Ammonites and Scientific Writing

I recently had to write a short piece for my tutor on the use of ammonites in palaeontology. It was a slight departure from my normal style of writing, as he prefers things to be succinct and straight to the point.

Had I been allowed to write in my usual self-indulgent style, my essay may have started something like this:

Ammonites: More Than Just a Paperweight.

The iconic fossil ammonite, elegantly simple in outward morphology, provides lucid prose for palaeontologists perusing what lays written in the rocks. Their Mesozoic ubiquity, with a characteristic evolutionary pace (which appears geologically hasty, almost eager) renders them excellent index fossils. The gradual evolution their lithified remains display is the bread and butter of biostratigraphy, facilitating dating for even shy, phlegmatic geologists.

As it stands, that is not what I wrote, here is my essay:


The Applied Palaeontology of Ammonites

Introduction

The iconic ammonites are a group of cephalopods of Subclass Ammonoidea which are invaluable in their application in palaeontology. ‘Ammonites’ is the vernacular term (after Order Ammonitida) for the Mesozoic forms of Ammonoidea, a Subclass which spans approximately 325 Ma from the Devonian to the Cretaceous. The major use of ammonoids lies in biostratigraphy where they are regularly used for zoning the rocks in which they are found, particularly the Mesozoic forms which have allowed for zones to be erected equivalent to less than a million years.

Ammonoid Biostratigraphy

Good index fossils need to have a wide distribution with high abundance, a high rate of evolution and be easily identified; all of which are characteristics of the ammonoids. Shortly after their first appearance in the Devonian, the ammonoids rapidly spread world-wide (House, 1981) and maintained this distribution until their demise in the K/T extinction. They are ubiquitous, particularly in Mesozoic strata, rendering them a highly effective tool for stratigraphy in the field. Although some orders, such as Order Phylloceratida, display little evolution over millions of years (Clarkson, 1998), the vast majority of ammonites show the characteristic rapid morphological change and high speciation rate preferred in index fossils. The identification of an ammonite (excluding heteromorphic ammonoids) is relatively easy, hence their iconic status, and they can easily be used to quickly identify whether the rock is Palaeozoic or Mesozoic with little inspection by an amateur.

The ammonoids display many evolutionary trends and diverse morphological characters readily identifiable in single specimens which allow for their specificity in dating rocks. Ammonoids are commonly found preserved as internal moulds which display the sutures between septa and shell. Fossils with preserved suture lines demonstrate a clear stratigraphical trend from the relatively simple Devonian and Carboniferous sutures, to the extremely complex and flamboyant Mesozoic sutures. Changes in sutures can be used to quickly differentiate between the two eras; Palaeozoic ammonoids have generally zigzagged sutures, whereas Mesozoic ammonites possessed sutures with complex lobes and saddles. Although some ammonoids do not easily fit into this trend – some Permian ammonoids have similar sutures to Mesozoic ammonites – these deviations can be identified using other morphological characters, allowing suture morphology to be utilised for high stratigraphical accuracy when studied in detail and can also be used in the study of ontogeny.

A famous example of a useful lineage of ammonites in biostratigraphy is the Jurassic Family Cardioceratidae, which spanned 20 Ma and can be traced through 28 zones and 62 subzones. They have been described in monospecific assemblages, making them easily identifiable, and they rapidly diversified, allowing for many easily observed trends and accurate dating. These include easily identified changes in the compression of the whorl, in rib shape and in the ornamentation of the keel.

During the Mesozoic, the abundance and diversity of ammonites has even allowed for accurate stratigraphy during extinctions in conjunction with other techniques (Guex et al, 2004).

Other Uses

Aside from biostratigraphy, their global distribution and rapid diversification has allowed ammonites to be used in determining the position of continents during continental drift (Kennedy et al, 1975) along with facilitating the dating of these events.

Conclusion

Ammonites form a group with a basic common shell plan along with a propensity for fossilisation, exceptional diversity, rapid evolutionary change and wide distribution, making them easily recognisable and one of the most useful fossil groups for application in biostratigraphy to a high degree of resolution; they are an invaluable tool for any palaeontologist studying the Palaeozoic and Mesozoic.

References

Clarkson, E.N.K. (1998). Invertebrate Palaeontology and Evolution (4th ed.). Oxford: Blackwell Science.

Guex, J., Bartolini, A., Atudorei, V., & Taylor, D. (2004). High-resolution ammonite and carbon isotope stratigraphy across the Triassic-Jurassic boundary at New York Canyon (Nevada) [Electronic version]. Earth and Planetary Science Letters, 225(1-2), 29-41.

House, M.R. (1981). Early Ammonoids in Space and Time. In M.R. House, & J.R. Senior (Eds.), The Ammonoidea. The Evolution, Classification, Mode of Life, and Geological Usefulness of a Major Fossil Group (pp. 359-367). London: Systematics Association Special Volume No. 18, Academic Press.

Kennedy, W.J., & Cooper, M. (1975). Cretaceous ammonite distributions and the opening of the South Atlantic [Electronic version]. Journal of the Geological Society, 131(3), 283-288.