Page 231 - Introduction to Paleobiology and The Fossil Record

218 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

was a monument to our ignorance. Although Acritarchs consist of vesicles composed of
many more taxa have been described since, various polymers combined to form sporopol-
and their value in biostratigraphic correlation lenin (Fig. 9.12). They range in shape from
has been proved, uncertainty still surrounds spherical to cubic and in size from usually 50

the origin and affinities of the group. Similar- to 100 μm, although some specimens from
ity, however, with the cyst stages of modern the Triassic and Jurassic are as small as 15–
prasinophytes and dinofl agellates suggests a 20 μm. Many lose these morphological details

relationship to primitive green algae. However, when preserved as fl attened films in black
because they are very useful in hydrocarbon shales. There is a huge variety of basic shapes
exploration, perhaps the minor issue of their (Fig. 9.13). Acritarchs can have single- or
identity can be left for future generations! double-layered walls; the wall structure is
often useful taxonomically. The central cavity
Morphology and classifi cation or chamber can be closed or open externally
through a pore or slit called the pylome. The
The composition and broad morphology of opening or epityche presumably allowed the
the acritarchs suggest similarities with the escape of the motile stage and may be modi-
dinocysts; like the dinocysts, acritarchs are fied with a hinged fl ap.

also often found in clusters. The group prob- On the outside the acritarch may be smooth
ably had a similar life cycle to that of the or, for example, have granulate or microgran-

dinoflagellates, single-celled protists that ulate ornament. Moreover, the vesicle may be
mainly live in the marine plankton today. modified by various extensions or processes

Acritarchs seem to show encystment struc- projecting outwards from the vesicle wall. If
tures, or cysts – protective devices similar to an acritarch has a set of similar processes,

those of modern dinoflagellates, in which the they are termed homomorphic, and if it
organism can survive drying out or lack of has a variety of different projections it is
food for long periods. When conditions return heteromorphic.
to normal, usually when the cyst is covered Over 1000 genera of acritarchs are known,
with water again, the organism “escapes” by defined mainly on shape characteristics (Box

bursting through the watertight skin of the 9.7). All acritarchs were aquatic with the vast
cyst, and resumes feeding and reproducing. majority found in marine environments. The
A number of escape structures have been classification of the group is based on the wall

described including median splits, pylomes structure, the shape of the body vesicle,
and cryptopylomes, that would have allowed pylome type and the nature of the extensions
material to seep out. and processes.

Box 9.6 Ernst Haeckel, art and the radiolarians

The link between art and paleontology has always been strong, with many images fi nding their
inspiration in the beauty of the fossil form. Ernst Haeckel (1834–1919), the German evolutionary
biologist, responsible for such terms as “Darwinism” and “ecology”, the phrase “ontogeny recapitu-
lates phylogeny” and the first detailed tree of life (see p. 128) was also an accomplished artist; he

believed in the esthetic dimension of morphology (Fig. 9.11). His giant opus Art Forms in Nature
(1899–1904) is considered to be one of the most elegant, artistic works of the 19th century, his
illustrations being a paleontological precursor to the Art Nouveau movement. His style is nowhere
better presented than in his monograph on the Radiolaria (Haeckel 1862). Unfortunately his attempts
to associate science with art may have damaged his career, but current interest in the tree of life has
generated a Haeckel renaissance. His illustrations are even available now as an attractive
screensaver!
You can see these beautiful images at http://www.blackwellpublishing.com/paleobiology/.