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Grades and Clades Among Rodents

Albert E. Wood
Evolution
Vol. 19, No. 1 (Mar., 1965), pp. 115-130
DOI: 10.2307/2406300
Stable URL: http://www.jstor.org/stable/2406300
Page Count: 16
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Grades and Clades Among Rodents
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Abstract

Rodent evolution can be envisioned as involving three relatively clear-cut evolutionary levels, here called Grades One, Two, and Three. The first involves welldeveloped gnawing animals, with a primitive mammalian jaw musculature. Grade Two includes those animals that have modified the jaw musculature in one of several ways that formerly were used as the basis for rodent subordinal classification. There were also changes in the dentition, especially in the development of cheek teeth with five transverse crests, rather than ones with no more than four crests as in Grade One. Changes occurred in numerous other parts of the skeleton and dentition, although these were probably not correlated with each other. Grade Three includes those rodents with very high-crowned or even ever-growing cheek teeth, in which there is sometimes the same type of limitation of the enamel that occurred during the Paleocene on the incisors. Grade Three also includes forms in which there has been a marked secondary increase in the length of the cheek teeth. A hypothetical Grade Zero is imagined for the rodents of the second half of the Paleocene. Only Grade One comes close to approximating a clade. The Protrogomorpha, as here defined, include the members of Grade One and some forms that have reached Grade Three without going through Grade Two. The cladal classification of the rodents still requires the recognition of numerous independent lines, showing no evidence of interrelationship later than in members of Grade One. Only two or possibly three clades can be recognized that require units larger than the superfamily-the Protrogomorpha, the Caviomorpha, and perhaps the Myomorpha. The other rodents fall into nine familial or superfamilial clades.

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