Society of Systematic Biologists

Home , Asinus, Cormohipparion, Dinohippus, Equus stenonis, Neohipparion, Plesippus

Stripes Do Not a Zebra Make, Part I: A Cladistic Analysis of Equus Author(s): Debra K. Bennett Source: Systematic Zoology, Vol. 29, No. 3 (Sep., 1980), pp. 272-287 Published by: Taylor & Francis, Ltd. for the Society of Systematic Biologists Stable URL: http://www.jstor.org/stable/2412662 Accessed: 20-05-2015 19:24 UTC

Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at http://www.jstor.org/page/ info/about/policies/terms.jsp

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected].

Taylor & Francis, Ltd. and Society of Systematic Biologists are collaborating with JSTOR to digitize, preserve and extend access to Systematic Zoology. http://www.jstor.org

This content downloaded from 196.21.233.72 on Wed, 20 May 2015 19:24:25 UTC All use subject to JSTOR Terms and Conditions Syst. Zool., 29(3), 1980, pp. 272-287

STRIPES DO NOT A ZEBRA MAKE, PART I: A CLADISTIC ANALYSIS OF EQUUS

DEBRA K. BENNETT

Abstract Bennett,D. K. (Department of Systematics and Ecology, The Universityof Kansas and Mu- seum of Natural History,Lawrence, Kansas 66045) 1980. Stripes do not a zebra make, part I: A cladistic analysis of Equus. Syst. Zool., 29:282-287.-Living and extinctspecies in Equus have not been reviewed fornearly a century.More than twentymorphological, mostlycranial characters of Equus and Dinohippus are here explained; synapomorphies uniting Equus and Dinohippus and autapomorphies within Equus are discussed. Significantoutgroups compared are Dinohippus Quinn, Astrohippus Stirton,and Neohipparion Gidley. Cladistic analysis indicates that Equus is a monophyletictaxon closely related to Dinohip- pus, and that Equus can reasonably be divided into two (and only two) subgenera, Equus (Equus) and Equus (Asinus), each of which is characterized by a suite of autapomorphic features. The NorthAmerican fossil record contains close relatives of every living species of equid except E. quagga. An examination of the zoogeographic implications of the cladistic hypothesis here presented indicates a complex pattern of migrationfrom North America to Eurasia during Blancan through late Pleistocene time, and a strongzoogeographic relationship between Africa and North America demonstrated by the equids. [Cladistics; Equus; zoogeography; fossil horses; phylogeny.]

The living and extinct species of the present differentiatingcharacters of all genus Equus have not been reviewed living species of Equus, and of those fos- since Gidley (1901), whose work was lim- sil species for which sufficientmaterial ited to tooth structureand variability in is available. Qualitative characters,most- all living and fossil Equus known at the ly cranial, are employed. A morphometric time. Catalogs and partial revisions (e.g., analysis of these characters will appear Gray, 1825; Allen, 1909; Lydekker, 1916) elsewhere. 2) To present derived and were attempts to organize the prolifera- primitive character states forfeatures ex- tion of names for the living members of amined (Table 1), as determined by out- this genus (Bennett, MS). Taxonomy of group comparison. The significant out- fossil representatives of this genus and group compared is the genus Dinohippus near relatives is a vexing problem, in part Quinn. Systematicrelationships of Equus because of a lack of correspondence be- have been studied by the basic method tween the neontological and paleontolog- of Hennig (1966). The resulting hypoth- ical viewpoints (Skinner and Hibbard, esis of evolutionary relationships is pre- 1972). Gazin (1936), McGrew (1944), and sented in a cladogram (Fig. 1). 3) To dis- Howe (1970) have studied the skull and cuss some of the zoogeographic skeleton in definable fossil populations implications of the cladistic hypothesis of Equus. Nevertheless, these and other here presented in light of recent work by studies of Blancan and Pleistocene horse Eisenmann (1979), and of the extensive material have not resolved systematicre- North American fossil record of Equus. lationships among Equus species. This A revised taxonomyof the genus Equus study identifies synapomorphic charac- is presented elsewhere (Bennett, MS), in ters for the genus Equus. These horses which nine living or recently extinct are systematically related to the earlier species are recognized. All living and Hemphillian form, Dinohippus Quinn most of the fossil Pleistocene and Blan- (Lance, 1950), with which they share can Equidae are placed in the genus some synapomorphic characters. Equus, in two subgenera, Equus and As- The purposes of this paper are: 1) to inus. Previously recognized subgenera, 272

This content downloaded from 196.21.233.72 on Wed, 20 May 2015 19:24:25 UTC All use subject to JSTOR Terms and Conditions 1980 STRIPES DO NOT A ZEBRA MAKE 273

TABLE 1. CHARACTERS AND CHARACTER STATES OBSERVED BY TAXON. TAXA ARE CODED AS FOLLOWS: A, DINOHIPPUS. B, EQUUS ASINUS. C, E. MEXICANUS. D, E. KIANG. E, E. CONVERSIDENS. F, E. ONAGER. G, E. FRANCISI. H, E. SCOTTI. i, E. CALOBATUS, j, E. HEMIONUS. K, E. ZEBRA. L, E. HATCHERI. M, E. CABALLUS. N, E. QUAGGA. O, E. OCCIDENTALIS. P, E. BURCHELLI. Q, E. ANDIUM. R, E. STENONIS. S, E. GREVYI. T, E. SHOSHONENSIS. CHARACTERS ARE CODED AS FOLLOWS: 0 = PRIMITIVE CONDITION. 1 = DERIVED CONDITION. 2 = MORE DERIVED CONDITION. "WAGNER 78" ANALYSIS OF THIS MATRIX PRODUCED SAME CLADOGRAM AS IN FIGURE 1.

Taxa Character Number a b c d e f g h i k I m n o p q r s t 1 11 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 1 1 11 1 11111 1 1 1 111 111 1 1 1 3 0 1 11 11 11 11 11 11 1 11 1 1 1 4 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 5 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 6 0 2 2 2 2 2 2 2 2 2 0 1 1 1 0 0 0 0 0 0 7 0 0 0 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 8 0 0 0 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 9 0 0 0 0 0 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 10 0 0 0 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 11 0 0 0 0 0 0 0 0 1 1 0 0 2 2 0 0 0 0 0 0 12 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 13 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 14 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 15 0 0 0 0 0 0 0 0 0 0 1 1 1 1 0 0 0 0 0 0 16 0 0 0 0 0 0 0 0 0 0 1 1 0 0 0 0 0 0 0 0 17 0 0 0 0 0 0 0 0 0 0 0 0 1 1 0 0 0 0 0 0 18 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 19 0 0 0 0 0 0 0 0 1 1 0 0 0 0 1 1 1 1 1 1 20 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 0 0 0 21 0 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1

