Page 418 - Introduction to Paleobiology and The Fossil Record

DEUTEROSTOMES: ECHINODERMS AND HEMICHORDATES 405

Box 15.6 Into the deep: but not until the Late Cretaceous?

A number of animal groups common in the Paleozoic evolutionary fauna, such as the brachiopods
and crinoids, are common in deep-water environments. This reinforces the view that the deep sea is
some sort of refuge for archaic taxa that have been forced down the continental slope by predation
or unsuccessful competition on the shelf. Andrew Smith and Bruce Stockley (2005) have developed
a quite different model, however, based on molecular clock estimates and the phylogeny of echinoid
taxa. Results show that the modern deep-sea omnivore fauna appeared gradually over the last 150–
200 myr; detritivores, however, were in place during a much shorter time span between 75 and
50 Ma (Fig. 15.13). This 25 myr window of seaward migration appears to be associated with marked
increases in seasonality, continental sediment discharge and surface productivity. The increased avail-
ability of organic carbon and nutrients in the deep sea provided the means for habitat expansion
rather than an escape from competition and predation on the shelf.

Paleoproductivity K/T
–2
–1
(gCm yr )
160
140
120
detritivores
100
20
carnivores
80
60 15 Number of deep-sea lineages
OAEs 10

5

200 150 100 50
Time (Ma)
Figure 15.13 Events in the deep sea: cumulative frequency polygons for maximum and minimum
times of origin of 38 clades of extant, carnivore and detritivore deep-sea echinoids (Smith &
Stockley 2005). K/T, Cretaceous–Tertiary boundary; OAEs, oceanic anoxic events.

abundant. The sparse early record of the
time

Turonian M. gibbus Coniacian – Santonian group might reflect a relatively fragile skele-
inferred depth of burrowing M. leskei M. decipiens not a common element of the Paleozoic
ton that quickly disintegrated after death; on
the other hand the echinoids were probably
sea bottom
benthos. The enigmatic Bothriocidaris,
described from the Ordovician of Estonia and
from southwest Scotland, has been variously
classified as an echinoid, cystoid or holothu-

M. coranguinum
rian. Some authorities consider that Bothrio-
Figure 15.14 Evolution of the Late Cretaceous cidaris and Eothuria might be unclassifi able
heart urchin, Micraster. (Based on Rose, E.P.F. & echinoids, hopeful monsters that arose during
Cross, N.E. 1994. Geol. Today 9.) the rapid Ordovician radiation of the group.