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Hippidion No More?
[Sent to DML for purpose of exploring gene studies revising morphological
studies and its relevance to topics of phylogeny and study materials in
Weinstock, J., E. Willerslev, A. Sher, Tong W.-f., S. Y. W. Ho, D.
J. Storer, J. Burns, L. Martin, C. Bravi, A. Prieto, D. Froese, E. Scott,
Lai X.-l., and A. Cooper. 2005. Evolution, systematics, and phylogeography
of pleistocene horses in the New World: A molecular perspective. _Public
Library of Science Biology_ 3(8) [August, 2005] (doi:
"The rich fossil record of horses has made them a classic example of
evolutionary processes. However, while the overall picture of equid
is well known, the details are surprisingly poorly understood, especially
the later Pliocene and Pleistocene, c. 3 million to 0.01 million years (Ma)
ago, and nowhere more so than in the Americas. There is no consensus on the
number of equid species or even the number of lineages that existed in these
continents. Likewise, the origin of the endemic South American genus
*Hippidion* is unresolved, as is the phylogenetic position of the
"stilt-legged" horses of North America. Using ancient DNA sequences, we show
that, in contrast to current models based on morphology and a recent genetic
study, *Hippidion* was phylogenetically close to the caballine (true)
with origins considerably more recent than the currently accepted date of c.
10 Ma. Furthermore, we show that stilt-legged horses, commonly regarded as
Old World migrants related to the hemionid asses of Asia, were in fact an
endemic North American lineage. Finally, our data suggest that there were
fewer horse species in late Pleistocene North America than have been named
morphological grounds. Both caballine and stilt-legged lineages may each
comprised a single, wide-ranging species."
Based on sequences from the HVR1 and HVR2 portions of the mtDNA control
region, the following phylogeny (simplified) was produced:
Included in this phylogeny of diverse fossil caballines was Przewalskii's
horse, which was apparently well nested among fossil *E. caballus* as the
sister-taxon of domestic *E. caballus*, rather than something far more zebra or
donkey-like. The presence of onagers and kiangs, originally part of the asses,
as the outlying groups of a zebra-horse clade, offers the possibility that
stripes in zebra are an exaptation of a feature more broad in horse phylogeny
than previously suspected. The entire genus *Hippidion* is subsumed within
*Equus* by default.
Based on exclusive mtDNA coding from HVR1 control region, the following
phylogeny is produced:
Domestic horses are spread throughout the caballine distribution, and
Przewalskii's horse formed a nested group within this group and Eurasian
caballines, whereas in the previous tree they were grouped next to a German
The paper includes this comment:
"Size has been used as one of the main morphological criteria for defining
species of Pleistocene equids , and the body size of the Late Pleistocene
North American caballines we sampled did exhibit marked regional
e.g., horses in Alberta (Canada) were larger than their eastern Beringian
Wyoming counterparts (see Figure 1). The DNA evidence strongly suggests,
however, that all of these large and small North American caballine samples
belong to a single species (Figure 5 and Table S3). The presence of a
morphologically variable caballine species widely distributed north and
of the Pleistocene ice sheets raises the tantalizing possibility that, in
spite of the many taxa named on morphological grounds [1,2,4], most or even
all North American caballines were members of the same species."
References noted are:
1. Winans, M. C. 1989. A quantitative study of North American fossil species
of the genus *Equus*, pg. 262-297, in Prothero. D. R. and R. Schoch, eds.
_The evolution of perissodactyls_. (Oxford: Oxford Univ. Press.)
2. Azzaroli, A. 1998. The genus *Equus* in North America. _Palaeontographica
4. Kurtén, B and E. Anderson. 1980. _Pleistocene mammals of North America_.
(New York: Columbia University Press.) 442 p.
Jaime A. Headden
"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)
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