including Dolichohippus,Onager, Hem- nohippus. Types or casts of types were ionus,Hippotigris, Plesippus, and others examined whenever possible. are shown by this cladistic analysis to be For purposes of simplicity and clarity, paraphyletic, polyphyletic, or unneces- charactersdiscussed in this paper are list- sary. The various affinities,within the ed below in the order in which they ap- subgenus Equus, of the living, striped pear on Figure 1 (the number assigned to equids explain why stripes do not a ze- each character is referredto in Table 1 bra make; these subjects are treated in and all figures): 1. Number of functional detail elsewhere (Bennett, MS). Subge- digits (see McFadden, 1972); 2. Degree neric assignments of recent and fossil of isolation of protocone and hypocone Equus species are listed below. (Fig. 2D); 3. Presence and size of preorbital facial fossa (Fig. 2A); 4. Presence MATERIALS AND METHODS and degree of development of secondary A total of 96 skulls and 57 skeletons of infundibularfold on '2 (Fig. 2E); 5. Size living or extinct equids were examined; and position of inferiorcanines (Fig. 2B); those examined include Equus andium, 6. Presence and degree of development E. asinus, E. burchelli,E. caballus (in- of isthmus on inferiormolars (Fig. 2C); cluding E. caballus alaskae, originally 7. Proportionsof metacarpal and metatar- named E. niobrarensisalaskae by Hay, sal III (see Skinner and Hibbard, 1972); 1915), E. calobatus, E. conversidens,E. 8. Shape of lambdoidal crest (Fig. 5B); 9. francisi,E. grevyi,E. hatcheri,E. hem- Relative length of snout (Fig. SB); 19. ionus,E. kiang,E. onager,E. occidental- Relative depth of rostrum(Fig. 5F); 11. is, E. quagga, E. scotti,E. zebra, and Di- Presence and degree of development of

This content downloaded from 196.21.233.72 on Wed, 20 May 2015 19:24:25 UTC All use subject to JSTOR Terms and Conditions 274 SYSTEMATIC ZOOLOGY VOL. 29

Ci) 0 C C co a) - ? o 0 0 . 0 \ \ co e C 0 \L (cu - c C.)cu() ./CarrEw0 U) .0 ONE co C > cm)C0 CA .- o * C > \ 0g{

ee rostru m oa isthmus6i 3

ullmo r i mussUnolded mastoid-paramastoid 3 Reduced cranial flexion 12 Cranial broadening complex

4 2 12 with m.w.infundibulum; large anterior canines

3 Facial lossa reduced or absenl 2 Superior c.t. with connected pc.and deep h.c.g. 1 Monodactylous

FIG. 1.-Cladogram showing systematicrelationships of all living and some fossil species (daggered) of Equus. Characters numbered on cladogram are keyed to Table 1 and are more fully explained in text. Abbreviations: l.c. = lambdoidal crest. w/ = with. Infundib. = infundibulum. Hz. = horizontal. p.o. postorbital. m.w. = middle wear. c.t. = cheek teeth. p.c. = protocone. h.c.g. = hypoconal groove. secondary infundibular fold in 13 (Fig. and interpopulational (Eisenmann, 1975, 2E); 12. Presence of "cranialbroadening 1976; Eisenmann and deGiuli, 1974a, complex"(see textand Fig. 3A); 13. De- 1974b; Eisenmann and Turlot, 1978) gree of "cranial flexion"(see text and variabilityin cranial and skeletal features Figs. 5E, F, G, H); 14. Relationshipsof of Equus. This study does not document mastoid,paramastoid, and mastoidpart of the limits of variability for any species temporalbone (Fig. 3B); 15. Orientation analyzed. I have instead presented the of postorbitalbars relativeto horizontal usual condition seen in mature individ- plane (Fig. 4); 16. Presence of supraor- uals for each character discussed. Skull bital boss; 17. Size and presenceof post- characterswere consistent in every spec- glenoid notchin cristatemporalis, or ear imen examined. Tooth characters are size (Fig. 3A); 18. Presence of "frontal those seen in middle wear (e.g., in sup- doming"(see textand Fig. 5H); 19. Rel- posed 5 to 12 year old animals). ative skull breadth(Figs. 5A, B, C, D); 20. Presenceof flattened incisors, lacking SYSTEMATICS OF EQUUS the secondaryinfundibular fold, which Although he did not exclude the pos- also lack the posteriorcommissure (Fig. sibility of a polyphyletic origin for 2E); 21. Presence of opisthoticdolicho- Equus, Stirton(1942) regarded "the pos- cephaly (Fig. 5). These charactersare sibility of any Equus species being de- presented in Table 1 in termsof their rived from Hipparion as ... remote." transformationseries by taxon,and Fig- However, like most other North Ameri- ure 1 is a cladogramshowing character can workers,Stirton conceded,the possi- polarityand hierarchicalposition. bility of an origin for differentparts of Several studies have soughtto docu- Equus out of such genera as Pliohippus; mentthe morphometricintrapopulation- or Astrohippus. Dalquest (1978) perpet- al (Howe, 1966, 1970, 1979; Gazin, 1936) uated this viewpoint, and derived E.

This content downloaded from 196.21.233.72 on Wed, 20 May 2015 19:24:25 UTC All use subject to JSTOR Terms and Conditions 1980 STRIPES DO NOT A ZEBRA MAKE 275

| A D,; 3

E Half infundibulum Fused Infund. Fuil Infund.

| 1 ~~~~~~~~~~~~~~2 / 3

No Posterior Commissure

Linguallexid Molar Isthmus 2 Infund. ,N Inf1und. C Linguaflexid Molar Isthmus 4 5 6

Ectoflex id

Metaconid Metastylid ANTERIOR-,'

3 7-

FIG. 2.-Skull and dental characteristics.A (Char. 3), preorbital facial fossae. Al, pocketed-type naso- maxillary fossa as seen in Cormohipparion. A2, Dinohippus, showing lack of preorbital facial fossa. B (Char. 5), relative size and placement of canines. Bi, Dinohippus, with small, more posteriorlyplaced canines. B2, Equus, with larger, more anteriorlyplaced canines. C (Char. 6), the molar isthmus. Cl, occlusal view of inferiormolar enamel showing lack of molar isthmus. C2, "Intermediate type" molar isthmus (see text). C3, full molar isthmus. C4, very elongate molar isthmus as seen in Neohipparon. D (Char. 2), degree of isolation of protocone and hypocone. Dl, occlusal and posteriorviews of "Nannippus" lenticulare, showing disconnected protocone and deep hypoconal groove (arrow). D2, Dinohippus, natural occlusal, sectioned, and posterior views, showing connected protocone and deep hypoconal groove. D3, Astrohippus ansae, occlusal and posterior views, showing connected protocone and shallow hypoconal groove. E (Chars. 4, 11, 20), inferiorincisors at middle wear. Dl, Dinohippus, with full infundibulum in I1, half-infundibulumin I2, and no infundibulumin I3. E2, E. grevyi,E. kiang, E. onager, E. shoshonensis, E. francisi, E. scotti, or E. conversidens, with fused infundibulum in '2 and half-infundibulumin I3. E3,- E. hemionus or E. calobatus, with fused infundibulumin both 12and 13.E4, E. caballus, with infundibulae in all lower incisors. E5, E. quagga, with full infundibulae in all lower incisors and I3 broadened. E6, E. burchelli, E. occidentalis or E. andium, with compressed lower incisors which lack infundibulae. E7, proposed sequence of development of either full infundibulum as in E. caballus, or flattened incisor without infundibulumas in E. burchelli.

