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The Tale of the Sharks

Megalodon tooth with two great white shark teeth (Wiki Commons)

Text: Dorien Schröder, Dutch Shark Society

Recently a paper was published showing that the structures supporting fossil shark’s gills more closely resemble those of modern-day bony fishes than those of modern-day sharks. This shows that sharks, in contrast to what we previously thought, are evolutionary advanced. The general opinion used to be that the internal jaw structure of modern sharks looks very similar to those in primitive shark-like fish. Now it seems like the textbooks need to change; the modern shark is very specialized, not primitive. So where do our modern sharks come from?
Tracing the evolution of sharks is very difficult, as their cartilaginous skeleton does not fossilize very well. Scales, spines and vertebra sometimes get preserved, but the most fossilized shark body part is their teeth.

The oldest undisputed shark-like scales (denticles) date back about 420 million years and can be found in what is now Siberia. They have been assigned to the genus Elegestolepis, although we don’t know what the rest of the shark looked like. The earliest fossil shark teeth date back 400 million years, are two-pronged and only 3-4 millimeters long. They belong to a shark species named Leonodus, possibly a freshwater shark.
The ancient sharks share certain structural features with our modern sharks, such as jaws, replaceable teeth, tooth-like scales (denticles), paired fins, internal fertilization and a cartilaginous skeleton. However, even in the early days, there were a lot of variations to these shark characteristics. Unlike modern sharks, the jaws were fixed to the braincase front and back, so they weren’t able to protrude their jaws as much as modern sharks. The pectoral fins were triangular and rigid, so they weren’t as manoeuvrable and agile. Their brain was also smaller, indicating that their senses were less acute than they are now.

 

Reconstruction of Helicoprion by paleo-artist Christopher David Reyes

Over hundreds of millions of years, sharks were living in a highly changeable environment with changes in sea level, temperature, salinity and circulation leading to mass extinctions of those species who couldn’t cope with the changes. After each mass extinction, shark species radiated to expand into the new niches that were opening up. The first shark radiation was during the Carboniferous Period, 360-286 million years ago, when there were 45 families of shark species (compared to 40 now). The species living back then were know for their weirdness, including the Helicoprion with a whorl of teeth in the front of the jaw. About 250 million years ago, something called ‘The Mother of All Mass Extinctions’, wiping out 99% of all marine life, but some shark species squeaked through and gave rise to what eventually would be the modern sharks. The second radiation was during the Jurassic Period, 208 to 144 million years ago, in which time the common ancestors of our modern sharks lived. At the end of the Cretaceous, 65 million years ago, a global catastrophe wiped out the dinosaur dynasty, along with a lot of other species, but again the sharks survived.

 

Paleocarcharias stromeri

The neoselachians (new sharks) were radiating rapidly, some lineages still alive today, such as the cow and frilled sharks of the orders Hexanchiformes and Chlamydoselachiformes. Cow shark remains can be found in the Jurassic period, 150 million years ago. The frilled shark dates back at least to the Cretaceous period, 95 million years ago. During this time the rays and skates started appearing too, collectively termed the batoids. One of the more recent adaptations is that of the filter-feeding sharks and rays, which occurred during the Tertiary period 65 to 35 million years ago. The lamnid sharks (a group to which the goblin, sandtiger, tresher, megamouth, basking and great white shark belong) have their common ancestor in the Jurassic Period, a wobbegong-like shark Paleocarcharias stromeri that lived about 155 million years ago. The great white shark appeared on the scene about 11 million years ago.
And not to worry, the 15 meter, 52 ton Megalodon has been extinct for 1.6 million years!

 

 

 

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