COLLECTING FOSSIL PLANTS IN FLORIDA
Steven R. Manchester
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
*DIATOMS / DINOFLAGELLATES
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
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.
If the wood is well preserved, it is usually possible to determine
whether it is a conifer (softwood) or angiosperm (hardwood) by examining
a freshly broken cross section with a hand lens. Conifer woods are
composed mainly of narrow, elongate cells (tracheids) that function
both in structural support and conductance of water. Conifers do not
possess vessels (pores); thus, their wood has a smooth, even appearance.
Angiosperm woods (including the dicots, or hardwoods) are composed
of both vessels and fibers. The latter are narrow, elongate, thick-walled
cells functioning in structural support. Vessels are water-conducting
tubes that in cross section are distinguished from fibers and tracheids
by their larger diameter and round or oval outline; each vessel is
composed of moderately thick walled cells stacked end to end and connected
by perforated end walls.
Woods of temperate conifers and angiosperms typically show growth
rings in cross section; i.e., the diameter of their tracheids and
vessels is large in wood laid down in spring and then decreases to
end in smaller-celled, thicker-walled summer wood, leading to concentric
rings. In contrast, palm stems (belonging to the monocots) have a
distinctive stem structure that lacks growth rings. The "wood"
of palm is composed of numerous fibrovascular bundles (small groups
of vessels and associated cells) scattered in a matrix of unspecialized
tissue, giving a speckled appearance to the cross section.
Some woods, such as oak and palm, are so distinctive in their anatomy
that it is possible to identify them simply with a 10x hand lens and
without preparatory procedures. Usually, however, it is necessary
to prepare thin sections so that the wood can be studied by transmitted
light with a microscope at magnifications of 10 to 400 times. Modern
wood sections may be prepared by boiling a small block of wood in
water for some hours to soften and then slicing with a razor blade
to make paper-thin shavings that permit observation with transmitted
light. Because of their hardness, petrified woods must be sectioned
with techniques suitable for cutting stone, which involve sawing,
grinding and polishing. Although expensive equipment is available
for preparing rock thin sections, it is possible with some practice
to prepare quality thin sections using standard lapidary equipment
that is commercially available for cutting and polishing rocks and
gems. The methods for preparing thin sections with lapidary equipment
are presented in the appendix.
Identification of fossil woods is accomplished through comparison
with modern woods and with other known fossil woods. There is a good
literature on the anatomy of commercial timbers including keys for
quick identification and descriptions and illustrations for thorough
comparison (Gregory, 1980; Hoadley, 1990). Computerized keys are also
available to help in determining modern and fossil woods (Wheeler
et al., 1986). These works also provide an introduction to the terminology.
Because of the three-dimensional nature of wood, it is usually necessary
to examine both cross (transverse) and longitudinal (tangential and
radial) sections for a complete study of the anatomy. Of the two kinds
of longitudinal sections, the tangential section, which is cut through
the wood along a plane approximately parallel to the bark, is particularly
important for showing the structure of the rays. Rays are ribbons
of tissue that run perpendicular to the grain and radiate outward
from the central core of the tree. The radial section, which is cut
in a plane along the radius of the stem, is helpful for showing the
rays in lateral view and for showing pits and perforations of the
PRELIMINARY SURVEY OF FOSSIL WOODS FROM FLORIDA
The Paleobotanical Division of the Florida Museum of Natural History
maintains collections of fossil leaves, woods, flowers, fruits and
seeds of various geologic ages and from many regions of the world.
Approximately 30 specimens of petrified woods from Florida reside
in the collection, most of them having been contributed through the
courtesy of private collectors. An analysis of these woods, and other
samples that I have examined in private collections, indicates that
the most common genera of petrified wood to be found in the Miocene
and Pliocene of Florida are Pinus (pine) and Quercus (oak). The fossil
woods that I examined are derived from three areas: Bone Valley, Santa
Fe River and White Springs.