This content downloaded from 196.21.233.72 on Wed, 20 May 2015 19:24:25 UTC All use subject to JSTOR Terms and Conditions 276 SYSTEMATIC ZOOLOGY VOL. 29

A~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ... . . - I 1 1....'.4t..*,.',...... -1 ...... W- ..

EA.M. posterior; ... E A.M.close behind glenoid| turned posterolaterallyl /turned anteriorly Living E.Asinus) / \ E.caballus,E.quaqgg| Dinohippups V Other E.(Equus)

Cranium. l Crista t mporalis . Cranium

Nlvlarrow Ct. Broa0ad CC |

Tm EAM Tm I EAMTXEM

X 3~~~~~~~~~~~~~~~~~~.

l ~~~~PMP| M M

Dinohippus i|E.(Asiu) |E.(Equusl

- ~1- 2 3

FIG. 3.-Primitive and derived skulls in Equus. A (Chars. 12 and 17), dorsal views of posterolateral quadrants of two Equus skulls. Al, primitive condition, with posteriorlyplaced external auditory meatus (E.AM.), narrow crista temporalis (C.T.), and lamhdoid-alcrest formedhy posteriorlyconvergent lines of bone. A2, dRerivedcondition, with anteriorlylocated E.A.M., broad C.T., and lambdoidal crest formed by nearly parallel lines of honle. B (Char. 14), lateral views of mastoid:regions of three horse skulls. Anterior is to the right.BL, primitive condition as in Dinohippus with mastoid (MP), paramastoid.(PMP), temporal (TM) "fan" tightlyfolded, anterior tip of PM far anterior to line. B2, conditilonas in E:. (Asinus), "fan" more open but mastoid:with little or no visible(Equus). contact with C.T. B3, derived condition of E. "fan"7opened greatly,mastoid process has broad visible contact with C.T. Slightly diagrammatic.

This content downloaded from 196.21.233.72 on Wed, 20 May 2015 19:24:25 UTC All use subject to JSTOR Terms and Conditions 1980 STRIPES DO NOT A ZEBRA MAKE 277 grevyiand E. simplicidensfrom Dino- 1-3) presented cladistic hypotheses of hippus, but E. asinus and E. hemionus, systematic relationships in the form of and "the bulk of American Pleistocene several alternativegraphic arrangements. horses" fromAstrohippus. Despite Stir- Each of her cladograms is generally at ton's (1942) suspicion of polyphyletic variance with conclusions presented in origins, he preferred to retain a single this study,with the exception of a sister- genus, Equus, for the living forms,and group relationship between E. asinus thereforealso argued against separate ge- and E. hemionus. However, in her short neric status for obviously similar forms (1979) paper she did not state which liv- such as Plesippus. ing or fossil groups were used for out- Many authors (e.g., Gidley, 1901; Ga- group comparison. The bases forher de- zin, 1936; Schultz, 1936; Stirton, 1940, terminationsof transformationseries and 1942; Hay, 1915; Savage, 1951; Simpson, their polarity are obscure. Furthermore, 1945) cited Pliohippus as the ancestor of since the "characters" Eisenmann used Equus. Their concept of the genus Plio- to construct her cladograms are largely hippus included forms as diverse as P. based on multivariate morphometric fossulatus and "P." interpolatus. analyses, they are difficultto compare Schultz's (1936) assertion that characters with the systematichypothesis here pre- within Pliohippus (as then understood) sented. were "quite consistent" must have been based on only a few comparisons. In their CHARACTER ANALYSIS search for ancestor-descendant relationships, other authors found themselves in SynapomorphiesUniting Equus and the contradictoryposition of citing one Dinohippus.-A survey of the extinct species for one character (such as P. su- equids reveals that possession of a facial premus for "strongly curved upper mo- fossa (Fig. 2A) may be primitive for the lars") and another species for another subfamilyEquinae (Simpson, 1945), and character (such as "P." interpolatus for is certainly primitive for the group of "narrow protoconal isthmus"). Paraphyly equines otherwise most similar to Dino- within Pliohippus caused many such hippus and Equus. The reduction or confusing "diagnoses." Stirton (1942) elimination of the preorbital facial fossa and Savage (1951) criticized non-diag- is therefore a derived feature for this nostic descriptions and suggested many group. The monodactylous condition (see synonymies. Quinn (1957) eliminated McFadden, 1972 and McFadden and much of the confusion about which Skinner, 1979), common to Equus and "Pliohippus" are relevant to Equus' an- Dinohippus, is a derived condition. cestry by naming and diagnosing Dino- Stirton (1942) pointed out that the hippus (formerlyall Dinohippus species "connection of the protocone to protose- had been included in Pliohippus). Based lene is evidently the primitive arrange- primarily on dental similarities, Lance ment seen in all early and middle Ter- (1950) was the firstto postulate Dinohip- tiary horses." Since the equid tooth pus as the ancestor of Equus. Skinner has began in a bunodont state and progressed documented the condition of the facial fromthere to lophodonty, it would seem fossae in many species of fossil equids that all disconnected cusp states are (Skinner and Hibbard, 1972; Skinner and primitive. Indeed, Stirton's conclusion McFadden, 1977; Skinner, pers. comm., applies only to primitiveteeth in middle 1977-1980). Only by this means could and late wear. [In early wear, all equid the reduction in facial fossae in Dinohip- teeth show the protocone separate from pus and Equus be documented as a any other structure.] In early Tertiary shared-derived feature. genera which show protocone connected Among several published analyses of to protoloph, the unworn and separate Equus, only Eisenmann (1979:498, figs. protocone may connect to the protoloph

This content downloaded from 196.21.233.72 on Wed, 20 May 2015 19:24:25 UTC All use subject to JSTOR Terms and Conditions 278 SYSTEMATIC ZOOLOGY VOL. 29

A - Line of Postorbital Ba

FIG. 4.-Condition of postorbitalbars and lambdoidal crest in living and extinctEquus and Dinohippus (Chars. 8 and 15). A, primitive conditions. Solid lines in postorbital region show condition in E. asinus, E. mexicanus, E. kiang, E. conversidens, E. hemionus, E. calobatus, E. onager, E. francisi, and E. scotti. Dashed line in postorbital region shows condition in E. zebra zebra. Solid line in lambdoidal region shows the short,squared lambdoidal crest of E. onager, E. kiang, and their fossil relatives. Dashed line in lambdoidal region shows condition in all other E. (Asinus) and Dinohippus. B, derived conditions. Solid line in postorbital region shows condition in E. zebra hartmannae, E. hatcheri, E. caballus, and E. quagga. Dashed line in postorbital region shows bulbous frontalsof E. burchelli and E. grevyi and their fossil relatives. Solid line in lambdoidal region shows squarest occiput in E. (Equus) observed; dashed line shows least square occiput in E. (Equus) observed.

