Fossil horses from Eohippus (Hyracotherium)

evolution North Biological Journal of the Linnean Soczeb (1988), 35: 37-48. With 5 figures Fossil horses from Eohippus (Hyracotherium) to Equus, 2: rates of dental evolution revisited BRUCE J. MACFADDEN Florida State Museum, University of Florida, Gainesville, Florida 32611, US. A. Received 16 Nouember 1987, accepted for publication I1 March 1988 Rates of dental evolution are calculated for four upper first molar (M ) characters of 26 ancestral-descendant species pairs of Cenozoic horses from North America. On average, crown height evolved significantly more rapidly (a = 0.104 darwins, d) than did occlusal dimensions (length and width; R = 0.045 d and 0.047 d, respectively). As might be expected, low-crowned Eocene and Oligocene horses (Hyracotherium through A4esohippu.r) exhibit relatively slow rates of dental evolution. During the major early Miocene adaptive shift from browsers to grazers (Parahippus to Merychippus), only crown height evolved rapidly. Advanced Miocene-Pliocene threetoed hipparions and one-toed equines are genrrally normal, or horotelic, in their rates of dental evolution. The most rapid rates are exhibited in Miocene browsing anchitheres and the dwarf genus Pseudhipparion. Horses do not show the very high rates of dental evolution reported elsewhere for Paleogene mammals. The traditional notion of horses being a prime example of rapid morphological evolution as seen from the fossil record is not corroborated by the data presented here. KEY WORDS:- Horscs - palaeontology - dentitions ~ ~ America. CONTENTS Introduction..... Materials and methods... Results and discussion.,. Patterns of dental evolution Ratrs ofdental evolution. Concluding comments... Acknowledgements... References Appendices....... 37.. 40.. 41.. 41.. 42.. 45.. 46.. 46.. 47 INTRODUCTION Throughout much of their 55 million year (Myr) history, horses (family Equidae) have been biogeographically widespread and, particularly in North America during the late Cenozoic, they are very common in terrestrial deposits (Fig. 1). It is therefore not surprising that this group is one of the best known and most frequently cited examples of evolution as interpreted from the fossil record. Y/ 0024-4066/88/090037 + I2 $03.00/0 0 The Linnean Society of London

38 B. J. MACFADDEN M Y R 0 S. AMERICA PHYLOGENY OF THE EQUIDAE NORTH AMERICA 1 OLD WORLD 5 10 15 20 25 30 35 40 45 5c! 5: Figurc 1. Phylogeny or fossil horses (family Equldae). Modified from MacFadderl (1985) and rrproduced with the permission of the Palroritological Society. The teeth of fossil horses are the most commonly preserved skeletal elements. Therefore much of our knowledge of the evolution of this group (as with most fossil mammals) is actually the sequence of morphological change of the dentitions. In recent years there has been considerable interest in evolutionary rates of fossil organisms (e.g. Gingerich, 1983; Stanley, 1979, 1985). However, very little original data and interpretations have been presented on the dental evolution of horses since the early interest in quantification of evolutionary rates (Stirton, 1947; Romer, 1949; Simpson, 1949; Haldane, 1949; Simpson, 1953). Those studies, although classics in palaeontology, did not benefit from the theoretical and geochronological advances as well as the new fossil discoveries available today that make similar studies so attractive. Four decades

FOSSIL HORSE DENTAL EVOLUTION 39!-?Edson Hemphill I ( faunaunnamei 1) OnnCkL j-t Niobrara I Sheep Crk. n 2r P GarvinGuUey z Figure 2. Chart from Stirton (1947: 34) depicting horse evolution as a gradual, progressive sequence of morphological change of cheek-tooth crown height. This pattern is now interpreted as an oversimplification resulting from the level of geochronological control then available to workers just after the advent of absolute dating techniques. Reproducrd with the permission of the Society Ibr the Study of Evolution. ago Simpson (1949: 207) wrote: Knowledge of absolute geochronology is so imperfect that such rates are always rough approximations and can only rarely at present be usefully determined for geologically short periods of time. With the lack of a precise absolute time scale, most early studies depicted horse evolution as principally a trend of linear or gradual increase of dental characters (e.g. Romer, 1949; Stirton, 1947; Fig. 2). This interpretation has continued through to the recent general evolutionary literature (e.g. Stanley, 1979). We can now resolve the biochronology of Tertiary mammals to about 1 Myr or less (Flynn, MacFadden & McKenna, 1984). Accordingly, some of the previously

