This post was originally written by me and published on the PLOS Paleontology Community Blog on October 30, 2015. The original post can be accessed here.
Happy Halloween everyone! Still looking for a Halloween costume? Instead of dressing as a serial killer with a chainsaw, might I suggest dressing as a sawfish? Maybe not as scary as Leatherface, but just as deadly…if you are a fish.
Now that you have those two groups down, let’s add a third, the Sclerorhynchoidea. These are Mesozoic forms that include Schizorhiza, and are closely related to rays. As I mentioned before, sclerorhynchids like Schizorhiza also have elongated snouts with “saw-teeth.” They are a wholly extinct group, present in the Cretaceous and Paleogene in epicontinental seas.
In fact, the rostrum saw evolved at least 5 times in different groups of sharks and rays . But what do sawfishes, sawsharks, and Schizorhiza have to do with actual teeth?
Ever had the chance to touch a shark or ray? They’re skin is not scaleless, but covered in minute, hook-shaped denticles (placoid scales). If you brush your hand away from the head, the skin feels smooth, but if you reverse and brush towards the head, your skin will catch on these tiny hooks, like velcro. The internal structure and composition of these hooks is not dissimilar to that of teeth, and it has been long hypothesized that in the earliest jawed vertebrates these denticles migrated into the jaw region, and eventually evolved into oral dentition. I was taught this “outside in” hypothesis, that teeth are a specialized, derived form of those dermal denticles that cover the skin of sharks, rays, and other cartilaginous fishes. But this hypothesis has been called into question by several researchers, including Underwood and his colleagues. They have published a series of papers (linked below) that have been testing and comparing how teeth develop during ontogeny, and how that compares to they way dermal denticles and “saw-teeth” develop.
So far, the team has examined the morphology of teeth and denticles and several cartilaginous fishes, as well as the genetic controls that dictate the order and development of oral dentition, and they have determined that dermal denticles and oral dentition may not have the same evolutionary origin. Chondrichthyans are polyphyodonts, meaning that their teeth are
continuously replaced in a “many-for-one” style of replacement, and sharks and rays are noted for their batteries of teeth, which develop lingually and move to the functional surface (i.e., the mouth margin for munching on prey) and, if not lost, will continue to rotate outside of the functional surface. Batoids also often display a variety of unusual morphologies to exploit different food sources .
In contrast, denticles do not follow this ordered pattern of replacement seen in the dentition. Rather, denticles develop as space becomes available (i.e., as the organism grows), and are only replaced as denticles are lost, in contrast to chondrichthyan teeth that grow regardless if its predecessor is lost or not.
So, does “saw-tooth” development resemble what is observed in dermal denticles or oral dentition? Welten et al.  examined this and noted two differences between sawfishes, sawsharks, and sclerorhynchids. Sawsharks and sclerorhynchids share a similar pattern, despite not being closely related: the saw-teeth develop under the skin of the embryo before “swinging” laterally into place along the rostrum, and are only replaced when the predecessor is lost. In sawfish, however, the prominent saw teeth fit into sockets within the rostrum and grow continuously. If a saw-tooth is lost, it is not replaced (bad luck, sawfish). The development of saw-teeth is associated with rostrum growth, and replacement is space-dependent, thus “saw-teeth” aren’t teeth, at least in the classical sense, and are more comparable to dermal denticles.
We’re back to Schizorhiza, the unusual ray from the Cretaceous of Morocco that was the subject of Underwood’s talk at SVP.
When I asked Underwood about collecting specimens of Schizorhiza, he told me, “The specimens all came from the phosphate mining area near Khouribga, Morocco. In this area sedimentary phosphorites form a unit about 10 meters thick but representing maybe 15–20 million years from latest Cretaceous to Early Eocene. The whole rock, including fossils, is ground up to make fertilizer, but locals, mostly the quarry workers, try to collect larger fossils as they emerge, especially mosasaurs in the Cretaceous and crocodiles, turtles, and large sharks in the Paleocene and Eocene. In the process, some rare specimens are collected, such as pterosaurs, birds, land mammals and Schizorhiza. These fossils are purchased by local wholesalers who prepare the fossils and sell them on to museums and collectors. Whilst a lot can be said against commercial trade in fossils, this is really a rescue operation, and it only happens because there is a market for the fossils.”
Multiple specimens were examined via volume-rendered and segmented micro-CT scans and histological thin sections. The saw-teeth of Schizorhiza differ from what is observed in sawfish and sawsharks; the saw-teeth form a continuous battery of staggered structures to create a functional saw-edge, with smaller teeth nearer the far end of the snout but not reaching the very tip of the rostrum. Each tooth has a small crown and a root with four deep lobes that would have extended into cartilage along the rostrum edge. New saw-teeth develop internally at the rostrum edge and point towards the skull, and as they develop they rotate laterally and finally end up in below older teeth, within the root space left by the older teeth. These teeth would then remain there waiting to replace their predecessor, a process that is more common in bony fishes but otherwise unknown in chondrichthyans.
Schizorhiza provides a great example of why scientists should continuously question and test ideas that are long taken for granted. The specimens are absolutely stunning and allow Underwood, Johanson, and their team to demonstrate that the origin of saw-teeth, oral teeth, and dermal denticles in sharks and rays, don’t represent as “cut and dry” a story as we all have assumed. Read their papers below and follow along as they continue to research the mystery of the “saw-slashing” fishes.
 Underwood CJ, Johanson Z, Welten M, Metscher B, Rasch LJ, Fraser GJ, Smith MM (2015) Development and Evolution of Dentition Pattern and Tooth Order in the Skates and Rays (Batoidea; Chondrichthyes). PLoS ONE 10(4): e0122553. doi:10.1361/journal.pone.0122553
 Welten M, Smith MM, Underwood C, Johanson Z. (2015) Evolutionary Origins and Development of Saw-Teeth on the Sawfish and Sawshark Rostrum (Elasmobranchii; Chondrichthyes) Royal Society Open Science 2: 150189. doi:http://dx.doi.org/10.1098/rsos.150189
 Smith MM, Riley A, Fraser GJ, Underwood C, Welten M, Kriwet J Pfaff C, Johanson Z. (2015) Early Development of Rostrum Saw-Teeth in a fossil ray tests classical theories of the evolution of vertebrate dentitions. Proceedings of the Royal Society B 282: 20156128. http://dx.doi.org/10.1098/rspb.2015.1628