Equus sp.—Horses // Equus sp. (small)—Small Horse // Equus sp (large)—Large Horse // Equus calobatus—Stilt-legged Onager // Equus conversidens—Mexican Horse // Equus cumminsii—Cummings's Ass // Equus francisci—Stilt-legged Onager // Equus fraternus—Fraternal Horse // Equus occidentalis—Western Horse // Equus scotti—Scott's Horse // Equus simplicidens—Western Horse // Equus Species A—Unidentified Equus
Horse nomenclature is a mess and has been for many years. The situation in the Southwest was discussed at some length by Harris and Porter (1980). A later study by Winans (a 1985 dissertation and the resultant, somewhat altered 1989 publication) either clouded the issue or clarified it, depending on viewpoint.
Winans (1989) recognized one group in the late Rancholabrean of North America possessing elongated metapodials (stilt-legged horses: Equus francisci group) and two groups of stout-legged horses: large horses which she assigned to the Equus laurentius group and a small horse assigned to the E. alaskae group. She did note (p. 295) that "It is possible that some of the groups which I have defined encompass more than one species, . . ." Harris and Porter (1980) recognized three species at Dry Cave: E. scotti (later re-identified as E. occidentalis by Harris ), E. niobrarensis, and E. conversidens. Winans (1989) recognized none of these latter three names, assigning horses generally assigned to E. occidentalis to the E. laurentius group, E. niobrarensis as a synonym of the E. scotti group, and E. conversidens as a synonym of her E. alaskae group. The names used herein are used for convenience, having been widely used for Southwestern species; no attempt at solving nomenclatural issues is implied. For further comments on the validity of E. conversidens, see Scott (2004).
Based on tooth enamel patterns, Barrón-Ortiz and Theodor (2011:148) found three morphological groups of stout-legged Equus in the late Pleistocene: ". . . a medium stout-legged equid (specimens previously identified as E. conversidens, E. fraternus, and E. lambei), a large stout-legged horse (specimens previously identified as E. complicatus, E. niobrarensis, and E. mexicanus) and E. occidentalis." In contrast, Weinstock et al. (2005) suggested that based on molecular data, a single species of caballine (=stout-legged) horses and a single species of stilt-legged horses may have have made up the entire North American late Pleistocene equine fauna. It should be noted, however, that their sample is highly biased toward northern North America with only one specimen (from Gypsum Cave, Nevada) from south of Wyoming.
The conclusions of Weinstock et al. (2005) do not hold up with the morphological data from the Southwest. Weinstock et al. imply that the morphological differences seen within North American stout-legged horses are adaptations of a single species to different environments. However, occurrences of two to three "morphological species" in what basically is geographic and chronologic sympatry (as at Dry Cave) renders this unlikely since interbreeding would quickly break down the morphological differences. On the other hand, it is easy to visualize different species morphologically adapted to different environments occasionally overlapping in space and time.
In current attempts to characterize the equine fauna of Dry Cave, measurements of horses from McKittrick and Rancho La Brea were taken as representing E. occidentalis and those from San Josecito Cave as representing E. conversidens. Seemingly no suite of measurements suitable for multivariate analyses for Dry Cave horses intermediate in size were available to me except those from Dry Cave (I consider the summary measurements of Winans'  Rancholabrean "big horses" to include both E. occidentalis and the Dry Cave intermediate-size horses).
Separation of the Dry Cave horses (plus a few from other sites in New Mexico) into multivariate categories equivalent to the samples of E. occidentalis and E. conversidens left an intermediate category hypothesized as a third species, that recognized as E. niobrarensis by Harris and Porter (1980). This is shown especially well in a cluster dendrogram (Gower paired-group) of second phalanges (Fig. 1).
