Geology Reference
In-Depth Information
results in having cells nested be-
tween each other in successive
rows (Figure 21.16b). Further-
more, the biochemistry of muscle
activity and blood proteins, and
the larval stages, are similar in
both echinoderms and chordates.
The evolutionary pathway
to vertebrates thus appears to
have taken place much earlier
and more rapidly than many sci-
entists have long thought. Based
on fossil evidence and recent ad-
vances in molecular biology, one
scenario suggests that vertebrates
evolved shortly after an ancestral chordate, probably re-
sembling
Yunnanozoon
, acquired a second set of genes. Ac-
cording to this hypothesis, a random mutation produced a
duplicate set of genes, letting the ancestral vertebrate animal
evolve entirely new body structures that proved to be evolu-
tionarily advantageous. Not all scientists accept this hypoth-
esis, and the origin of vertebrates is still hotly debated.
Dorsal hollow nerve cord
Notochord
Dorsal fin
Tail
Muscles
Anus
Oral
tentacles
Pharynx
Midgut cecum
Atrium
Ventral fin
Velar
tentacles
Pharyngeal
(gill) slits
Atrial opening
(excurrent)
◗
Figure 21.14
Three Characteristics of a Chordate The structure of the lancelet
Amphioxus
illustrates the three characteristics of a chordate: a notochord, a dorsal hollow nerve cord,
and gill slits.
The most primitive vertebrates are fish, and some of the
oldest fish remains are found in the Upper Cambrian
Deadwood Formation in northeastern Wyoming. Here,
phosphatic scales and plates of
Anatolepis
, a primitive mem-
ber of the class Agnatha (jawless fi sh), have been recovered
from marine sediments. All known Cambrian and Ordovi-
cian fossil fi sh have been found in shallow, nearshore ma-
rine deposits, whereas the earliest nonmarine (freshwater)
fi sh remains have been found in Silurian strata. This does
not prove that fi sh originated in the oceans, but it does lend
strong support to the idea.
As a group, fi sh range from the Late Cambrian to the
present (
◗
Figure 21.15
Yunnanozoon lividum
Found in 525-million-year-old
rocks in Yunnan Province, China,
Yunnanozoon lividum
, a 5-cm-long
animal, is one of the oldest known chordates.
◗
Figure 21.16
Cell Cleavage
Figure 21.17). The oldest and most primitive of
the class Agnatha are the
ostracoderms
, whose name means
“bony skin” (Table 21.2). These are armored, jawless fish
that fi rst evolved during the Late Cambrian, reached their
zenith during the Silurian and Devonian, and then became
extinct.
The majority of ostracoderms lived on the seafloor.
Hemicyclaspis
is a good example of a bottom-dwelling
ostracoderm (
◗
Figure 21.18a). Vertical scales allowed
Hemicyclaspis
to wiggle sideways, propelling itself along the
seafloor, and the eyes on top of its head allowed it to see
such predators as cephalopods and jawed fi sh approaching
from above. While moving along the sea bottom, it prob-
ably sucked up small bits of food and sediment through its
jawless mouth.
The evolution of jaws was a major evolutionary ad-
vance among primitive vertebrates. Although their jawless
ancestors could only feed on detritus, jawed fi sh could chew
food and become active predators, thus opening many new
ecological niches.
◗
Radial cleavage
a
Arrangement of cells
resulting from radial
cleavage is characteristic of
chordates and echinoderms.
In this confi guration, cells
are directly above each
other.
Spiral Cleavage
b
Arrangement of
cells resulting from
spiral cleavage. In this
arrangement, cells in
successive rows are
nested between each
other. Spiral cleavage
is characteristic of all
invertebrates except
echinoderms.
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