40 B. J. MACFADDEN proposed evolutionary patterns of fossils, like those of horses, can now greatly profit from renewed investigations in a modern context. In a classic paper, Haldane (1949) quantified rates of morphological change. He coined the unit darwin which is calculated by the following equation: r = (In ( x2) -In (.,))/At, (1) where r = rate of change (in darwins, d), x, = initial dimension of a character, x, = final dimension of a character, and At = amount of time involved. It cannot be disputed that this equation is the foundation of all quantitative studies of morphological change as interpreted from the fossil record. This subject has been subsequently amplified in the literature (e.g. Simpson, 1953; although he did not use the unit darwin ), and with the recent renaissance in systematics and evolution, studies of rates have also become more common (for a recent review, see Stanley, 1985). The purpose of this paper is to present new data on rates of dental evolution for 26 presumed ancestral-descendant pairs of fossil horses spanning from the Eocene to the Pleistocene. Part 1 of this series (MacFadden, 1987) presented a study on the rates of body size evolution for fossil horses. In the current paper I do not recalculate rates of dental evolution for the three-toed Neogene hipparion horses because they are presented elsewhere in a similar study (MacFadden, 1985). However, I do compare the results of the new data in this paper with those of hipparions (MacFadden, 1985) and Pseudhipparion (Webb & Hulbert, 1986). MATERIALS AND METHODS Four characters of the upper first molar (MI, Fig. 3) were measured for 28 species of Eocene to Pleistocene horses (Appendix A). These characters were chosen because they seem to characterize adequately the important changes seen in fossil horse teeth. The study sample comes from 408 specimens housed in the ten natural history museums listed in Appendix B. These species were chosen because they represent the morphological diversity observed in fossil horses and because they are represented by adequate samples to measure all four dental characters. Fossil horses, particularly the later Cenozoic hypsodont taxa, are notoriously variable in dental characters because of morphological changes that occur during ontogeny (e.g. MacFadden, 1984; MacFadden, 1988). Therefore, in this study the following procedures were followed: ( 1 ) For occlusal dimensions (M 1 APL, M 1 TRNW and M 1 PRTL) of all highcrowned and selected low-crowned taxa, only individuals in middle wear were used. Ontogenetic changes are less of a problem in the primitive, low-crowned horses and, if available specimens were few, a pooled ontogenetic sample was used if coefficients of variation (Vs) were c. 10% or less, which seems to be a reasonable amount in a single species (Simpson, Rowe & Lewontin, 1960; Yablokov, 1974). (2) Within a given horse species, crown height (MlMSTHT), varies greatly with ontogenetic age. Therefore, for this character, specimens were generally used only if they were in juvenile or early maturity wear stage, thereby

