In this chapter, Shubin describes how Tiktaalik was discovered and how it had a shoulder, elbow, and wrist. He previously began by explaining why having a wrist, shoulder, and elbow is advantageous to humans. But if having limbs was used to get out of the water (to avoid predators), how were these things helpful to fish before they were able to move from water to land? What functions did a wrist, shoulder, or elbow have in fish that helped them survive from a structure/function perspective specifically focusing on why it was advantageous? (Although this is a broad topic, focus on one piece in particular and expand on how this specific structure allowed the fish to have a certain function that was more efficient than it was previously able to perform).
-Michelle Layvant, mlayvan2@students.d125.org
Shubin doesn't just talk about the development of limbs as a single achievement, but as a slow evolutionary process involving many organisms, millions of years, and environmental pressures. The earliest form of a limb had the beginnings of the modern bone pattern (Owen's "one bone - two bones - lotsa blobs - digits") in limbs was in the Eusthenopteron. This lunged-animal showed very early signs of limbed animals, being the proud owner of "one bone - two bones". But it ended there, and these three bones existed in a true fin, not yet a limb. The next organism, Icthyostega soderberghi, a key organsim in the transition from water to land, already had fully formed limbs, and therefore didn't contribute much to their development. The biggest breakthrough occured in Acanthostega gunnari, which had fully formed limbs, but in a fin. This explained that the structure of the primitive limb led to the function of helping the animal to swim, not walk. It was this distinction that allowed Shubin to come to the conclusions he did about Tiktaalik's use for its limbs.
ReplyDeleteIn the book, Shubin describes the environment in which Tiktaalik lived: the floors of shallow ponds and streams, which he discerned from the animal's flat head, eyes on top, and ribs. He describes how other fish in the environment were almost "twice the size of the largest Tiktaalik", presenting a significant environmental pressure on Tiktaalik's ancestors. From Shubin's point of view, an animal in that situation had to either evolve into a larger animal, one with armor, or simply had to find a way to leave the environment. In his essay entitled "The Great Transition", Shubin uses the previous organism, Acanthostega gunnari, to show that limbs developed in fish, as opposed to animals that had already made the transition. Tiktaalik's unique adaptations allowed it to more easily maneuver the environment, and eventually to leave the water for the safer land. Daniel Kane from MSNBC says that the rivers where Tiktaalik lived were filled with "organic muck" from the decaying plant biomass. This gunky, thick river bottom provided a big challenge to the animals living there, and having weight-bearing fins would provide a significant advantage.
http://www.msnbc.msn.com/id/4638587/ns/technology_and_science-science/
http://www.edge.org/3rd_culture/shubin06/shubin06_index.html
Jeremy Solomon
imabum14@gmail.com
The discovery of Tiktaalik was so extraordinary and important because it was one of the first species found to have features of both fish (webbed fins and scales) and amphibians (joints and limbs). Shubin's studies on the Tiktaalik revealed that it had "a shoulder, elbow, and wrist composed of the same bones as an upper arm, forearm, and wrist in a human" (39). The structure of the joints in Tiktaalik provided a very extraordinary function: "it was capable of doing push-ups" (39). Tiktaalik, having a flat head, eyes on top, and ribs, was "built to navigate the bottom of shallows of streams or ponds, and even to flop around on the mudflats along the banks" (40). Having fins that were capable of doing push-up like movements, this was very advantageous to Tiktaalik because the fins were able to support the body and help it maneuver through streams, ponds, and mudflats. Shubin also states that the rocks which had Tiktaalik fossils embedded within them showed evidence that Tiktaalik did actually live in shallow streams that were surrounded by seasonal mudflats. Because Tiktaalik was structured to have a shoulder, elbow, and wrist, it was able to navigate the floors of shallow streams and mudflats - the main function of this push-up like motion that Tiktaalik was able to perform, aside from being able to leave the water to escape its predators.
ReplyDeleteFish without the advantage of having limbs and joints like Tiktaalik move through the water by contracting muscles called myomeres in front-back and side to side movements. Because these fish don't have fins structured to have shoulders, elbows, and wrists, they are not able to navigate the floors of water, since they do not have as much control over their movements with only fins and myomere muscles. Also, many fish would have trouble navigating shallow streams since the current could be strong and the volume of water would be shallower and smaller than other bodies of water. Fish without joints would not be able to navigate through mudflats like Tiktaalik.
Sujin Ko (sujinko93@gmail.com)
Sources: Shubin - Chapter 2 (pages 38-40)
http://www.fish.state.pa.us/anglerboater/2002/S-O02web/fishswim.pdf