202 Evolution? The Fossils Say YES!
which is reduced to two smooth button-shaped shields for the head and thorax and almost
no tail spine.
Still not macroevolutionary enough? What about the “missing links” between major body
plans and classes and phyla? Here, the problem is more difficult, because the great radiation of
animals in the Cambrian was largely among phyla with soft, unfossilizable bodies before large
shells finally appeared in the late Early Cambrian, or Atdabanian. We have excellent molecu-
lar, anatomical, and embryological evidence demonstrating how the major animal phyla are
interrelated (fig. 5.7). This evidence shows that the mollusks are most closely related to the
segmented worms among the living animal phyla. How about a transitional form between
a worm and a mollusk? Ask and ye shall receive. Among the earliest fossil mollusks known
from the Cambrian are simple cap-shaped shells that had been given names (such as Pilina),
even though we had no evidence of their soft anatomy. Then, in 1952 a dredge brought up
specimens from the deep waters off Costa Rica that included another classic “living fossil”
named Neopilina galatheae (fig. 8.15). Neopilina is clearly a mollusk with a cap-shaped shell
secreted by a mantle, as well as a mouth, digestive tract, anus, and gills.
But it is unlike any other mollusk alive today because it still retains the segmentation of
its wormlike ancestors. Arranged around the body on the margin of the mantle and below
FIGURE 8.15. The “living fossil” Neopilina, a relict of the early Cambrian and a transitional form between
segmented worms and mollusks. (A) Diagram showing the segmented paired gills on each side of the body
(there are also segmented paired retractor muscles as well). This demonstrates that normally unsegmented
mollusks evolved from segmented ancestors such as the annelid worms (confirmed by the molecular data).
(B) Photograph of the underside of the living animal, showing the relict segmentation. (C) Top view of the
cap-shaped shell. (Courtesy J. B. Burch, University of Michigan)
(A) (B)
(C)