Horse evolution



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Fossil Species of Equus
As if the abundance and diversity of living members of the genus Equus were not enough, we also have an extensive record of this animal in the Northern Hemisphere beginning with its origin at the end of the Tertiary Period. The genus immediately ancestral to Equus is Dinohippus. This animal is similar to Equus in every respect – so much so that I cannot find any characteristics to distinguish them. I continue to use the term Dinohippus, therefore, merely as a way of distinguishing certain species that lived in North America during the Hemphillian and Blancan Land Mammal Ages, between 7 and 5.5 million years ago. (This is a good example of the sometimes uneasy marriage between the logic trees generated by cladistic analysis and the older method of drawing “phylogenetic trees” showing the descent of a species through time. Properly, some day, it will probably be necessary to re-classify Dinohippus by making it a sub-genus, and then we will write, for the best-known species in the genus, “Equus (Dinohippus) interpolatus”).
Dinohippus gave rise to a species, Equus francescana, that migrated from California across the Bering Land Bridge during the earliest phase of the Pleistocene when sea-level was lowered. Fossils of this species found in the Old World are called Equus stenonis, and in this form it spread throughout that continent and into Europe and Africa. In Europe, it gave rise to such species as Equus sussenbornensis, the ancestor of Equus mosbachensis, and Equus bressanus – the latter being the probable ancestor of asses. In western Asia and north Africa, Equus stenonis appears to have given rise to Equus hydruntinus, another ass-like form, and Equus teilhardi, which is probably an onager. In Africa, it ultimately gave rise to Equus grevyi.
In North America, Equus francescana gave rise to Equus simplicidens. Equus francisi, an onager-like species, also appears very early; it may be derived from E. francescana or from the back-migration of a Eurasian form. Likewise, E. simplicidens is a form which in many respects parallels Equus grevyi, having a very long head, big body, stout legs, and a relatively simple dental pattern. The “stilt-legged” Equus francisi, on the other hand, is not only longer in the shanks but smaller and lighter-bodied, with a more complex enamel pattern in the teeth.
From Equus francescana, and also possibly from forms back-migrating from the Old World, there came an enormous proliferation of North American species of the genus Equus. There really are quite a few of these, but those of you wishing to read the techical literature should be forewarned that paleontologists eager to make their mark in this fertile field of investigation have, over the past two centuries, almost hopelessly muddled matters by naming hundreds of species, most of which must be regarded, for a variety of reasons, as invalid “duplicates”. I therefore discuss here only those species that I think are really unique and valid.
Bloodlines of the genus Equus continue through three Land Mammal Ages: the Blancan, which is the tail-end of the Tertiary; the Irvingtonian, which is the earlier part of the Pleistocene; and the Rancholabrean, which is the later part. In each of these time periods, Equus species can be classified into either “stout-legged” (horse or zebra-like) or “stilt-legged” (onager-like) forms. (Ass-like forms are difficult to distinguish, and apparently rare, in the fossil record from the New World):
Blancan
Stout-legged forms: long head, relatively simple teeth, large size
Equus francescana

Equus simplicidens (old names for this form that you might still see in a museum exhibit are Plesippus, Equus shoshonensis). This form is commonly exhibited because a large number of skeletons have been recovered from the famous Hagerman Quarry near Hagerman, Idaho. There is a complete skeleton of this species on exhibit in a small Museum run by the National Park Service in Hagerman, and at some seasons you can also take a bus tour to view the actual quarry itself. Equus simplicidens skeletons are also on exhibit at the U.S. National Museum of Natural History/Smithsonian Institution in Washington, D.C.).
Stilt-legged form: shorter head with broader snout, relatively complex tooth structure, smaller size

Equus francisi (first appearance)

Typically Irvingtonian (though some range from late Blancan up into the Rancholabrean):


Stout-legged forms:

Equus niobrarensis

Equus hatcheri

Equus scotti (See a complete skeleton of this form on exhibit at the American Museum of Natural History in New York City).
Stilt-legged forms:

Equus francisi (later occurrences)

Equus arellanoi

Equus calobatus

Equus zoytalis
Typically Rancholabrean (though some originate in the Irvingtonian):
Stout-legged forms:

Equus caballus

Equus occidentalis (See a complete skeleton and many skulls of this form on exhibit at the Page La Brea Tar Pit Museum in Los Angeles, California, and at the U.S. National Museum of Natural History/Smithsonian Institution in Washington, D.C.)

