Tertiary

Tertiary The stratigraphical term ‘Tertiary’ is one of the oldest still in use, though it has in fact been superseded largely by ‘Palaeogene’ and ‘Neogene’ combined or by ‘Cenozoic’ which embraces a slightly longer time-span. It was originally coined by Giovanni Arduino (1714–95), a professor at the University of Padua. In 1758 he recognized three major divisions in the rock succession of the Apennine mountains of central Italy, naming them Primitive, Secondary and Tertiary. The last of these was a group of very fossiliferous sedimentary rocks, not very strongly indurated and hence sharply different from the hard strata of the Secondary and the crystalline rocks of the Primitive categories. It is the only one of Arduino's terms to survive, but its meaning is now different from the original.

Charles Lyell (1833), writing in London, adapted the word for the post-Mesozoic strata of western Europe; his ‘Tertiary Period’ was subdivided into Eocene, Miocene, and Pliocene (Older and Newer), and was followed by his ‘Recent Period’. Lyell's division of this new unit was based on a new concept: that of dividing the Tertiary on the basis of the percentage of living species to be found among the molluscan fossils at each level. The fossils had been carefully collected by the French conchologist Paul Gerard Deshayes (1797–1875), and Lyell was able to reach a conclusion similar to that of Deshayes, on the basis of about 8000 species allotted to over 40 000 specimens from the Paris basin. Lyell was really intent upon establishing the use of molluscan fossils as a sort of palaeontological ‘clock’. By finding the percentage of modern species in a sample from anywhere within the Tertiary rocks, a geologist would, he thought, be able to establish the relative age of the sample. Subsequent events were to prove that local geologists found it anything but easy to put the concept to work. Disputes and mistaken correlations were soon all too common. It was a nice idea, but unfortunately the rates of extinction and replacement are neither uniform nor constant. The process is decidedly episodic rather than smoothly gradual. Recent work on these rocks and fossils makes out a rather different picture of the taxonomy at each stage, and much of the fossil mollusc fauna of the Paris basin is restricted to that region. Lyell's terms have by now all been redefined, but remain in use as part of the Cenozoic.

Invertebrate fossils apart, the Tertiary strata to which so much attention was given by Lyell and his contemporaries are also remarkable for their mammalian remains and also, locally for their teleost fishes. The mammals include many marsupials and large placental forms which arrived on the scene abruptly after the extinction of the dinosaurs. The remains of mammals are, in fact, so numerous and stratigraphically sensitive in parts of the equivalent continental rocks in North America that biostratigraphic mammal zone successions have been drawn up there.

The Global Stratigraphic Chart published by the International Union of Geological Sciences (1989) uses the term ‘Cenozoic’ for all the post-Cretaceous part of the column. Arduino's Primitive (Primary), Secondary, and Tertiary categories have all dropped from modern use. The term ‘Quaternary’ remains as a strange relic of past geological debate, but it is well defined and understood, despite the fact that some geologists wish to make it redundant. Ironically, it was never used by Lyell. Today ‘Tertiary’ is being replaced by the terms ‘Palaeogene’ and ‘Neogene’, which denote two sub-periods that are justified on the basis of both physical and biostratigraphy in a way that would have won Lyell's approval.

During the Tertiary times the ocean ridges diminished in volume to a marked degree, and this has probably been responsible for most of the withdrawal of ocean waters from continental areas. Tertiary stratigraphy is thus dominated by regression and fall of sea level. This is shown in Fig. 1a; areas of sea floor created during this time are shown in Fig. 1b.

D. L. Dineley

Bibliography

Pomerol, C. (1986) The Cainozoic Era; Tertiary and Quaternary. (Trans. D. and and E. E. Humphries .) Ellis Horwood, Chichester.
Savage, R. J. G. and and Long, M. C. (1986) Mammal evolution; an illustrated guide. British Museum (Natural History), London.