FOSSIL HORSE DENTAL EVOLUTION 41 YAPL --I 1 4-4 M 1 PRTL 0 1 2cm t----- Figure 3. Dental characters of upper 1st molar measured in this study. MIAPL, greatest anteriorposterior length of occlusal surface (excluding cement); M ltrnw, greatest transverse width of occlusal surface (excluding cement); MI PRTL, greatest protocone length; M 1 MSTHT, greatest mesostyle crown height in unworn or little-worn individuals (i.e. juvenile or early maturity ontogenetic stage). representing or closely approaching maximum crown height (MacFadden, 1984: 20). Using the mean dimension and median biochronological range for each species, rates of morphological evolution were calculated (using Haldane s (1949) equation above) for 26 inferred or close to ancestral-descendant species pairs (Fig. 4). When recent studies were unavailable for a particular clade of horses, then the phylogenetic interpretations are generally taken from Stirton (1940) or MacFadden (unpubl. obs.). Because of the possible problems of comparing ancestral-descendant taxa with Ats of different orders of magnitude (the denominator in Haldane s equation, see discussion in Gingerich, 1983; Gingerich, 1984; Gould, 1984), fossil horse species pairs were chosen to minimize the range of At; in 25 of the 26 cases, this value is between 1 and 10 Myr. The time scale used to calibrate the temporal distributions of species is taken from Berggren, Kent & Flynn (1985) and Tedford, Galusha, Skinner et al. (1 988). All computations were made using SAS (Statistical Analysis Systems) Programs available at the University of Florida computer centre. The nonparametric statistical tests used here follow Siege1 (1956) and Sokal & Rohlf (1981). RESULTS AND DISCUSSION Patterns of dental evolution The mean dimension for each of the four dental characters is plotted against median geological age for the 28 horse species studied, plus, for comparison, selected three-toed hipparions (data taken from MacFadden, 1985). Regression

42 B. J. MACFADDEN 0 5 1 Eqws comphcofus Equus scott~ 1 Equus simphodens Dinohippuq m exicanus On0 hippi dium g alushai DInohlDDUS leidvonus 4 4 I 45 Epihippus uintensis Epihippus grociiis \ / Orohippus pumulis I Hyrocotherium voccossiense Hyrocotherium topirinum \ / 55 Hyrocotherium angustidens Figurc 4. Synoptic diagram of the 26 inferred or close to ancestral--descendant species pairs of fossil horses used to calculate rates of evolution. Although not analysed in this report, the phylogenrtic positions of hipparions and P.reudhzpparion, which are dealt with elsewhere (MacFadden, 1985; Webb & Hulbert, 1986; respectively), are indicated because these latter clades arc compared with the fossil horses studied here. equations were calculated for 21 of the 28 species (the anchitheres were excluded because of their highly specialized browsing dentitions) using linear Cy = x ), squared Cy = x ) and exponential (in logarithmic form, lny = x) models. For the occlusal characters M 1 APL, M 1 TRNW, and M 1 PRTL, although the squared model gives the poorest fit (as determined by the sum of squared residuals, R ), these R2 values seem not to indicate a significantly better fit between the linear and exponential models. Crown height (M 1 MSTHT), however, has a far better fit using the exponential models (R2 = 0.911, a fact evident from the bivariate plot in Fig. 5D. The significance of these regression patterns, which is related to the morphological rates of character evolution, is discussed below. Hayami (1978) proposed that selection acts to limit the body size of an evolutionary lineage. He stated ( 1978: 252) that: Because the selection pressure must decrease as the average body size of an evolving population approaches the limit, some sigmoidal curve should be regarded as more appropriate for the model of size increase than an exponential curve as in the case of population growth. The phyletic pattern of MlMSTHT for fossil horses, which is well correlated to estimated body size (r = 0.82 using body mass data from MacFadden, 1987) suggests an exponential, not sigmoidal pattern of character evolution. Rates of dental evolution Table 1 presents the results of using equation 1 above to calculate rates of morphological evolution in darwins (d). A Wilcoxon matched-pairs signedranked test (Siegel, 1956) for the 26 species pairs in Table 1 indicates that the