Fig. 1. Dendrogram showing clustering of second phalanges, interpreted as representing E. conversidens (red), an intermediate-sized horse (blue-green), and E. occidentalis (magenta). Specimens with labels starting with 192/ are from San Josecito Cave; those starting with CIT are from McKittrick; the remainder are from UTEP specimens. All measurements are by myself. Based on measurements 1-6 of Harris and Porter (1980).
Utilizing a variety of univariate measurements, the specimens used as representing E. occidentalis and E. conversidens agree well with Winans' (1985) summary univariate statistics for Rancho La Brea and San Josecito, respectively, indicating correct assignments. The Southwestern specimens hypothesized by Harris and Porter (1980) as E. niobrarensis, which would be placed in the "big horses" category by Winans (1985), differ in various measurements (e.g., proximal metacarpal width) from Winans' Rancho La Brea sample (of Winans' "big horses" category) by p = <0.001; that sample also differs from Winans' San Josecito sample ("little horses") by p = <0.001. The overall result of both univariate and multivariate analyses is division into three groups best considered as separate species. It thus appears that the late Rancholabrean large, stout-legged horses of Winans include at least three species.
This confirmation that the Dry Cave fauna included three stout-legged species led to looking more closely at the horses identified by Harris and Porter (1980) as E. niobrarensis. Utilizing the data in Winans (1985), it became apparent that the Dry Cave measurements fit closely with those of E. scotti, though Winans (1989) had listed its chronologic range as late Blancan and early Irvingtonian. This fit between E. scotti and various samples of late Rancholabrean horses has been noted by others (e.g., Lundelius 1984). In addition to limb measurements, late Rancholabrean horses that approximate E. scotti in size approach the so-called zebrine condition of the lower molars, wherein in the first two molars the ectoflexid tightly approaches or inserts itself into the isthmus connecting the metaconid-metastylid unit to the rest of the tooth (see Fig. 2, E. scotti account). To a degree, this is the case, for example, in the Dry Cave and the Cueva Quebrada samples; other species of North American, late Rancholabrean horses lack this character (Winans 1985). Presence of an infundibulum in the lower incisors further separates this taxon from E. conversidens and E. occidentalis. On the basis the characters seen, I now recognize E. scotti rather than E. niobrarensis as present in the Southwest and I accept E. niobrarensis as a synonym of E. scotti. However, since this decision has not reached general acceptance, identifications have been changed only for specimens examined by myself; separate lists are given in the E. scotti account for specimens seen by myself and judged to represent E. scotti and for those listed in the literature as E. niobrarensis and not examined by me.
Fig. 2. Principal components plot of Equus metacarpal measurements. "All large Dry Cave", "All E. scotti", "All E. alaskae", "All E. mexicanus", "All E. francisci", and "Cueva Quebrada Large" are based on data from Winans (1985). "Dry Cave Intermediate" and "31-57" are from my data. Measurements 1-8 of Harris and Porter (1980).
Winans (1989) recognized only one stilt-legged horse, E. francisci, Irvingtonian to Rancholabrean. However using her data, Coleman (listed as E. francisci) clusters with E. conversidens in a principal components plot, and the Arkalon small horse, the type of E. calobatus, and the Silverton specimen plot far from the otherwise tight cluster, leading to the suspicion that more than one species is involved. Likewise, such statistics as the coefficient of variation and the standard error of the mean are much larger than in a comparable sample of E. conversidens.
Fig. 3. Principal components plot of metacarpals considered by Winans (1989) to represent E. francisci showing the extreme separation from the stout-legged taxa. Measurements 1-8 of Harris and Porter (1980).
Fig. 4. Navicular (central tarsal) of three species of fossil horse (left to right): E. occidentalis, E. scotti, E. conversidens. UTEP locality 22 is the Animal Fair complex of Dry Cave; locality 75 is Dark Canyon Cave.
In the late 1970s, I made measurements of a number of equine elements at the Los Angeles County Museum and the Philadelphia Academy of Natural Sciences, which I gratefully acknowledge. Records of occurrence of horses with the source given as "Harris data" are based on those studies.
