Page 450 - Introduction to Paleobiology and The Fossil Record

FISHES AND BASAL TETRAPODS 437

Sarcopterygii

100
90 Conodonta Placodermi
80
Number of families 60 Cephalochordata Petromyzontida Heterostraci Galeaspida Osteostraci Chondrichthyes Actinopterygii
70
50
40
30
20
10 Tunicata Myxinoidea Anaspida Thelodonti Acanthodii
0
Stem Gnathostomes Crown

Duplication

Figure 16.7 Phylogeny of the basal fishes. One major genome duplication event was apparently

associated with the origin of jaws. When the fossil groups (open lines) are omitted, there is a

large morphological and genomic leap from jawless lampreys and hagfishes; when the fossil
groups are included, as here, the transition appear much more gradual. The timing of the genome
duplication events is uncertain, and falls within the area of the gray box. The number of families
within each living and fossil group is shown by the shaded vertical bars. (Courtesy of Phil
Donoghue.)

spaced rows on their undersides (Fig. 16.8b). lobefins, the sarcopterygians, that had thick,

Acanthodians are often found preserved in muscular, limb-like fins. Today, the lobefi ns
vast numbers in the rock layers, so they prob- are rare, being represented by only three

ably swam in huge shoals in open water, species of lungfishes and the rare coelacanth.
perhaps feeding on small arthropods and The coelacanth Latimeria is a famous “living
plankton. They escaped predators by rapid fossil”. Until 1938, coelacanths were only
darting from side to side in their shoals, and known as Devonian to Cretaceous fossils, but
perhaps their exceptional spininess made them in 1938 the world was astounded to hear that
diffi cult to swallow. a living coelacanth had been fi shed out of
deep waters off East Africa, and more have
been caught since then.
Bony fi shes: ray fi ns and lobefi ns
The ray fins of the Devonian include Chei-

The osteichthyans, or bony fi shes, also ap - rolepis (Fig. 16.8c), which had a fl exible body
peared in the Devonian. There are two groups: covered with small scales and a plated head.
(i) those with ray-like fi ns, the actinopteryg- This was an active predator that may have fed
ians, ancestors of most fishes today from carp on acanthodians. The Devonian lobefi ns

to salmon, and seahorse to tuna; and (ii) the include both lungfi shes and “rhipidistians”.