As
discussed in "Morphotyping Process",
we are using the Manual of Leaf Architecture [1]
to determine specimen uniqueness and assign morphotype numbers. However,
sometimes categorizing fossil leaves using the morphotype process
can be difficult. With some specimens, the category information defined
cannot
be obtained. In the example at left, it is impossible to provide
information about the apex (top) of the fossil leaf. It's simply
not there. However, there is enough other information present that
the specimen can be shown to be unique from other morphotypes. So,
the leaf is assigned a morphotype number, in this case "CR026" and the
best example of this type of leaf is called the holomorphotype. If
another more complete specimen is found which shares all other characteristics
of this leaf shown here, it may actually replace this holomorphotype.
It's also possible that fossil leaves from the same plant species may actually receive different morphotype numbers due to variations in architectural characteristics occurring on the same tree. There are species today which exhibit variations such as the long-shoot versus short-shoot leaves of Cercidiphyllum japonicum. These would get different morphotype numbers. It often takes years of collecting to find leaves which are "attached" to show that two different morphotypes belong to the same species. Although morphotypes do not directly represent species, most of the time, each morphotype is a different species.
Although the vast majority of the ~100,000 leaves on a tree are very similar, researchers have found that some characteristics of leaves are highly variable such as size, shape, and apex. Characteristics that are usually consistent include higher order veination patterns and teeth.
In an experiment to demonstrate some of the challenges of morphotyping, I took a camera into my own backyard to photograph architectural differences in leaves from a single tree - an autumn purple ash (Fraxinmus americana). The photos are shown below. The vast majority of the leaves on the tree look like photo #1. Compare the patterns created by this leaf's veins to the remaining photos and notice how similar they are. If these leaves were fossilized and later dug up, leaves #1 and #4 might be given different morphotype numbers until the attached leaves in photo #6 were discovered.
Sometimes a fossil does not preserve enough characteristics to place it into a morphotype category. These are usually left in the field unless they have an unusual feature such as insect damage, as show in photos 7 and 8. When we dig fossils, we collect all evidence that we can of insect damage, even when we know we will never be able to identify the morphotype.
Consider the variations of these living leaves known to be from a single tree. Then, imagine the challenges posed when looking at 64.1 million year old fossils where preservation quality adds further complexity.