FOSSIL HORSE DENTAL EVOLUTION 43 IB *._ ' * E 6 I 1 1 I 0 10 20 30 40 50 60 14 1 c o I D 1 I c 100 - m- r E 12. U E '0-. 0 0. 8.- 0 6 -", *,.O - 4- E 2- a- 0,'. 8. - 0 1 I 1 I I ::. l l years ago 0: Species used for regression equations (see text) o = Specialized species excluded from regression equations i. '...., "... I Figure 5. Bivariate plots of dental character evolution versus geological age for the horse species studird here and, for comparison, those presented in MarFadderi (1985) and Webb & Hulbert (1986); MlAPL (A), MlTRNW (B), MlPRIL (C), MlMSTHT (D). mean MIMSTHT evolved significantly faster than either MI APL (T = 52, P= 0.05) or MlTRNW (T= 57.5, P= 0.05) and MlPRTL evolved significantly faster than M1 APL (T = 88.5, P = 0.05). Rates of dental evolution are compared for six selected clades of fossil horses in Table 2. Several interesting results are apparent from the data: 1. MlAPL and MlTRNW These two characters provide a rough index of the occlusal surface area that functions during mastication (Van Valen, 1960). As indicated by the patterns graphed in Fig. 5, there seems to be a more uniform overall evolutionary rate for the occlusal dimensions than MlMSTHT. For the groups compared in Table 2, the fastest rates are exhibited in the anchitheres and Pseudhipparion. It is interesting that those represent respectively, size increase and size decrease. (See further discussion below.) 2. MlPRTL: The evolutionary rate of this character does not seem linked to either M 1 APL, M 1 TRNW, or M 1 MSTHT. Whereas the occlusal surface area is related to the quantity of food that can be processed, the enamel part of the protocone, being a resistant dental tissue in hypsodont horses (where cusp shearing has been lost relative to primitive mammals), limits increased wear as a result of a more abrasive food source (Van Valen, 1960). Rensberger, Forsten & Fortelius (1984) have also shown that the amount of enamel is related to dental processing efficiency. Increased enamel complexity in horses (e.g. fossettes) is frequently observed with increased protocone length, for example in Neohipparion and Equus. 3. MIMSTHT. Increase of crown height is one of the most frequently cited

44 B. J. MACFADDEN TABLE 1. Rates of morphological evolution (in darwins, d) for the four characters of M' for 26 hypothesized ancestral-descendant species pairs of North American Equidae. Negative sign indicates decrease in size. For calculation of mean evolutionary rates, the absolute values were used. The AT values are the amount of time (in million years) separating the midpoints of the ancestral and descendant species known temporal ranges Species pair Hyracothrrium angustidens-hyracotherium tapirinum Hyracolherium angustidens-hyracotherium uaccassiense Hyracotherium vaccassiense-orohippus pumulis Orohippus pumulis- Epihippus gracilis Orohippus pumulis-epihippus uintensis Epihippus gracilis-mesohippus bairdii Mesohippus bairdii-miohippus quartus Miohippus quartus-parahippus tyleri i2liohippu.r quartus-a rchaeohippu.r blackbergi Parahippus tyleri-parahippus leonensis Parahippus leonensis- Merychippus gunteri Parahippus leonensis-meryehippus isonensis Parahippus leoneusis-merychippus insignis Anchitherium clarencei-hypohippus large sp. Anchitherium clarrncei-megahippus mckennai Me,cahippus mckennai-megahippus matthewi Merychippus iaonesus-pliohippus pernix Dinohippus leidyanus-onohippidium galushai Dinohippus leidyanus-dinohippus mexicanus Dinohippus mexicanus-equus simplicidens Equus simpliciden3-equus scotti Mesohippus bairdi-mesohippus barbouri MiohippuA quartus-anchitherium clarencei Parahippus leonensis-protohippus simus Parahippus laonensis-merychippus primuj Equus simplicidens-equus complicatus Mean species pair evolutionary rate* AT MlAPL MITRNW MlPRTL MIMSTH'T (Myr) (4 (4 (d) (d) -. 