THE LINEAGE OF MAN
known lines were then near the end of their career; second, the shark group, at that time relatively small, which was to become highly diversified in the next period and then to be crowded almost to the wall by the higher fishes; third, the true ganoids (actinopts), ancestors of the sturgeons, spoon-bills, bony gars, and eventually of the higher or teleost fishes; fourth, the dipnoans or lung-fishes, and fifth, the lobe-finned or crossopterygian ganoids, to be described presently.
Among which, if any, of these groups are we to seek for the ancestors of the higher vertebrates, including man? The known ostracoderms of Devonian time were already specialized side branches, all far too late in time to be the actual ancestors of the higher vertebrates. The Devonian sharks were already giving rise either to the specialized side branches or to the ancestors of the modern sharks; the actinopt ganoids were already on the line of advance to the higher fishes; the dipnoans were already highly specialized in their dental apparatus and skull characters and well along on the line to their modern descendants. Only certain of the lobe-finned fishes seem to have the right combination of characters to be even nearly related to the direct ancestors of the land-living vertebrates.
What, then, are the broad characteristics of these interesting fossil fishes, some of which may lie relatively near to the line of our own ancestry? Some of them were long-bodied, pike-like fishes, with great, strong jaws armed with sharp teeth. Others were stout and heavy-bodied, somewhat like a sea-bass, with shorter jaws. One very specialized group that lasted for many millions of years was short-bodied, propelled by fan-shaped paddles and by a very broad tail. Of all these, only the first lot, including Osteolepis, Megalichthys, Eusthenopteron and their allies, appear to be near the line of ascent to the land-living vertebrates. In
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