199 — Correlations of Climate and Plant Ecology to Leaf Size and Shape: Potential Proxies for the Fossil Record

The size and shape of leaves — leaf physiognomy — is often used for paleontological climate reconstructions. For over a century it has been accepted that mean annual temperature (MAT) correlates with the percentage of dicotyledonous (many types of leaf’d trees) species with margins that are untoothed. “Dicotyledonous” trees — “dicots” — are most woody, leafy trees that flower. “Toothed” here means exaclty what you’d expect: The leaf has little pointy “teeth” shapes along its edge.

But leaf margin analyses look only at the toothiness of the leaves, and ignores the rest of the leaf physiognomy. Another method, called “Cimate-Leaf Analysis Multivariate Program,” which is A Bit Of A Stretch if ever I did hear one, but is also called “CLAMP” for short which I dig, does incorporate leaf shape, size, and more — but in practice CLAMP doesn’t seem to provide higher fidelity MAT predictions than margin analysis alone.

In 2003, Huff et al proposed a digital imagery method aptly named “Digital Leaf Physiognomy” that used computer vision to better measure the fossil leaves. This paper expands further upon that pilot, adding new features and new sites to improve the resolution of the survey.

For example: Insect herbivory correlates with high nitrogen content (and generally with short lifespans); Mass per area is relatively consistent between leaves all over the world which can be used to refine the model.

These refinements enable us to better understand the climate of sites in the fossil record. These echos of plant matter enable us to understand the climate and the ecology of sites many millions of years ago without relying on chemical methods that can be lossy or inaccurate. This enables far greater understanding of the prehistoric world than if we relied on more direct methods alone.