Tracing the origin of flowering plants has long been a challenge for evolutionary researchers, as discussed in this month's Origins essay. Paleobotanist David Dilcher thinks part of the reason is that researchers in his field misidentified fossil plants as members of modern groups. Back in 1979, he and a colleague reanalyzed fossil leaves collected from 45-million-year-old clay pits in Tennessee. Careful cleaning revealed previously unnoticed stipules, small outgrowths from the base of the leaves, calling into question the fossils’ supposed identity as modern corkwood. A close examination of the venation pattern and the cuticle of the leaves convinced Dilcher that these leaves were an extinct group belonging to the coffee family. Dilcher, now at the Florida Museum of Natural History in Gainesville, talks about the impact of this and subsequent work by others on understanding flowering plants.
Often the phrase “the origin of the flowering plants is an abominable mystery” can be read in popular and scientific literature. This phrase is credited to Darwin and comes from a letter that Darwin wrote to J. D. Hooker on 22 July 1879 (Darwin, 1905). It represents Darwin’s frustration with the paleobotanical record of his time. The literature available to Darwin in the 1870s shows that when flowering plants are first found in the fossil record, they are nearly all given names of extant genera. At the time of Darwin, there was no evolution that could be demonstrated from the fossil record of flowering plants. This record was based almost entirely upon impressions and compressions of fossil leaves, and paleobotanists of the 19th and first half of the 20th centuries looked for “matches” or similar leaf types with the leaves of living flowering plant genera. This ”leaf matching” can be done if one does not look closely at detailed characters of fossil and living leaves. When characters such as fine venation, epidermal cell patterns, trichome types, and stomatal complexes are examined carefully (Dilcher, 1974), a different view of early flowering plants emerges.
Such was the case for Paleorubiaceophyllum eocenicum (far left), which was once classified as Leitneria floridana (near left) but which really represents an extinct group of plants.
Darwin’s “abominable mystery of the origin of the angiosperms” can be understood when careful observations of characters are made. With the study of detailed leaf venation and leaf epidermal cell characters, it is clear that many of the earliest flowering plants represent extinct species, extinct genera, extinct families, and perhaps even extinct orders (Dilcher 1974, 2000). This paradigm change has caused a revolution in the study of fossil flowering plants which only in the past 40 years has begun to present a realistic record of extinct flowering plants.
It seems to be human nature that when a fossil leaf is found, the first question asked is what is its living counterpart. When fossil leaves are examined only as hand specimens, using gross form, it is easy to find leaves of trees living today that “match” the fossils. The success of early paleobotanists depended upon making such matches. It has taken a philosophical shift in angiosperm paleobotany in order for researchers today to strive to understand relationships between fossil and living plants, based upon detailed characters, rather than feeling the need to find a living genus to which they can name a fossil. Using character analyses, we now have an emerging new fossil record of flowering plants with many extinct taxa that would have delighted Darwin. This new record is one he could have understood because it demonstrates the evolution of flowering plants, a major group of organisms on Earth. We do not yet know all the details, but there is no longer any “abominable mystery” to the origin of flowering plants.
Darwin, F. (Ed.). More letters of Charles Darwin. Vol. 2. (Murray, London, 1905).
Dilcher, D. Approaches to the identification of Angiosperm leaf remains. The Botanical Review, 40:1, 1 (1974).
Dilcher, D. "Toward a new synthesis: Major evolutionary trends in the Angiosperm fossil record. pages." Variation and Evolution in Plants and Microorganisms: toward a new synthesis 50 years after Stebbins. F. J. Ayala, W. M. Fitch, and M. T. Clegg, Eds. (National Academy Press, Washington, D.C., 2000), pp 255-270.
Credits: Paleorubiaceophyllum eocenicum: H. Wang and D. L. Dilcher; Leitneria floridana: J. S. Peterson, USDA NRCS NPDC. Missouri Botanical Garden.