Historical Geology: Paleontology, part 4

Alessandro Grippo, Ph.D.
FOSSILS AND THE ENVIRONMENT - PALEOECOLOGY

part 4: Diversity, Fossils as Environmental Indicators, Ecology & Paleoecology

diversity

Last Updated • September 29, 2015

Diversity can be measured at different taxonomic levels by the number of families, genera, or species within a group.

This means that if, for example, a genus has 10 species and another has only 2,
the second is much less diverse than the first.

During geologic time the diversity of many groups has changed vastly,
reaching diversity peaks (many species) at some times and sinking to diversity lows at others.

The following figure shows an example of diversity variation over time:
the width of the yellow field is proportional to the number of different species of trilobites
(a geologically very important class of Arthropods).

Trilobites appeared in the Cambrian period and "boomed", or expanded very quickly,
(with different species, not simply with several individuals of the same species),
until the Ordovician, when their maximum evolutionary explosion occurred.

After the Ordovician, trilobites' diversity started a slow decline;
they eventually became extinct during the Permian
having lived (as a group) for a total time span of about 250 million years.

fossils as environmental indicators

Last Updated • September 29, 2015

As we have seen previously, fossils are also very important from an (ancient) environment point of view:
it is possible to get information about the ecological conditions
that existed at the time the rocks containing the fossils formed

Heavy-shelled organisms are often associated with high-energy environments,
such as swift rivers and streams and waves
while delicately shelled organisms point towards a quiet, low-energy environment, such as a lagoon or a pond.

Trying to get information about how a certain organism behaved is more difficult
and it becomes necessary to work on modern, analog life forms, applying the principle of uniformitarianism.
(for instance, the study of contemporary Nautilus has provided information about the extinct Ammonites)

But what happens if a fossil is eroded, transported and reburied
in sediment pertaining to a different sedimentary environment?

Both temporal and environmental straight interpretations would be not true
and the paleontologist would have to look for signs or erosion, such as abrasion or wear,
or look for an assemblage of fossils, (that is a group, or a suite of fossils),
that would provide much more significant results in terms of probability to have been deposited in situ
(that is, in the place where it has been found).

ecology and paleoecology

Last Updated • September 29, 2015

Modern ecology studies how living organisms are interrelated to the chemical and physical conditions
of their environment and to other organisms that share that environment

Paleoecology deals with the same problems for ancient organisms
(that is, we are trying to reconstruct ancient ecological systems by studying fossils).

Paleoecology relies heavily on the knowledge of contemporary organisms' ecology.
Organisms today live mainly in the atmosphere (the air),
in the hydrosphere (the water in oceans, rivers, lakes, etc.)
and in the lithosphere (at the surface, but also in some cases, underground).

Many species spend some time in the atmosphere, but none spends its entire life cycle in the air.
Species that live on land can either move around on the ground, fly,
burrow in the soil or, in the case of plants, be rooted in the soil.
Microbes (bacteria) can actually spend their whole life in the soil.
Organisms that live in the ocean can be divided into three categories:

Plankton
These organisms simply float in the ocean, or can very weakly swim.
They are limited by waves and ocean currents
They include phytoplankton and zooplankton
Nekton
These organisms are able to swim (strongly)
They are not subject to physical limits such as waves and ocean currents
Benthos
These organisms live at the bottom of the ocean (or a lake, or a river)
They can move at the bottom (vagrant benthos), or they can be anchored in one fixed position (sessile benthos); of these,
some are physically attached to the bottom (epifauna), like a tree on land, while others live underground, in burrows or boreholes (infauna).

how materials and energy move within an ecosystem

Last Updated • September 29, 2015

Every single ecosystem contains nutrients, organic and inorganic molecules
that work as fertilizers for producers (plants or algae, that is photosynthetic organisms).

Producers, as you remember, also use carbon dioxide and sun energy to create, generally speaking,
carbohydrates (a source of energy or, more simply, food):

6H₂O + 6CO₂ ---> C₆H₁₂O₆ + 6O₂ (photosynthesis)

Animals and other non-photosynthetic organisms (consumers)
must feed on producers in order to be able to sustain themselves.
Consumers might be herbivores, which feed directly on plants or algae;
carnivores, which eat the herbivores or other carnivores;
omnivores, which eat both plants (producers) and animals (consumers);
scavengers, which eat dead animals;
and parasites, which feed on a living organism.
Decomposers (bacteria and fungi) recycle dead plants and animals back to nutrients,
which in turn become newly available for the cycle of life as fertilizers.

References: - Vittorio Vialli, Notes from the Paleontology Lessons, Pitagora, Bologna
- Jon M. Poort and Roseann J. Carlson, Historical Geology, Interpretations and Applications (6^th ed.), Pearson/Prentice Hall, NJ