Equus amerhippus

Equus excelsus (See a complete skeleton of this form on exhibit at the Nebraska State Museum of Natural History in Morrill Hall on the campus of the University of Nebraska at Lincoln).
Stilt-legged forms:

Equus altidens

Equus quinni

Equus conversidens (See a complete skeleton of this form on exhibit at the American Museum of Natural History in New York City).
Subspecies of Equus caballus
In a recent publication, my co-author Robert Hoffmann and I recognize seven subspecies within the species Equus caballus. (This paper is posted in its entirety at the Equine Studies Institute website, www.equinestudies.org, under “Knowledge Base”).
The first item to take care of under this heading are all the synonyms for Equus caballus – “duplicate names” proposed, as previously mentioned, by paleontologists eager to name horse types. Here is a list of published synonyms of Equus caballus with their namer and the publication date; you can see that this tendency for the proliferation of names goes right back almost to the beginning of the science:
Equus ferus (Boddaert, 1785)

Equus sylvestris (Brincken, 1828)

Equus przewalskii (Polyakov, 1881)

Equus mosbachensis (Reichenau, 1903)

Equus hagenbecki (Matschie, 1903)

Equus gmelini (Antonius, 1912)

Equus laurentius (Hay, 1913)

Equus niobrarensis alaskae (Hay, 1913)

Equus abeli (Antonius, 1914)

Equus mexicanus (Hibbard, 1957)

Equus midlandensis (Quinn, 1957)

