Living Fossil: Shark

Its shark week.  Lets talk about sharks!

I feel like in some ways sharks have become the poster child for the "living fossil" concept.  When teaching a group of children at my job at the zoo and the subject of prehistoric animals pops up at least one kid will raise a hand and mention sharks to me without fail.  Although not as old as some other animals I have spoken of before, sharks are indeed a highly successful animal that has been around on the planet long before the first dinosaurs.  Some of the oldest shark fossils exist from about 400 million years ago during the Devonian Period.  These first sharks are not as large and imposing as some of the ones we are familiar with today.  In fact, scientists speculate they would have more closely resembled their relatives, the rays in some ways.

Fossil of Doliodus promlematicus, the oldest known shark. 

From that point on sharks never stopped flourishing.  They diversified into so many different forms, some of which we are familiar with today and others that we may not even recognize as being an actual shark.  Stethacanthus was an extinct genus of shark that lived about 360 million years ago during the Carboniforous period.  It more or less resembled some modern sharks despite a few subtle differences including differences in tail fluke proportions but what really sets it apart is the fact that it had a strange platform growing out of its back lined with bony teeth called denticles.  Denticles are essentially teeth that cover a shark's entire body (all sharks are covered with them) but they were exceptionally large on the top of Stethacanthus.  

Stethacanthus fossil.

Another interesting kind of shark that existed slightly later in during the Permian era, almost 300 million years ago (not long after the first dinosaurs), called the Xenacanthids resembled an eel with because of its body shape.  These sharks commonly sported spikes on their dorsal sides and existed in fresh water.

Xenacanthid fossil.

By the time the Cretaceous rolled around some of the sharks wouldn't have been that different looking from the sharks we are familiar with today despite being a different family.  One such shark was called Cretoxyrhina which could grow to about twenty feet long (same size as a really big modern Great White).  Despite their impressive size, these sharks wouldn't have been the top predators of their environment and probably fell prey to giant prehistoric marine lizards, called Mosasaurs, which existed at that time as well.  

Cretoxyrhina fossil.

Oh yeah and then millions of years later, closer in time to the present day, there was this one species of modern-style shark called Charcharodon megalodon that got pretty big.  Its extinct now though, despite what a fictional documentary aired on the Discovery Channel may have misled some people to believe.

I made another meme!

No, seriously there is zero chance megalodons are alive still.  No,they couldn't be hiding in the dark parts of the ocean we haven't explored yet.  If they were still out there they probably would be migrating around the world, eating whales and junk all near the surface of the water so...yeah somebody would have seen them.  Sorry.

megalodon life reconstruction by Christopher DiPiazza.

So what is it about sharks that made them stand the test of time since the Devonian?  Well. they certainly have a number of unique characteristics like denticles for scales and skeletons almost entirely made of cartilage (stuff inside our noses and ears) which aids in them being light and flexible.  Some sharks can even thermo-regulate their own bodies thanks to muscle friction despite being cold-blooded animals.  I really think they are successful because they managed to diversify into so many different forms and fill so many aquatic niches early on though.  Keep in mind many sharks have gone extinct over the millennia but since they were so diverse, there were always successful species to survive when others died.  Lets hope they continue to flourish to fascinate us for years to come! 

Sit tight.  I'll have a prehistoric shark in store for you to check out this Sunday as a farewell to shark week.  Remember, just because the official week is coming to an end doesn't mean you should stop loving and learning about sharks!

References

Bruner, J. C. (Sept.-Oct. 1997). "The Megatooth shark, Carcharodon megalodon: Rough toothed, huge toothed". Mundo Marino Revista Internacional de Vida (non-refereed) (Marina) 5: 6–11

Egerton, Philip De Malpas Grey. "XLII.—Pleuracanthus, DiplodusXenacanthus,Xenacanthus Decheni ()." Journal of Natural History Series 2 20.120 (1857): 423-24. Print.

Everhart, Mike. "A GIANT GINSU SHARK (Cretoxyrhina mantelli Agassiz) From Late CRETACEOUS Chalk of KANSAS"

Haven, Kendall (1997). 100 Greatest Science Discoveries of All Time. Libraries Unlimited. pp. 25–26. ISBN 1-59158-265-2.

Miller, Randall F., Richard Cloutier, and Susan Turner. "The Oldest Articulated Chondrichthyan from the Early Devonian Period." Nature 425.6957 (2003): 501-04. Print.

Pimiento, Catalina; Dana J. Ehret, Bruce J. MacFadden, and Gordon Hubbell (May 10, 2010). Stepanova, Anna. ed. "Ancient Nursery Area for the Extinct Giant Shark Megalodon from the Miocene of Panama"