This is a drawing of Lonchidion, a hybodont shark from the Mesozoic Era. There were at least eleven different species, one of which was found in the Hell Creek Formation. I won’t get into all of the particulars regarding this genus or the Hell Creek species in particular (L. selachos). Their size depended upon the species, some being very small. Lonchidion selachos may have been three feet long, judging by the size of its dorsal spines. The drawing is based upon the preserved remains of other hybodont sharks, because specimens from the Hell Creek Formation consist mostly of teeth, well-preserved specimens of any Lonchidion species are very rare, and as far as I am aware, they looked more or less like other well-known hybodonts.
Hybodont sharks are identified by their large dorsal fin spines as well as the four large spines atop their heads, which are really overly-enlarged denticle scales found all over the rest of the body. Hybodonts first appeared during the Carboniferous Period, but it was during the Jurassic that they came into their own. However, by the Cretaceous Period, they were being replaced by so-called “modern” sharks very similar to the ones we see today. Lonchidion was one of the last surviving members of its kind before the whole hybodont group (the few species that remained, anyway) was completely wiped out at the end of the Mesozoic Era 65 million years ago.