quickly: it must do so, since the teeth are The hypoconal groove is an analogous brachydont.In species which show hyp- feature of the superior teeth, in that pos- sodont teeth,the connection of protocone session of a deep hypoconal groove is a to protoloph may occur at any point from primitive feature. Yet early species of the tooth roots to very near the tooth Dinohippus possess a shallower hypo- crown, but at some point in all equid conal groove than later Dinohippus or teeth, the protocone connects to the pro- Equus. Thus, I consider possession of a toloph (Fig. 2D). deep hypoconal groove derived for How, then, may a derived condition in Equus and Dinohippus.Astrohippus and this feature be identified? It is the point Pliohippus both possess one or more fa- within the hypsodont tooth at which con- cial fossae and both have lost the hypo- nection between protocone and protoconal groove. Neohipparion lacks the fa- loph occurs which is critical. The primi- cial fossae and possesses the hypoconal tive condition is to have protocone join groove, but shows protocone disconnect- protoloph early in wear; Stirtonwas not ed from protoloph until very late wear. wrong. Documentation of how early this Among these genera, only Dinohippus connection occurs in members of the and Equus are known to have become group in question (establishment of a monodactyl. Thus the distribution of transformation series) will reveal any primitive and derived features in these trends toward "more connection," "

This content downloaded from 196.21.233.72 on Wed, 20 May 2015 19:24:25 UTC All use subject to JSTOR Terms and Conditions 1980 STRIPES DO NOT A ZEBRA MAKE 279

A B- C D

E F G H FIG. 5.-Dorsal and lateral views of representative Equus skulls (Chars. 8, 9, 10, 13, 18, 19, 21). Dorsal views all reduced to same length and presented over 10% lines. A, E, Dinohippus interpolatus, type, UCMP 30200. B, F, E. onager onager, YPM 5097. C, G, E. caballus przewalskii, AMNH 32686. D, H, E. grevyi, AMNH 54247. A, B, C show converging lines of C.T. In A and B this represents the primitive condition; in C it is due to broadened interorbitaldistance. In B the convergence is mediated somewhat by squared lambdoidal crest. E.A.M. visible in A, B, but mastoid process visible instead in C, D. B shows short snout; D shows br6ad, square occiput and narrow skull. Notched C.T. in C, D. E-G show cranial flexion and condition of mastoid area of skull; cranial flexion decreases fromE to H. Primitive mastoid- paramastoid "fan" in E, F; derived condition in G, H. H shows frontaldoming.

This content downloaded from 196.21.233.72 on Wed, 20 May 2015 19:24:25 UTC All use subject to JSTOR Terms and Conditions 280 SYSTEMATIC ZOOLOGY VOL. 29

Some species of Dinohippus possess an fundibulum of Equus will be found. indentation along the rostrum,below and There do not seem to be any autapomor- anterior to the orbit, which is a remnant phies for Dinohippus; pending further of a facial fossa; the absence of a preor- study,the name will be retained. bital facial fossa in living Equus is the Autapomorphies of the Subgenus end result of a trendtoward its loss. Some Equus Linnaeus, 1758.-There are two fossil skulls which can readily be as- fundamentallydifferent skull conforma- signed to Equus on other grounds prob- tions to be found in Equus. The firsttype ably possessed these antorbital grooves representsthe primitivecondition, which, (see Gazin, 1936, plate 27, E. shoshonen- coincidentally, is almost universally sis). Species of Equus are on the average found in the subgenus Asinus (see be- larger (as indicated by skull measure- low), and in the genus Dinohippus. The ments) than those in Dinohippus, but conformationof the skull is a complex of this too is a gradational feature. functionally interrelated features affect- Individuals of Equus examined have, ed by the conformationof the entire an- in every case, a full or at least a fused imal, but especially by the length and infundibulum (Fig. 2E) on '2 in middle characteristicorientation of the animal's wear. This means thatboth a primaryand neck. a secondary infundibularfold are present There are seven major differencesbe- and that the posterior commissure of the tween the primitive skull type and the incisor (if any) disappears before middle derived skull type. The firstconcerns the wear (Fig. 2E). The I2 in Dinohippus dis- breadth of the lambdoidal crest, occiput, plays a posterior commissure in middle and occipital condyles compared to the wear; only in late wear (if ever) does the zygomatic breadth. Primitively,the por- commissure disappear. The 13 in Dino- tion of the skull posteriorto the zygomata hippus shows a strongposterior commis- and postorbital bars is narrow and this sure in all wear stages in most individu- narrowness is generally expressed as pos- als; but a few individuals lose the teriorconvergence of the lines formedby posterior commissure in very late wear. the lateral edges of the squamosal bones These, too, are gradational features; they and their posterior prolongation into the reflecta functionaltrend to increase the lambdoidal crest (Fig. 3A). The foramen amount of enamel present in the incisive magnum is rounded and the distance be- series. This trendalso affectsthe size and tween the occipital condyles is short. placement ofthe canines. In Dinohippus, Primitively,the tube of the externalau- the superior and inferior canines are ditory meatus projects beyond the crista more posterior in their placement within temporalis, a feature whose presence in the P2-I3 diastemata, and they are rela- asses, kiangs and hemiones was noted by tively smaller than they are in Equus Flerov (1937). Primitivelyin Equus, the (Fig. 2B). Differences in the placement crista temporalis is formedas a thin shelf of the bullae of the two genera also exist. which stretchesfrom the posteriorpart of In Dinohippus, the bullae are located far the zygomatic arch to the anteriorpart of back on the basicranium, nearer the oc- the lambdoidal crest. The tube of the ex- cipital condyles than to the glenoids. In ternal auditory meatus simply extends Equus, the bullae are located at least fartherlaterally than this shelf as it rises midway between these two points (Fig. fromits ventromedial attachment in the 3A). I expect that, as the fossil record of petrosal bone. No excavation of the crista late Tertiary(latest Hemphillian and ear- temporalis is necessary to permit this re- liest Blancan) horses becomes more com- lationship because of the narrowness of plete and better known, skulls which in the cranium. Hence, the zygomata and their small size, small facial fossae, and lambdoidal crest are joined by fairly earlier occurrence would be called "Di- straight,posteriorly convergent lines of nohippus" but which possess the 12 in- bone (Fig. 3A). In lateral view the zygo-

