Page 110 - Introduction to Paleobiology and The Fossil Record

PALEOECOLOGY AND PALEOCLIMATES 97

Box 4.3 Ecological interactions

Animals and plants have participated in a wide range of relationships throughout geological time.

Ecologists have classified these arrangements in terms of gain (+), loss (−) and neutrality (0). Antago-
nistic arrangements include antibiosis (−,0), exploitation (0,+) and competition (0,0) whereas sym-
biosis involves both commensalism (+,0) and mutualism (+,+).

Antibiosis is difficult to demonstrate although mass mortalities of fi shes have been
ascribed to dinoflagellate blooms. Some paleontologists believe that the twisted skeleton of a

Late Cretaceous Struthiomimus from Alberta may show the animal died from strychnine
poisoning.
Exploitation includes predation and parasitism. There are many records of bite marks, particularly
by marine reptiles on mollusk shells, while the stomach contents of Jurassic ichthyosaurs have
revealed a diet of belemnites. Moreover a wide variety of nibble marks have been reported from
fossil leaves. The relationship between the Devonian tabulate coral Pleurodictyum and the worm
Hicetes fooled many paleontologists. Was this a bizarre compound organism? In fact the worm was
probably a parasite; the association is common throughout Europe and virtually every specimen of
Pleurodictyum has a parasitic worm at its core.

Competition is often difficult to observe directly in the fossil record. Encrusting bryozoans,
however, commonly compete for space and food resources on the seabed. Competition between the
cyclostome and cheilostome clades (see Chapter 12) may have influenced the post-Paleozoic history

of the phylum in favor of the latter. Encrusting bryozoans can also faithfully replicate their substrate,
recording the imprint of a soft-bodied animal or aragonitic mollusk. This process of bioimmuration
(“biological burial”) is a useful means of preserving an organism that otherwise may have escaped
detection.
Commensalism is one of the most common relationships apparent in the fossil record, where small
epifauna or epibionts use larger organisms for attachment and support. Small and immature pro-
ductoid brachiopods are often attached by clasping spines to crinoid stems while microconchids,
previously thought to be Spirobis worms (see Chapter 12), are commonly attached near the exhalent
currents of Carboniferous non-marine bivalves. Some of the most spectacular examples have been
reported from the shells of Devonian spiriferide brachiopods. Derek Ager (University of Wales,
Swansea) reported a succession of epifauna, commencing with Spirobis (microconchids) followed by
Hederella and Paleschara and finally the tabulate coral Aulopora, clustered near the inhalent current

of the brachiopod (Fig. 4.18).

Spirorbis sp.
Hederella filiformis
Paleschara incrustans
(a) (b) Aulopora elleri

Figure 4.18 Commensalism between (a) the gastropod Platyceras and a Devonian crinoid and (b)
Spinocyrtia iowensis with an epifauna primarily located on the fold of the brachial valve adjacent
to inhalant or exhalent currents. (Based on Ager 1963.)