Moorad asks --
>Talk is cheap. What is the evidence that one came from the other, or that
>both came from a common descent. Moorad
The horse belongs to a group of hoofed animals called perissodactyls. If
we trace the fossil record of horses backward in time, the first
perissodactyl identified as a horse is Hyracotherium. This was a small
generalized herbivore (probably a browser) with teeth very unlike modern
horses, including small canine teeth, simple tricuspid premolars and
low-crowned simple molars. Its narrow elongated skull had a relatively
small brain and its eyes were placed well forward in the skull. It had 4
toes in front and 3 in the back.
The significance of the anatomy of Hyracotherium is that it is very
similar, in fact almost indistinguishable, from the earliest members of the
other perissodactyl groups that existed at the same time. These include
the rhinos, tapirs, and titanotheres. To summarize, as we trace all the
distinct groups of perissodactyls back in time they converge in appearance
such that members of the different groups can be distinguished only with
Moving further back in time, the earliest hoofed animals (ungulates) belong
to an informal collection of highly generalized animals referred to as
condylarths. Interestingly, these hoofed animals include groups with teeth
suggesting carnivorous habits.
Now let's trace back the dog family. The earliest representatives of the
families of dogs and weasels (of the superfamily Caniformia) and the cats
and civets (superfamily Feliformia) are very similar to each other. Of
these early carnivores, Romer (1966) states: "Were we living at the
beginning of the Oligocene, we should probably consider all these small
carnivores as members os a single family." These earliest members of the
living carnivore groups were also very similar to the miacids, a primitive
group of carnivores. Furthermore, the miacids were sufficiently similar to
some condylarths that some taxa have had their assignments to these orders
changed. The taxonomic uncertainty associated with early "stem groups"
such as the condylarths and miacids is a consequence of their position in
time at the initial diversification of the ungulate and carnivore groups.
Thus, the fossil record provides strong evidence for dogs and horses having
descended from a common ancestor.
Romer, A.S., 1966, Vertebrate Paleontology (Univ. of chicago Press), p.232.
Keith B. Miller
Department of Geology
Kansas State University
Manhattan, KS 66506
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