Equus algericus (Bagtache, Hadjonis and Eisenmann, 1984)
Dr. Hoffmann and I recognize seven subspecies within the species. These are forms that we distinguish on the basis of their morphology as well as their mapped geographic occurrence. The seven valid subspecies are:
Equus caballus alaskae – the Lamut or Alaskan wild horse. Became extinct about 10,000 years ago, near the end of the Pleistocene. Once ranged in northeastern Asia and across the Bering Land Bridge into Alaska.
Equus caballus mexicanus – the American peri-glacial horse. Became extinct at the end of the Pleistocene. Once ranged from the glacial margin west of the Mississippi, south into northern Mexico.
Neither of these forms were ever domesticated, as they were already extinct by the time horse domestication began.
Equus caballus przewalskii – the Przewalski horse or Mongolian wild horse. Its last known range was a narrow strip in the Gobi Desert, but it once ranged broadly across Russia and Siberia from the Ural Mountains to northeastern Asia. You may frequently see this animal in zoos. It became extinct in the wild in 1947, but survives in zoos and preserves today, all living animals being the descendants of only 13 wild-caught ancestors. The Przewalski horse has contributed in a minor way to a restricted number of Asian breeds in Mongolia, northern China, and eastern Tibet. It is, as I have explained in both the “Mammalian Species” and “Origin of the Mustang” papers posted in our Knowledge Base, most definitely NOT the ancestor of the domestic horse.
Equus caballus ferus – the Tarpan. This animal became extinct in 1913, a herd being kept up until that time in the Bialovesh Forest Preserve in Poland. Once ranged from eastern Poland east to the Ural Mountains. This form was the first wild horse to be domesticated. Its characteristics are preserved within several Russian, eastern European, and west Asian breeds including the Konik, Hucul, and Akhal-Teke and related Turkmenian breeds. Through these in turn, Tarpan characteristics have been incorporated into the Thoroughbred.
Equus caballus pumpelli – the Afro-Turkic horse. This is the immediate ancestor of our “Oriental” breeds, including the Arabian, Old Hittite, Persian, Bashkir, Lokai, Marwari, and Barb.
Equus caballus mosbachensis – the Central European horse. The most ancient and primitive form of Equus caballus, the Mosbach horse is the immediate ancestor of the Warmblood breeds, including the Latvian, Grönigen, Friesian, Cleveland Bay, and all the German Warmbloods and their derivatives now bred in Sweden, Denmark, and the Low Countries.
Equus caballus caballus – the West European horse. This is the cold-and-wet adapted subspecies, the form that has the stoutest, most rounded body, the shortest ears, the broadest and deepest head, the shortest legs, the broadest feet, and the thickest fur, mane, and tail. It is native to those parts of Europe that lie west of the Rhine and Rhone rivers, from Norway and Sweden in the north through the Low Countries, the British Isles, western France, and Spain. E. c. caballus is the immediate ancestor of all the draft breeds, such as the Belgian, Clydesdale, Brabant, and Suffolk Punch and their derivatives now bred in other countries, such as Russia and the Americas. It is also the immediate ancestor of all the pony breeds, large and small, including the Fjord, Shetland, Exmoor, Dartmoor, Mehrens, Galician and Asturian. Ponies and draft horses all derive from a single ancestry, the overall size of a given form being remarkably concordant with the area of land upon which the form developed, i.e. Continental forms are the largest, those occurring on the main British island next biggest, and those occurring on offshore islands like Man, the Shetlands, and the Orkneys being the smallest.
INTRODUCTION TO THE FAMILY-LEVEL REVIEW
The following text is reprinted almost exactly as it originally appeared in the Elsevier World Encyclopedia of Animal Science, Volume C-7, “Horse Breeding and Management”. It is a review, written in semi-technical language, of the horse family. It includes a cladogram with characters. The interested student will want to carefully study this logic-diagram, noting that only ONE “shared-derived” character is sufficient to define any given clade (though more such characters are always desirable).
This paper contains many citations – that is the custom in peer-reviewed scientific reporting. All of the citations are listed in the bibliography which follows. In addition to works cited in “The Evolution of the Horse Family”, the bibliography also lists a large number of other papers with annotations as to which fossil species may have been discussed or illustrated in those papers. Many of these works are very old – you will note the long series by the prolific 19th-century American paleontologists Joseph Leidy and Edward Drinker Cope. These men lived at a time when many fossil horse remains first came to light. They named and were the first to describe most of the species. That most of these species have now been placed in genera other than the one originally assigned does not, by the rules of taxonomy already explained, invalidate the type or the species name. It certainly does not invalidate the work of these brilliant and dedicated men, either.
