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The dental development of modern bears does not follow the typical pattern seen in most mammals. The reason lies millions of years ago in the history of bear evolution. SNSB zoologists have identified two phases in bear evolution that are responsible for the differences in bear dentition. The researchers have now published their findings in the journal Boreas.
Mammalian teeth show an astonishing diversity that has developed over almost 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. SNSB zoologist PD Dr. Anneke van Heteren and her doctoral student, Stefanie Luft, investigated the origin of this phenomenon. They searched for clues in the evolutionary history of bears and actually found two breaks in bear history, indicating when and in which bear species tooth development deviates from the general pattern. 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 in the course of 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 PD Dr. 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 the 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.
PD Dr. Anneke van Heteren
SNSB – Zoologische Staatssammlung München
Tel.: 0049 151 6516 1715
E-Mail: vanheteren@snsb.de
van Heteren A. H. and Luft A. S. (2025), Fossil bears break free from inhibitory cascade constraints at least twice (Ursus minimus and Ursus deningeri) caused by dietary adaptations. Boreas. https://doi.org/10.1111/bor.70044
https://www.snsb.de - Staatliche Naturwissenschaftliche Sammlungen Bayerns (SNSB)
Ursus deningeri, an early cave bear, had a larger third molar (right) compared to the second molar ( ...
Source: Anneke H. van Heteren, SNSB
In this young cave bear, the third molar is just erupting, its size determined by the second tooth. ...
Source: Katja Henssel, SNSB
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