The evolutionary history of mammals can be reviewed as the evolutionary history of tooth loss. The early mammals had many teeth, and every now and then in evolutionary time, a tooth is lost wiht subsequent species arriving from that n-1 toothed form having that smaller number of teeth. With ver few exceptions, no mammals have added a tooth during the history of mammals. (Excepting maybe the very very earliest period, but probably not.)
Well, the loss of enamel itself is also an evolutionary trend in mammal history, and recent research published in PLoS Genetic associates genetic changes over time with what is known of the morphological evolution of mammals.
Researchers newly report correlation of the progressive loss of enamel in the fossil record with a simultaneous molecular decay of a gene involved in enamel formation in mammals. Details are published in the September 4 issue of the open-access journal PLoS Genetics.
Enamel is the hardest substance in the vertebrate body. One of the key proteins involved in enamel formation is enamelin. Most placental mammals have teeth that are capped with enamel, but there are also lineages without teeth, such as anteaters, pangolins and baleen whales, or with enamelless teeth, such as armadillos, sloths, aardvarks and pygmy and sperm whales. All toothless and enamelless mammals are descended from ancestral forms that possessed teeth with enamel.
Given this ancestry, the researchers hypothesized that mammalian species without teeth or with teeth that lack enamel would have copies of the gene that codes for the enamelin protein, but that the enamelin gene in these species would contain mutations that render it a nonfunctional pseudogene. To test this hypothesis, they sequenced most of the protein-coding region of the enamelin gene in all groups of placental mammals that lack teeth or have enamelless teeth.
In every case, they discovered mutations in the gene that disrupt the proper reading frame that codes for the enamelin protein. The results link evolutionary change at the molecular level to morphological change in the fossil record.
"Currently, we are actively engaged in deciphering the evolutionary history of other genes that are involved in enamel formation," said Mark Springer, team leader. Springer collaborated with colleagues based at the University of California, Riverside, Texas A&M University, and the San Diego Zoo's Institute for Conservation Research.
(By the way, one of the authors of this article is a person in The Congo Memoirs!)
Meredith, R., Gatesy, J., Murphy, W., Ryder, O., & Springer, M. (2009). Molecular Decay of the Tooth Gene Enamelin (ENAM) Mirrors the Loss of Enamel in the Fossil Record of Placental Mammals PLoS Genetics, 5 (9) DOI: 10.1371/journal.pgen.1000634