Page 111 - Introduction to Paleobiology and The Fossil Record

98 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

can exist for tens or even hundreds of millions association, and fi nally a monospecifi c assem-
of years (Sheehan 2001). blage characterized by a late successional
Despite the fantastic potential to test models dominant (Wilson 1985). Environmental dis-
for community change through time there turbances, such as the tipping over of the
have been relatively few rigorous attempts. cobbles, allowed a recolonization of the
Some paleontologists have recognized a hardground, thus maintaining high diversity
pattern of coordinated turnover followed by within the paleocommunity. Clearly in some
stasis in which many species disappear over a cases an Eltonian model may be applicable
short time interval and are replaced by other and in others Gleasonian paradigms rule.
functionally and taxonomically similar species.
The new assemblages may retain their struc-
ture for 2–8 million years (Brett et al. 1996), Evolutionary paleoecology
although in some cases this appears to occur Biodiversity trends through time for the
through repeated reassembly following dis- majority of animal and plant groups have
turbances. By contrast Ordovician marine been documented in some accuracy and detail
assemblages from the Appalachians (see p. since the late 1970s (see p. 534). However, it
38) show a strong relationship between envi- is clear that there are a series of ecological
ronmental fl uctuations and uncoordinated changes underpinning this incredible taxo-

changes in the composition and dominance of nomic diversification and such changes were
animals in their assemblages. Even life at a probably decoupled from each other. For
small scale shows these patterns. An Ordovi- example, there have been marked changes in
cian hardground paleocommunity constructed the use of ecospace through the evolution of
by encrusters, mainly bryozoans and edrioast- new adaptive strategies (Bambachian mega-
eroids, on cobbles shows fi rst a low-diversity guilds), an escalation in the number of guilds
pioneer community, then a high-diversity and accelerated tiering both above and below

Box 4.4 Chemosynthetic environments

Amazing new discoveries in the modern oceans have revealed some of the most bizarre living crea-
tures that survive in the dark, cold depths, clinging onto the life support provided by hydrocarbon
seeps and hydrothermal vents. Uniquely, these bizarre organisms never see the light, and their food
chains are not based on sunlight and carbon, but on sulfur from hydrothermal vents. The search is
on to find their fossil counterparts. Kathleen Campbell, together with a range of colleagues, has been

exploring the distribution of these types of weird communities through time (Campbell 2006). She
has identified 40 fossil examples, recognized on the basis of key types of faunas, specifi c biomarkers

and their geochemical and tectonic settings. Such communities associated with Precambrian vents
were populated by microbes and it was not really until the Silurian that we fi nd our fi rst groups of
metazoan chemosynthetic organisms. These organisms are strange. Gigantic non-articulated brachio-

pods are associated with large bivalves and worm tubes in a massive volcanic sulfide in the Ural
Mountains (Fig. 4.19). In general terms, pre-Jurassic vent faunas were dominated by extinct groups
of brachiopods, monoplacophorans, bivalves and gastropods, and post-Jurassic faunas were popu-
lated by extant families of bivalves and gastropods. The modern vent-seep fauna is endemic and may
either have evolved from a collection of Paleozoic and Mesozoic relics or perhaps some invertebrate
groups periodically migrated into the gloom during the Phanerozoic to set up their own communi-
ties. Although unusual, the chemosynthetic world was yet another ecosystem with its own set of
rules and evolutionary paradigms, contributing to the past and present biodiversity of our planet.
Perhaps, also, during times of major and rapid environmental change, this ecosystem provided a
stable refugia where at least some organisms could escape fluctuations in those other ecosystems that

rely on light and organic nutrients.