Principles of Protein Structure Assignment 1996

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Classification of the Rodents

In terms of adaptive radiation, the numbers of species, and the number of individuals within a species, the rodents are by far the most successful mammals. The earliest and most primitive of the known rodents is the Paramys whose fossils have been found in Paleocene and Eocene sediments in North America. The Paramys was like a large squirrel, with clawed feet for grasping and climbing and a long tail for balancing. The subsequent evolution of the rodents has been difficult to study because of their many and complex adaptations; they have been the most difficult mammalian order to classify and their history is very incompletely known.

Traditionally, the rodents have been divided into three sub-orders based on the comparitive physiology of their masseter (chewing) muscle:

Sciuromorphs (squirrels and their relatives)
Myomorphs (mice and their relatives)
Caviomorphs (South American rodents)

However, the discovery of new fossils over the past fifty years have made this classification unsatisfactory; some rodents simply will not fit into the three-fold classification:

"The current status of rodent phylogeny is such that anyone can point out inconsistencies in anybody else's classification."
A.E. Wood (1966)(World authority on rodents)

One solution has been to retain the three original sub-orders and place the leftovers in a series of new sub-orders. Although untidy, this classification expresses with honesty the lack of knowledge concerning rodent relationships, and is open to adjustments as we learn more about the troublesome rodents.

Click here for an illustration of the rodent evolutionary tree according to comparitive physiology. The three basic sub-orders are highlighted in red. The new sub-orders are in black.

Index to Assignments

Index to Assignment 1

Last updated 2nd Apr '96