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What is morphotyping and why do we do it?

When confronted with a group of fossils from an excavation, it is much easier to answer the question. “How many species do I have in this collection?” than it is to answer the question, “What are they?” The first question simply implies that you need to carefully observe the fossils and sort them into groups. The second question implies that you need to know if any of them have ever been described or named before. The first question can be answered by a careful observer with a minimum of training while the second question requires a specialist with access to literature and the type specimens.

The process of sorting groups of unknown fossils into groups based on their characteristic features is known as morphotyping. In 1989, Kirk Johnson began to standardize the “Morphotype Method.” Here’s how it works. First, a collection of fossil plants is sorted into groups of similar morphology. Then, the best example of each group is designated as the holomorphotype (This is analogous to the type specimen of the Linnaean System but the big difference is that the morphotype system is an informal system with common practices while the Linnaean System is a formal one with laws. The holomorphotype, unlike the holotype, can be replaced by a better specimen). The holomorphotype is given a number that consists of two letters and three numbers (for example, PC105). This morphotype number then stands as an informal label for that holomorphotype specimen (and by comparison, all of the fossils in that group).

Once morphotyped, a flora can be analyzed to yield information about paleotemperature, paleorainfall, and floral diversity.

How to morphotype a fossil flora (Step-by-step)

1.  Number each specimen with a locality number and prepare it in the lab so that the features of the leaf are as visible as possible. It is useful to place each specimen in a cardboard specimen tray so that labels remain with the specimen.

2.  Select a two-letter morphotype prefix for the material being morphotyped based on the stratigraphic unit and research area (e.g., HC for Hell Creek Formation of Montana or RS for Rock Springs, Wyoming). A master register of prefixes is kept by Kirk Johnson (kjohnson@dmns.org) although it is not necessary to register them in order to use the system.

3.  Begin to sort the leaves into groups based on shared leaf-architectural characteristics.  As each group is defined, select the best specimen (most complete and well preserved) of that group to be the “holomorphotype.” Assign a unique morphotype number to the holomorphotype (e.g., HC1) and sequester the type where it is accessible for comparison. This specimen should be assigned a museum specimen number and its status as the holomorphotype noted on the specimen tag. It is also useful to maintain a running list or spreadsheet that records the information about the individual holomorphotypes. One major distinction between holotype specimens (the formal name-bearing specimen Linnaean taxonomy) and holomorphotype specimens (the informal numbering-bearing specimen in this system) is that holotypes are permanent where holomorphotypes may be replaced with better specimens or sunk into other morphotypes.

4.  Create specimen tags that have space for morphotype number, your initials, and the date. Then proceed to identify all of the remaining specimens that can be referred to the morphotype based on the holomorphotype specimen and label their tags accordingly. Using these tags allows for the morphotype decisions to be recorded and revisited. It is usually best to start with the best-preserved and most abundant morphotypes and work toward the poorly preserved and less common types. In practice, as work proceeds on a fossil flora, some of what were originally recognized as sharply delineated morphotypes will be shown to belong to a continuum that more nearly represents the spectrum of variation in the natural species, while others will remain as discrete entities.

4. The initial sorting of a collection is usually done on the basis of toothed versus entire margins, primary and secondary vein patterns, and the presence and types of  lobes.  These characters are usually stable within morphotypes. The least reliable characters are leaf size and shape. Once the fossils are grouped into broad categories, it is much easier to separate them by higher-order venation pattern and tooth type [see Hickey 1973, 1979 and Hickey and Wolfe, 1975]. To highlight the characters that define your groups, it is very helpful to sketch and/or photograph the holomorphotype and note diagnostic features and the range of variation. It is useful to print photos or scanned slides as full page images that can be mounted on the wall of you work area. This allows increases familiarity with the various morphotypes. In one variation on this technique, Kirk Johnson makes two sets of holomorphotype images. The first set is mounted on the wall in numerical order and the second set is placed in manila folders in the following categories: pinnate toothed leaves; pinnate entire leaves; palmate toothed leaves; palmate entire leaves; palmately lobed leaves; pinnately lobed leaves; fruits, seeds, and cones; gymnosperm leaves; ferns and fern allies. This allows a large number of images to be searched visually or by major architectural group.

5. Describe the morphotype using the holomorphotype as the basic reference. Expand the circumscription, when necessary, using additional specimens that show clear overlap in their morphological characters with the holomorphotype specimen. Use the fossil-leaf database and this manual as a guide in this process.