The Neogene deposits of the Bone Valley region are well known for
their vertebrate fauna (Morgan, 1994; Webb, 1990). These sediments
range in age from Middle Miocene to Early Pliocene, and a careful
study of the woods according to their stratigraphic position in this
area has not been done.
Numerous woods from the Mobil Big Four Mine collected and donated
by D.B. Crissinger in 1992 were found to represent the pine family
(Pinaceae; Figs. 1, 4).
Three genera have been recognized in a suite of woods from the Cargil
Mine collected and donated by Jim Toomey in 1995: Quercus, Juglans,
and a conifer (probably Pinus or Taxodium although the preservation
is not fine enough to be certain). The Juglans sample (Figs. 3, 6)
is the only specimen of walnut wood that has been recovered so far
from the fossil record of Florida. Its occurrence indicates that walnuts
extended farther south into Florida in the Neogene than they do today.
Although the black walnut (J. nigra) is widespread in eastern North
America today, its modern distribution in Florida does not extend
south of the Panhandle region (30š North latitude).
Santa Fe River
Petrified woods have been collected from several underwater sites
in the Santa Fe River bed bordering Columbia and Gilchrist Counties.
The age of the sites vary, and there is a potential problem of mixing
because the bed of the river carries woods from different Pliocene
to Pleistocene horizons. Most specimens have been found to be Quercus.
In addition, one specimen of an unidentified hardwood, and one of
palm have been recovered.
The Santa Fe 1B vertebrate fauna site (Morgan and Ridgway, 1987,
pp. 15-16) of Late Pliocene (late Blancan) age, has yielded a single
piece of palm (Figs. 7, 8) in addition to Quercus.
A nice chunk of Quercus wood shown in the examples was collected
by Eric Taylor and Eric Prokopi from the from the Occidental Chemical
Suwannee River Mine, Hamilton County, is significant in being associated
with late Miocene (middle Clarendonian) vertebrates (Morgan, 1989,
The main genera of fossil wood so far found in Florida are briefly
described below as an aid to collectors wishing to identify the woods
in their collections. Woods with structure different from those discussed
here are likely to be new records for the state of Florida. Transverse
and tangential views of each wood type discussed here are shown at
Wood of the Pinaceae (pine family) has been found to be among the
more common types found in the Bone Valley deposits. Pinus wood is
very fine-grained, composed mainly of tracheids, and narrow rays (1-2
cells wide). In addition, it is distinguished by the presence of vertical
and horizontal resin canals (Fig. 1). Taxodium (bald cypress) is another
conifer that might be expected to be preserved as fossil wood in Florida.
However, the wood of Taxodium is readily distinguished from that of
Pinaceae by the lack of resin canals. Big Four Mine, Bone Valley,
Polk County, Florida, X20 magnification.
Figure 1. Pinus transverse section showing uniformly small tracheid
cells and occasional circular resin canals.
Figure 4. Pinus tangential section showing uniseriate rays. Same specimen
as figure 1, X 25 magnification.
Oak wood is ring-porous, with very large pores at the beginning
of each growth ring that taper off in flame-like tracks to the outer
part of each year's growth (Fig. 2). The wood has two kinds of rays:
narrow (one cell wide), and very broad (many cells wide)(Fig. 5).
Such wide rays, in combination with the ring-porous pattern, make
it easy to identify this genus with a hand lens.
Figure 2. Quercus (oak) wood transverse section showing an extremely
wide ray down the middle and large vessels at the beginning of each
growth ring. Occidental Chemical Suwannee River Mine, Hamilton County,
Florida, X20 magnification..
Figure 5. Quercus, tangential section with narrow uniseriate rays
and a very broad multiseriate ray in the middle. Same specimen as
figure 2, X 25 magnification.
