Friday, August 6, 2010

A remarkable story, and another croc -- of teeth!

Nature reports what appears to be a remarkable find, and this time the remarkableness isn't a crock.  It's a roughly 150 million year old Tanzanian crocodile fossil that has a dentition that is remarkably like modern carnivorous mammals--Lassie and Tabby.

Most reptiles have a row of continuously replaced peg-like teeth.  They may vary in size but as a rule they don't vary much in shape.  This is called a homodont dentition--all the same.  There are no separate sets of adjacent teeth of a given type, like our heterodont incisors, canines, premolars, and molars.

Mammal teeth vary depending on diet and other uses, such as threatening displays and fighting.  Incisors are used and presumably evolved by and for nipping, canines for goring or fighting, molars for grinding.  Mammals that don't fight with their teeth, like grazers, don't have much if anything in the way of fang-like canines.  Dogs and cats have shearing molars rather than the flat, bumpy grinding molars of grazing animals.

But the O'Connor group found that their crocodilian fossil has threatening canines, followed by a set of small grasping teeth, followed by larger, sharp shearing teeth.  This appears to be a clearly heterodont dentition and one associated with carnivorous diet.  If the findings hold up, this is very interesting because on first glance, at least, it shows that complex, highly ordered segmentation can evolve repeatedly in very similar ways and, presumably, in response to similar causes--a predatory diet.

As we point out in Mermaid's Tale, complexity of this kind can be made simply, and not requiring tons of new genes to arise--maybe not any new genes, just changes in gene regulation.  It instead seems to be achieved by altering the parameters of the patterning process, that sets up spots where teeth (or hair, or feathers, or scales, or color spots, or mammary glands) will grow, separated by spots where they won't.  The parameters have to do with the timing and level of expression of genes that serve as signaling factors for cells, in this case along the future tooth rows.  Altering of a small number of genes' expression patterns to set up regions of differential growth and growth rate, regions of different lengths or widths, can do the trick. Whether the same genes that many of us have studied in mammalian teeth (in our case, in mice) are involved, or different genes, remains to be determined.  But our bets would be on it being largely the same genes.

Very interesting and very revealing, yet again, of the nature of evolutionary processes.  Here's one croc that's not a crock.

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