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What is homeomorphy?

I first contemplated homeomorphy in an exam hall, answering a question set by John Haynes for the end of year master’s exams at the University of Aber­ystwyth. The question was something along the lines of, ‘Compare and contrast the alveolines and the fusulines’. Simple. Both are foraminifera but the fusulines are calcareous and Carboniferous and the alveolines are porcellaneous and Cenozoic. But of course the question went deeper than that.

Why? Both lineages have larger forms that are fusiform in shape, but one was from the Carboniferous and the other from the Cenozoic. Why should, or even could, nature repeat itself in two such distinct groups? And why do larger forms want to get so large? What are the mechanics that allow a single-celled animal to become so large and cigar shaped that you can imagine picking it up and smoking it?

Closer examination reveals the differences in the ontogeny of the families. Fusu­lines have secondary septae which divide the chambers. They have no aperture on the last chamber but an apertural face with septal pores. In alveolines the initial proloculum is followed by a spiral passage that becomes planispirally coiled. Secondary septae are formed parallel to the direction of coiling and the test walls are imperforate. Apertures are round pores in the apertural face cor­responding to the position of the chamberlets.

But these are details. My favourite paper explaining homeomorphy in the planktonic foraminifera is Neagu (2005). He observed that two similar environmental events — intervals of global maximum sea temperatures — correspond to two principal times of evolutionary radiation in planktonic foraminifera. First the Late Albian rotaliporids and hedbergellids at the time of maximum sea levels at the Albian–Cenomanian boundary. Then a second period of maximum temperature in the Coniacian which resulted in explo­sive radiation of the marginotruncanids and rugoglobigerinids. Neagu con­cluded that the same causes lead to the same results during the evolution of certain groups: ‘Owing to similar environmental changes… the organisms react also alike.’

Caron (1985) described examples of homeomorphy in her clear and precise summary of the Cretaceous planktonic foraminifera which many of us refer to constantly. She explained: ‘The interaction of form and function forces the tests of the Globigerinacea to develop morphological characters that reoccur several times during the Cretaceous in members of different lineages’. Her taxonomic overview helps the biostratigrapher recognize this convergence of form and illustrates several examples.

Another example of homeomorphy was described by Keen (1988) in his de­tailed review of the cytherettid ostracods. He suggests that the Tethys Ocean was a barrier to migration and restricted their distribution to the northern hemi­sphere, mostly Europe, until the Late Palaeogene. Species occurring in South America, South Africa, and southeast Asia assigned to the family are homeo­morphic and not true cytherettids. He also warned us that within the family the development of a tricostate exterior ornament has occurred independently several times.

So beware when you have to invoke reworking or caving to explain the juxta­position of the fossils you see. Reflect a little on the possibility of homeomorphy and look a little closer if the preservation allows it — and the strength of your microscope, the steadiness of your hand, and most of all your patience. Detailed analysis by the taxonomists may reveal different lineages; but usually the indus­trial (or industrious) micropalaeontologist doesn’t have the time.

For the non-micropalaeontologists who wonder why we sometimes change a species name and subsequently the age and correlation, homeomorphy may be the explanation. Only the fossils are perfect.


References

Caron, M (1985). Cretaceous planktonic foraminifera, in Bolli, H, J Saunders, and K Perch-Nielsen (eds.). Plankton Stratigraphy. Cambridge University Press, Cambridge. 17–86.

Glaessner, M F (1972). Principles of Micropalaeontology. Hafner Publishing Company, New York. 297 p.

Keen, M C (1988). The evolution and distribution of Cytherettine Ostracods. In: N Ikeya, K Ishizaki, and T Hanai eds. Evolutionary Biology of Ostracoda: Its Fundamentals and Applications. Elsevier Science. 967–986, DOI 10.1016/S0920-5446(08)70233-4.

Neagu, T (2005). Albian Foraminifera of the Romanian Plain. Acta Palaeontologica 5, 311–332.

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