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Comparison of dental characters of fossil horses in two Pleistocene local faunas.

ABSTRACT. -- Lower molars of Pleistocene horses from sites in Aguascalientes, Mexico (Cedazo), and Texas (Slaton) were compared. Several combinations of dental characteristics were noted. A deep ectoflexid and V-shaped linguaflexid separates the zebras from other horses in the two collections studied. The pattern of shallow ectoflexid and U-shaped linguaflexid seems to be consistent in the caballine group. However, whereas the stilt-legged horses apparently are characterized by shallow ectoflexid and V-shaped linguaflexid, the character combination apparently is not restricted to that group. No zebras were recovered at Slaton, and zebras were rarely found at Cedazo. Other than one Equus calobatus, only caballine horses were found at Slaton. Besides zebras, stilt-legged horses and caballines occurred at Cedazo, and also an undetermined species with shallow ectoflexids and V-shaped linguaflexids. Key words: Pleistocene local faunas; fossil horses; Texas; Mexico.

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Pleistocene horses ranged through Europe, Asia, Africa, North America, and part of South America. Their supraspecific relationships remain speculative, though recent works have clarified the position of some species groups. Winans (1989) found, on the basis of skulls, mandibles, and limb bone proportions, that North American horses fall into five species groups. European writers (Eisenmann, 1978, 1985; Azzaroli, 1982, 1984; Forsten, 1986) have recognized somewhat similar groupings of Old World Pleistocene horses. Recognition of species within these groups is difficult, for the extent of individual, chronological, geographic, and sexual variation within most species is unknown. Most of the fossils available consist of cranial and mandibular fragments, isolated teeth, and limb bones. Some dental characters, however, appear to be widely applicable and diagnostic of species groups, even allowing for the variation in morphological features prominent in Pleistocene horses. This is especially true of characters of the lower molar teeth.

Many European paleontologists (cited above) accept the shape of the linguaflexid of the lower molar as a major character separating caballine (domestic horse type) from the zebrine (stenonine) horses. They usually consider the nature of the ectoflexid to be a less important character. Some North American students (Skinner, 1972, and Dalquest, 1988, for example) have found the ectoflexid to be of major value and the shape of the linguaflexid to be of less value. Proportions of metapodials are especially important in identifying the stilt-legged horse group, but the extremely long, slender limb bones found in some North American fossil horses apparently do not occur in the Old World.

Value of the linguaflexid versus the ectoflexid, in combination, was determined in two local faunas, one early Pleistocene (Irvingtonian Land Mammal Age), the Slaton Quarry, Lubbock Co., Texas (Dalquest, 1967), and the other late Pleistocene (Rancholabrean Land Mammal Age), the Cedazo local fauna, Aguascalientes, Mexico (Mooser and Dalquest, 1975). Collections from these local faunas are large enough for comparison and species appear to be relatively diverse.

From the Slaton Quarry, 18 lower jaws with m1-m2, or sets of teeth found in direct association during collecting, are available. Seven of the jaws are of senile individuals, with teeth too worn for meaningful study. From Cedazo there are 95 lower jaws or sets of associated lower teeth, of which only 46 are young enough for use. There are five complete metatarsals from Slaton and 18 from Cedazo.

Linguaflexids were judged to be V-shaped (rare specimens with a trapezoidal pattern are included here) or U-shaped. The molar isthmus was considered deep (Fig. 1A) if it penetrated a line connecting the floors of the preflexid and postflexid (Dalquest, 1988). One jaw from Cedazo was rejected as aberrant because it had the linguaflexid of m1 U-shaped and that of m2 V-shaped. A few specimens had the ectoflexid extending almost to the line connecting preflexid to postflexid, and if the tip of the fold was broad it was listed as deep; if slender, it was listed as shallow.

More metatarsals than metacarpals were present in both collections; only metatarsals were studied. Total length and proximal, midshaft, and distal breadths were measured. If the midshaft breadth divided by the total length times 100 exceeded 14.5, the bone was considered stilt-legged. However, metatarsals of the stilt-legged horses are so long and slender as to be immediately obvious to the eye.

Results of dental analysis are seen in Table 1. None of the jaws from Slaton shows the zebrine condition but six jaws from Cedazo (13 percent) belong to zebras (Fig. 1A). None of the jaws from either site show the reverse of the zebrine pattern (deep ectoflexid and U-shaped linguaflexid). This suggests that the zebrine pattern is not a variant of another pattern or one would expect the reverse condition to occur in numbers equal to the zebrine pattern. The metatarsals of zebras probably cannot be distinguished from those of other than the stilt-legged horses. The six zebra jaws are all of medium size and probably belong to Equus parastylidens Mooser.

[FIGURE 1 OMITTED]

Three jaws (27 percent) from Slaton have the pattern found in the lower molars of the few specimens in which the limb bones and lower jaws of stilt-legged horses are certainly associated (see Lundelius and Stevens, 1970). One moderately large metatarsal from Slaton is of a stilt-legged horse. The three lower jaws are all of moderate size, and jaws and metatarsal probably belong to Equus calobatus Troxell.

