The world's formost evolutionary paleontologist, George Gaylord Simpson.
In his book, "Tempa and Mode in evolution," states that nowhere in the
world is there any trace of a fossil that would close the considerable gap
between Hyracotheerium, which most evolutionist assume was the first horse,
and its supposed ancestral order Condylarthra. He goes on to say:

        This is true of all thirty-two orders of mammals
        ....the earliest and most primitive members of every
        order already have the basic ordinal characters, and
        in no case is an approximately continuous sequence from
        one order to another is known. In most cases the break
        is so sharp and gap so large that the origin of the order is
        speculative and much disputed.

Later on (p.107), Simpson states:

        This regular absence of transitional forms is not
        confined to mammals, but is an almost universal
        phenomenon, as has long been noted by paleontologists.
        It is true of almost all orders of all classes of animals,
        both vertibrate and invertibrate. A fortiori, it is also
        true of the classes, and of the major animal phylia,..