3.0 0.101 3.0 0.010 2.5 0.011 4.5 0.026 4.5 0.047 14.0 0.023 6.0 0.022 5.0 0.067 7.0-0.017 2.0-0.062 2.0-0.023 3.0 0.030 3.0 0.053 8.0 0.062 4.0 0.083 2.0 0.036 2.5 0.072 2.0 0.033 2.0-0.004 2.0 0.064 2.0 0.040 1.0 0.157 7.0 0.065 6.0 0.051 3.0-0.009 2.0 0.000 0.045 0.106 0.026 0.029-0.010 0.052 0.037 0.018 0.052-0.018-0.039-0.107 0.014 0.01 I 0.072 0.096 0.022 0.101 0.027-0.026 0.081 0.005 0.146 0.057 0.034-0.026-0.014 0.047 0.091 0.096-0.020 0.000 0.012 0.068 0.006 0.020 0.042-0.007 0.032 0.036 0.012 0.022 0.066 0.091-0.029-0.021-0.179 0.088-0.116 0. I72 0.028 0.339 0.038 0.228 0.077 0.077 0.094 0.109 0.064 0.219 0.185 0.180 0.030-0.074 0.074-0.054 0.088 0.142 0.162 0.097 0.136 0.050 0.049 0.046 0.073 0.247-0.012 0.154 0.1 15 0.054 0.0690 0.104 *Absolute values are used to calculate these values examples of evolution observed from the fossil record (e.g. Stirton, 1947: fig. 2). As presented in Fig. 5 and Table 2, MlMSTHT evolves most rapidly. It is not surprising, nor it is a new idea, that the lowest evolutionary rates are exhibited by the early horses and that among the highest rates are observed at the base TABLE 2. Comparisons of mean rate of morphological evolution (in darwins, d) for the same MI characters for selected groups of North American Equidae Group I. Early horses (Hyracotherium to Mesohippus, Fig. 4) 8 2. Anchitherinrs (Anchitherium, Hypohippus, and 4 Megahippus, Fig. 4) 3. Parahippus and Merychippus (Fig. 4) 6 4. Hipparionines (MacFadden, 1985) 8 5. Pseudhippariun (Webb and Hulbert, 1986) 4 6. Advanced monodactyl equines (Dinohippus and 4 Equus, Fig. 4) 7. All horses (1-6 above) 43 N (species MlAPL MI'I'RNW MlPRTL MIMSTHI pairs) (dj (d) (d) (d) 0.049 0.053 0.044 0.037 0.061 0.062 0.07 I 0.113 0.038 0.039 0.074 0.204 0.040 0.030 0.040 0.080 0.070 0.087 0.113 0.242 0.027 0.032 0.110 0.086 0.048 0.051 0.075 0.127

FOSSIL HORSE DENTAL EVOLUTION 45 of the major cladogenesis of hysodont horses between Parahippus and Merychippus. However, two very interesting rates of MlMSTHT evolution are as follows; (1) Because anchitheres are generally stated to be browsing horses, I would have expected only the occlusal dimensions to have evolved rapidly, yet crown height also increased rapidly (mean rate of 0.113 d, Table 2), particularly within Megah$pus (Table 1). (2) The highest rate of MIMSTHT evolution for horses is observed in Pseudhipparion. This is perplexing because (excluding anchitheres) increased MlMSTHT seems correlated with increased body size (r = 0.82, N = 32, this study plus hipparionines; body masses taken from MacFadden, 1987). Yet for the four ancestral-descendant species comparisons within Pseudhz$parion, the trend is decidedly toward dwarfism (Webb & Hulbert, 1986). In this evolving genus one might predict a different strategy of decreased longevity (Eisenberg, 1981, has demonstrated a relationship between body size and individual lifespan) and possibly a trend towards R-selection, i.e. higher fecundity. A problem also arises with the case of increased crown height in Pseudhipparion, which Webb & Hulbert ( 1986) have demonstrated to be incipiently hypselodont (ever-growing, e.g. like some advanced rodents). It might be expected that there would not be strong selection for the evolution of this character in dwarfing lineages. There are at least two possible explanations for rapid MIMSTHT evolution in Pseudhipparion. (1) An increase in the abrasive characteristics of the food resources taken by these horses. Although this is possible, their is no evidence available from relevant fossil localities to test this hypothesis. (2) A decreased amount of resistant dental tissues may compose the teeth. As pointed out to me by S. D. Webb, this is seen in Pseudhipparion, particularly in the late Miocene and early Pliocene (Hemphillian) forms from Florida, where there is both decreased enamel thickness and during wear (in the hypselodont phase) a loss of enamel parts (fossettes). As previously hypothesized (e.g. Van Valen, 1960), loss of any portion of enamel surfaces available for mastication of abrasive food stuffs would result in stronger selection for higher crowned teeth. Pseudh$parion, was the only clade within the Equidae to show experimentation with hypselodonty, a character otherwise associated with the feeding strategy and extraordinary evolutionary success of many advanced groups of grazing rodents. CONCLUDING COMMENTS The results presented here corroborate those for hipparions (MacFadden, 1985) in which generally horses are characterized by normal, average or, as Simpson (1953) proposed, horotelic evolution. It is perhaps surprising that Eocene horses do not show the high rates on the order of 1-10 d that Gingerich ( 1982) found for primitive, contemporaneous mammals radiating into new adaptive zones. A productive line of future research might be to examine rates of evolution for the same characters in the closest outgroup of horses, i.e. the phenacodontid condylarths, and between the latter group and primitive Equidae. Even during the major adaptive radiation of horses during the early Miocene shift from browsers to grazers, represented by Parahippus- Merychippus, the rate of dental change, although high within the Equidae, is an order of magnitude

