Thursday, March 10, 2011
Tiktaalik: the Holden Caulfield of Biology
Tiktaalik: Part Fish, Part Land-Animal In 2004, Neil Shubin and his team discovered a new creature which they named Tiktaalik, or “large freshwater fish”. Tiktaalik exhibited traits of both a fish and a land animal. With scales and webbed fins, this new animal also had a flat head, neck, and the beginning of limbs and joints. While Tiktaalik was not a land-animal himself, his adaptations allow us to better understand the traits needed for animals to transition to life on land. Explain how exhibiting all of these traits could give Tiktaalik a competitive advantage in both his natural habitat and a land environment. Explain how these characteristics could give Tiktaalik a disadvantage. Be sure to consider Tiktaalik’s relationship with his environment or habitat, as well as other animals that may have been living at that time.
Subscribe to:
Post Comments (Atom)
This comment has been removed by the author.
ReplyDeleteThis comment has been removed by the author.
ReplyDeleteThe tiktaalik that resides mostly in the water needs gills to breath and fins for movement. However, there are other features in this animal that represent tetrapods. The tiktaalik has a flathead like a crocodile and a jaw for predation. The benefit for having the flathead is the versatile jaw that can be used to catch just about any predator. The structure of the flathead provides the powerful elongated jaws that crocodiles have and the powerful snapping motion to completely crumble the prey. This exhibits one theme of biology: structure and function. The tiktaalik has an evolutionary advantage to other fish by having this jaw that is more powerful. Another advantage could have been the space a flat head would create in the mouth. More space would allow more food to enter the tiktaalik, but another benefit would be to ensure that the prey was caught. This structure gives the tiktaalik an advantage in catching food. A possible reason for this evolution in fish could be due to the rise of other dangerous fish in the sea. The Devonian period, the time period when tiktaalik is thought to have lived, had ancestors of shark and armored placoderms that also had powerful jaws lined with teeth for hunting (National Geographic). The Devonian period was known as the “Age of Fishes” when a huge variety of fish rose and the most formidable of them were the armored placoderms, a group that first appeared during the Silurian with powerful jaws lined with bladelike plates that acted as teeth” (National Geographic). Competition for food could have provided the tiktaalik with the structure like a flathead for more efficient hunting.
ReplyDeleteThe reason for the neck would allow further enhancement in feeding and hunting for the tiktaalik. The neck provides mobility for the jaws to move in the revelant direction as the prey in sight, and the neck would allow the head to move around which is turn would allow the eyes move and give the fish more visibility in the water. The tiktaalik like most other fish have scales necessary for protection against predators. The scales on the outside could have been used as protection from fish like the armored placoderms and shark ancestors. This brings up the theme of evolution. The tiktaalik clearly shows the primitive outline of the reptiles we have today. A crocodile now most likely came from the tiktaalik from millions of years ago. The flathead and moveable neck of the tiktaalik was a selective advantage and it lasted until now because it was a beneficial change for the animal.
The benefit of having a shoulder and a hard bone to allow the fish to prop up would help in hunting and survival. One way to use the shoulder would be to “crawl” on the seafloor which would reduce the use of the fins and tail for more vibrations. Fish have a lateral line system which allows them to detect water vibrations, and by reducing water vibrations the tiktaalik can quietly crawl to its prey to secure its food (mongabay). Another method to use the shoulder would be to crawl out of the water in case it detects a predator nearby. The shoulder and a bony fin would allow the tiktaalik to crawl and move about on land to escape the dangers of the predators in the water as well as another opportunity to hunt for food (mongabay). If the tiktaalik had shoulders and were able to crawl out of water, that means they must have had some kind of other adaptation like a lung for a source of oxygen. That could have also been true because it is said that they had an enlarged and thicker ribcage. All these adaptations like “the sturdy wrist bones, neck, shoulders, and thick ribs of a four-legged vertebrate” resemble the similarities to a terrestrial animal (Berkeley). Another example of evolution that shows the early use of wrists and shoulders to stand and move on land instead of using fins to move in water.
http://science.nationalgeographic.com/science/prehistoric-world/devonian.html
http://fish.mongabay.com/anatomy.htm
http://evolution.berkeley.edu/evolibrary/news/060501_tiktaalik
To add on to what Benny has previously said about structure and function, I would like to introduce a specific article from National geographic which reveals the discovery of a new crocodile which was thought to have lived during the Cretaceous period (http://news.nationalgeographic.com/news/2011/03/pictures/110330-new-dinosaur-crocodile-cousin-brazil/). Examining the fossil reinforces the theme of evolution: qualities that are necessary to survive the land on life are effectively passed on to its offspring. Just like its ancestors, the fossil holds a large jaw which aids in the digestion of other prey. Although Benny mentions the benefits of having unique structures that are found in Tiktaalik, such as the jaw, the neck, and the shoulders, he does not mention the overall advantages that a terrestrial organism has over an organism that lives in an aquatic environment.
