Okay, folks, it’s that time of year again. The week that either fills you with elation or dread. It’s….SHARK WEEK.
This week brings an opportunity for people to celebrate an oft misunderstood group of organisms, even if it is in the form of “Must-See-TV.” As I mentioned last year in my first Shark Week post, I want to veer away from the “Big Baddies” of sharkdom and instead focus on lesser-known sharks in the fossil record. This year, I asked folks on social media to nominate sharks for me to cover, and the folks on Twitter did not disappoint. I even inspired some fantastic conversation about what exactly qualifies as a “shark” within a phylogenetic context (a conversation begun by our own Andy Farke nominating Helicoprion, even though Helicoprion is now considered a holocephalan).
After receiving a few nominations, I’ve selected three that follow a general theme: they are all true sharks that are found in the Cretaceous Western Interior Seaway*! These are this year’s contenders for the Fossil Shark Hall of Fame, as nominated by members of the Paleo Community!
I love durophagous sharks. There are many small and adorable hybodontiform sharks in the Triassic deposits I’m familiar with, but Ptychodus, well, Ptychodus was big. At around 10 meters (33 feet) long, Ptychodus was likely chomping on large bivalves and crustaceans on the sea floor. Though first found in the 1820s in England, fossil teeth of Ptychodus were first reported in Kansas the 1860s by Leidy, and later on, full palates of teeth, arranged like cobbles in a pavement, were discovered in Kansas and elsewhere in the Western Interior Seaway deposits (see Mike Everhart’s informative website on Ptychodus mortoni).
Ptychodus is mostly known from its teeth, as is often the case with fossil sharks. In fact, its name is derived from its teeth — “Folded Teeth” referring to the many rugose ridges that look like folds on the surface of each tooth. Most reconstructions of Ptychodus depict it similar to modern, benthic, durophagous sharks, such as the Port Jackson Shark, nurse sharks, or carpet sharks. So, picture one of these modern cuties, sitting on the sea floor, only picture it 30 feet bigger. And be glad you are not a bivalve.
Swimming above Ptychodus in the Western Interior Seaway* was Cretolamna, a typical example of what people imagine when they think of a shark. Cretolamna was around 2–3 meters in length, known from its teeth and vertebrae found in the fossil record, and lived in the pelagic (open waters) of the ocean, feeding opportunistically on fish and marine reptiles. As the name suggests, Cretolamna (“Cretaceous Fish of Prey”) represents one of the earliest examples of lamniform sharks (also known as mackerel sharks) in the fossil record (Lamniformes includes the heavy hitters Megalodon as well as the great white shark). Cretolamna, like Ptychodus, wasn’t restricted to the midwest US, but are found globally from the Cretaceous to the Miocene, thus surviving the Cretaceous extinction event and being one of the most successful lineages of sharks ever.
Everything about Cretolamna is Shark 101, from its teeth to its appearance to its habitat and feeding. It was just into the scene first (sorry Megalodon). But it just goes to show you that being a basic badass never disappoints, and can help you survive for millions of years
Our final contender is Squalicorax, another lamniform shark found in the Cretaceous Western Interior Seaway*!
Okay okay, I know that featuring two similar Cretaceous lamniform sharks is kind of anti-climatic, but what appears similar on the outside can be quite different once you begin to look at the morphological specifics. For example, the teeth of Squalicorax are just one of the things that set it apart from other mackerel sharks. Rather than tall teeth like those of Cretolamna pictured above, the teeth of Squalicorax were wide and stout, recurved and heavily serrated — more similar to the tooth morphology seen in modern-day tiger sharks. If similar diets can be inferred based on the teeth, Squalicorax was probably a predatory consumer of a large variety of organisms, similar to modern tiger sharks (which feed on crustaceans, fish, seals, birds, turtles, dolphins, and even other sharks).
Given that tiger sharks are considered one of the most dangerous modern shark species, this extinct analogue Squalicorax might not have been one to mess around with, either.
Thanks to everyone that nominated their personal favorite sharks, and continue to let me know which sharks you’d like to see in the Fossil Shark Hall of Fame! List them in the comments below or message us on Twitter @PLOSPaleo
*as well as other places, too, but I’m biased
Featured Image: From Carrillo-Briceño et al. (2015) CC-BY.