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Written by at Wesleyan University, Middletown, CT USA for Paleoecology (E+ES 340) Prof.Carmela Cuomo March 28th,1997
Rodents are generally described as "diversified, small, terrestrial mammals, specialized for gnawing, and with a high rate of reproduction"(Hartenberger, 3). Constituting one half of mammalian diversity, rodents are taxonomicly distinct since multiple uniquely derived characters "corroborate the monophyly of the group"(Harteneberger,10). However, while rodents are easily identifiable, their origin has remained almost as elusive as material on this subject. The evolutionary origin of rodents is obscured by the group's sudden and highly transformed first appearance in the fossil record in the latest Paleocene. Thus, this paper makes use of scant recent English-language research concerning rodent origins, and attempts to explicate the importance of the study of early rodents in the fossil record, describe the most sound theory concerning rodent origins, and to explore the implications for this theory.
Fossil rodents provide vital information in several important areas. A result of the rapidity with which generations succeed one another, rodents enable study of "evolutionary modes and processes of speciation"(Chaline,44). It is possible to use what is known about this speciation for stratigraphic dating. While a single rodent tooth can be used to "asses the age of a human skull or prehistoric settlement" it can also throw light on the environment and climate of the time(Chaline,44).
Since rodents are closely adapted to their biotopes and live under well defined climatic conditions, "they are extremely valuable for reconstructing the landscape and climates of prehistory"(Chaline,44). After accepting that the ecology of a certain species has not changed, present-day ecological assumptions can be extrapolated from the fossil species. Climatic fluctuations often lead(and led) to important changes to areas of rodent distribution(i.e. migrations). The demonstration of rodent migrations within the fossil record "appears to be one of the most reliable sources for reconstructing climatic changes in the Pleistocene [and Holocene] period[s]"(Chaline, 44). Thus, while the Tertiary, particularly Paleocene, fossil record of rodents is scarce, it is hoped that ultimately it will be possible to extrapolate climatic conditions as more specimens are found. This would be difficult since it would often involve species no longer extant, but if possible, would prove especially useful in the reconstruction of changes that occurred within ancient environments. It should be noted that landscapes reconstructed from the evidence provided by fossil rodents "caught by birds-of-prey [usually] will not represent the complete landscape of the region, but only the hunting territory of the bird of prey"(Chaline,48).
Chaline explains that rodents, usually nocturnal, often become the prey of nocturnal "birds-of-prey" such a owls. The rodents' fur and skeletal remains, incapable of being digested, agglomerate in the bird's stomach into "an ovid mass, which the bird regurgitates after a few hours"(Chaline,44). These "rejection pellets" accumulate at the foot of, or near, the birds' nest over time. Most rodent remains are found in large deposits, often corresponding to ancient accumulations of rejection pellets from various birds of prey(Chaline,44). Teeth represent the most important fossil remain, due to the pronounced differences between one species and another. Thus while early rodents evolved very rapidly, the sites at which their teeth are found often can be dated from the degree of evolution attained by the teeth, and new finds are often comprised of teeth which are then compared to those of known species. Hartenberger asserts that characters related to "the dentition and occlusal pattern, are the most important derived characters shared within Rodentia"(Hartenberger,38). Thus while dental formula from the scarce, older rodent-fossils cannot be used for dating, it is usually the morphological characteristics of these fossil teeth, often preserved in bird feces, from which hypotheses concerning rodent evolutionary origins emerge.
The "Glires cohort", designating small gnawing mammals, was first proposed by Linnaeus, with "clear taxonomic value"(Hartenberger, 15). Gradually, studies divided Glires into two major groups, rodents and lagomorphs. Since Rodentia(an order) was proposed in 1821, there have been numerous attempts to place the origins of rodents, all of which were mostly without much success(Li and Chow,15). The fragmentary late Paleocene rodent fossil record does not allow identification of all the distinctive rodent characters for each specimen, in most cases only dental remains are preserved. However, "rodent paleontologists, using a now qualified gradualistic approach, have for a long time recognized lineages from good middle Eocene records(often with skulls or even complete skeletons), which took root in taxa from the late Paleocene beds."(Hartenberger,10). The origin of rodents has been, and still is, a subject of debate. While some have left the rodents in "an essentially incertae sedis position. Others, at one time or another, have favored an origin from primitive primates, or close relatives of them. In the past decade or so, increased knowledge of the eurymylids, presumed members of the Asiatic anagalids, has provided a possible answer [to the question of rodent origins]"(Wilson,3). Traditionally, as will be seen, authors have not attempted to imagine an animal that resembled a proto-rodent; instead, they typically analyzed the relationship of fossil rodents with other mammals "by evaluating the occurrence of advanced characters in fossil and living groups"(Hartenberger,18).