46 B. J. MACFADDEN lower than for Eocene mammals. An interesting corollary to these results is that for the Miocene horses (Merychippus primus and Pseudhipparion gratum), Van Valen (1964: 106) found that: A quantitate estimate shows that weak natural selection is adequate to account for the most rapid evolutionary change in the Equidae. Therefore the standard notion of horses exemplifying rapid evolutionary change of dentitions under a regime of strong selection is not corroborated by the current data. However, recent work (Hulbert & MacFadden, unpubl. obs.) suggests that, although there was not rapid morphological change, Miocene horses underwent rapid taxonomic evolution (Simpson, 1953) resulting from explosive cladogenetic speciation. With the results of the present study I do not claim any radical departure from the overall general pattern or rate of dental evolution presented in the important studies some 40 years ago (Romer, 1949; Haldane, 1949; Simpson, 1953). However, the new results provide a more robust data base that can be used to quantify the sequence of horse evolution and provide a basis for comparison with other taxa. ACKNOWLEDGEMENTS I have greatly profited from discussions and helpful comments on this study by Richard C. Hulbert, Jr. and S. David Webb. I thank the following persons for access to relevant research specimens (See Appendix B for institutional abbreviations); Philip J. Bjork, SDSM; Walter W. Dalquest, MSU; Robert J. Emry, USNM; Linda Gordon, USNM-M; Farish A. Jenkins, Jr., MCZ; Wann Langston, TMM; Everett H. Lindsay, UALP; Samuel McLeod, LACM; Guy Musser, AMNH-M; Miriam Schwartz, YPM; Chuck Schaff, MCZ; Richard H. Tedford, AMNH; Richard Thorington, USNM-M; David P. Whistler, LACM; John A. Wilson, TMM. Dr Philip D. Gingerich kindly provided me access to his measurements on the important Castillo Pocket sample of Hyracotherium. Dr Donald R. Prothero allowed me access to his unpublished research on MeJohippus and Mioh$pus. Ms Wendy Zomlefer and assistants skilfully prepared the illustrations. Computing for this research was done using the Faculty Support Center and Northeastern Regional Data Center at the University of Florida. This report is the University of Florida Contribution to Paleobiology number 246. This research was partially supported by U.S. National Science Foundation grant BSR-8515003. REFERENCES BEKGGREN, W., KENT, D. V. & FLYNN, J. J., 1985. Cenozoic geochrotiology and chronostratigraphy, Bulletin qfthe Geological Sociely of America, 96: 1407-1418. EISENBERG, J. F., 1981. The Mammalian Radiations: An Analysis of Trends in Evolution, Adaptation, and Behavior. Chicago: Ihc IJniversity of Chirago P FLYNN, J. J., MACFADDEN, B. J. & M ENNA, M. C., 1984. Land-mammal ages, faunal hcterochrony, and temporal resolution in Cenozoic terrestrial sequences. j ournal qf GeoloSy, 92; 687-705. GINGERICH, P. D., 1982. Time resolution in mammalian evolution: sampling, lineages, and faunal iurnover. Proceedings 3rd North American Paleontological Convention, I: 205-2 10. GINGERICH, P. D., 1983. Rates of evolution: effects of time and temporal scaling. Science, 222: 159-161. GINGERICH, P. D., 1984. Smooth curve of evolutionary rate: A psyrhological and mathematical artifact. (Reply to Gould.) Science, 226: 995-996. COULD, S. J., 1984. Gingerich s smooth curve of cvolutionary rate: a psychological and mathematical artifact. Sciencp, 226: 994-995. HALDANE, J. B. S., 1949. Suggcstions as to quantitative measurements of rates of evolution. Enolution, 3: 51-56.

