Research Part 2: Leaf Margin Analysis

Previously, we talked about using the fossil record to glimpse what ancient ecosystems might have looked like. Today, we’ll look at another handy research tool that builds on the fossil record: Leaf Margin Analysis.

Leaf margin analysis is a way of telling average temperature by looking at the shape of leaves. Scientists have observed that in warm tropical areas, you find a higher percentage of leaves with smooth edges (aka “margins”) like in figure A; the opposite is true in cooler climates, more like figure B. Think of magnolia leaves as opposed to oak leaves.

smooth edges vs. rough edges in fossils

A closer look at leaf margins: smooth edges vs. rough edges in fossil leaves from Germany in the Pliocene. Notice the points on the edges of leaf B (Image from Palaeontology Online)

Smooth and rough margins in modern-day species

Smooth and rough margins in modern-day plants. The leaves on the left come from warmer climates, the ones on the right from colder climates. (Image from Palaeontology Online)

Why would this be? It has to do with how plants transpire (a process similar to evaporation; think of it as plant sweat). Among other things, transpiration is a vital part of photosynthesis. Little pores on the leaves called stomata allow carbon dioxide gas to enter the plant while water vapor exits. This carbon dioxide gas is one of the raw ingredients for making the sugars that give plants their energy; the carbon in the gas gets “fixed” into a solid state. Having toothed edges increases a leaf’s surface area, which means more transpiration can occur, which in turn means more photosynthesis. If your growing season is short because of cold temperatures, it’s advantageous to have jagged margins that let you begin photosynthesis earlier in the growing season — stock up on sugars while you can! On the other hand, if you’re in a warm climate, you don’t want jagged margins: the increased surface area and high transpiration rate would make you very dehydrated.

So, if leaf margins correlate strongly with temperature, then by looking at a collection of fossil leaves and comparing the number of smooth margins to jagged margins, you can actually determine an average annual temperature. Here’s the process:

comparing leaf shape to mean annual temperature

Leaf margin analysis involves comparing leaf shape to annual mean temperature. (figure from http://droyer.web.wesleyan.edu/research.htm)

  1. Scientists calculate the percent of smooth edged leaves to the amount of toothed edged leaves in a sample.
  2. This percent is then put into a linear function. The function itself is made from data collected from samples of temperature vs. the percent of smooth to rough edges. For that, you just have to go to an area where you know the annual mean temperature and the species composition.
  3. You place your initial percent of smooth vs. rough edges on axis with the percentages, and look at the corresponding temperature according to the function. In the example graph above, an 80% entire margined sample means the mean annual temperature was around 26°C.

Using this method, paleontologists can look at leaf fossils and figure out what the average temperature was in that area. And that’s how we know Wyoming was so much warmer back in the Eocene! For more information, I highly recommend checking out the Smithsonian’s lesson plans and online interactive game.

For more on leaf margin analysis:

Seyfullah, L. (2012). Fossil focus: Using plant fossils to understand past climates and environments. Palaeontology online, 2(7): 1-8.  http://www.palaeontologyonline.com/articles/2012/fossil-focus-plant-fossils/

Royer, D. and Wilf, P. (2006). Why do toothed leaves correlate with cold climates? Gas exchange at leaf margins provides new insights into a classic paleotemperature proxy. International Journal of Plant Science, 167(1): 11–18.  http://droyer.web.wesleyan.edu/leaf_teeth(IJPS).pdf

Wilf, P. (1997). When are leaves good thermometers? A new case for leaf margin analysis. Paleobiology, 23(3): 373–390.  http://www3.geosc.psu.edu/~pdw3/Wilf1997Paleobiology.pdf

Smithsonian Education website: Prehistoric Climate Change and Why it Matters Today
http://www.smithsonianeducation.org/educators/lesson_plans/climate_change/index.html

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