A question of size


Ever heard of the Bone Wars? Probably not, but this was the name given to a period of intense academic rivalry between American anatomist and palaeontologist Edward Drinker Cope and his erstwhile academic partner Othniel Charles Marsh. This 20-year fued at the end of the 19th century saw each man trying to out-compete the other by naming as many new Paleocene vertebrate species from North America as possible. And it was during this intense period of fossil collecting that Cope noticed something remarkable…

Edward Drinker Cope (1840-1897)

Observing many specimens and publishing an astonishing number of academic papers (over 1,300 in his lifetime, still the highest number by a single individual), Cope uncovered what appeared to be a tendency towards larger body size in a population’s lineage over evolutionary time. This was, he suggested, because new groups are commonly founded at smaller sizes, but it was generally advantageous to be larger. This became known as Cope’s rule.

A classic example of a group of organisms that conforms to the rule is that of horses, or the Equidae lineage. Early ancestors of modern horses were no bigger than dogs during the Eocene period (56-37 million years ago), as illustrated below.

Late Eocene: 37-33.7 million years ago; Middle Miocene: 16 – 11.5 million years ago; Late Miocene: 11.5 – 5.3 million years ago. Image: H. Zell

Cope’s rule now has an extensive history of research, with some studies supporting a trend of size increase and others countering it. The notable palaeontologist Stephen J. Gould proposed that the best way to test Cope’s rule would be to study all lines of ancestry within large groups with excellent data over substantial geological time. Not so easy – much of the fossil record is just too poor to support this approach.

That’s where the little fellas you can see in the picture at the top of the post come in. At the Museum I am investigating the validity of Cope’s rule by using the fossil record of something called planktonic foraminifera. These are single-celled organisms that make a hard shell no bigger than a grain of sand. But when viewed under a microscope these shells display a wide variety of shapes, as you can see in the photograph, making it possible to identify different species.

They live in our modern oceans, but have existed for over 100 million years and can be found from the poles to the equator. Crucially, they have the best-documented species-level fossil record of any group for the last 65 million years and a well-constrained family tree (phylogeny).

This means we can drill deep-sea cores and collect countless planktonic foraminifera specimens from all the world’s oceans. These specimens can then be sorted into different species and measured. And that’s what I am doing. With over 30,000 specimens measured, the project I’m running at the Museum will be the largest and most robust test for Cope’s rule anyone has ever attempted. Hopefully it will shed some light on this fascinating phenomenon.

Tracy Aze – Research fellow

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