Page 65 - Introduction to Paleobiology and The Fossil Record

52 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

mapping metamorphic zones in orogenic
belts. Conodonts in particular (see p. 429) are
useful thermal indicators. They change color
from light amber to gray to black and white,
and eventually translucent, on a scale of con-
odont alteration indices (CAI values) from 1
to 8, through a temperature range from about
60 to 600˚C. Carbonaceous organisms, includ-
ing the graptolites (see p. 412), also show
color changes, as does vitrinite derived from
plant material. These changes have also been
documented in detail for acritarchs (see p.
Figure 2.19 Strained Cambrian trilobites from 216), where acritarch alteration indices (AAI
Himalaya. (Courtesy of Nigel Hughes.) values) range from 1 to 5. Spores and pollen
have spore color indices (SCI values) ranging
from 1 to 10, with colors ranging from color-
less to pale yellow through to black. Other
Can the actual color of fossils help us groups such as phosphatic microbrachiopods
understand the geological history of an area? and chitinozoans show similar prospects, but
The investigation of thermal maturation is their color changes have yet to be calibrated
now a routine petroleum exploration tech- with precise paleotemperatures. Paleotemper-
nique. A number of groups of microfossils atures can also help predict the oil and gas
change color with changing paleotemperature window, usually located at depths between
(Table 2.2). The upper end of the thermally- 2.5 and 3.5 km, and thus have important
induced color range has proved useful in application to hydrocarbon exploration.

Box 2.8 Scandinavian Caledonides

Mountain belts are a source of all sorts of exciting and significant fossil assemblages. The Scandi-

navian Caledonides are no exception. This mountain belt stretches for some 1800 km from north
to southwest Norway, never exceeding a width of 300 km. It developed during a so-called Wilson
cycle (the opening, closing and subsequent destruction of an ancient ocean, named after J. Tuzo
Wilson) culminating in the collision of the Baltic plate with those of Avalonia (England, Wales and
parts of eastern North America and north central Europe) and then Laurentia (cratonic North
America). During its transit from high to low latitudes in the Early Paleozoic, Baltica rotated anti-
clockwise and first captured terranes adjacent to the craton itself with Baltic faunas, followed by

island terranes from within the Iapetus Ocean, with endemic taxa, and finally island complexes that

were marginal to the Laurentian plate with North American faunas (Harper 2001). The mountain
belt in its pile of thrust sheets thus stores much of the biogeographic history of the Iapetus Ocean
and its marginal terranes (Fig. 2.20). Moreover during the Late Silurian-Devonian, as the mountain
belt continued to rise, marginal basins contained remarkable marine marginal biotas with spectacular
eurypterid faunas. Adjacent basins, for example in Scotland, contain some of the earliest land arthro-
pods and plants. So the collision of plates and the generation of a huge mountain belt was not
entirely a destructive process. It has helped preserve key evidence for an ancient ocean with diverse

and endemic faunas that helped contribute to the great Ordovician biodiversification event (see p.
253) while its later non-marine basins hold critical information on the early development of life on
land (see p. 442).