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For Day 4 of Prehistoric Shark Week, I would like to mention another modern day shark that has been around since the Cretaceous: the sand tiger sharks. Tomorrow, I will discuss a couple of Cretaceous sharks that may be the ancestors of the two most famous sharks in the world – the Great White and the giant Megalodon.
The Sand tiger is a common shark in the Cretaceous sediments, or at least, their teeth are, which means they were probably pretty common back then. The teeth tend to be long and thin, with two small cusps on either side of the large, center blade. Elasmo-branch.org reports that the center blade is smooth-edged with a strongly bilobed root, large bulge in the center of the root (aka lingual protruberance), and nutrient foramen in the center.
There are actually two sharks that are often called sand tigers in the Cretaceous rocks. One is Carcharias holmdelensis, the Cretaceous version of Carcharias taurus, the modern day sand tiger shark. Also going by the name grey nurse shark, amid several others, sand tigers are large-bodied sharks that will eat pretty much anything, but since it is a fairly slow and placid shark most of the time, it doesn’t seem to go after anything that requires a lot of effort. They are known for gulping air to allow themselves to float in the water column without expending much effort. So although they look scary, they appear to be too lazy to live up to appearances.
The other shark that gets called a sand tiger, is Odontaspis aculeatus, one of the ragged toothed sharks, which also go by the name sand tiger. These sharks were until recently in the same family as Carcharias, but have since been pulled out into their own family. They are very similar, as one might has guessed from the numerous times these sharks have been grouped and split over the years. As Elasmo-research.org put it, “Chaos reigned until Leonard Compagno examined museum specimens from all over the world, corrected misidentifications and sorted out synonyms.”
For Day 2 of Prehistoric Shark Week on paleoaerie, we are going to take a look at my personal favorite shark. In the late Cretaceous, it was called Scapanorhynchus, the spade snout. But its closest living relative is called Mitsukurina owstoni, also known as the goblin shark. The perfect shark for Halloween.
Scapanorhynchus means spade snout, so named for the elongated, flat snout, the same feature which got the modern shark named goblin. Most of them are small, less than one meter, but can get in excess of four meters. Spade snouts were some of the earliest sharks in Neoselachii, the modern sharks. One of the things this means is that they did not just have straight cartilaginous skeletons, they calcified most parts of the skeleton to reinforce the cartilage. They didn’t make true bone, but the calcium spicules provided more strength for the cartilage.
Goblin shark teeth are long and thin, looking like a mouth full of curved needles. But what most people are fascinated by is the amazing length to which they can protrude their jaws. Modern sharks have what is known as hyostylic jaws, meaning that the jaws are not directly connected to the skull. Instead, they are attached at the back of the jaws on an intermediary bone that allows the jaw to swing forward. All sharks can do this to an extent, but the goblin shark is expecially known for it.
The modern goblin sharks are generally only found in deep water. Its Cretaceous cousins, on the other hand, were widespread in shallow marine areas. Like many fish in the Cretaceous, they seem to have survived the mass extinction even at the end of the Mesozoic by going deep.