Creature Feature #2: Jurassic Jaws

Happy #FossilFriday! On this lovely day, we are bringing it back to our Creature Feature series for a special fossil Friday edition. However, this time, it's more like Fantastic Beasts and When to Find Them! Today I want to talk about a freaky group of extinct sharks within the genus Helicoprion......

Buckle up for this one, because "you're gonna need a bigger boat...." Okay okay no more Jaws quotes (probably). It would have felt like a missed opportunity if I couldn't slip that in somewhere. 

Helicoprion is a genus of extinct "shark-like"fish. Their skeletons are composed of cartilage like modern-day sharks, but little is known about this genus and its close relatives, so most researchers agree to call them "shark-like" instead of sharks. These fish first appeared in the early Permian period approximately 290 million years ago. They persisted until the early Triassic when they went extinct approximately 225 million years ago. Compared to a lot of other extinct groups, they were not around very long. However, this group is particularly impressive because it is one of the very few that survived the Great Dying, the largest mass extinction in history that marked the end of the Permian and the beginning of the Triassic. Approximately 96% of all marine species and 70% of all terrestrial species were wiped out during this time. However, Helicoprion lived on (for a little bit longer at least)! So, technically my title for this post is wrong because this genus did not make it to the Jurassic, but I'm a fan of alliteration and if Jurassic Park can do it, so can I. 

Reconstruction of Helicoprion bessonovi 
Image courtesy of Dmitry Bogdanov / CC BY (https://creativecommons.org/licenses/by/3.0)

What is the first thing you notice about this fish? The weird teeth, right? Helicoprion is the largest genus in a group called the eugeneodonts, fish with this unique dental situation called a tooth whorl. The end of the bottom jaw is arranged in a weird spiral of teeth. I will get into the paleontology of this group in just a second, but very little is known about this tooth whorl and what purpose it could have served. Recent studies have determined that Helicoprion is most closely related to the living Chimaeriformes, another weird group of cartilaginous fish that include ghost sharks, rat fish, and spookfish. 

Hydrolagus alberti, a shortnose ratfish, one of the closest living relatives of Helicoprion.
Image courtesy of SEFSC Pascagoula Laboratory; Collection of Brandi Noble, NOAA/NMFS/SEFSC - NOAA photo library or NOAA Photo Library: fish4287, Public Domain, https://commons.wikimedia.org/w/index.php?curid=10683941

Paleontologists estimate that Helicoprion could reach lengths of about 40 feet. For a size comparison, most great white sharks can't even reach 20 feet in length and Megalodon averaged between 40 and 50 feet. Beyond a general estimate of size, there is not much we know about Helicoprion. Fossils have been found in the Ural Mountains of Russia, Western Australia, China, and the western United States. Over 50% of the fossils found are from Idaho, USA and another 25% are from the Ural Mountains. Locations of these fossils suggest that this genus lived off the southwestern coast of Gondwana. 

Supercontinents of Laurasia and Gondwana, ca. 225 million years ago
Image courtesy of Benoit Rochon - Image:Laurasia-Gondwana.png, Public Domain, https://commons.wikimedia.org/w/index.php?curid=3176140

Fossils of Helicoprion are only known for the tooth whorls, that's why this group is such a mystery. Because the skeleton of sharks if made of cartilage instead of bone like ours, it takes incredibly specific and special circumstances to fossilize; the cartilage begins to decay as soon as decomposition sets in. However, the skull of a closely related genus was eventually discovered and led to a much better understanding of the anatomy of Helicoprion. The teeth in the whorl represent a growth ring, similar to a tree; as new teeth are formed throughout the shark's life, they push the older teeth farther into the whorl, elongating it through time. A study in 2013 found that the tooth whorl most likely sat at the back of the lower jaw. 

Fossilized Helicoprion bessonovi tooth whorl
Image courtesy of Ghedo - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=85736559

While we still have a lot to learn about these monstrous fish, they represent an incredible evolutionary mystery. Could the tooth whorl have been used for catching prey? Attracting mates? Territorial disputes between rivals? Comment what you think below!

Thanks as always for reading. Please send me a message or drop a comment if you have a suggestion for my next post, want to write a guest article, or have a question about anything I've written. Until next time, get out there and discover something!