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For Day 3 (a little late, yes) of Prehistoric Shark Week, I want to bring to your attention the diversity of chondrichthyans that have opted for a flatter bauplan.
Sharks are generally split into two groups, the galeomorphs, which are mostly the more typical torpedo-shaped sharks, including the sharks that most people think of when they envision a shark. The other group is the squalimorphs. These sharks lack an anal fin and many of them have developed a penchant for flatter bodies and broad pectoral fins, and in some cases pelvic fins as well (although not all, such as the dogfish and frilled sharks). Up until recently, the batoids, otherwise known as skates and rays, were considered part of this group, the consensus being that they were a more specialized type of squalimorph shark that had taken flat to an extreme. But the most recent molecular studies have indicated that they are a group unto themselves. The batoids have a long fossil history, with a number of ray teeth found in the Cretaceous deposits of Arkansas, particularly the eagle ray family Myliobatidae. Their teeth are typically flat rectangles on top with a comb-like surface below. Another type of ray that can be found are the guitarfish, or Rhinobatos casieri. These pectoral fins of these fish extend to their head, giving them a triangular shaped front end of a more traditional shark-like back end.
Skates and rays are generally very docile and would not be very threatening, spending their time scrounging about on the sea floor for benthic (living in or on the sea floor) invertebrates and the occasional fish. The same can’t be said for the last member of this group, the sawfish. Armed with a rostrum (its elongated snout) with teeth out to the side, the fish looks like it has a chain saw for a nose. The sawfish will swim into a school of fish and thrash its rostrum rapidly back and forth, spearing and stunning several fish, which it can then gobble up. They can also use it to dig up clams and crabs from the sediment. While they won’t attack humans, any human who provoked one may easily wind up perforated by the rostrum, probably not deadly but certainly painful. Most modern sawfish reach a respectable two meters, but the largest species, the green sawfish (Pristis zijsron) can top seven meters (24 feet). This is as large as the Cretaceous versions. Modern sawfish are typically put into the family Pristiformes. The Cretaceous ones are in their own family, called Sclerorhynchiformes and are not directly related, in that the Cretaceous ones are not thought to be ancestral to the modern ones. They are both put into the group Pristirajea, so they are thought to at least be related. But with the uncertainties in the relationships of the modern fish, the relationships with fossil forms are necessarily less certain. In any case, Arkansas sports several different species from this group, including Schizorhiza stromeri, Sclerorhynchus sp., Ischyrhiza mira, Ischyrhiza avonicola, and Ptychotrygon vermiculata. We were postively awash in sawfish.
The true squalimorph sharks that have shown up in the Arkansas Cretaceous rocks are best represented by the Angel shark (Squatina hassei), which looks like an early rendition of a skate, so it is little wonder that most researchers viewed skates and rays as simply more specialized versions of these sharks. Nevertheless, it appears this is case of convergence, not homology (similarity due to relationship). If it is homologous, it isn’t directly so. It is possible both groups had a common slightly flattened ancestor and each took their own route from there.
All of these fish are pretty docile hunters, scrounging around the sea floor for benthic organisms, all those animals that make their home in or on the sea floor sediments. They spend their time digging around the sand for crabs, clams, and other invertebrates, the occasional fish. When threatened by the presence of a predator, they hide on the bottom, using their shape to help them blend in with the seafloor. Neither the ones today or the ones in the Cretaceous would have bothered a human swimming around them.
Becker, Martin A., Chamberlain, John A., Wolp, George E. 2006. Chondricthyans from the Arkadelphia Formation (Upper Cretaceous: Upper Maastrichtian) of Hot Spring County, Arkansas