Page 448 - Introduction to Paleobiology and The Fossil Record

FISHES AND BASAL TETRAPODS 435
CONODONT ZONES STAGES

Skinnerbukta Pseudolonchodina fluegeli Pterospathodus celloni Aulacognathus bullatus Pseudooneotodas tricornis Pterospathodus amorphognathoides

Ireviken Event Icriodella discreta Ozarkodina hassi O. oldhamensis O? kentuckyensis Pranognathos tenuis Distomodus spp. Carniodus carnulus Pterospathodus
amorphognathoides
Snipklint Primo Telychian
Vik Episode Pterospathodus
celloni

Rytteråker Malmøykalven staurognathoides
Secundo
Distomodus
Episode
Sandvika Event
Jong Primo Episode Aeronian

Solvik Spirodden Distomodus
Secundo
Episode kentuckyensis
Rhuddanian

Figure 16.6 The use of conodont assemblages in stratigraphy: alternation of primo and secundo
oceanic states correlated with part of the Lower Silurian succession of the Oslo region, Norway.
In the stratigraphic column, limestone is shown by a blocky pattern and mudstone by gray.
(Courtesy of Dick Aldridge.)

anterior comb-like ramiform elements proba- linked by tiny muscles. The transition cannot
bly grasped prey items that were sucked be followed in fossils because the gill skeleton
towards the mouth, and the posterior pectini- of jawless fi shes was not mineralized. Molecu-
form elements may have chomped the food lar biologists have even suggested that the
before swallowing. One the greatest mysteries origin of jaws was so profound that it must
in paleontology had been solved. have been associated with a dramatic genome
duplication event – but the fossils say no (Box
16.4).
JAWS AND FISH EVOLUTION Some of the oldest jaw-bearing fi shes were
the placoderms, such as Coccosteus (Fig.
The fi rst jaws
16.8a), which had an armor of large bony
The basal vertebrates, including conodonts, plates over the head and shoulder region, as
lacked jaws, and jaws probably evolved during in the ostracoderms, and a more lightly
the Ordovician. Study of the anatomy of armored posterior region. They swam by
modern vertebrates suggests that jaws may beating this tail region from side to side. The
have evolved from the strengthening bars of edges of the jaws did not carry teeth, but
cartilage or bone between the gill slits, each instead sharp bony plates that would have
of which consists of several elements, all been just as effective in snapping at prey.