Walnut wood is diffuse-porous, with medium-sized vessels pores distributed
evenly across the growth rings, and has wavy, thin bands of parenchyma
(Fig. 3). The tangential section shows medium-sized rays (2-4 cells
wide) rays (Fig. 6). A single sample of this genus is known from the
Figure 3. Juglans (walnut) wood, transverse section showing medium-sized
vessels, thin wavy bands of parenchyma and medium-sized rays. Note
the growth ring boundary oriented horizontally. Cargil Mine, Polk
County Florida, X 20 magnification.
Figure 6. Juglans, tangential section showing medium-sized (2-4 seriate)
rays. Same specimen as figure 3, X 25 magnification.
Wood of the many genera of living palms is difficult to distinguish
from one another, but it is easy to identify them to the family Palmae.
Fossil woods of the palm family are generally assigned to the fossil
genus Palmoxylon. A single small stem of palm was recovered from late
Pliocene sediments in the Santa Fe River locality 1B (Figs. 7, 8).
The stem shows numerous fibrovascular bundles in a ground mass of
Figure 7. Palmoxylon, small stem of palm, transverse section showing
numerous fibrovascular bundles. Late Pliocene (late Blancan), Santa
Fe River locality 1B, Gilchrist County, Florida, X 15 magnification.
Figure 8. Same as figure 7, further enlarged, showing vessels within
the vascular bundles and the ground mass of small parenchyma cells,
X 30 magnification.
Bartholomew, R.L., L.C. Matten, and E.F. Wheeler. 1970. Staining
silicified woods. Journal of Paleontology, 44: 905-907.
Berry, E.W. 1916. The physical conditions and age indicated by the
flora of the Alum Bluff Formation, U.S. Geological Survey Professional
Hoadley, R.B. 1990. Identifying wood; accurate results with simple
tools. Taunton Press, Newtown, CT 224 pp.
Gregory, M. 1980. Wood identification: an annotated bibliography.
International Association of Wood Anatomists Bulletin n.s. 1:3-41.
Ivany, L.C., R.W. Portell, and D.S. Jones. 1990. Animal-plant relationships
and paleobiogeography of an Eocene seagrass community from Florida.
Palaios, 5: 244-248.
Lumbert, S.H., C. den Hartog, R.C. Phillips, and S.F. Olsen. 1984.
The occurrence of fossil seagrasses in the Avon Park Formation (late
middle Eocene), Levy County, Florida (U.S.A.) Aquatic Botany, 20:
Morgan, G.S. 1989. Miocene vertebrate faunas from the Suwannee River
Basin of North Florida and South Georgia. Pp. 26-55 in G.S. Morgan
(ed.), Miocene paleontology and stratigraphy of the Suwannee River
Basin of North Florida and Georgia. Southeastern Geological Society
Morgan,G.S. and R.B. Ridgway. 1987. Late Pliocene (late Blancan)
Vertebrates from the St. Petersburg Times site, Pinellas Co., Florida,
with a brief review of Florida Blancan faunas. Papers in Florida Paleontology,
Rich, F.W., and L.A. Newsom. 1995. Preliminary palynological and
macrobotanical report for the Leisey Shell Pits, Hillsborough County,
Florida, Pp. 117-126 in Hulbert, R.C., Jr., G.S. Morgan, and S.D.
Webb (eds.), Paleontology and geology of the Leisey Shell Pits, Early
Pleistocene of Florida. Bulletin of the Florida Museum of Natural
History, 37: pp. 1-660.
Smith, F.H. and B.L. Gannon. 1973. Sectioning of charcoals and dry
ancient woods. American Antiquity, 38: 468-472.
Webb, S.D. 1990. Historical Biogeography, Pp. 70-102 in R.L Myers
and J.J. Ewel (eds.), Ecosystems of Florida. University of Central
Florida Press, Orlando.
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1986. Computer-aided wood identification. North Carolina Agricultural
Research Service Bulletin, 474: 1-160.
This article was a contribution to Papers In Florida Paleontology,
No. 8, November 1996, published by the Florida Paleontological Society.