Circumventing a Constraint - the Case of Thylacoleo (Marsupialia, Thylacoleonidae)

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Abstract
Marsupial carnivores of the order Dasyurida are more uniform in molar morphology and jaw geometry than are their placental counterparts. This difference is related to the difference in tooth replacement between marsupials and placentals. In Carnivora, the permanent carnassial can erupt in its (geometrically) permanent position, and the post-carnassial teeth are free to evolve in various ways. In the Dasyurida, each erupting molar in turn functions as carnassial before being pushed forwards (relatively) in the jaw by the next erupting molar, which in turn becomes the carnassial. Thus, in the Dasyurida, all molars come to have carnassiform morphology. One group of Australian fossil carnivores has avoided this constraint: the Thylacoleonidae, 'marsupial lions'. In this group, P3/3 are the teeth functioning as carnassials, having been coopted for this function from the presumed sectorial P3/3 of the herbivorous ancestors of Thylacoleonidae. This has made molar reduction possible in this group, but has brought about some incidental effects. P3/3 lie far forward in the jaw, and the muscle resultant has been shifted forwards to compensate for this, reducing gape, but increasing bite force at I1/1, teeth which function as canines in Thylacoleonidae.