The mere fact that rodents appear full blown at their earliest appearance suggests that searching for a source of origin would be problematic, whether by the fossil record or systematic analysis(Li,Wilson et al 98). The Eurymylidae, from the early Tertiary of Asia, are the latest candidates for rodent ancestors. Seven morphological characteristics of rodents were defined by Hartenberger(Li and Ting, 15). Li and Ting pointed out 27 features of eurymylids. Except for some tiny structure of cheek teeth, their description concurs with almost all of Hartenberger's definition(Li and Ting, 15).
While recent discussions of eurymylid-rodent affinities do not reach a consensus, several authors support this hypotheses, and it is usually presented as the most feasible theory, other theories(such as primate origins) having been ruled out in recent years by new fossil evidence(Li,Wilson et al 98). Li asserts that the genus Eurymylus, of the family Eurymylidae, from the latest Paleocene of the Gashato Formation, Mongolia, "has been a subject of great scientific interest from the time of its original description"(Li and Chow 35). These Asiatic eurymyloids were very diversified at the generic and species levels during Paleocene and early Eocene times and many of characters of this genus seemingly show rodent-affinities.
During the 1970s, Chinese paleontologists obtained new and better eurymylid material, including specimens older than those of Eurymylus, probably from the late Paleocene. An important advance in terms of rodent origins occurred in 1977 with the discovery of the eurymylid genus Heomys from the Middle and Late Paleocene of Anhui, China. Li described Heomys as a genus which possessed many rodent-like characteristics. Li was not sure at the 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, Hartenberger and many others(Li and Chow,15, Hartenberger, 35). Most authors are presently in agreement with Li concerning this apparent relationship between Heomys and early rodents(Li and Ting, 35). Via comparison of dental composition, array, implications for jaw and cheek position, and enamel, a number of features are presently in the known material of Heomys which suggest a condition equivalent to that in early rodents, or more primitive(Wilson,5).
Currently, after subsequent finds, four genera, Eurymylus,Heomys, Matutinia, and Rhombomylus, are referred to the family Eurymylidae(Li and Ting,35). Wilson asserts that while Heomys appears too late in time and too advanced in various morphological details to be in the ancestry of rodents, "it seems likely that [Heomys] belongs to the stock out of which the rodents originated"(Wilson,4). Li asserts that all of the character analyses of eurymylids lead to the conclusion that a reasonable ancestor of rodents should "arise form a primitive Cretaceous(?) eurymylid, even if Heomys seems to be somewhat too specialized to be a direct ancestor of rodents"(Li and Chow, 16). Hartenberger, and the members of his symposium conclude "we would like to emphasize again the important value of the dentition and masticatory apparatus in assessing the phylogenetic relationships of the Eurymylidae. Heomys is, perhaps, very close to the ancestral stem of Rodentia. We still believe that 'there is no reason not to think that as yet unknown members of the Eurymylidae actually are ancestral' to rodents"(Hartenberger,43).
Mostly defined by dental characters, the oldest fossil rodent specimens have since been assigned to two families, the Eurymylidae(with genus Heomys) and the Mimotonidae(with genus Mimotona). While, as discussed above, the former genus and its family suggest rodent affinities, the latter suggests lagomorph affinities(Wilson,4). New evidence based on the post-cranial material of indicates that eurymylids are different from lagomorphs. Mimotonids, however, as a sister group of eurymylids, may have given rise to lagomorphs. During recent years, several partial skulls of Mimotona were collected from Late Paleocene of Anhui. The abundance of parallel character items show the affinities of Mimotona to the lagomorphs. Yet the mimotoniods and eurymylids, specificly Eurymylus, are very similar in cheek-tooth pattern and other characters. Li and Chow therefore assert that they[mimotonoids and eurymylids] "should be derived form a common Cretaceous ancestor" thus establishing a connection between the Lagomorphs and Rodentia(Li and Chow, 16).