ReplyDeleteOne of the greatest differences between aquatic and terrestrial organisms is the way in which they fertilize its eggs. Aquatic organisms, such as fishes and frogs, generally exhibit external fertilization (meaning that the egg is fertilized externally from the body of its parents). External fertilization provides for a large number of offspring to be made. On the other hand, terrestrial organisms, such as humans, exhibit internal fertilization (meaning that the egg is fertilized inside body of its parents). Although organisms that exhibit internal fertilization produce fewer zygotes, it allows for a higher probability for survival compared to external fertilization. This is because the zygote is protected from environmental hazards (predation, weather, etc.), and therefore are not susceptible to factors that may harm it. In addition, internal fertilization also does not require water, an important adaptation for organisms that must live by areas without water. Terrestrial organisms also have a reliable source of nourishment in its location of development (the placenta, yolk, etc.), giving them an advantage over zygotes in an aquatic environment which must find its own food to grow and survive.
Another part of the question that Benny did not mention are the disadvantages that terrestrial organisms have over aquatic organisms- parental care. The amount of parental care provided for its offspring is inversely proportional to the number of offspring that a species produces at one time. Therefore biggest disadvantage to an organism that exhibits internal fertilization is the amount of parental care that they must provide for its children. Because the mother must physically bear the fetus during its stage of development, there is a significant amount of environmental stress that it must go through to protect and nourish its child.
However, terrestrial organisms have developed several mechanisms to cope with the environmental stress that comes from the transition from sea to land. Because the conditions were significantly different, Tiktaalik and other descendants of terrestrial organism had to develop new ways of reproducing. One reproductive adaptation that terrestrial animals have made is parthenogenesis. Parthenogenesis is a form of asexual reproduction in which an egg develops without being fertilized. Practicing parthenogenesis will allow for more offspring to be made in comparison to sexual reproduction methods. However, as with all methods of asexual reproduction, this will lead to less genetic variation, a disadvantage for species seeking to survive a severe, rapidly changing environment. Another reproductive adaptation that terrestrial animals have made is hermaphroditism. Hermaphroditism is when an organism has both male and female reproductive systems. Hermaphroditism proves useful and advantageous when the organism is sessile (stationary) our does not encounter many other mates during its lifetime.
ReplyDeleteFurthermore, terrestrial organisms have developed additional structures to aid their survival on land. Its embryos have extra-embryonic membranes which help them to make life on land possible. Extra-embryonic membranes include the amnion, yolk sac, chorion, and allantois (715) . The amnion forms a sac filled with amniotic fluid and protects the embryo of an amniote. This is important for species that are oviparous. Because the development of the embryo occurs outside the body, any sort of environmental hazard could lead to the death of the embryo. To cover for this, the amniotic fluid cushions any external pressures which may potentially break the egg shell. In addition, terrestrial organisms have a yolk sac that contains yolk, which will provide nourishment for the growing embryo. The amount of yolk is related to the environment in which the animal develops. Frogs have less yolk because they have swimming larvae which can obtain their own food. Birds are terrestrial species with eggshells to prevent desiccation, so they are unable to obtain their own food. Therefore, they require more yolk to provide them with more nutrition. Because the offspring of terrestrial organisms cannot acquire their own food while they are in its embryonic developmental stage, the yolk sac is an essential adaptation for life on land. Terrestrial organisms also have the chorion, which lines the inner surface of the shell and participates in the exchange of CO2 and O2 between the embryo and the outside air. This essentially facilitates gas exchange within the developing embryo. Finally, the allantois stores metabolic wastes and eventually participates in gas exchange.
Works Consulted
Campbell Biology
Your Inner Fish
http://news.nationalgeographic.com/news/2011/03/pictures/110330-new-dinosaur-crocodile-cousin-brazil/
(Keigo Tanaka;tanakarus3@hotmail.com)
ReplyDelete