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| Lower jaw of a polar bear The polar bear has a second molar that is only slightly larger than the first. Although the polar bear is a carnivore, it is descended from the omnivorous brown bear. Photo Credit: © Katja Henßel, SNSB |
Mammalian teeth show an astonishing diversity that has developed over 225 million years. One approach to describing the development of mammalian teeth is the so-called “Inhibitory Cascade Model”, short ICM. The ICM describes the growth pattern of molars in the lower jaw. According to the model, the following applies to many mammals: The front molars in the lower jaw influence the growth of all the teeth behind them.
Certain molecules inhibit or activate tooth growth in the animal's dentition according to the same pattern. Which molars become small or large depends on the size of the first molar, which depends on the animal's diet. In carnivorous mammals, the first molar is usually larger than the third. In herbivores, it is the other way around: the first molar is small, while the third is large.
This is not the case in modern bears, whose tooth development does not follow the ICM pattern. In almost all modern bears – regardless of their diet – the second molar is the largest of all molars. PD Dr. Anneke van Heteren from the Faculty of Biology at LMU and the Bavarian State Collection of Zoology (ZSM) and her doctoral student, Stefanie Luft, investigated the origin of this phenomenon. They searched for clues in the evolutionary history of bears and found two breaks in bear history, indicating when and in which bear species tooth development deviates from the general pattern. The results have now been published in the journal Boreas.
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| Lower jaw of a young cave bear In this young cave bear, the third molar is just erupting, its size determined by the second tooth. Cave bears were herbivores and have second and third molars that are approximately the same size Photo Credit: © Katja Henßel, SNSB |
For their work, the researchers compared the jaws of fossil and modern bears with the ICM model – going far back in bear history; the oldest jaw examined dates from the Miocene and is at least 13 million years old. The zoologists identified the first fundamental break in tooth development around 3.6 million years ago. In Ursus minimus – the common ancestor of most modern bears – the second molar grew disproportionately large. The second break occurred somewhat later, around 1.25 to 0.7 million years ago, in Ursus deningeri, the predecessor of the classic cave bear. In this species, the third molar grew larger than expected according to the model.
"Apparently, the balance of chemical compounds that inhibit or activate the growth of the different molars shifted during these periods. These shifts are probably associated with dietary adaptations of bears during their evolution. On their way from carnivores to omnivores or herbivores, bears adapted to a changed food spectrum, but without following the ICM pattern. Their spectrum still ranges from pure carnivores to pure herbivores, with most bears today being omnivores," says Anneke van Heteren, responsible for the mammal collection at the Bavarian State Collections of Natural History (SNSB).
The researchers explain the two breaks in the tooth development model by environmental changes during the evolutionary history of bears. The first break between the early and late Pliocene correlates with climate changes that led to changes in habitats from subtropical rainforests to shrubland and steppes. The second break occurred between the late Pliocene and the middle Pleistocene and is associated with the development of extensive grasslands and a cooling of the climate.
Published in journal: Boreas
Authors: Anneke H. van Heteren, and A. Stefanie Luft
Source/Credit: Ludwig-Maximilians-Universität München
Reference Number: ebio120125_01
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