Page 281 - Introduction to Paleobiology and The Fossil Record
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268 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD
Stromatoporoid morphotypes
laminar low domical low domical low domical
high domical
extended domical bulbous
side
view
Terminology
ragged
Mamelons
Morphotype classes: non-enveloping
smooth
Laminar: V/B up to 0.1
Low domical: V/B 0.1–0.5
High domical: V/B 0.5–1 V
Extended domical: V/B > 1 enveloping
B non-enveloping
Figure 11.8 Stromatoporoid growth modes. (Based on Kershaw, S. 1984. Palaeontology 27.)
poroid skeleton; both bivalves and brachio-
pods have been seen in borings within Archaeocyatha
stromatoporoids that may have provided The Archaeocyatha or “ancient cups” are one
some of the fi rst cryptic habitats for Phanero- of only a few major animal groups that are
zoic biotas. entirely extinct. They appear to have been an
Animals with a stromatoporoid grade of evolutionary dead end. The group exploited
organization have been identifi ed from rocks calcium carbonate during the early part of the
of Botomian age; however these forms were Cambrian radiation to construct porous cup-
apparently short lived. Pseudostylodictyon or cone-like skeletons, usually growing
from the Middle Ordovician of New York together in clumps and often living with stro-
and Vermont may be the oldest true stromato- matolites to form reefs. The Archaeocyatha
poroid, derived from a soft-bodied, sponge- dominated shallow-water marine environ-
like ancestor in the Early Ordovician. ments, usually in tropical paleolatitudes. From
Stromatoporoids formed the basis for reef an Early Cambrian origin on the Siberian
ecosystems during the Silurian and Devonian, Platform, the group spread throughout the
becoming largely extinct during the end- tropics, forming the fi rst Paleozoic reefs.
Frasnian (Late Devonian) event. The group However, by the end of the Early Cambrian
revived in the Mid and Late Jurassic when and the start of the Middle Cambrian,
stromatoporoids again participated in reef archaeocyaths are known only from Austra-
frameworks. Nevertheless, most groups dis- lia, the Urals and Siberia. They disappeared
appeared at the end-Cretaceous mass extinc- at the end of the Cambrian.
tion. However, some living sponges have a Current studies suggest that the Archaeo-
stromatoporoid grade of organization; cyatha have a grade of organization similar
Astrosclera and Calcifi mbrospongia are both to poriferans; in fact most authorities
calcified demosponges with a stromatoporoid would place the group fi rmly within the
architecture. sponges as a separate class. Because no living