~~~ ~ FOSSIL HORSE DENTAL EVOLU TION 47 HAYAMI, I., 1978. Notes on the ratrs and patterns of size change in evolution. Paleobiology, 9: 252-260. MACFADDEN, B. J., 1984. Systematics and phylogeny of Hipparion, Neohipparion, Nannippus, and Cormohipparion (Mammalia, Equidae) from the Miocene and Pliocene of the New World. Bulletin American Museum Natural History, 179: 1-196. MACFADDEN, B. J., 1985. Patterns of phylogeny and rates of evolution in fossil horses: Hipparions from the Miocene and Pliocene of North America. Paleobiolou, If: 245-257. MACFADDEN, B. J., 1987. Fossil horses from Eohippus (Hyracotherium) to Equus: Scaling, Cope s Law, and the evolution of body size. Paleobiology, 12: 355-369. MACFADDEN, B. J., 1988. Character variation in fossil horscs (Equidae): a trst of the paleopopulation and morphospecies concepts. In D. R. Prothero, R. M. Shoch & J. L. Franzen (Eds), The Evolution of Perissodactyls. Courirr Forsrhung-Institut Senkenberg; in press. RENSBERGER, J. M., FORSTEN, A. & FORTELIUS, M., 1984. Functional evolution of the cheek tooth pattern and chewing dircction in Tertiary horses. Paleobiology, IO, 439-452. ROMER, A. S., 1949. Time series and trends in animal evolution. In G. L. Jepsen, G. G. Simpson & E. Mayr (Eds), Genetics, Paleontology, and Evolution: 103-120. New York: Atheneum. SIEGEL, S., 1956..Nonparametric Statisticsfr the Behavioral Sciences. New York: McGraw-Hill Book Company. SIMPSON, G. G., 1949. Rates of evolution in animals. In G. L. Jepsen, G. G. Simpson & E. Mayr (Eds), Genetics, Paleontology, and Euolution: 205-228. New York: Atheneum. SIMPSON, G. G., 1953. The Major Featurfs of Evolution. New York: Columbia University Press. SIMPSON, G. G., ROE, A. & LEWON TIN, R. C., 1960. Quantitative ~oology. San Francisco: Harcourt, Brace and Co. SOKAL, R. R. & ROHLF, F. J., 1981. Biometry: The Prinriples and Practice of Statistics in Biological Research. San Francisco: W. H. Freeman & Company. STANLEY, S. M., 1979. Macroeuolution: Pattern and Process. San Francisco: W. H. Freeman & Company. STANLEY, S. M., 1985. Rates of evolution. PaleobioloQ, II: 13-26. STIRTON, K. A,, 1940. Phylogeny on North American Equidae. University California Publications, Bulletin Department Geological Sciences, 25: 165-198. STIRTON, R. A,, 1947. Observations on evolutionary rates in hypsodonty. Evolution, I: 32-41. TEDFORD, R. H., GALUSHA, T., SKINNER, M. F., TAYLOR, B. E., FIELDS, R. W., MarDONALD, J. R., RENSBERGER, J. M., WEBB, S. D. & WHISTLER, D. P., 1988. Faunal succession and biochronology of the Arikareean through Hemphillian interval (late Oligocene through earliest Pliocene epochs), North America. In M. 0. Woodburne (Ed.), Vertebrate Paleontolo,g as a Discipline in Geochronology. Berkeley: University California Press, 153-2 10. VAN VALEN, L., 1960. A functional index of hypsodonty. Euolution, 14: 531-532. VAN VALEN, L., 1964. Age in two fossil horse populations. Acta Zoologica, 54: 93-106. WEBB, S. D. & HULBERT, R. C., Jr. 1986. Systematics and evolution of Pseudhipparion (Mammalia, Equidar) from the late Neogene of the Gulf Coastal Plain and the Great Plains. In K. M. Flanagan & J. A. Lillegraven (Eds), Vertebrates, Phylogeny, and Philosophy: 237-272. Contributions to Geology, University of Wyoming, Sperial Paper 3. YABLOKOV, A. V., 1974. Variability of Mammals. New Delhi: American Publishing Company Limited. APPENDIX A Fossil horse species studied, localities, land-mammal age, institution in which specimens are contained (see Appendix B for institutional abbreviations), and total number of specimens (N). 