200 Evolution? The Fossils Say YES!
if they are supposedly unchanged through millions of years, they would have no transitional
forms that could produce the “horseshoe crab kind” from some other kind? Wrong! When
I collected in the Upper Cambrian beds of Wisconsin, one of the most spectacular fossils
I encountered were the plates of the primitive arthropods known as aglaspids (fig. 8.14A).
These large creatures don’t look like the modern horseshoe crab, but still they are either close
FIGURE 8.13. The evolution of sand dollars. (A) Living sand dollars in feeding position, half-buried obliquely
into the sand like shingles, with their mouths on the bottom facing into the food-bearing water currents.
(Photo by the author) (B) Evolution of flat sand dollars from the biscuit-shaped cassiduloid sea urchins and
slightly flatter oligopygoids through the transitional Paleocene fossil Togocyamus from western Africa and
concluding with progressively flatter and more specialized sand dollars. The petal-shaped areas on the top of
the shell bear the tube feet, while the mouth is on the bottom of the shell and gradually shifts from the center
of the bottom to the lower front edge. Meanwhile, the anus shifts from the center of the top of the shell to the
back edge of the shell as burrowing ability becomes more specialized and improved. (After Mooi 1990; used
with permission of the Paleontological Society.)
(A)(B)
“CASSIDULOIDS” OLIGOPYGOIDA Togocyamus CLYPEASTERINA LAGANINA SCUTELLINAA B CDE F