Macroevolution of
Selachimorpha
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Dr. Fet
Austin Swayde Carpenter
Sharks, (SuperOrder Selachimorpha of the Domain Eukarya, Kingdom Animalia,
Phylum Chordata, Class Condrichtyes, and Subclass Elasmobranchii) are one of evolution’s most enduring success tales. Sharks have prowled all of Earth’s seas over the globe for almost 400 million years, making them older than most insects, amphibians, reptiles, and even flowers. Even at an older origin, their success is represented in somehow outlasting all five major mass extinctions over the Geological
Time Scale and the different vast of amount of species lost as a result of each extinction. Today, sharks are included of approximately 8 orders, 40 families, 170 genera, and 430 species (Discovery Channel,2013). Some 2,000+ plus rich fossils have been analyzed and described since their origin, even though their cartilaginous skeleton
(which does not preserve well) and lack of bone have left very few clues for the number of sharks that may have lived. An idea of what the most ancient of sharks might have been like has been a serious challenge to due to this actual physical nature of the beast. The most common fossils of a shark are obviously their teeth, which would include denticles from the very first sharks(Arnold,2013). These types of fossils are most frequently found since sharks grow and shed tens of thousands of teeth from birth until death. These fossils and other pieces of history left behind from the beasts in the sea has given us some sort of evidence of how sharks may have lived over the years, essentially unchanged over time(Dando,2005).
The first evidence of existence of sharks appear from the Paleozoic Era, near the end of the Ordovician Period and the beginning of the Silurian Period, approximately
440 million years ago(Gilbert,2011) Some paleontologists tend to argue about the actual classification of these first forms of evidence. Only scales, not teeth, have been recovered from the earliest “sharks.” The argument still exists whether these scales belonged to the very first sharks in the marine sea or the variety of jawless fish also establishing niches in the same waters. The oldest form of an actual shark fossil ever accepted were calcium phosphate teeth, originating from about 418 million years ago.
The oldest complete shark fossil found is that of Doliodus problematicus, which was dug up in full-body form, including brain case, jaws, and fins, at a New Brunswirk, Canada site. Estimated to live about 409 million years ago and growing to be between only
50-70 centimeters, this ray-like bottom dweller would have been found in most temperate and tropical oceans during the Devonian Period (Markey,2003).
One of the most important ancient genus of sharks arose near the beginning of the Devonian Period around 407 million years ago. Most of the best Cladoseloche remains have been dug up from the Cleveland Shales on the south bank of Lake Erie, signifying that these early, agile predators roamed the North American oceans (Gilbert,
2011). At about 1.8 meters in length, Cladoseloche’s body consisted of anywhere between 5-7 gill slits and a streamline body with sturdy, light fin spines, enabling them to swim quick and easily in the water (e,g, Figure B). A short,rounded snout with smoothedged teeth were probably used to seize prey by the tail to swallow them whole, instead of sharp teeth for tearing and chewing. Reproduction of this first order of sharks remains a mystery, because no species has been found with claspers, which are a male shark’s sex organs used for internal fertilization (Gilbert,2011).
Another ancient shark arose during the Carboniferous Period about 360 million years ago, named Stethacanthus(Gilbert,2011) (e,g, Figure C). Fossils have been found both in Scotland and North America, mostly around length measurements between 2
and 2.5 meters. Stethacanthus looked like the sharks we see today in most respects, besides its “anvil” protrusion of a