This content downloaded from 196.21.233.72 on Wed, 20 May 2015 19:24:25 UTC All use subject to JSTOR Terms and Conditions 1980 STRIPES DO NOT A ZEBRA MAKE 281 matic part of the temporalbone has a temporalisin all E. (Equus); it is smallin characteristicstraight or slightlycurved E. caballus, a littlelarger in E. quagga, outline (Fig. 3A). Primitively,the exter- and quite large and deep in the long- nal auditory meatus and bullae are eared E. (Equus) (Fig. 3A). This arrange- placed midwaybetween the paramastoid ment allows space forthe ear cartilage processand the glenoidprocess (or even and permitsthe animal to hold its ears fartherback, in Dinohippus), and the erectinstead of out to the side, cow-like. tube ofthe meatuspoints to some degree Thus the bony lip extendingfrom the posterodorsally(Fig. 3A). posterior zygomata to the lambdoidal The subgenusEquus (Equus) contains crestin E. (Equus) is nota straightline; thefollowing living and fossilspecies: E. it shows a characteristichooked outline zebra,E. hatcheri,E. quagga, E. caballus in lateralview (Fig. 3A). The tube of the (includingE. c. alaskae), E. burchelli,E. externalauditory meatus points laterally occidentalis,E. andium,E. shoshonensis, or anteriorlyand its terminusis quite E. stenonis,and E. grevyi.More species close to the back of the glenoid,so that maybe added in subsequent'workif they the ear emergesdirectly behind the pos- can be demonstratedto sharethe follow- teriorzygomatic arch (Fig. 3A, and see ing autapomorphies which define E. photographsof livingequids in Groves, (Equus). 1974; Willoughby,1967). The skull of species withinthis sub- Primitively,basicranial-basifacial flex- genus has undergonea markedposterior ion in the genus Equus is considerable, broadening, which is evident in the althoughdifficult to quantifyby measure- greaterbreadth of the lambdoidalcrest, ments,especially on fossilskulls. Osborn occiput,and occipitalcondyles compared generalizescranial flexion (and some oc- to the zygomataand to the ball-likecra- cipitalcharacters, see below) in his "oc- nium.In thederived forms, the lines con- ciput vertexangle" (1912:91) and in his nectingthe posterior zygomata to thecor- paper and summarydiscussion of cyto- ners of the lambdoidal crest do not cephaly (Osborn, 1902, 1912:95). When converge nearly as stronglyas in the the skullis viewed fromthe rearand ori- primitiveskulls (except in E. caballus, entedso thatthe lambdoidalcrest is held where the foreheadhas also undergone just at eye level and the line of observa- broadening-see Fig. 4B). In derived tion parallels the basisphenoid, in the skulls,the paramastoid processes and the primitivecondition the facial axis falls mastoidparts of the temporalbone are away too steeply forthe frontalsto be laterallywide apart and can clearly be seen over the "ball" of the cranium.In seen lateral to the edges of the cristae the derivedcondition, when the skull is temporaliin dorsalview, despitethe fact so orientedand viewed, the frontalsare thatthe line of the cristatemporalis has visible beyondtheir line ofjuncture with also moved away fromthe "ball" of the the line formedby connectingthe post- cranium, thus' forminga much wider orbitalbars. This reflectsthe lesser de- shelfthan in the primitiveskulls. gree of basicranial-basifacialflexion in In the derivedforms which are known these equids (Fig. 4B). In E. burchelli, to have shortears (E. caballus and E. E. grevyiand theirfossil relatives,the quagga), thetube ofthe external auditory domed frontals(Fig. 5H) are especially meatusis not or onlyslightly visible be- visible. Anotherway of observingthis neaththe lip of the cristatemporalis. In flexionis to view the skull laterally;it the derived formswhich are known to will be seen thatthe lambdoidalcrest is have long ears (E. zebra, E. burchelli, apparently stronglydownflexed in E. and E. grevyi),the external auditory mea- (Asinus) skulls.This is notdue to differ- tus is visible throughthe cristatempo- entialmovement of the lambdoidalcrest ralis. This view is permittedbecause an on the cranium,nor to bending of the excavationhas developed in the crista lambdoidal crest itself,but simply re-

This content downloaded from 196.21.233.72 on Wed, 20 May 2015 19:24:25 UTC All use subject to JSTOR Terms and Conditions 282 SYSTEMATIC ZOOLOGY VOL. 29 flects the acuter angle of the basifacial- expressed the difference differently:he basicranial axes. The dashed lines of Fig- noted that a vertical line dropped from ure 5 represent cranial flexion. the occipital crest to the plane of the base The dark trianglesin Figure 5 illustrate of the mandible passes well behind the occipital angle, which is related to but occipital condyles in an ass, while in a not the same as cranial flexion. In all horse, this line passes between or touch- Equus skulls, the mastoid portion of the es the condyles. Osborn (1912) also noted temporal bone, which is located posterior thathorse skulls can be balanced on their to the opening of the external auditory occipital regions but most domestic ass meatus, partially overlies the mastoid skulls cannot. More vertically oriented process. The mastoid process in turnpar- occiputs are observed to occur together tially overlies the paramastoid process. with reduced cranial flexion, and addi- All three bones reach posterodorsallyand tively the two changes produce a large meet the cristatemporalis in a point. This change in the way the skull is oriented arrangement can be thought of as a sort on the neck. of Chinese fan, whose apex is the point Another major change in skull confor- at which these bones meet at the crista mation concerns the appearance of the temporalis, and which can (in an evolu- postorbital bars in posterior view. In the tionary sense) be opened or collapsed. primitive condition, the transverse line In the primitive condition, the fan is formedby the postorbital bars and zygo- rathertightly folded. In Dinohippus, the matic arches stretchesdownward and lat- ventralmostpoint of the mastoid process erally froma point in the median frontal is ventral to the opening of the external area. In the derived condition, the line auditory meatus, and the ventralmost formedby the postorbital bars and zygo- point of the paramastoid process is ante- matic arches remains horizontal as it rior to the meatus opening (Fig. 3B). In moves laterally away fromthe midline of E. (Asinus), the fan has opened a little the skull. Horizontal postorbital bars are wider, so that the paramastoid process is seen in E. zebra,E. hatcheri,E. caballus, on a line with or slightlybehind the ex- and E. quagga (Fig. 4B). ternal auditorymeatus. The mastoid pro- Primitively, Equus skulls are dorso- cess in these formsis wide only ventrally, ventrallyshallow. The distance between and quickly disappears between the mas- the plane of the palate and the plane toid part of the temporal and the para- formedby connecting the facial cristae is mastoid. The mastoid process reaches the especially short in primitive skulls, but crista temporalis only as a very thin line the distance between the plane of the fa- (Fig. 3B). cial cristae and the nasal plane is also In the derived condition, the fan has shorter in the more primitive forms. In opened conside'rably(Fig. 3B). The mas- some E. (Asinus), this distance becomes toid narrows much less quickly in lateral very great (Fig. 5). In the primitive con- view, and the paramastoid process is far dition, Equus skulls are also moderately behind the opening of the external au- broad; the decided narrowingseen in E. ditorymeatus. In opening this fanlike ar- hemionus on the one hand and E. bur- rangement of bones, the angle of the oc- chelli and E. grevyi on the other are par- ciput has been affected in the same allel derived features (Fig. 5). degree: in primitive skulls, the ventral The last major difference between occiput is located more anteriorly,and primitive and derived Equus skulls lies the occipital plane is slanted forwardand in the proportions of the incisor to pre- downward. In derived skulls, the ventral molar diastema, tooth row, and basicra- occiput has moved posteriorly and the nial length. Primitively the proportions occipital plane has become more vertical of diastema to total length of tooth row to (Fig. 5). Osborn (1912) summarized this basicranial length are about 20:50:30 differencebetween horses and asses, but (Fig. 5). Short snouts are present in E.

