THE ORIGIN OF RODENTS

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HISTORICAL REVIEW Since Bowdich first proposed the term of Rodentia in 1821,there have been a number of attempts to place the origin of rodents : e. g. Proglires (Osborn, 1902), Multituberculata (F. Major, 1893), Primates (McKenna, 1961,Wood, 1962), and Palaeoryctoid (Szalay, 1977), etc. Unfortunately, all of these ideas were mostly without much success, even no fossil evidences to identify it. In 1977,Li described a new genus of Paleocene eurymylid, Heomys from Anhui, China, which possesses most rodent-like characters, but Li was not sure at that time whether or not the resemblance was phylogenetic or the result of convergence. Following the description of Heomys, a rodent relationship was suggested by Chaline and Mein (1979), Hartenberger (1980), Gingerich and McKenna (1980), Dawson et al. (1984), Korth (1984), Li and Ting (1985), Luckett (1985), Novacek (1985), Szalay (1985), and Wilson (1989). However, Wood (Patterson and Wood, 1982) did not think that "eurymylids have had any special relationship to the rodents." THE DEFINITION OF THE RODENTS Seven morphological characteristics of rodents were defined by Hartenberger (1985) as follows : 1,one upper and one lower enlarged, ever-growing incisor (I 2/2); 2,long diastema between incisor and premolar; 3,incisor enamel restricted to anterior face; 4,paraconid lost; 5,orbital cavity lying just dorsal of cheek teeth; 6,ramus of zygoma lies anterior to first cheek tooth; 7,glenoid fossa appears as an anterior-posterior trough. Functionally, the gnawing with the incisors occurs independently of chewing with the cheek teeth and the movement of the mandible is propaline-palinal acting. IS THE EURYMYLID AN ANCESTOR OF RODENTS? Li and Ting (1985) pointed out 27 features of eurymylids. Except for some tiny structure of cheek teeth, e. g. large hypocone shelf and compressed trigonid, it concurs almost all of Hartenberger's definition. Some new evidences based on the post-cranial material of Rhombomylus indicate that eurymylids are different from lagomorphs, but similar to the primitive rodents, such as paramyid (Li and Ting, 1993). So, all the character analyses of eurymylids lead us to think that a reasonable ancestor of rodents should arise from a [figure] primitive Cretaceous (?) eurymylid, even if Heomys seems to be somewhat too specialized to be a direct ancestor of rodents. MIMOTONIDS, AS A SISTER GROUP OF EURYMYLIDS, MAY HAVE GIVEN RISE OF LAGOMORPHS During the recent years, several partial skulls of Mimotona were collected from Late Paleocene of Anhui. It shows clearly the affinities of Mimotona to the lagomorphs. Li (1989) mentioned 31 character items of Mimotona. Most of them are the synapomorphies shared with those of lagomorphs (only 6 items represent as primitive features and 2 as autopomorphies!). The mimotonids and eurymylids are very similar in cheek-tooth pattern. They should be derived from a common Cretaceous ancestor.
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THE ORIGIN OF RODENTS

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