Ursus spelaeus, dentition

[divingwolf]

Measurements taken of M2 (lower molars) of Ursus spelaeus and Ursus arctos show covariation between protoconid height, where the protoconid is one of the main cusps of a molar, and length of the molar. To put it in simple terms, shorter molars have lower protoconids, longer molars have higher protoconids. This is what one would expect.

The pleistocence Ursus spelaeus specimens from Odessa were bigger than the recent Ursus arctos speciments from Finland, so the two distributions conveniently fit on the same graph without overlap.

Source: On Evolution and Fossil Mammals, by Björn Kurtén, 1988

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[divingwolf]

A population of Ursus spelaeus from the late Pleistocene of Drachenhöhle at Mixnitz in Austria shows positive allometry of 1.62 between the lengths of M3s and M2s, where M3 is a lower molar right at the back and M2 is a lower molar one position closer to the front.

This means that there isn't a linear relationship between the lengths of M3s and M2s, but rather the lengths of M3s increase at a non-linear rate in a comparison of bears with different length M2s.

However, if logarithms are taken of the measurements of of the lengths of M3s and M2s, then a linear relationship results with slope 1.62.

Source: On Evolution and Fossil Mammals, by Björn Kurtén, 1988

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[divingwolf]

This graph shows an allometric relationship of 1.62 between y and x. If the logarithms of y and x are plotted, a straight line with slope 1.62 will result, similar to the trend line in the graph above by Kurten.

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[divingwolf]

This graph shows the allometric relationship between the hypoconid heights and crown lengths of second lower molars (M2) in recent brown bears, Ursus arctos, and late Pleistocene cave bears, Ursus spelaeus from Odessa.

The hypoconid is one of the two main cusps of a lower molar.

An allometric relationship results in a curve when raw data are plotted. But when the logarithms of the data are plotted, the trend line is straight.

In the brown bears, the allometry coefficient of the hypoconid heights relative to the crown lengths is 1.72. In the cave bears, it is 1.31. This means that in both species the hypoconid heights increase more than linearly with an increase in the crown lengths, and this is more pronounced in brown bears.

"One specimen from a cave bear is clearly off the main regression line, and instead lies on the extension of the brown bear line; this Odessa specimen appears to have formed in accordance with the growth pattern of Ursus arctos and is, therefore, a morphologic monstrosity."

Source: On Evolution and Fossil Animal, by Björn Kurtén 1988.

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[divingwolf]

This figure shows a normal and an aberrant lower molar M2 from an Odessa cave bear. The excessively high hypoconid of the aberrant tooth puts it on the extension of the brown bear line in the preceding graph.

Source: On Evolution and Fossil Mammals, by Björn Kurtén, 1988

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[divingwolf]

This is the graph of my December 27 posting, but with additional labeling.

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[divingwolf]

This graph shows the allometric relationships between the heights of paracones and the crown lengths of first upper molars in Ursus arctos (recent) and Ursus spelaeus (late Pleistocene, Odessa). For Ursus arctos, the allometric constant is 1.45, for Ursus spelaeus it is 1.34, so there isn't much difference.

Note that a paracone is one of the cusps of an upper molar.

The positive allometric constants mean that paracone heights increase more than linearly compared to crown lengths. If you had two different size teeth where the crown is 1.5 times as long in the bigger tooth than in the smaller tooth, then the paracone height of the bigger tooth would be more than 1.5 times the paracone height of the smaller tooth.

Source: On Evolution and Fossil Animal, by Björn Kurtén 1988.

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