This content downloaded from 196.21.233.72 on Wed, 20 May 2015 19:24:25 UTC All use subject to JSTOR Terms and Conditions 1980 STRIPES DO NOT A ZEBRA MAKE 283 onager and E. kiang as derived condi- dial plane of protoconidand hypoconid tions.Long, narrowsnouts and long basi- is lengthened,the posteriorcorner of the craniaare presentin E. burchelliand E. preflexidand the anteriorcorner of the grevyias derivedconditions. postflexid are no longer apical but Autapomorphiesof the Subgenus Asi- U-shaped,and the apex of the ectoloph nus Gray,1825.-The subgenus E. (As- is foundlateral to theplane ofprotoconid inus) containsthe followingliving and and hypoconid. fossil species: E. asinus, E. mexicanus, Molar isthmuseshave evolved in pop- E. scotti,E. francisi,E. onager,E. con- ulationsof the genus Equus in the same versidens,E. kiang,E. calobatus, and E. way that infundibulae in the incisors hemionus.More species maybe added in have developed: by gradual enhance- subsequent workif theycan be demon- ment,from the base of the toothto the stratedto sharethe synapomorphiesdis- crown,and fromone toothin a series to cussed below. the rest.When a molaristhmus first ap- While the skullsof species in the sub- pears in Equus (and in otherfossil horse genus Equus are generallymore derived genera,e.g., Neohipparion)it appears at thanthose seen in the subgenusAsinus, the base ofthe tooth on whichit appears, some derived featuresappear in all E. and is thereforenot evident until late (Asinus), and it is these features,rather wear. The firsttooth on which this fea- than the coincidentalcranial symplesio- tureappears in Equus is the M3; it then morphiesof E. (Asinus),which argue for appears on M2 and M1 in formswhich the monophylyof this taxon. have the featurein morethan one tooth. Primitively,the lowermolars of Equus Isthmusescan be foundon the M3 in at do notdisplay isthmnuses (Fig. 2C). "Mo- least middle wear in all E. (Asinus) and lar isthmuses"are formedwhen the lin- in E. (Equus) quagga, E. (E.) caballus, gual apices of the centralvalleys of the some E. (E.) zebra hartmannae,and E. ectolophidsmove away fromtheir prim- (E.) hatcheri.E. hatcheriand some E. itivecontiguity with the metaconid-meta- caballus, E. andium,E. zebra hartman- stylidcolumns toward the buccal borders nae, (and E. quagga?) occasionallyshow ofthe teeth.The degree ofbuccal move- an "intermediateisthmus" on M2 or M, mentvaries with species and wear stage, (Fig. 2C) whichmay begin to be evident but teethin whichthe apices of the ec- as earlyas middle wear.There is a trend tolophidsare never foundmore lingual in E. (Equus) to increase the amountof thanthe buccal lines of protoconid and hy- inferiormolar enamel by acquiringthe poconidare nothere considered to possess isthmusstructure. But the synapomorphy "molaristhmuses" (see Skinnerand Hib- thatunites members of E. (Asinus)in one bard,1972). An intermediatecondition is subgenusis thatstrong isthmuses can be sometimesfound, in which the ectolo- foundon all lowermolars in all stagesof phid apices lingerclose to the posterior wear. cornerof the preflexid from about middle SynapomorphiesDefining Taxa within wear onward (actually,this "intermedi- the Subgenus Equus Linnaeus, 1758.- ate" conditionis a stagein whichthe dis- Characteristicof E. zebra (and its North tance between the labial reach of meta- Americanfossil relative,E. hatcheri)is conid-metastylidand the medial plane of a raisingof the anterodorsalquadrant of protoconid and hypoconid is being the orbitover the plane of the moreme- lengthened.Since the primitiveposition dial part of the frontals,so thatthe ani- of the apex of the ectolophis at the pos- mals appear to have a dished forehead terolabialcorner of the metaconid,in the and a bump over each eye. This bump "intermediate"condition, the apex ofthe may be elaboratedin some individuals ectolophstill has not moved). In the de- intoa bony boss. Some E. zebra and E. rived condition,the distance between hatcheripossess intermediate-typemolar metaconid-metastylidcolumns and me- isthmuses(Fig. 2C).

This content downloaded from 196.21.233.72 on Wed, 20 May 2015 19:24:25 UTC All use subject to JSTOR Terms and Conditions 284 SYSTEMATIC ZOOLOGY VOL. 29

Equus caballus and E. quagga possess internal,secondary fold, and then closed an M3 isthmus in middle wear. Horizon- the commissure,these formshave simply tal postorbitalbars are also characteristic. closed the commissure withoutacquiring These are the only equids known to have the secondary enamel fold.) Equus grev- very short ears (a condition reflected in yi seems to have retained a primitive theirpetrosal and temporal structure;see skeletal conformation(see Skinner and above). E. quagga displays the derived Hibbard, 1972). Living E. grevyi resem- feature of broadened I3-I3 distances, and ble E. (Asinus) in having a minimally also displays extraordinarilybroad inci- haired tail and large ears, but these may sors, especially the I3. An infundibulum represent parallel desert adaptations was present in middle wear in its I3. (Bennett, MS). Equus grevyi is the least Equus caballus shows the derived fea- hairy of the living Equidae. ture of broadened forehead,and also pos- Synapomorphiesfor Taxa within E. sesses the I3 infundibulum.Quaggas and (Asinus).-The only instance within the horses have achieved the greatest degree genus Equus where skeletal conforma- of incisive area in all of Equus. It is note- tion seems systematicallyimportant is in worthythat E. caballus is the only mem- the clade composed of E. kiang, E. ona- ber of the genus Equus which is almost ger and E. hemionus and their fossil rel- unstriped, which has dark legs and ven- atives. There is a tendency here to elon- ter, and has a fully haired tail. Quaggas gate the metapodials (and other distal and horses are the least striped of all E. appendicular elements). Equus kiang and (Equus), and quagga tails seem to have E. conversidens display deep rostra, been almost fully haired (see photo- a derived feature which distinguishes graphs in Willoughby, 1967 and Lydek- them. Equus hemionus and E. calo- ker, 1916). batus have developed an infundibulum Equus burchelli,E. grevyi,E. andium, on the I3 which parallels the condition E. stenonis,E. shoshonensis,and E. oc- found in E. caballus. Whereas the sec- cidentalis all display frontal inflation, ondary infundibular fold of this tooth narrow skulls with long, narrow snouts, in E. caballus is at middle wear separate and dolichocephaly. Osborn (1912) dis- fromthe primaryfold, in E. hemionus at tinguished between proopic dolicho- middle wear the secondary fold is still cephaly and opisthopic dolichocephaly. contiguous posteriorly with the primary While all Equus are pro6pically doli- fold (Fig. 2E). The skull of E. hemionus chocephalic compared to anchitherines and E. calobatus is distinguished by its (Gregory, 1945), E. grevyi, E. bur- small, light cranium (Skinner and Hib- chelli, and their fossil relatives are es- bard, 1972) and its overall narrowness pecially so. Equus grevyi and E. (Fig. 5). Equus onager and E. francisi shoshonensis display both formsof doli- are distinguished by markedly short- chocephaly, while E. burchelli, E. sten- ened snouts and by short, square lamb- onis, E. andium, and E. occidentalis doidal areas. Some fossil onagers were show less relative cranial length. E. bur- very large (E. scotti), while E. on- chelli and its fossil relatives have very ager hemippus is quite small, smaller broad, square, vertically oriented occi- than some Dinohippus. Changes in skel- puts (the extreme condition is seen in E. etal conformation associated with in- andium), and they all lack even the be- creased size in fossil equids are very like- ginnings of a secondary infundibularfold ly to mask phylogenetically useful and posterior commissure. (This is not a characteristics, since the limb bones character reversal; the primitive infun- must alter their proportions in order to dibular condition contains only a primary bear the increased weight (see Willough- fold, which is open posteriorly at the by, 1967). Thus, E. scotti either shows commissure [Fig. 2E]. While otherequids parallel adaptations in the skull, or is have elaborated the primaryfold into an closely related to E. onager, having

