Common descent of horses and dogs

From: Keith B Miller (
Date: Fri Oct 12 2001 - 18:31:46 EDT

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    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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