There continues to be large interest in research on the horse family. The genus Equus actually has its own “fan club” (a technical interest-group) within the Friends of the Pleistocene of the Society of Vertebrate Paleontology. I have been welcomed as a member of this group and for that I am grateful. Thanks to the continuing, diligent efforts of paleontologists -- professionals and amateur fossil-hunters alike – new discoveries continue to be made which add to the thousands of skeletons already known. From these comes our only hope of correctly recording and interpreting diversity, environmental adaptation, and change through time in the horse family.
THE TECHNICAL READING LIST
The following is a list of publications from which information used to compile this article comes. If you want to pursue the technical literature, you should print this bibliography out and take it to the library with you, where you will find that it saves you a great deal of time. The ordinary public library probably will not have or be able to locate most of these listings for you; you will have to access either a University library or a library within a large Museum of Natural History, such as the American Museum in New York City, the British Museum in London, the National Museum of Canada in Toronto, the National Museum of Mexico in Mexico City, or the U.S. National Museum/Smithsonian Institution in Washington, D.C. Online Internet listings of the U.S. National Library may also be of assistance; go to http:/catalog.loc.gov or do a Google search for “Library of Congress catalog”.
Abusch-Siewert, S. 1983. Gebissmorphologische Untersuchungen an eurasiatischen Anchitherien (Equidae, Mammalia) unter besonder Berücksichtigung der Fundstelle Sandelzhausen. Courir Forschungsinstitut Senkenberg 62:1-361.
Agassiz, L. and A. Gould. 1851. Principles of Zoology. Gould and Lincoln Publishers, London.
Akersten, W.A., H.A. Lowenstam, and A. Walker. 1984. “Pigmentation” of soricine teeth: composition, ultrastructure, and function, in Am. Soc. Mammal. 64th Ann. Meeting, Abstracts. Humboldt State University, Arcata, California, no. 153, p. 40.
Alberdi, M.T., L. Caloi, and M.R. Palombro. 1992. Pleistocene equids from Western Europe: their biostratigraphy and palaeoecological characteristics, in F. Spitz et al., eds., Ongulés/Ungulates 91 (Toulouse, France). Société Francaise pour l’Etude de la Protection des Mammifères, Paris; and Institut de Recherche sur les Grands Mammifères, Toulouse, pp. 31-35.
Albritton, C.C. 1980. The Abyss of Time. Freeman, Cooper, and Co., San Francisco.
Alexander, R. N. 1988. Elastic Mechanisms in Animal Movement. Cambridge University Press, New York, 141 pp.
Alexander, R.N. 1989. On the synchronization of breathing with running in wallabies (Macropus spp.) and horses (Equus caballus). Jour. Zool. London, 218:69-85.
Alexander, R.N., N.J. Dimery, and R.F. Kerr. 1985. Elastic structures in the back and their role in galloping in some mammals. Jour. Zool. London, 207:467-482.
Allen, J.A. 1878. The geographical distribution of mammals. U.S. Geological Surv. 4:313-376.
Ambrose, S.H., and M.J. DeNiro. 1989. Climate and habitat reconstruction using stable carbon and nitrogen isotope ratios of collagen in prehistoric herbivore teeth from Kenya. Quaternary Research, 31:407-422.
Anderson, Elaine. 1984. Who’s who in the Pleistocene: a mammalian bestiary, in P.S. Martin and R.G. Klein, eds., Quaternary Extinctions: A Prehistoric Revolution. The University of Arizona Press, Tucson, pp. 40-89.
Antonius, O. 1912. Was ist der "Tarpan"? Naturw. Wochenschr. 2(11):27.
Antonius, O. 1914. Equus abeli nov. spec. Beitr. Pal. Geol. Ost.‑Ung. Or. 26:1913‑1915.
Antonius, O. 1919. Untersuchung uber den phylogenetischen Zusammenhang zwischen Hipparion und Equus. Zeitsch. Fur induktive Abstammungs‑und Vererbungslehre 20(4):273‑295.
Antonius, O. 1929. Streitfragen zur Phylogenie der Equiden. Verh. Zool.‑Bot. Ges. Wien (1928).
Arambourg, C. and J. Opieveteau. 1929. Les vertébrés du Pontian de Salonique. Ann. Paléont. 18:59-138.
Archibald, J.D., P.D. Gingerich, E.H. Lindsay, W.A. Clemens, D.W. Krause, and K.D. Rose. 1987. First North American Land Mammal Ages of the Cenozoic Era, in M.O. Woodburne, ed., Cenozoic Mammals of North America: Geochronology and Biostratigraphy. The University of California Press, Berkeley, pp. 24-76.
Arroyo-Cabrales, J., E. Johnson, and R.W. Ralph. 1993. New excavations at San Josecito Cave, Nuevo Leon, Mexico. Current Res. in the Pleistocene, 10:91-94.
Axelrod, D.I. and H.P. Bailey. 1969. Paleotemperature analysis of Tertiary floras. Palaeogeography, Palaeoclimatology, Palaeoecology, 6:163-195.
Ayala, F.J. 1988. Can “progress” be defined as a biological concept? In M.H. Nitecki, ed., Evoljutionary Progress. The University of Chicago Press, Chicago, pp. 75-96.
Azzaroli, A. 1982. On Villafranchian Palaearctic Equus and their allies. Palaeontographica Italica, new ser, 72:74-97.