Barrón-Ortiz and Theodor 2011; Harris and Porter 1980; Scott 2004; Weinstock et al. 2005; Winans 1985, 1989.
Fossil horse material is relatively easy to identify to genus on many elements, but often very difficult to place to species. As a result, much equine material is recorded only as "horse" or "Equus sp."
The Pleistocene material earlier noted as from Skull Ridge (e.g., Harris 1993c) has been included in the Santa Cruz site, following Morgan and Lucas (2005).
Pleistocene: Encino Blowouts (Agogino 1972); Floyd (Morgan and Lucas 2005); Fresnal Canyon (Morgan and Lucas 2006); Laguna (Morgan and Lucas 2005); Lazy E Ranch (Morgan and Lucas 2005); Moriarty (Morgan and Lucas 2005); Pecos River near Roswell (Morgan and Lucas 2005); Terlingua (Cockerell 1930).
Several areas of the map have far too many localities to be able to show them all.
Late Blancan: Cal Tech (Lindsay 1984); California Wash (Morgan and White 2005); Curtis Ranch (Lindsay 1984); Kelly Canyon (Morgan et al. 2011); Prospect (Johnson et al. 1975); San Simon Fauna (Morgan and White 2005); Snowflake (Lindsay and Tessman 1974).
Irvingtonian: El Paso (UTEP).
Early Irvingtonian: Domingo (Morgan and Lucas 2005); Fite Ranch (Morgan and Lucas (2003); Inman Gravel Pit (Morgan and Lucas 2005); Rincon Arroyo (Morgan and Lucas 2005); Tijeras Arroyo (Morgan and Lucas 2005).
Late Irvingtonian or Early Rancholabrean (?): Eighteenmile Bend (Morgan and Lucas 2005).
Rancholabrean: Alkali Flat (Morgan and Lucas 2005); Arizpe (White et al. 2010); Bachoco (White et al. 2010); Bajimari (White et al. 2010); Black Rock (Morgan and Lucas 2005); Boquillas Station (Mead et al. 2005); Cedros (White et al. 2010); Cerro Colorado (Mead et al. 2005); Cerros Negros (Mead et al. 2005); Charley Day Spring (Lindsay and Tessman 1974); Chinobampo (White et al. 2010); Choate Ranch (Mead et al. 2005); Costa de Hermosillo (White et al. 2010); Davies Tank (Morgan and Lucas 2005); Dragoon Mountains (Mead et al. 2005); El Carrizal (White et al. 2010); Empire South (Mead et al. 2005); Fenn Site (Mead et al. 2005); Fresnal Canyon (Harris 1993c); Guadalupita Canyon (Gillette et al. 1985); Hermosillo (White et al. 2010); Hord Rock Shelter (Smith 1934); Joseph City (Mead et al. 2005); Jusibampo (White et al. 2010); Keams Canyon (Lindsay and Tessman 1974); La Angostura (White et al. 2010); La Botana (White et al. 2010); Lake San Agustín (Weber 1994); La Playa (White et al. 2010); La Puercera (White et al. 2010); Lewis Site (Mead et al. 2005); Lindsay Ranch (Mead et al. 2005); Llano Prieto (White et al. 2010); Lynx Creek (Pasenko and Agenbroad 2012); Mesa Redonda (Morgan and Lucas 2005); Naco (Sonora) (White et al. 2010); Palomas Creek Cave (Harris 1993c); Placitas (Morgan and Lucas 2005); Playa San Bartolo (White et al. 2010); Quiriego (White et al. 2010); Quitovac (White et al. 2010); Rancho Aigame (White et al. 2010); Sahuaro (White et al. 2010); Santa Ana (White et al. 2010); Santa Cruz (Morgan and Lucas 2005); San Rafael Aqueduct (Mead et al. 2005); Seff (Mead et al. 2005); Socorro (Morgan and Lucas 2005); Sonoita (Mead et al. 2005); South of Charleston (Lindsay and Tessman 1974); Springerville (McDonald et al. 2004); Térapa (Mead et al. 2006); Tooth Cave (DeSaussure 1956); Tucson Brickyard (Mead et al. 2005); Whipple Gravel (Mead et al. 2005); Whitewater Draw (Mead et al. 2005); Willcox (Mead et al. 2004); Yeso Creek (Agogino and Stevens 1972).
