Page 416 - Introduction to Paleobiology and The Fossil Record
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DEUTEROSTOMES: ECHINODERMS AND HEMICHORDATES 403
nerve ends divide externally to form a sensory strates and in caves and crevices on the sea-
net. floor; these sea urchins may have been
omnivores, carnivores or herbivores. Irregular
The regular to irregular transition forms display a range of adaptations appropri-
ate to an infaunal mode of life where burrows
Regular echinoids, the sea urchins, with a were carefully constructed in low-energy envi-
compact, symmetric morphology are most ronments. Extreme morphologies were devel-
basal. Their shape contrasts with the irregu- oped in the sand dollars or Clypeasteroidea,
lars, with marked bilateral symmetries and permitting rapid burial just below the sedi-
specialized for forward movement. The irregu- ment–water interface in shifting sands. Echi-
lar echinoid morphotype evolved rapidly and noids generally lived in shallow seawaters, but
apparently involved some large architectural some went deeper; the timing of this move off-
changes to adapt the animal to burrowing. shore has been controversial (Box 15.6).
Plesiechinus hawkinsi is one of the fi rst irregu-
lar echinoids, appearing early in the Early Life modes and evolution: microevolution of
Jurassic (Sinemurian) with an asymmetric test, Micraster One of the classic case studies of
short numerous spines, large adapical pores evolutionary patterns in fossils is seen in
and a posteriorly placed periproct together Micraster, an infaunal, irregular echinoid.
with presumed keeled teeth. Ten million years Paleobiologists have repeatedly used this
later, by the Toarcian, much of the “toolkit” of example to test phyletic gradualist and punctu-
adaptations had evolved for a burrowing life ated equilibria models (see p. 121) and as the
mode. Secondary bilateral symmetry was raw material for the rigorous statistical analy-
superimposed on the existing pentameral sym- sis of both ontogenetic and phylogenetic
metry to form a heart-shaped or fl attened ellip- change. In the best-known lineage, M. leskei–
soidal test. The periproct migrated from a M. decipiens–M. coranguinum, the following
position on the apical surface to the posterior morphological changes occurred (Fig. 15.14):
side of the test to eject waste laterally. By the
Early Cretaceous, one of the ambulacral areas 1 The development of a higher, broader
had become modified to form a food groove (heart-shaped) form associated with an
and a series of tube feet were extendable with increase in size and thickness of the test.
flattened ends to assist respiration. 2 The peristome (mouth) moved anteriorly
One of the earliest sand dollars, the and the posteriorly situated periproct
clypeasteroid Togocyamus, appeared during (anus) had a lower position on the side of
the Paleocene, and some 20 million years later the test with a broader subanal fasciole.
in the Eocene more typical sand dollars had 3 The madreporite increased in size at the
evolved to command a cosmopolitan distribu- expense of the adjacent specialized plates.
tion. The flattened test was adapted for bur- 4 More tuberculate and deeper anterior
rowing, whereas the accessory tube feet could ambulacra evolved.
encourage food along the food grooves and 5 More granulated periplastronal areas
draped the test with sand. The highly accentu- developed.
ated petals helped respiration by providing an
increased surface area for the tube feet, and These morphological changes are associated
the development of a low lantern with hori- with life in progressively deeper burrows. But
zontal teeth signaled changes in feeding there is a lack of associated trace fossils that
patterns. might prove this. On the other hand, the
adaptations may have been geared to greater
Ecology: modes of life burrowing efficiency, probably in shallow
depths in the chalk where such traces were
The regular Echinus and the irregular Echino- destroyed by reworking of the sediments.
cardium probably mark the ends of a spectrum Microevolutionary trends have been tested
of life modes from epifaunal mobile behavior in other echinoid lineages. The irregular Dis-
to a number of infaunal burrowing strategies coides occurs abundantly through an Upper
(Fig. 15.12). Mobile regular forms such as Cretaceous section at Wilmington, south
Echinus grazed on both hard and soft sub- Devon, England. The height and diameter of
