital city); while a round-trip is half the cost. From Udon Thani you can take a 40-minute minivan ride that will take you to the Friendship Bridge that crosses the Mekong river and unites Thailand and Laos. From the border, a 100-Thai baht ($3) bus ride will take you to the center of the city in approximately 20 minutes.

Vientiane could be defined as one of the world’s unique capitals. As contrasting as the city is, you will find European sedans parked alongside rusty tuk-tuks (three-wheeled bicycles that serve as taxis), people talking on cell phones, monks in robes playing online video games at an internet café, and fine dining restaurants sharing the block with the popular and inexpensive ones. This tendency Laos is following from neighboring Thailand’s model is regarded as both dangerous to its natural resources and helpful to its economy. On one side, the further exploitation of forests and waterways may signify the loss of a gentle way of life; on the other, the aperture to the international market may boost the country’s economy. On the verge of this decisive position, the Lao government has been working with UN agencies to control the rural development in order to protect the vital resources at stake. Either way, if you decide to go to Laos and experience its natural state, do it with the utmost interest of being an attentive observer.

Laotians are extremely generous, friendly, and easy-going, and music and dance are part of the daily Lao character as well as their religion (60% of Lao people are practicing Buddhists). Foreigners are encouraged to do their part to preserve the peaceful environment, so keep in mind these social rules when you leave your hotel room.
- One of the main tourist attractions are the hundreds of Buddhist temples (wats) and shrines dispersed across the country. Never go shirtless, wear miniskirts or revealing shorts, and take off your shoes when entering one; breaking these rules is regarded as not polite and a lack of respect for their religious beliefs. Also, never climb a Buddha statue and keep your head lower than the monks. If you’re a woman, refrain yourself from touching or speaking directly to a monk, unless inside a temple. To your surprise, you will find that Lao people will change seats on the buses so that monks can sit only near men.
- A kiss or embrace from a stranger is considered insulting. Traditionally, Lao people use the nop to greet someone. Raise your hands in praying position; the higher you go, the more respect you show. Perform it sincerely and will win you instant friends.
- The head is believed to be very important to the souls and purity, while the feet are considered filthy. Never touch someone’s head, not even a baby’s, or point with your feet as it is considered barbaric. If you’re a woman seated on the floor, tuck your legs to one side; only men may sit with the legs crossed. Needless to say that you have to keep your feet off furniture when in public.
- Body hygiene is of the highest standards. Shower often and you will avoid being embarrassed.
- Show respect for the elderly and for the Lao culture.
- If you’re traveling with your partner, keep any display of affection private; public displays are taboo.
- Smile a lot, and use your eyes and common sense to maximize your chances of social success.

Laos is very much a cash country, especially outside of Vientiane. Keep always small USD notes with you that will be easier to change. Travelers cheques are almost impossible to change outside the city, and credit cards are only accepted in fine restaurants and luxury hotels. Although US Dollars are widely accepted, it’s always better if you carry some Thai baht or Lao kip, the national currency (the conversion goes US$ 1 to 10,000 kip). Accommodation-wise, you will find from luxury to backpacker and inexpensive deals. Either way, make sure you book ahead and ask for a discount if you go during the rainy season. $20 a day will suffice for all your needs, although it’s possible to get by on less than $10 depending on your expectations.

Outside of Vientiane toiletries can be hard to find, as well as are batteries, film and video tapes, so bring your own toilet paper or find a convenient tree. Even though trash cans are limited outside the city, pollution is almost inexistent, so don’t contribute to it and keep a plastic bag with you where you can dispose of your garbage until you make it back to the city. Keep in mind that Laos uses 220V AC, so have your own adapter if you’re taking electronics with you, and don’t forget to throw in a flashlight in your baggage as electricity is scarce outside the city (sometimes available for only a few hours a day), and power cuts are frequent. The country is extremely safe as a general rule, but petty crime does exist, so minding your valuables won’t hurt. If you find yourself out in the rural area, be cautious as little accommodations and services are available, and some recent robberies have been reported in those areas.

Vientiane will offer you the chance to experience first hand the Buddhist culture and more. Many temples have been beautifully reconstructed and allow access to visitors of any kind. That Luang is probably the most prominent one and the scene of an important festival in November. The Buddha Park -a sculpture garden full of Hindu and Buddhist statues- is awe-inspiring. You can’t miss the huge reclining Buddha and the opportunity to take the “me next to a giant Buddha” shot. The Morning Market (Talaat Sao) is where all the action takes place. Wake up early one morning and walk among the street vendors to breathe in some of the laid-back atmosphere. The city block is a sea of goods to explore, and your chance to put your bargaining skills to test. This is the Lao version of “mall culture” and a place you can’t ignore during your stay. The Plain of Jars is also a stunning and mysterious place with hundreds of stone jars of varying sizes. At the end of the day, you can relax by the Mekong sipping on Beerlao, one of the country’s few exports, and watch the sunset turn the water pink.

During your stay, you will understand the fascinating concoction this country is. Laos will ask you to never be in a hurry, and let the calm penetrate your very being. Make sure you leave your worries and prejudices at home when you travel to Laos, and you will find that the beauty of it exists not only along the Mekong at sunset, but in smiles at the market and the hospitality of its citizens.