In the chapter entitled “Teeth Everywhere”, Neil Shubin discusses the importance of teeth to the study of fossils and the study of evolution, overall. In our last unit of AP Biology regarding animal diversity, we had discussed that the factor that would determine if a fossil was a mammal versus another classification would be teeth. Shubin describes the “major patterns of chewing” over time as they had aided paleontology and evolutionary research (73). The mouth became more specialized as we evolved to have incisors to cut food, canines to puncture food, and molars to shear or mash food (73). He discusses that this “new tooth row” first appeared in small mammals about 150 million years ago (73).
This change in jaw structure clearly became a quick identifier to the developmental stage of fossils discovered for paleontologists. What factors caused teeth to become a popular identifier in fossils for paleontology? And if, hypothetically, this evolutionary change did not occur within the jaw, then what may be another structure-identifier to determine the evolutionary status of the organism due to its evident and obvious increased specialization over time?
Sonia Doshi soniadoshi7@gmail.com
Teeth are probably the most useful thing for paleontologists to use in identifying characteristics of organisms because they “are often the best-preserved animal part we find in the fossil record for many time periods” due to their high concentration of hydroxyapatite, which makes them extremely hard and durable (61). However, teeth are not only useful because they are so durable, but also because of what they can tell paleontologists about the organisms they came from. Teeth can describe how old an animal was at death by different levels of wear, or distinguish carnivores from herbivores based on their shape - flat teeth in herbivores versus conical or triangular teeth in carnivores (http://www.suite101.com/article.cfm/paleontology/40769). In fact, teeth and jaw structure can be used to create a morphological phylogenetic tree to show how mammals developed. The progress from early tetrapods to mammals can be traced by looking at jaw structure and tooth characteristics: synapsids, which had multiple bones in their lower jaw and single-pointed teeth, evolved from early tetrapods. These evolved into theraspids, which had specialized teeth - large canines - and large dentary bones; these evolved into the cynodonts, whose teeth had several cusps, and soon these evolved into later cynodonts, with complex cusp patterns and two jaw hinge locations, and finally very late cynodonts, with only one jaw hinge (Campbell 513). Teeth developed in so many different ways that just looking at their structure alone can tell an organism’s age and type just by looking at the fossils.
ReplyDeleteWhile there are many structures that would display a certain stage of evolution in organisms, most clearly identifying structures are made of soft tissue, and thus would not survive for long enough to be found cleanly in fossils millions of years later. As Karl Reichart discovered in 1837, “the same gill arch that formed part of the jaw of a reptile formed ear bones in mammals” (160). The bones that make up the jaw in reptiles evolved to make up bones of the inner ear in mammals, as evidenced by the exaptation (change in function of a structure due to evolution) of some jaw bones into middle ear bones (Campbell 513), which is why jaw structure can be helpful in discerning animal evolutionary stage. As animals evolved from tetrapods to mammals, the change in jaw structure led directly to the change in middle ear structure; most scientists explain this by showing that the middle ear bones all originated from the hinge between jaws (http://www.post-gazette.com/pg/07074/769686-115.stm). If the evolutionary change of structure from the jaw into the ears did not occur, paleontologists would be stuck just looking at the tooth structure of animals; however, since ear development was essential in the development of animals, perhaps the middle ear bones would have originated from other parts of the skull, and paleontologists would be looking at four different bones instead of the three we have now! Possible alternatives could then be the carpal (wrist) or tarsal (ankle) bones. In the transition from tetrapods to the more specific mammals, the many carpal and tarsal bones have been modified by fusion or loss. Animals normally have three rows of each, but the number of individual bones varies depending on the animal; in primitive amphibians and tetrapods, there were many carpals and tarsals, but through evolution many of them fused together or been lost, as seen in mammals that lack their fifth distant carpal and have only one centrale carpal, the latter of which is even missing in humans. There are probably a few other morphological variations that exist in evolution from tetrapods to mammals, but the wrist and ankle bones serve as a nice possibility due to the fact that as animals got closer to mammals, the bones fused together to serve different functions, and paleontologists could easily just count how many (and of what shape) the carpal or tarsal bones of an animal were to see what kind of animal it was.
Eugene Bulkin (doubleaw002@gmail.com)
I think Eugene did a really good job explaining alternative identifiers for organisms in fossils, but I think he could have gone a little more in depth in describing the actual importance of the teeth in determining the lifestyle of an organism. He's definitely right in mentioning that the structure of teeth can identify an organism's diet. Teeth are indicative of an organism's diet. As human ancestor species moved from the water to the land, their diet also changed. For example, in bony fish, teeth consist of an enamel coating over dentine, surrounding a pulp cavity filled with nerves, blood vessels, and connective tissue. In carnivorous fish, teeth are sharper, just as in carnivorous terrestrial animals. Herbivores that live on land tend to have large, mostly flat round teeth. They lack the incisors and canines of the carnivorous creatures (http://www.blm.gov/id/st/en/prog/wildlife/herbivores.html). This idea ties to the theme of evolution. Over time, the carnivorous terrestrial population developed sharper teeth so that those species of animals could engage in mechanical digestion of their food. This is because individuals with sharper teeth as a result of genetic mutation were provided a selective advantage allowing them to survive and reproduce more effectively.
ReplyDeleteOnce a researcher can determine the diet of an organism based on the structure of its fossilized teeth, he or she can go on to determine other aspects of that organism’s lifestyle. For example, most carnivorous animals that live in marine ecosystems are fast, because they need to be able to catch their fast-moving prey (Exploring Life Science - Marshall Cavendish). As for the actual convenience during identification, Eugene is absolutely correct in pointing out that the hydroxyapatite’s durability as a major contributing factor for why paleontologists often examine teeth. Shubin discusses the reasons that hydroxyapatite arose in the first place. Interestingly, the presence of hydroxyapatite first arose in teeth, not just for protection, but for the consumption of other animals (Shubin 76). Terrestrial animals developed hydroxyapatite in their teeth because of their changing diet. This difference in lifestyle can explain the disparity between human teeth and the calcium carbonate or chitin of other organisms, which have vastly different diets. Hydroxyapatite allows animals to mechanically digest the tough, resistant defenses of the animals that they eat. Thus, a more hydroxyapatite would not only preserve the fossil, but also shed more light on the lifestyle of the organism.
Vickram Pradhan vickram.pradhan@yahoo.com