Page 447 - Introduction to Paleobiology and The Fossil Record

434 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Despite the mystery of their identity, con- animal with a conodont apparatus at its front
odonts became key tools in biostratigraphy end. Detailed examination showed that the
(Box 16.3). In addition, because color changes elements were in place and, this time, had not
of the elements can be related to changing been merely eaten by the animal. Ten con-
temperature, conodonts are important indica- odont animals have now been found, as well
tors of thermal maturation. Now paleontolo- as examples from other localities (Aldridge et
gists believe they know what conodont al. 1993a). The Scottish conodont animal is
animals looked like, but it took 150 years to up to 55 mm long, and has a short, lobed
work this out. head with large goggling eyes that are fossil-
The solution came in 1983, when the fi rst ized black, perhaps a stain produced by the
complete conodont animal was found in the visual pigments. Below and behind the eyes is
Granton Shrimp Bed, a dark Carboniferous the conodont apparatus, clearly located where
mudstone on the seacoast near Edinburgh, the mouth should be, showing that conodont
Scotland (Fig. 16.5b). This was an eel-like elements really did function as teeth. The

Box 16.3 Conodonts and biostratigraphy

Detailed biostratigraphic schemes based on conodonts have been established for many parts of the
Paleozoic and Triassic. For example, over 20 conodont zones have been determined for the Ordovi-
cian System, while the Upper Devonian is the most congested interval, with over 30 biozones, each
less than 500,000 years long. In northwest Europe the Carboniferous is routinely correlated on the
basis of conodont zones.
Remarkable precision is now available in some zonal schemes. This has permitted the development
of models for global environmental change during the Early Silurian (Fig. 16.6) tied to a tight con-
odont zonation (Aldridge et al. 1993b). Two oceanic states are recognized: those with oxygenated
cool oceans that had a good vertical circulation and adequate supplies of nutrients (termed “primo”),
and those with warm stratifi ed oceans that had deep saline levels and poor nutrient supplies (termed
“secundo”). Sudden changes between ocean states altered the vertical circulation and nutrient supply
dramatically, perhaps causing extinction events.
These kinds of stratigraphic schemes may depend on geographic zonations. Cambrian conodont
faunas were divided into equatorial (low latitude) warm-water associations and polar (high latitude)
cool-water associations. During the Early Ordovician, these low- and high-latitutde assemblages
further divided into six discrete provinces. Conodonts evolved independently at high latitudes, and
there were only a few incursions from lower-latitude faunas. Towards the end of the Ordovician
high-latitude, cold-water faunas migrated into lower latitudes. Thus Late Ordovician equatorial
mid-continent assemblages originated in polar and subpolar regions and themselves formed the
foundation for the Silurian fauna. During the Mid and Late Paleozoic, conodonts were mainly
restricted to tropical latitudes. Devonian and Carboniferous faunas show some biogeographic dif-
ferentiation among shelf associations. These differences among the geographic provinces can affect
the stratigraphic schemes and the possibility of correlation from area to area.
Conodonts occur in a wide range of marine and marine-marginal environments, although the
group is most common in nearshore carbonate facies, commonly in the tropics. Distinct environ-

ment-related conodont paleocommunities have been identified in many parts of the Paleozoic, and
statistical analysis may discriminate, for example, deeper-water from shallow-water assemblages. It
is important to be aware of the infl uence of depth and other factors on the distribution of communi-
ties before they are used in establishing biostratigraphic zones. It would clearly be a mistake to
identify distinctive depth-determined conodont assemblages and then to interpret them as indicators
of different time intervals.
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