This content downloaded from 196.21.233.72 on Wed, 20 May 2015 19:24:25 UTC All use subject to JSTOR Terms and Conditions 1980 STRIPES DO NOT A ZEBRA MAKE 285

AFRICA| EUASIA E.asinus Eqguagga E.zebra E.burchelli w E.grevyi z E o LU nus kiang / o-J 0

ElconversidensUX rJ AMERICA Eab allus w zm El |~~~~~~~~~~~~~EcalballuonagemCa Eshsonsi7 / Z w Effrancisi E occienftais E. hatcheri < 0 ZO0 Z w CL_ E.calobatus E.scotti Eandium E.shoshonensis

S. AMERICA ,stenonis z E. mex~~~~~~~~inohpus' z ~~~~~~~~~NORTH AMERICA -j Dinohippus'A" (E.asinus-like) Dinohipp "E" (E.zebra-like)

w r ~~~~~~~~~~~Dinohipp~

FIG. 6.-Zoogeographic phylogramcontaining part of the informationcontained in Figure 1. failed to develop or having reversed the ed. A zoogeographic phylogram (Fig. 6) trend to narrow metapodials. is presented whose systematic informa- Equus asinus and E. mexicanus may tion content is the same as that in Figure be distinguished by a rather long crani- 1 (note that the terminal branches of the um, paralleling the condition seen in E. cladogram all consist of unresolved poly- grevyi and its relatives. They are, outside chotomies; in the phylogram, ancestor- of the possession of full molar isthmuses, descendant relationships,as one possible the most primitive of living Equus. resolution, are presented). The zoogeographic phylogram shows that the North DISCUSSION American fossil record contains either To posit a particular sister-species re- closest common ancestorsor sister-species latio-nshipis to imply thatthe two species of every living species of Equus, with the named shared a closest common ances- exception of E. quagga. The cladistic tor. For this reason, every cladistic hy- analysis also implies thatat least E. grev- pothesis entails zoogeographic implica- yi, E. zebra, E. caballus, E. burchelli, E. tions about all organisms studied whose asinus, E. onager, E. kiang, and E. hem- fossil or recent distribution is known. ionus should be found in the Pleisto- Early consideration of the- stratigraphic cene or subrecent fossil record of Eur- position of fossil formscan bias a cladistic asia. Equus stenonis is believed to analysis which initially should be based represent the Eurasian ancestor of E. wholly on morphological information. grevyi. Strong zoogeographic ties be- However, once a cladogram has been tween North America and Africa are in- constructed, the stratigraphicas well as dicated by E. (Equus). If various mem- spatial occurrence of fossil forms be- bers of E. (Equus) which are now comes a matterof importance. endemic to Africamigrated throughEur- The North American fossil record of asia to Africa,they now represent relict Equus is excellent and well-document- populations preserved in the once much

