While fossil plant remains are nowhere as abundant as the remains of fossil vertebrates, or invertebrates in Florida, they are nonetheless, an important part of Florida’s fossil record.

In fact, the fossil sea grasses found in the limestones of the Middle Eocene Avon Park Formation of south-central Florida are among the oldest fossils known from Florida.

Other leaf impressions are seen in Miocene, Pliocene, and Pleistocene clays. Petrified woods, represented by several different trees, are sometimes found in the same layers as vertebrate fossils. A good example is the vertebrate rich Bone Valley Formation of Polk, Hillsborough, and Hardee Counties which has produced sections of petrified tree trunks weighing hundreds of pounds.

At the microscopic level, Florida sediments are rich in fossil pollen, diatoms, and phytoliths. Although these fossils are too small to be seen with the unaided eye, their unique beauty and scientific importance as environmental indicators make these fossils an important resource.

Petrified Woods in Florida

Florida has a rich Cenozoic fossil record that is known mostly from studies of animal fossils. Plant fossils, however, have not received much attention. A few paleobotanical studies have dealt with leaf impressions (e.g., Berry, 1916; Lumbert, et al., 1984; Ivany et al, 1990) and dispersed pollen (e.g., Rich and Newsom, 1995). Some of the best preserved fossil plant remains found in Florida are fossil woods; however, to my knowledge, nothing has been published about them in the scientific literature. Petrified wood is sometimes found in the same strata as vertebrates and can be important in reconstructing the vegetation and probable climatic conditions inhabited by the extinct fauna.

Fossil woods sometimes retain exquisite details of the original cell structure that can be investigated with the aid of a microscope. Such fossils may be identified by comparing their anatomy with that of modern woods or other fossil woods. This method of identification is similar to that used in the identification of timber woods in the forest products industry. Anatomical features such as presence of vessels, vessel size and grouping, distribution of parenchyma, ray size and distribution, are used in wood keys aiding in the determination to family, genus, and in some cases, species.

In Florida, particularly well-preserved fossil woods have been found in the Bone Valley phosphate mining operations of Polk County south of Bartow, in the beds of the Suwannee and Santa Fe Rivers in Columbia and Gilchrist counties northwest of Gainesville, and in phosphate mining operations north of White Springs in Hamilton County. Undoubtedly, many other sites exist in the state that remain to be documented in the literature. Although petrified woods are relatively common at various sites in Florida, it is likely that many specimens are overlooked by those seeking other kinds of fossil remains. With this article I would like to remind fossil collectors of the potential scientific value of petrified woods and provide an introduction to the techniques of collecting, preparing and identifying them.

Preservation of Fossil Wood

Fossil woods can be preserved in a variety of ways. Sometimes the wood tissue is preserved in more-or-less original condition, remaining soft and unmineralized, and typically compressed by the weight of overlying sediment. This kind of fossil wood is common at some localities, such as the Pleistocene Leisey Shell Pit (Rich and Newsom, 1995). Woods preserved in this way are best stored in a 50/50 mixture of glycerine and ethyl alcohol to prevent fungal growth and/or drying and crumbling. Such woods, if sufficiently soft, may be studied by sectioning with razor blades, or may be allowed to dry so that small fragments can be studied with the Scanning Electron microscope. In addition, techniques exist for embedding ancient wood or charcoal in epoxy resin to allow sectioning as if they were petrified (Smith and Gannon, 1973). When collecting such unmineralized fossil woods, one must be careful to avoid modern wood imposters, i.e., contaminants of recent origin.

Petrified wood (wood turned to stone) falls into two categories: casts and permineralizations. The first step in formation of both of these types of fossils involves quick burial in sediment so that fungal and bacterial decay is retarded due to low oxygen levels. A cast is formed as minerals or other sediments fill and harden within the sedimentary cavity formed as the original wood deteriorates. Casts show the external form of the fossil but do not preserve internal cell structure, and consequently cannot be identified to genus or species. Permineralized woods are formed when minerals dissolved in ground waters infiltrate the wood, filling the spaces within and between cells, gradually embedding and preserving the entire tissue. Permineralized woods retain the original cellular structure and therefore can be identified by anatomical study.

What to Collect

For purposes of identification, a small hand sample (measuring perhaps one to three inches on each side) is usually sufficient. It is not necessary and usually not desirable to collect an entire log, but it is advisable to collect pieces that are from the mature wood (outer portion of the stem or trunk). If a hand sample is being selected from a larger specimen in the field, it is best to select a piece that most clearly shows the grain of the wood. A 10X hand lens can be useful at this stage. Usually the darker colored portions maintain more detail than the lighter colored portions.

As in all fossil collecting, the most important step is field documentation. The location of the specimen should be noted as accurately as possible. Photographs and/or diagrams of the specimens as found may also be useful. It is helpful to note whether the pieces collected are from a single log or stump, or whether several different sources were collected. It is also important to note whether the wood appears to have originated in place or whether it may have been relocated by slope movement, stream action, or human activities.

This article was a contribution to Papers In Florida Paleontology, No. 8, November 1996, published by the Florida Paleontological Society.