I finished the first of the essays, which was an assignment for a tutorial session on Monday. It is not formally marked, so I doubt I would get in trouble for putting it here as well. It should be simple enough for most people to understand anyway :)
The impact of animals on geological processes may not be intuitively
appreciated, but even large-scale differences in the outlook of our modern
world compared to the times where no animals were around could be attributed to
animals, directly through erosion and biomineralisation, and indirectly
through their effects on nutrient cycles, plant distribution, and their various
ecological roles.
This
essay will discuss the role of ancient animals on the stratigraphic record –
the sequence of preserved rock strata – in particular the impact of their
diversity. The implied focus is on animals preserved in the fossil record, but
the role diverse of animals in weathering and erosion will also be considered.
Biogenic
strata consist of rocks derived from parts of organisms, typically chemically
altered into rock (lithified), and comprise the clearest connection between
animal remains and the rock record. Limestones and dolomites are familiar
examples, their calcium carbonate component often derived from biomineralising
organisms, many of which are animals that secrete calcareous shells or
exoskeletons. These include corals, poriferans, bryozoans, echinoderms,
brachiopods, most molluscs, and some marine arthropods (e.g. trilobites). The
diversity of calcareous animals may affect the precise composition of the
limestone or dolomite. Presumably, differences in precipitation rate of these
diverse animals also influence the rate of carbonate rock formation. It should,
however, be noted that biomineralising microbes, such as diatoms,
foraminiferans and some dinoflagellates, also play a significant part in the
formation of biogenic rocks.
Some
of the mentioned phyla also include forms that compose their exoskeleton of
other compounds, such as chitin (e.g. some bryozoans, many arthropods,
craniiform brachiopods). Chitin is a polysaccharide lacking the preservation
potential of calcium carbonate, and is especially susceptible to degradation in
marine settings (Stankiewicz et al.
1998) . Therefore, a high proportion of animals with chitinous exoskeletons,
relative to calcareous ones, may influence the carbonate content of the derived
rock. The chitinous organisms are also less likely to be preserved, which would
manifest as an underrepresentation of diversity in the fossil record, possibly
quantitatively proportional to the carbonate content in the rock – in other
words, there may be a numerical relationship between animal diversity and the
chemical composition of biogenic rocks.
Biostratigraphy
is the subdivision of the stratigraphic record into units defined by their
fossil content. The definitions are made on the basis of biozones: assemblages
of index fossils – which ideally (1) are common, and have (2) distinct
diagnostic features and (3) rapid speciation rates. Thus, high diversity is an
implied requirement for a good index fossil taxon. Animal examples include
graptolites and belemnite cephalopods. Their diversity is helpful for our
cataloguing (and later our understanding) of the stratigraphic record.
Identical or similar fossil assemblages also play a considerable role in
correlating strata chronically across their spatial distribution around the
world, as many taxa provide a quick field estimate of rock age, and thus a
context for interpretation of other features under study.
In
addition, a comprehensive section in the fossil record may assist in the
identification of paraconformities (unconformities where the strata above and
below the unconformity plane are parallel). A diverse record of fossils would
improve the chances of discovering a gap in the sequence, and multiple fossils
may corroborate the interpretation.
Moreover,
this relationship may be viewed from the opposite perspective: an incomplete
stratigraphic record influences the apparent diversity of fossil in the rock
record. Missing strata equates with missing fossils; the relationship is
two-way. For example, the Late Cretaceous hiatus (paucity in the fossil record)
of sauropods in North America may be attributed to preferential preservation of
coastal sediments (Lucas & Hunt 1989, Mannion & Upchurch 2011), while
North American sauropods of that age may have preferred inland habitats (Mannion & Upchurch 2010). Thus, what
appears as an extinction of sauropods on the North American continent may
simply be due to bias in the stratigraphic record.
The
role of animals in rock weathering and sediment erosion may not be obvious, but
it is nonetheless significant. For example, aeolian erosion requires sediment
particles to be ejected into the air before the wind can transport them, as
wind speed at the ground surface is negligible; animals can assist in this, be
it by small bioturbating vertebrates and insects, or large ungulate herds
stirring up dust as they migrate (Tarbuck et
al. 2011).
Animals
may also be significant for chemical weathering, as they can be regarded as
mobile digestive systems. Their guts maintain optimal conditions for catabolic
reactions, albeit in particular of organic molecules (Beerling &
Butterfield 2012), and their mobility enables animals to transport the material
away from the site of ingestion before egesting the weathered products into a
potentially different environment. Examples include a diversity of migratory
aquatic animals, such as diadromous fish, sharks, eels and cetacean mammals.
In
addition, soil-living animals play an important role in soil formation, which
involves the chemical weathering of sediment, and thus an alteration of the
stratigraphic record. Their metabolic diversity determines the range of
transformation processes that are possible. However, soil microbes and fungi
arguably play a greater part in soil formation than animals.
Finally,
the sheer size and mobility of animals enables them to function as agents of
physical weathering and transport. Benthic animals rarely break bedrock, but
stir loose sediment and may leave distinct traces, burrowers in particular.
Terrestrial animals, free from the ancestral bond of the water, may assert
their influence over inland environments, expanding the range of bioturbation.
Terrestial animals also have great potential for transport of sediment, even
between aquatic and terrestrial environments. However, the likelihood of
leaving noticeable marks in the stratigraphic record may be negligible.
It
should be noted once again that the role of animals as erosional agents only
has one foot in the implicit scope of this essay. Fossil animals are unlikely
to influence weathering or erosion, as they are long dead. However, the
persistence of fossil-supported (a variant of clast-supported) strata may be
determined by the resistance of the fossil material. Also, as food for thought,
it is not impossible that fossilised animals may have influenced younger strata
– e.g. by eroding them or assisting in soil formation – in their lifetime!
References
· Beerling, David J., Butterfield, Nicholas J. 2012.
Plants and animals as geobiological agents in Fundamentals of Geobiology ed.s Knoll, Andrew J., Canfield, Don E.,
Konhauser, Kurt, O. Blackwell Publishing Ltd. Chapter 11. Pp. 188-204
· Lucas, Spencer G., Hunt, Adrian P. 1989. Alamosaurus and
the sauropod hiatus in the Cretaceous of North American Western Interior. in Paleobiology of the Dinosaurs ed.
Farlow, James O. Boulder. Colorado. Pp. 75-85
• Mannion, Philip.
D., Upchurch, Paul. 2010. A quantitative analysis of environmental associations
in sauropod dinosaurs. Paleobiology 36(2). Pp. 253-282
• — 2011. A
re-evaluation of the ‘mid-Cretaceous sauropod hiatus’ and the impact of uneven
sampling of the fossil record on patterns of regional dinosaur extinction. Palaeogeography, Palaeoclimatology,
Palaeoecology 299. Pp. 529-540
· Stankiewicz, B. A., Briggs, D. E., Evershed, R. P.,
Miller, R. F., & Bierstedt, A. 1998. The fate of chitin in Quaternary and
Tertiary strata. In ACS Symposium Series. American Chemical Society. Pp.
211-225
· Tarbuck, Edward J. Lutgens, K., Tasa, D.
2011. Earth – An Introduction to Physical
Geology. 10th ed. Pearson Education Inc. New Jersey.
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