Shark Week: Fossil Shark Hall of Fame

As this week is Shark Week, it would be remiss of me, as the resident fishy person, not to jump on the bandwagon of “sharkery” and share with you some contenders for the weirder shark fossils out there, and some of the stories behind them. Now, unlike Shark Week, which often “jumps the shark” (see what I did there?) in terms of content, these are not the biggest, baddest sharks out there. The All-Stars of the fossil shark world include heavy hitters like Megalodon and  Helicoprion, but rather, I bring you three examples of some of the lesser knowns that still have a rightful place in the Shark Fossil Hall of Fame (if that were to exist). So kickback, relax, maybe make yourself a Piña Colada, read this list, and for the love of Darwin, stay out of the water!


Glossopterae

Stenoshark
Steno’s famous woodcut interpretation of a shark head, comparing fossil shark teeth to modern shark teeth. Public Domain.

Not a specific shark per se, but let et us begin with glossopterae, aka ‘tongue stones.’ Long before modern science or paleontology, when Pliny the Elder (AD 23–79) was trucking around the known world compiling all sorts of “very useful” information into the 37 volumes of Natural History, he came across fossil shark teeth. His best interpretation of these unusual artifacts was that they had obviously fell from the sky during lunar eclipses. Later, people interpreted them to be the the tongues of serpents turned to stone, aka the “tongue stone” name, and that they were imbued with magical powers, most notably to counteract toxins from venomous snakebites. Simply rub the the tongue stone on your snakebite and be cured! Many people took to wearing shark teeth like amulets, to ward off anything intending to hurt you.

In 1611, Fabio Colonna was the first to suggest that these were not the tongues of serpents turned to stone, but rather the teeth of sharks. In 1666, when Nicolas Steno was presented with the head of a great white shark to dissect, that he too realized that glossopterae were actually shark teeth, but noted the difference in composition. It puzzled Steno to no end how shark teeth became embedded in rocks (as he had no knowledge of what a fossil really was, and believed the Earth to be merely thousands of years old), as the origin of modern paleontology was still a few hundred years to come.

Read more about glossopterae here.


Xenacanths

Orthacanthus_BW
Orthacanthus seckenbergianus, by Nobu Tamura. CC BY 2.5
Orthacanthustooth
A tricuspid Orthacanthus tooth from Bolsovian shale at Whitehaven, Cumbria, England. Found by K & C Paxton. CC-BY-SA 3.0. From Wikimedia Commons

Sure, you’ve heard of famous extinct sharks, like Megalodon, but have you heard of xenacanths? These sharks hardly resemble sharks at all, and were one of the first groups of sharks to enter the freshwater realm. Found in deposits from ranging from the Carboniferous to the Triassic, these sharks had evolved a more eel-like, elongated, slender body and lacking that characteristic shark dorsal fin. One xenacanth, Orthacanthus grew upwards of 3 meters in length.

These sharks resided in swamps, lagoons, and bayous, and were probably some of the top predators of their ecosystem. The teeth of these sharks were unusual, with huge double fangs making a ‘V’ like shape on the cusp of each tooth.


Stethacanthus

In the realm of evolutionary oddities, may I present one of the weirder sharks to have ever evolved during the Paleozoic, and have rightly earned the title of “anvil sharks” because of an unusual dorsal fin shaped like an anvil.

StethacanthusesDB_2
Two species of Stethacanthus. Courtesy Wikimedia Commons, CC-BY-SA 4.0

On top of this anvil were prominent little spikes, similar to the placoid scales on the body but larger. These spikes were also found on the top of the head. Many suggest that this was a sexually dimorphic feature found only on the males, and that a similar shark from the same period that lacked the brush on the back, Symmorium, may in fact be the female of the species. These brush-like fins on Stethacanthus may have been used by males to attach to females while mating. Stethacanthus is also one of the first sharks to have claspers, penis-like extensions the pelvic fins that are found on modern sharks, and are used for  internal fertilization. Older sharks, such as the Devonian Cladoselache, have never been discovered with claspers; either they are never preserved, or more likely Cladoselache had some other method of mating, such as external fertilization. Regardless, sharks like the Late Devonian Stethacanthus possessed claspers, allowing us to distinguish male and female sharks in the fossil record, and understand the evolutionary history of shark sex.

So there you go, my three candidates for the Shark Fossil Hall of Fame. Happy Shark Week, folks!

UPDATE: I want to hear what fossil sharks YOU would add to the Fossil Shark Hall of Fame! Message me @gombessagirl, or add your comments below!


Featured image: BROOK WARD/FLICKR (CC BY-NC 2.0)

Published by Sarah Z. Gibson

Dr. Sarah Z. Gibson is a paleontologist and science communicator based in Minnesota. Her research focuses on the evolutionary history of ray-finned fishes from the Early Mesozoic. https://orcid.org/0000-0002-6784-3980

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