A part of every landscape and the basis of the terrestrial food chain, plants are excellent indicators of changing environments. Because fossil leaves are common, they provide large samples for paleobotanists studying patterns of evolution and extinction. But reading the clues left by ancient plants means being able to discern the relationships between them.
Since the time of Linnaeus, plant relationships have been determined largely by looking at fruit and flower characteristics. This is a useful approach, but it doesn't always work. Botanists studying modern tropical plants sometimes have to wait years for a particular plant to flower. For paleobotanists, the problem can be even worse. Think of a tree in a temperate forest. Every spring, it sprouts a new set of leaves and flowers. Some of the flowers flutter away in the breeze. Of the flowers that become fruits, many get eaten, and all of them eventually fall off the tree. In the autumn, the tree sheds all its leaves. In other words, the tree spits out parts all year long but rarely do those parts come off together. Of all the parts plants shed, leaves are by far the most common, but they are seldom attached to the reproductive organs that botanists have traditionally used to classify plants.Because of their abundance, leaves are the optimal choice for identifying fossil plants, but until recently, this has been complicated by inconsistent terminology and use of leaf characteristics that aren't truly diagnostic. While working on his Ph.D. thesis in 1989, the DMNS paleobotany curator Kirk Johnson devised a new approach known as morphotyping. A morphotype is an informal taxonomic category independent of the Linnaean system, and it offers some very attractive advantages.
In 1993, Museum volunteer Beth Ellis initiated a six-year project based on Kirk's morphotyping method and leaf architecture terminology established by Leo Hickey of Yale University. Beth's goal was to standardize the terminology used by botanists and paleobotanists to describe leaves. The result is a 65-page Manual of Leaf Architecture, available in PDF format via http://www.peabody.yale.edu/collections/pb/MLA/ (a new browser window will open). Accompanied by a leaf architecture database entry form, this manual walks researchers through the steps of morphotyping leaves based on stable, diagnostic characteristics. Others who worked with Beth and Kirk on this manual were Amanda Ash and Scott Wing of the Smithsonian Institution, Leo Hickey of Yale University, and Peter Wilf of the University of Michigan. Collectively, these individuals are known as the Leaf Architecture Working Group.
A major part of Kirk's research focuses on Late Cretaceous and Early Paleocene leaves from North Dakota, from the Hell Creek and Fort Union Formations. The North Dakota vicinity where Kirk does much of his fieldwork preserves one of the world's best terrestrial records of the K-T boundary, the time when the last dinosaurs went extinct. In 1999 alone, Kirk and his Leaf Whackers identified and catalogued nearly 7,000 fossil leaf specimens, collected from eighty-four different localities between 1991 and 1999.
Leaf Architecture Working Group. 1999. Manual of Leaf Architecture - morphological description and categorization of dicotyledonous and net-veined monocotyledonous angiosperms. Washington, DC: Smithsonian Institution. Available on the Web via http://www.peabody.yale.edu/collections/pb/MLA/
Hickey, Leo J. 1973. "Classification of the architecture of dicotyledonous leaves" American Journal of Botany Vol. 60, no. 1, 17-33
Johnson, Kirk R. 1992. "Leaf-fossil evidence for extensive floral extinction at the Cretaceous-Tertiary boundary, North Dakota, USA" Cretaceous Research Vol. 13, 91-117