Saturday, 7 December 2013

What is the relationship between the stratigraphic record and animal diversity in the fossil record?


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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