This content downloaded from 196.21.233.72 on Wed, 20 May 2015 19:24:25 UTC All use subject to JSTOR Terms and Conditions 286 SYSTEMATIC ZOOLOGY VOL. 29 more widespread savanna environment "for use as needed"; it has been a guidepost forall of Africa. my work on fossil and living equids. I extend my thanks also to R. H. Tedford, E. 0. Wiley, H.-P. Several argumentscan be made against Schultze, R. S. Hoffmann,N. A. Neff,R. W. Wilson, such an apparently complex zoogeogra- and L. D. Martin forhelpful discussion and critical phy. First,some have argued thatE. sten- comments on the manuscript. Specimens used in onis of the early Villafranchianof Europe this study were loaned by S. Anderson, American all Museum of Natural History, Division of Mammals; gave rise to the later Equus. The de- R. Estes, Academy of Natural Sciences of Philadel- rivedness of its cranium makes it evident phia, Division of Mammals; R. Thorington,United that this would require the secondary States National Museum, Division of Mammals; loss of frontalinflation, the loss of "open" and by R. S. Hoffmann,University of Kansas Mu- mastoid-paramastoid complex, and the seum of Natural History, Division of Mammals. This study was supported by the Theodore Roose- loss of proopic dolichocephaly. Others velt Fund of the American Museum of Natural His- have argued that a form of Dinohippus tory, the Visiting Scholar Program at the United early migrated to Eurasia, to give rise to States National Museum, and the Graduate Teach- all or part (Dalquest, 1978) of the living ing Fund of the University of Kansas Department Equus, while its North American rela- of Systematics and Ecology. tives were becoming extinct. No Dino- REFERENCES in hippus have been found the European ALLEN, J. A. 1909. Mammals fromBritish East Af- fossil record. The literaturealso contains rica, collected by the Tjader Expedition of 1906. many other speculations about the origin Bull. Amer. Mus. Nat. Hist., 26:147. of Equus, but these constitute sugges- BENNETT, D. K. (MS). Stripes do not a zebra make, tions made before documentation of the part II: A taxonomic revision of Equus. DALQUEST, W. W. 1978. Phylogeny of American importance of the facial fossae. horses of Blancan and Pleistocene age. Ann. Zool. Finally, it can be argued that the sys- Fennici, 15:191-199. tematic relationships, based on charac- EISENMANN, V. 1975. Nouvelles interpretations ters in Figure 1, are in error: either in des restes d'equides (Mammalia: Perissodactyla) their distributionor in their assumptions de Nihowan (Pleistocene inferieur de la Chine du Nord): Equus teilhardi nov. sp. Geobios, of derivedness. Eisenmann's (1979) 2(8): 125-134. cladograms imply far fewer migrations EISENMANN, V. 1976.. Le protostylide: valeur sys- than here proposed. Assessment of the tematique et significationphyletique chez les es- most parsimonious cladogram, both from peces actuelles, et fossiles du genre Equus (Pe- rissodactyla, Mammalia). Sonderdruck aus Z. f. a morphological and froma zoogeograph- Saugetierkunde, 41(6):349-365. ical standpoint, requires: 1) documenta- EISENMANN, V. 1979. Characteres evolutifset phy- tion of the limits of variation for charac- logenie du genre Equus (Mammalia, Perissodac- ters used in this paper; 2) documentation tyla). C. R. Acad. Sc. Paris, T. 288, Serie D., Pa- of the transformationstates and leontologie, pp. 497-500. polarity EISENMANN, V., AND C. DE GIULI. 1974a. Charac- of all charactersused by Eisenmann (and teres distinctifsentre vrais zebres (Equus zebra) others), and assessment of the utility of et zebres de Chapman (Equus burchelli antiquo- multivariate"characters" and of ratios as rum) d'apres l1'tude de 60 tetes osseuses. Mam- "characters"; 3) thorough review of the malia, 38(3):509-543. EISENMANN, V., AND C. DE GIULI. 1974b. Char- Eurasian and African fossil record of acteres distinctifsdes premieres phalanges anter- Equus in light of the hypothesis pre- ieures et posterieures chez sertains equides ac- sented in this paper. tuels et fossiles. Bull. de la Soc. Geologique de France, 14(7e serie):352-361. EISENMANN, V., AND J. C. TURLOT. 1978. Sur la ACKNOWLEDGMENTS taxonomie du genre Equus. Les Cahiers de I gratefullyacknowledge the generous instruc- l'Analyse des Donees V., 3(2):179-201. tion I have received on the subject of fossil and FLEROV, K. K. 1931. Quelques donnees sur la crani- living horses from Morris F. Skinner, Frick Asso- ologie de la fam. Equidae. C. R. Acad. Sci. USSR, ciate Curator Emeritus at the American Museum of Moscow, pp. 269-272 (in Russian). Natural History. My copy of Dr. Skinner's manu- GAZIN, C. L. 1936. A study of the fossil horse re- scriptof May, 1974, entitled "Tentative Tribal Sub- mains from the upper Pliocene of Idaho. Proc. divisions of the Oligocene to Holocene Equidae U.S. Nat. Mus., 83(2895):281-325. (Subfam. Anchitherinaeand Equinae)," is inscribed GIDLEY, J. W. 1901. Tooth characters and revision

This content downloaded from 196.21.233.72 on Wed, 20 May 2015 19:24:25 UTC All use subject to JSTOR Terms and Conditions 1980 STRJPES DO NOT A ZEBRA MAKE 287

of the North American species of the genus OSBORN, H. F. 1902. Dolichocephalyand brachy- Equus. Bull. Amer.Mus. Nat. Hist., 14 (9):99- cephalyin the lowermammals, Bull. Amer.Mus. 141. Nat. Hist.,16 (7):77-89. GRAY,J. E. 1825. A revisionof the family Equidae. OSBORN, H. F. 1912. Craniometryof the Equidae. Zool. Jour.,5(1):241-556. Mem. Amer.Mus. Nat. Hist.,(new ser.),1(3):57- GROVES,C. P. 1974. Horses,asses, and zebras in 100. thewild. Ralph Curtis Books, Hollywood, Florida, QUINN, J. 1957. PleistoceneEquidae ofTexas. Bur. 192 pp. Econ. Geol. Univ.Texas, Rep., 33:1-51. HAY, 0. P. 1915. Contributionto the knowledge SAVAGE, D. E. 1951. Late Cenozoic vertebratesof ofthe mammals of the Pleistocene of North Amer- the San FranciscoBay Region. Univ. California ica. Proc. U.S. Nat. Mus.,48(2086):515-575. Publ. Bull. Dept. Geol. Sci., 28(10):215-314. HENNIG, W. 1966. Phylogeneticsystematics. Univ. SCHULTZ, J. R. 1936. Plesippus francescana IllinoisPress, Urbana, 274 pp. (Frick)from the late Pliocene, Coso Mountains, HOWE, J. A. 1966. Observationof changesin the California.Carnegie Inst. Washington,Publ., conformationand enamel patternof horse'teeth 473:1-13. due to wear. Compassof SGE, 44(1):10-18. SIMPSON, G. G. 1945. The principlesof classifica- HOWE, J.A. 1970. The rangeof variation in Equus tionand a classificationof mammals. Bull. Amer. (Plesippus) simplicidensCope fromBroadwater Mus. Nat. Hist.,85:1-350. Quarriesof Nebraska.J. Paleo., 44(5):958-968. SKINNER, M. F., AND C. W. HIBBARD. 1972. Early HOWE, J.A. 1979. PleistoceneEquidae fromSher- Pleistocenepreglacial and glacial rocksand fau- idan County,Nebraska. J. Paleo., 53(5):1228- nas of north-centralNebraska. Bull. Amer.Mus. 1236. Nat. Hist.,148(1):117-125. LANCE, J.F. 1950. Paleontologiay estratigrafiadel SKINNER, M. F., AND B. J. MCFADDEN. 1977. Cor- Plioceno de Yepomera,Estado de Chihuahuala mohipparion,n. gen. (Mammalia,Equidae) from Mexico.Inst. Geol. Bol., 54:1-81. the NorthAmerican Miocene (Barstovian-Clar- LYDEKKER, R. 1916. Catalogue of the ungulate endonian).J. Paleo., 51(5):912-926. mammalsin the BritishMuseum (Nat. Hist.). STIRTON, R. A. 1940. Phylogenyof North American 5:916-1107. Equidae. Univ.California Publ. Bull. Dept. Geol. McFADDEN, B. J. 1972. Cladisticanalysis of prim- Sci., 25:165-198. itive equids, withnotes on otherperissodactyls. STIRTON, R. A. 1942. Commentson the originand Syst.Zool., 25(1):1-14. genericstatus of Equus. J.Paleo., 16(5):627-637. McFADDEN, B. J.,AND M. F. SKINNER. 1979. Di- WILLOUGHBY, D. P. 1967. The empireof Equus. versificationand biogeographyof the one-toed A. S. Barnesand Co., New York,pp. 1-475. horses Onohippidiumand Hippidion. Postilla Yale Univ.,175:1-10. Manuscriptreceived September 1979 McGREw,P. 0. 1944. An earlyPleistocene (Blan- RevisedMay 1980 can) faunafrom Nebraska. Geol. Ser. Field Mus. Nat. Hist.,9(2):33-66.

This content downloaded from 196.21.233.72 on Wed, 20 May 2015 19:24:25 UTC All use subject to JSTOR Terms and Conditions