SAWTEETH IN CHONDRICHTHYAN ARENT REAL TEETH
A UK researcher team just figure out that toothlike skin denticles exhibited by many chondrichthyan are not likely related to real teeth.
Chondrichthyans - sharks and rays- are covered of external placoid scales calles skin denticles. Three groups in the class include those with sawlike growths on their heads: Pristiophoridae (sawsharks, Selachii), Pristidae (sawfish, Batoidea) and the extinct Sclerorhynchoidea (Batoidea).
The skin covering of many in the class are well known, but much less has been learned about the morphological diversity among the group. Researches sought to try to understand if the sawlike growths were related to actual teeth. To find out, they undertook a study of several specimens from embryos to adults. Scanning specimens researchers noted that the sawteeth of Pristiophoridae and Sclerorhynchoidea were morphically similar despite not being very close phylogenetically—in those, the sawteeth develop under the skin in embryos—and in adults are replaced as needed.
The sawteeth in Pristidae, on the hand, were inserted into sockets in the cartilage and grew continuously and thus never needed replacing.
Real teeth, in contrast, develop in an ordered sequence governed by genes and those that are lost are replaced by those that have been pre-formed in rows, for just that purpose.
Photo by Marty Snyderman/INNERSPACE VISIONS
Reference (Open Access): Welten et al. 2015. Evolutionary origins and development of saw-teeth on the sawfish and sawshark rostrum (Elasmobranchii; Chondrichthyes). Royal Society Open Science