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Evolution of Bull Sharks
Transcript of Evolution of Bull Sharks
Christopher Owens & Jaylen Smith-Sloane Fossil Evidence Problem is, sharks generally don't fossilize as well as other animals because their skeletons are made of cartilage, a softer and more flexible tissue. Fortunately, shark teeth do fossilize well, and sharks produce thousands of teeth a lifetime and also can fossilize their skin, scales, and over time body parts. The oldest fossilized evidence of prehistoric sharks comes from shark-like scales that date to 455 million years ago during the Ordovician Period, in Colorado. The scales found during the Silurian Period, aged 420 million years, are from sharks. More complete fossil sharks remain date to 380 million years old, including a fossilized shark brain case, possibly a Xenacanth, found in Australia. Influence of Natural Selection Although there are a handful of species that live in freshwater, we tend to think of sharks as being ocean dwellers. However, there is a very unique shark that can live in both ocean and fresh water estuaries/lakes, known as the Bull Shark. Osmoregulation is the ability of an organism to maintain a constant concentration of water in its body even though the organism is outside its environment , which would normally cause it to lose or gain water. Scientific name: Carcharhinus Leucas This species can be found primarily in shallow coastal waters and is common in lagoons, bays, and river mouths. Bull sharks can also be found in fresh water tat connects with salt water and have been caught in the Mississippi River as far upstream as Illinois. Geographic Distribution Bull sharks occur in tropical to subtropical coastal waters worldwide as well as in numerous river systems and some freshwater lakes. The bull shark has a short snout that is wider than it is long (hence its name). Its belly is off-white, its top surface is grey, and its eyes are small. The first dorsal fin is much longer and more pointed than the second dorsal fin. The females are larger than the males, on average, adult males are about 7 feet long and weighting in 200 pounds. Adult females are about 11.5 feet long and weighting in about 500 pounds. Environmental Factors Sharks adapt differently to their environments depending on the species of shark and the particular environment. For example, the bull shark can live both in salt and fresh water, at least for a time. Evolutionary Advantage Bull sharks have kidneys that are good at taking the salt out of the blood and putting it into urine. The kidneys also hold onto urea the waste product made from breaking down proteins. They also rectal glands that can remove salt from the body. If a bull shark slowly can switch over to deal with the fresh water. The kidneys get rid of the urea, but hold onto the salt. They produce large amounts of very watery urine to get rid of excess water. This what makes the bull shark more ferocious than other shark in the sea, lake, or even river cause of this advantage. Ancestry Even as long as 370 million years ago, sharks would have been recognizable to us as sharks, although many, such as the Cladoselache, had characteristics that would seem unusual to us now. Unlike most ancient and all modern sharks, Cladoselache lacked tooth-like scales that provide protection and allow muscles to attach to the skin more strongly. Cladoselache also lacked the claspers that male sharks use to mate with females. Mutated Genes/Chromosomes Earlier in March 2013, a fisherman caught a bull shark recently off the Florida Keys, but came across an unlikely surprise: one of the shark's had with it, a two-headed fetus. He reported his catch on March 25, 2013, and described it to the Journal of Fish Biology. It's one of the very few examples of a two-headed shark ever recorded. There about six instances in published reports, and the first time this has been seen in a bull shark. Technically called "axial bifurcation," the deformity is a result of the embryo beginning to split into two separate organisms, or twins, but doing so incompletely.