1. Hyracothfrium anpslidens; Big Horn Basin, Wyoming; San Juan Basin, New Mexico; Big Bend National Park, Texas; Clarkforkian or Wasatchian; AMNH, TMM, USNM. (N = 34) 2. Hyracotherium tapirinum; Castillo Pocket, Colorado; Wasatchian; AMNH, USNM (A = 21) 3. Hyracotherium uassucciense; Castillo Pocket, Colorado; Wasatchian; (N = 3) 4. Orohippus pnmulis; Bridger Basin, Wyoming; Bridgerian; AMNH, USNM, YPM (N = 12) 5. Epihippus uintensis; Uinta Basin, Utah, Wind River Basin, Wyoming; Uintan: AMNH, YPM (N = 2) 6. Epihippus gracilis; Uinta Basin, Utah; Uintan; AMNH, USNM (N = 2) 7. Mesohippus bairdii; Big Badlands, South Dakota and adjacent Nebraska, Wyoming; Orellan-Whitneyan; AMNH, USNM, YPM, SDSM, LACM (N = 3) 8. Mes0hippu.r barbouri; Harvard Fossil Reserve, Goshen Hole, Wyoming; Orellan-Whitneyan; MCZ (N = 10) 9. Miohippus quartus (and M. equiceps); John Day Basin, Oregon; Arikareean; AMNH (N = 3) 10. Archaeohippus blackbergi; Thomas Farm, Florida; Hemingfordian; UF, AMNH, MCZ, SDSM (N = 12) 1 I. Anchitherium clarenci; Thomas Farm, Florida, Hemingfordian: UF, FGS, (N = 2) 12. Hypohippus sp. (large); Ash Hollow Formation, Nebraska; Clarendonian; F:AM (N = 7)

48 B. J. MACFADDEN 13. A4gahippus mckennai; Barstow Formation, California; Willow Grove L. F., Nevada; Weld and Logan counties, Colorado and adjacent Nebraska; Barstovian-Valentinian; F:AM (N = 8) 14. Megahippus matthewi; Burge Quarry and equivalents, Nebraska; Valentinian; F:AM (N = 2) 15. Parahippus phi; Duulop Camel Quarry, Sioux County, Nebraska; Arikareean- Hcmingfordian; F:AM, UNSM (N= 8) 16. Parahippus leonensis; Thomas Farm, Florida; Hemingfordian; UF, FGS, AMNH, MCZ, SDSM (N = 26) 17. Merychippus gunteri; Hawthorne Formation, Florida; Barstovian; UF (N = 13) 18. Merychippus insignis; Echo Quarry, Sioux County, Nebraska; Barstovian; F:AM (N = 19) 19. Merychippus primus; Thomson Quarry, Sioux County, Nebraska; Hemingfordian, F:AM, AMNH, (X = 43) 20. Merychippus isonenus; Sheep Creek Beds, Nebraska; Salt L.ake Grp., Idaho; Mascall Formation, Oregon; Hemingfordian and Barstovian; AMNH, UF, (A = 4) 21. Protohippus szmus; Burge Quarry and equivalents, Nebraska; Valentinian; F:AM (N = 12) 22. Pliohippus pernix; Burge Quarry and equivalents, Valentine Formation, Nebraska; Valentinian; F:AM (N= 13) 23. Dinohippus leidyanus; Guymon Quarry, Oklahoma; Hemphillian; F:AM (N = 47) 24. Dinohippus mexicanus; Yepomera localities; Chihuahua, Mexico; Hemphillian, LACM (N = 32) 25. Onohippidium galushai; Bird Bone, Clay Bank and equivalent quarries, Wikieup, Arizona; Hemphillian, F:AM i.n= 17) 26. Equus szmplicidens (= E. shoshonenszs); Hagerman Horse Quarry, Idaho; Blancan, AMNH, TMM, USNM, YPM ( N= 45) 27. Equus complicatus, Ingleside Quarry, San Patricio County, Texas; Rancholabrean, TMM (N = 6) 28. Equus scotti; Rock Creek Quarry, Texas; Irvingtonian, AMNH, YPM (N = 2) APPENDIX B Institutional collections examined and abbreviations used in Appendix A AMNH Department of Vertebrate Paleontology, American Museum of Natural History, New York, New York. F:AM Frick: American Mammals (now part of AMNH). LACM Natural History Museum of Los Angeles County, Los Angeles, California. MCZ Museum of Comparative Zoology, Harvard University, Cambridge, Massachusetts. SDSM Museum of Geology, South Dakota School of Mines, Rapid City, South Dakota. TMM Vertebrate Paleontology Laboratory, Texas Memorial Museum, Austin, Texas. UF Division of Vertebrate Paleontology, Florida State Museum, University of Florida, Gainesvillc, Florida. UNSM Vertebrate Paleontology, Nebraska State Museum, University of Nebraska, Lincoln, Nebraska. USNM Department of Paleobiology, US. National Museum of Natural History, Washington, D.C. YPM Division of Vertebrate Paleontology, Yale Peabody Museum of Natural History, New Haven, Connecticut.