Page 233 - Introduction to Paleobiology and The Fossil Record

220 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Box 9.7 Classifi cation of the main organic-walled groups: acritarch form
classifi cation

The classification of acritarchs is based entirely on morphology and as such is merely a set of shape

and ornament categories with no phylogenetic status. The names of the main groups thus are also
used as morphological terms to define the variation in shape (see Fig. 9.12). Clearly such a classifi ca-

tion is rife with convergent morphotypes that may never be properly classified. Recent studies,
however, suggest that understanding the mode of encystment may be a step towards the development
of a more phylogenetic classifi cation.

ACRITARCHS WITHOUT PROCESSES OR FLANGES
Sphaeromorphs
• Spherical forms lacking processes but with ornamented walls. These morphs are often variably
ornamented
• Precambrian (Animikean) to Recent

ACRITARCHS WITH FLANGES BUT LACKING PROCESSES
Herkomorphs
• Subpolygonal or spherical with polygonal ornament defi ned by crests
• Cambrian (Lower) to Recent
Pteromorphs
• Forms equipped with an equatorial fl ange
• Ordovician (Caradoc) to Recent

ACRITARCHS WITH PROCESSES BUT WITH FLANGES
Acanthomorphs
• Spherical forms lacking an inner body and crests, with simple or branching processes
Polygonomorphs
• Polygonal forms with simple processes

excystment
opening process
vesicle

endophragm
periphragm

Figure 9.12 Descriptive morphology of the acritarchs.