Skinner (1972) thought the North American stilt-legged horses belonged in the subgenus Hemionus, along with the Asiatic kiang, living today in Mongolia. However, the American stilt-legged horses seem to be typified by lower molars with shallow ectoflexids and V-shaped linguaflexids. Skinner's figure of Equus hemionus Pallas shows shallow ectoflexids and U-shaped linguaflexids. His figures of E. calobatus Troxell show shallow ectoflexids and V-shaped linguaflexids. Nominal association of the stilt-legged horses and the hemiones continued until Winans (1989) showed that E. hemionus does not have the elongated metapodials like those of the American Pleistocene species. The American stilt-legged horses appear not to have close relatives in the Old World.

Three metatarsals from Cedazo (17 percent) are of stilt-legged horses. One is large, and probably represents Equus calobatus, one is small and probably belongs to E. tau Owen, and one is of intermediate size and cannot be assigned to a species.

The lower molar combination of shallow ectoflexid and V-shaped linguaflexid (Fig. 1B) is the common pattern at Cedazo (28 jaws, 61 percent). Only nine lower jaws have p2-m3. One is small (length 144 mm), but the other eight range from 149 to 165 mm. The largest probably belong to Equus calobatus, the smallest to E. tau. The collection may include some jaws of other stilt-legged horses. However, the number of jaws with shallow ectoflexids and V-shaped linguaflexids so outnumbers the number to be expected based on metatarsals that one or more species of horses that were not stilt-legged, but that had the "stilt-legged" dentition pattern, must be included in the Cedazo collection.

At Slaton eight (73 percent) of the jaws show the caballine horse pattern (shallow ectoflexid, U-shaped linguaflexid) (Fig. 1C) versus 12 (26 percent) at Cedazo. "Caballine" suggests resemblance to the domestic horse, Equus caballus, and this is unfortunate when applied to the present dental characters. Domestic horses may have virtually any combinations of dental characters. However, the combination of shallow ectoflexid and U-shaped linguaflexid has been termed caballine and the term is used here.

At the Slaton Quarry all horses except the stilt-legged Equus calobatus seem to be caballine. One large jaw may belong to E. scotti Cope and the remainder, small to moderate in size, to E. conversidens Owen and E. excelsus Leidy. The Slaton metatarsals, other than the stilt-legged E. calobatus fossil, must all belong to caballines, and range from small to moderate in size.

The Cedazo jaws with the caballine combination include large to small specimens. The presence of the undetermined species with the stilt-legged molar character combination makes the metatarsals useless for separation of species in the Cedazo collection and caballines, outnumbered by noncaballines, are only moderately common at Cedazo.
TABLE 1. Combinations of ectoflexid depth and linguaflexid shape in
lower molars of horses in two Pleistocene local faunas.

Number U, shallow U, deep V, shallow V, deep

Slaton Quarry
11 8 0 3 0

Cedazo
46 12 0 28 6


ACKNOWLEDGMENTS

We are indebted to R. S. Pfau for making the illustrations and Frederick B. Stangl, Jr., for aid in preparation of the figures and review of the manuscript. Appreciation is expressed to Jane Lindsey for technical assistance. We thank M. C. Winans and E. L. Lundelius for use of the collection housed in the Balcones Research Center at the University of Texas.

LITERATURE CITED

Azarroli, A. 1982. On Villafranchian Palaearctic equids and their allies. Palaeontographica Italica, 72(n.s. 42): 74-97.

_____. 1984. On some vertebrate remains of middle Pleistocene age from the upper Valdarno and Vad do Chana, Tuscany. Palaeontographica Italica, 73(n.s. 43): 104-115.

Dalquest, W. W. 1967. Mammals of the Pleistocene Slaton Quarry local fauna of Texas. Southwestern Nat., 12:1-30.

_____. 1988. Astrohippus and the origin of Blancan and Pleistocene horses. Occas. Papers. Mus., Texas Tech Univ., 116:1-23.

Eisenmann, V. 1978. Sur la taxinomie de genre Equus [Equides]. Les Cahiers de l'Analyses des Donnees, 3:179-201.

_____. 1985. Etude des cornets des dents incisives des Equus [Mammalia, Perissodactyla] actuels et fossiles. Palaeovertebrata, 10:127-226.

Forsten, A. 1986. Equus lambei Hay, the yukon wild horse. J. Mamm., 67:422-423.

Lundelius, E. L., and M. S. Stevens. 1970. Equus francisci Hay, a small stilt-legged horse, middle Pleistocene of Texas. J. Paleo., 44:248-253.

Mooser, O., and W. W. Dalquest. 1975. Pleistocene mammals from Aguascalientes, central Mexico. J. Mamm., 56:781-820.

Skinner, M. F. 1972. Order Perissodacatyla. Pp. 117-130, in Early Pleistocene pre-glacial and glacial rocks and faunas of north-central Nebraska (M. F. Skinner, C. W. Hibbard, E. D. Gutentag, G. R. Smith, J. G. Lundberg, J. A. Holman, J. A. Feduccia, and P. V. Rich) Bull. Amer. Mus. Nat. Hist., 148:1-148.

Winans, M. C. 1989. A quantitative study of North American fossil species of the genus Equus. Pp. 262-297, in The evolution of the Perissodactyla (D. R. Prothro and R. M. Schoch, eds.), Oxford Monogr., Geol. Geophys., 15:1-537.

TRACY S. SCHAFER AND WALTER W. DALQUEST

Department of Biology, Midwestern State University, Wichita Falls, Texas 76308
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Author:Schafer, Tracy S.; Dalquest, Walter W.
Publication:The Texas Journal of Science
Geographic Code:1MEX
Date:Feb 1, 1991
Words:1690
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