Early/Early-Mid Wisconsin: Lost Valley (Harris 1993c).
Mid Wisconsin: CC:5:6 (Mead et al. 2003); Canez Wash [age may be incorrect] (Mead et al. 2005); Kartchner Caverns State Park (Carpenter and Mead 2003); Lehner Ranch (mammoth site) (Haynes 2008); Shonto (Agenbroad and Downs 1984).
Mid/Late Wisconsin: Rampart Cave (Wilson 1942); Sandblast Cave (Emslie 1988).
Late Wisconsin: Bonfire Shelter (Frank 1968); Conkling Cavern (Conkling 1932); Hermit's Cave (Harris 1993c); Hord Rock Shelter (Smith 1934); Lake Otero (UTEP); Lehner Site (mastodon site) (Mead et al. 1979); Lucy Site (Agogino 1972); McCullum Ranch (Morgan and Lucas 2005); Murray Springs (Hemmings 2007b); Muskox Cave (Logan 1981); Naco (Mead et al. 2005); Placitas (Morgan and Lucas 2005); Sheep Camp Shelter (Gillespie 1985); Stanton's Cave (Harington 1984); U-Bar Cave 15-18 ka (Harris 1989).
Late Wisconsin/Holocene: Beyond Bison Chamber (Harris 1970a); Deadman Cave (Mead et al. 1984); Pratt Cave (Lundelius 1979).
Agenbroad and Down 1984; Agogino and Stevens 1972; Cockerell 1930; Conkling 1932; DeSaussure 1956; Emslie 1988; Frank 1968; Gillespie 1985; Gillette et al. 1985; Harington 1984; Harris 1970a, 1989, 1993c; Harris and Porter 1980; Haynes 2008; Hemmings 2007b; Lindsay 1984; Lindsay and Tessman 1984; Logan 1981; Lundelius 1979; McDonald et al. 2004; Mead et al. 1979; Mead et al. 1984; Mead et al. 2003; Mead et al. 2004; Mead et al. 2005; Mead et al. 2006; Morgan and Lucas 2005; Morgan and White 2005; Morgan et al. 2011; Pasenko and Agenbroad 2012; Smith 1934; Weber 1994; White et al. 2010; Wilson 1942.
Since the common late Wisconsin small horses were Equus conversidens, it seems likely that this is the taxon represented; however, this is speculation only, and the potential presence of a small horse having having lower incisor infundibulae (Scott 1996) makes speculation dangerous.
Early Irvingtonian: Pajarito Springs (Morgan and Lucas 2005); Tijeras Arroyo (Morgan and Lucas 2005),
Rancholabrean: Shelton Canyon (Morgan and Lucas 2005).
Harris 1993c; Morgan and Lucas 2005 .
Either E. scotti or E. occidentalis could be represented here, and possibly E. calobatus in the Irvingtonian.
Irvingtonian: El Golfo (Croxen et al. 2007).
Early Irvingtonian: Inman Gravel Pit (Morgan and Lucas 2005); Adobe Ranch (Morgan and Lucas 2003).
Rancholabrean: Shelton Canyon (Morgan and Lucas 2005).
Sangamon: La Brisca (Van Devender et al. 1985).
Mid/Late Wisconsin: Shelter Cave (Harris 1993c).
Literature. Croxen et al. 2007; Harris 1993c; Morgan and Lucas 2003; Morgan and Lucas 2005; Van Devender et al. 1985.
Last Update: 13 Nov 2013