The opinion of paleontologists and neomammalogists has long been that "the two major forms of gliriform mammals, the rodents and the lagomorphs...are not closely related."(Li, Wilson et al, 97). New evidence from these recent fossil discoveries, based heavily on features shown by the fossils(especially dental formula), yields the hypothesis that "the Lagomorpha and Rodentia can be traced to a common ancestral group [therefore making] Cohort Glires a valid taxonomic unit"(Li and Wilson, 98, 105). Wilson concurs, asserting that while separation of the Rodentia and Lagomorpha is clear-cut in all post-Eocene specimens, the eurymylids offer evidence for special affinities between the two orders. Li and Wilson conclude that these fossils-mimotonids and eurymylids-central Asiatic mammals ranging in age from middle or late Paleocene to ?Oligocene, thus far offer the best evidence among fossils for a relationship between rodents and lagomorphs, a relationship that has not been favored for many years, and one that was uncovered through an examination of the apparently close relationship of the eurymylids to rodent origins(Li and Wilson, 101. Wilson, 3).
Wilson maintains that punctuated equilibrium seems most promising in explaining the obscure origin of most rodent groups, but gradualism is evident in many specific lines of descent in rodents(Wilsonb,163). Most concur that it is difficult to determine whether evolution occurred rapidly during the early radiation of rodents(illustrating a so-called punctuated model), or if this is only "an artifact of both the fossil record and the way in which it is studied"(Hartenberger,28). Once established in the record, rodents "rapidly became common in fossil populations, as possible Paleocene competitors such as the multituberculates and the more rodent-like eurymylids decline." Hence, the eurymyloids appear in a time when "one would expect rodent morphology to be developing, whatever their role in actual fact"(Wilson,5). Wilson concludes that while Heomys, the genus closest to rodent origins(which also implicates a common ancestry with the lagomorphs), cannot be directly ancestral to the Rodentia "a somewhat older eurymylid than any now known could be such an ancestor. Such a pre-Heomys mammal would be closer to rodent origins than anything now known....What is needed is good material from the earlier Paleocene"(Wilson,5). It appears as if recent research has yielded a find that approaches Wilson's description.
It is necessary to conclude by discussing a recent find which could have an effect on the above mentioned theory concerning rodent origins. Meng et al report the discovery of nearly complete dental remains of a new primitive rodent from strata of transitional Paleocene-Eocene age in Inner Mongolia, China. The morphological features of this taxon, Tribosphenomys minutus, "substantially modify previous ideas about the ancestral rodent morphotype, which in turn has important implications for understanding the origin of rodents and their relationship to other eutherian mammals". Flynn concurs that this new fossil might shed more light on the "enigmatic origin of Rodentia" and rodent radiations(Flynn,97). These fossil teeth, found in coprolite(fossilized excrement) of unknown origin in rock dated in excess of 50Myr old, represent the complete dentition of an entirely new genus of rodents. Tribosphenomys, "appears to be quite primitive [and to] occupy a position basal to other Rodentia, its features can reasonably be evaluated as representing the primitive morphotype for the group"(Flynnn,97). Meng asserts that this new fossil [in conjunction with recent morphological and molecular evidence] indicates a sister-group position of the new taxon to other rodents, and supports the alliance of lagomorphs and rodents, reveal[ing] an unexpectedly complex pattern of character evolution near the ancestry of Rodentia"(Meng et al, 134).
Although its dental formula, incisor modifications, and lower cheek teeth are typical of early rodents, the upper teeth of Tribosphenomys show few of the features common to nearly all other members of the order. Its "primitive arrangement of principal crown cusps and the retention of many pleisomorphic features suggest a phylogenic position of Tribosphenomys peripheral to that of other rodents; this is crucial for establishing the primitive character conditions for the Rodentia"(Flynn,98) The inner enamel layer of rodent incisors shows one of three kinds of incisor enamel structure: pauciserial, multiserial, or uniserial. Pauciserial(single-layered) has been thought to be primitive, with the two other types derived from it(Wlison,170). From recent data, it appears as though all early rodents were pauciserial and that "derived conditions" appear later, indeed, no Neogene rodents are pauciserial(Flynn, 98). While single layer-enamel present in Heomys is only one of many characters that strengthens the argument that members of this genus are related to early rodents (if not members), single-layer enamel present inTribosphenomys seemingly confirms the basal position of this genus within the ancestry of Rodentia.