Azzaroli, A. 1990. The genus Equus in Europe, in E.H. Linsay, V. Fahlbusch, and P. Mein, eds., European Neogene Mammal Chronology, Plenum Press, New York, pp. 339-356.
Azzaroli, A. 1992. Ascent and decline of monodactyl equids: a case for prehistoric overkill. Annales Zoologica Fennici, 28:151-163.
Azzaroli, A. 1995. A synopsis of the Quaternary species of Equus in North America. Bolettino della Societa Paleontologica Italica, 34:205-221.
Azzaroli, A., and M.R. Voorhies. 1990. The genus Equus in North America: the Blancan species. Palaeontographica Italica, 80:175-198.
Azzaroli, A., C. De Giuli, G. Ficcarelli, and D. Torre. 1988. Late Pliocene to early mid-Pleistocene mammals in Eurasia: faunal succession and dispersal events. Palaeogeography, Palaeoclimatology, Palaeoecology, 66:77-100.
Bader, R.S. 1956. A quantitative study of the Equidae of the Thomas Farm Miocene. Bull. Mus. Comp. Zool. 115:47-78.
Badgley, C. and L. Tauxe. 1990. Paleomagnetic stratigraphy and time in sediments: studies in alluvial Siwalik rocks of Pakistan. Jour. Geol. 98:457-477.
Badoux, D.M. 1987. Some biomechanical aspects of the structure of the equine tarsus. Anatomischer Anzeiger, Jena, 164:53-61.
Bagtache, B., D. Hadjouis, and V. Eisenmann. 1984. Présence d’un Equus caballin (E. algericus no. sp.) et d’une autre espèce nouvelle d’Equus (E. malkeinsis n. sp.) dans l’Aterian des Allobroges, Algérie. Comptes-Rendus d l’Académie des Sciences, Paris, 298, série II: 609-612.
Barbour, E. H. 1914. A new fossil horse, Hypohippus matthewi. Bull. Nebraska Geol. Surv., 4(10):169‑173, pl. I (H. matthewi).
Barry, J.C., E.H. Lindsay, and L.L. Jacobs. 1982. A biostratigraphic zonation of the middle and upper Siwaliks of the Potwar Plateau of northern Pakistan. Palaeogeography, Palaeoclimatology, Palaeoecology, 37:95-130.
Benirschke, K., N. Malouf, R.J. Low, and H. Heck. 1965. Chromosome complement: difference between Equus caballus and Equus przewalskii Poliakoff. Science, 148:382-383.
Bennett, D.K. 1980. Stripes do not a zebra make, Part I: A cladistic analysis of Equus. Syst. Zool., 239(2): 271-294.
Bennett, D.K. 1984. Cenozoic rocks and faunas of north-central Kansas, with an appendix concerning taxonomy and evolution in the genus Equus. The University of Kansas, Dept. Syst. and Ecol., dissertation submitted for the degree of Doctor of Philosophy.
Bennett, D.K. 1988. The ring of muscles. Equus Magazine, 121:36-42.
Bennett, D.K. 1992. The evolution of the horse, in J.W. Evans, ed., Horse Breeding and Management, World Encyclopedia of Animal Science, Vol. C7. Elsevier Publ., Amsterdam, pp. 1-40.
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Bernour, R.L. and Hussain, S.T. 1985. An assessment of the systematic, phylogenetic and biogeographic relationships of Siwalik hipparionine horses. Jour. Vert. Paleo. 5:32-87.
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Berry, W.B.N. 1987. Growth of a Prehistoric Time Scale: Based on Organic Evoluton. Blackwell Scientific Publl, Palo Alto, California.
Bock, W.J. 1973. Philosophical foundations of classical evolutionary classification. Syst. Zool., 22:375-392.
Boddaert, P. 1785. Elenchus Animalium. C.R. Hake, Roterodami [Rotterdam], Sistens huc usque nota, eorumque varietates, I: 1-173.
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Boné, E.L. and R. Singer. 1965. Hipparion from Langebaanweg, Cape Province and a revision of the genus in Africa. Ann. South African Mus. 48:273-397.
Boulé, M. 1899. Observations sur queleques equides fossiles. Bull. Soc. Geol. France 3:27.
Bowler, P.J. 1986. Fossils and Progress: Paleontology and the Idea of Progressive Evolution in the Nineteenth Century. Science History Pubglications, New York.
Bowler, P.J. 1989. Holding your head up high: degeneration and orthogenesis in theories of human evolution, in J.R. Moore, ed., History, Humanity, and Evolution. Cambridge University Press, pp. 329-353.
Branco, W. 1883. Eine fossile Saugethier‑Fauna von Punin bei Riobamba in Ecuador. Pal. Abh. 1:2.
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Brincken, J. (Baron de). 1828. Mémoire Descriptif sur la Forêt Impériele de Bialowieza, en Lithuanie. N. Glücksberg, Imprmeur – Libraire de l’Université Royale, Varsovie [Warsaw], 127 pp.
Brooks, C.E.P. 1928. Climate Through the Ages: A Study of the Climatic Factors and their Variation. Yale University Press, New Haven.
Broom, R. 1909. On the evidence of a large horse recently extinct in South Africa. Ann. South Afr. Mus., Kapstadt, 7:281‑282.
Brown, T.M. and A.J. Kihm. 1981. Xenicohippus, an unusual new hyracothere (Mammalia, Perissodactyla) from lower Eocene rocks of Wyoming, Colorado, and New Mexico. Jour. Paleontol. 55:257-270.
Brown, W.L. Jr. 1987. Punctuated equilibria excused: the original examples fail to support it. Biol. Jour. Linnean Soc. 31:383-404.
Burmeister, H. 1875. Los Caballos Fòsiles de la Pampa Argentina. La Tribuna Press, Buenos Aires, Argentina.
Butler, P.M. 1952a. The milk-molars of Perissodactyla, with remarks on molar occlusion. Proc. Zool. Soc. London 121:777-817.
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