The only rodents displaying a molar crown pattern similar to the arrangement hypothesized to be primitive for Eutheria are Tribosphenomys, and two Asian forms and one North American form, further confirming a tribospheric[triangular array of principal crown cusps] stage in rodent evolution"(Meng et al 135). Meng et al conclude, "rodents are first known from many localities of the latest Paleocene-earliest Eocene age in Asia and North America. They are widely considered to have originated in Asia based on the occurrence there of eurymylids, their perceived nearest relatives. Tribosphenomys, now the best known example of an emerging suite of highly pleisomorphic early Cenozoic rodents, confirms the Asiatic origins for the Rodentia"(Meng et al,136). Thus Meng places Tribosphenomys in a position peripheral to the Eurymylidae genus' Heomys and Eurymylus, suggesting that all three share a common ancestor, while this particular Tribosphenomys find is seemingly situated closest to rodent origins due to conclusions based on the analysis of its dental array.
The search continues for more fossils and further hypotheses concerning the origin of rodents. It is evident that such information would be invaluable to studies concerning evolution and rapid climatic change within ancient environments. With the fossils found thus far, close examination of early Paleocene(and earlier) climatic fluctuations and rodent evolution remain difficult. However, the Eurymylidae seemingly offer a possible late Paleocene glimpse at rodent origins. The eurymylid Heomys and the new fossil Tribosphenomys are particularly basal to rodent ancestry, yielding the closest look at the proto-rodent and its derived(mostly dental) characters. Perhaps just as importantly, these fossils and their relationship to Eurymylus implicate the lagomorphs as an order which probably shares a common ancestry with Rodentia. Most of the studies mentioned, many of which were done more than a decade ago, were based on comparative anatomical analyses of a very limited number of mostly incomplete fossils. More than paleontological and anatomical approaches, observations on the biological, more specifically cellular and molecular level are needed, and indeed innovations in protein analysis have recently "opened new fields of investigation for systematic relationships"(Harenberger,24). Thus perhaps with new field research, and a re-appraisal of the fossil evidence at hand, it may be possible to draw closer to the origins of Rodentia than the distant past of the Eurymylidae and the Tribosphenomys.
Chaline, J. "Rodents, evolution and prehistory." Endevour New Series 1 (1977): 44-51.
Flynn, Lawrence J. "Paleontology; roots of rodent radiation." Nature 370;6485 (1994): 97-98.
Hartenberger, Jean-Louis "The order Rodentia : Major questions on Thier Evolutionary Origin, Reltionships, and Suprafamilial Syatematics" Evolutionary Reltionships among Rodents. Ed. Luckett, Patrick W. New York, NY: Plenum Press:1985. 1- 34.
Li, Chuankuei and Chow, Minchen. "The origin of rodents", Rodent and lagomorph families of Asian origins and diversification. Ed. Tomida-Yukimitsu. Tokyo, Japan: National Science Museum, 1994. 15-18.
Li, Chuankuei; Wilson, Robert W; Dawson, Mary R; and Krishtalka, Leonard. "The origin of rodents and lagomorphs." Current Mammology 1(1987): 97-108.
Li, Chuankuei and Ting, Su-yin "Possible Phylogenetic Reltionship of Asiatic Eurymylids and Rodents with comments on Mimotonids" Evolutionary Reltionships among Rodents. Ed. Luckett, Patrick W. New York, NY: Plenum Press:1985. 35-59.
Meng, Jin; Wyss, Andre R; Dawson, Mary R; and Zhai, Renjie. "Primitive fossil rodent from Inner Mongolia and its implications for mammalian phylogeny." Nature 370;6485 (1994): 134-136.
Wilson, Robert W. "The Paleogene record of the rodents : fact and interpretation", Vertebrates, phylogeny, and philosophy. Ed.Flanagan, Kathryn, M. Laramie,Wyoming: University of Wymoning, Department of Geology and Geophysics,1986. 163-173.
Wilson, Robert W. "Rodent Origins", Papers on fossil rodents; in honor of Albert Elmer Wood. Ed. Black, Crag C. Los Angelas, California: Natural History Museum of Los Angeles County, 1989. 3-6.
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