Era: 65.5 Million Years to Present
Oligocene Epoch: 33.9-23 Million Years Ago
Palmoxylon of the Catahoula Formation
plants or angiosperms (Magnoliophyta) make their first unmistakable
appearance during the Early Cretaceous 140 Ma (Kenrick & Davis
2004, p. 195). Traditionally, angiosperms have been divided
into monocots and dicots. Woody deciduous trees such as oak, elm,
and maple are familiar examples of dicots. Grasses and palms are
well known examples of monocots. Among angiosperms, dicots have a
more extensive fossil record than monocots.
to be the case since today dicots outnumber monocots six to one.
In addition to this fact, most monocots are herbaceous plants, which
may not as readily fossilize as the woody dicots (Stewart & Rothwell,
1993, pp 487 & 488). The
coastal states of Texas, Louisiana, Mississippi and Alabama are
therefore special in that they possess late Eocene
and Oligocene deposits in which the silicified remains of palm
wood are common (Berry, 1916, p. 233). In fact, petrified
palm wood or Palmoxylon is the state stone for Texas and
the state fossil for Louisiana. The state stone for Mississippi
is petrified wood and much of the fossil wood found in the state
genus name Palmoxylon is derived from the Latin
word for palm tree, palm, and the Greek word for wood, xylo (Borror,
1988, p. 69 &
111). Palm trees actually do not produce wood; although, they do
produce fibrous, wood-like stems. The woody cylinder stems of
angiosperm dicots and gymnosperms, such as sequoias, spruce and
pines, are produced from secondary
that adds girth to the stem and consists primarily of secondary
xylem made of cellulose and lignin. In fact, wood is often defined
as secondary xylem (Raven, Evert & Curtis, 1981, p. 664). Palm
tree trunks result from only primary
growth and reach their adult diameter near ground level.
tree trunks consist of individual vascular bundles embedded in
a groundmass of living
parenchyma cells. In
vascular bundles can give a spotted appearance to the palm fiber.
vascular bundle or fibrovascular bundle consists of a small vascular portion of one to four (usually two) large vessels surrounded by numerous fibers that thicken into a bundle cap on one end. Fibers provide structural support. Vessels conduct water. Phloem or food conducting tissue is found between the vessels and bundle cap.
the center of
the stem vascular bundles are spaced far apart. Towards the periphery
of the stem the vascular bundles become
more numerous and crowded. Longitudinal
cuts reveal that the vascular bundles form rod-like structures.
general, fossil palm fiber is easy to identify; however, monocot
fiber is fairly uniform in appearance and yields little specific
taxonomic information. Identification
of palm tree species is difficult because their appearance
is so generic. The
vast majority of collectors are happy to identify their fossil
specimen as belonging to the genus Palmoxylon.
palm specimens can be rare. Full rounds are unusual
with most specimens representing fragments of trunks. Palm
trunks are made from primary
growth and much of the stem is composed of living parenchyma
cells. The parenchyma tissue is not as resistant to decay as
of gymnosperms and dicot angiosperms. Thus, good preservation
of intact palm trunks is less likely. The lack of good preservation
combined with the generic appearance of the palm fiber explains
why there is less scientific systematic work on palms versus
and angiosperm dicots.
Still, to the keen observer differences
in vascular bundle structure and ground tissue can be observed
between specimens. Some species have fibrous bundles that appear as small roundish bundles composed of sclerenchyma cells or fibers (Tidwell, 1998, p. 248). Fibrous bundles are made of the same cells that make up bundle caps.
of the best if not the finest permineralized palm fiber, commonly
known as petrified palm wood, comes from the Catahoula Formation
in Louisiana, Mississippi and Texas. The Catahoula formation consists
of sandstones, sand, clays, and conglomerates. Rivers and streams flowing
across broad coastal plains 24 to 30 million years ago deposited sediments making up the Catahoula Formation (Matson,
1916, p. 226; Paine & Meyerhoff,
1968, p. 92; John, 2001, p. 6). These rivers were powered by an
uplift of the Rockies. Volcanic activity during the Oligocene in
types of sediments
found in the Catahoula. Palms along with other tropical plants
grew along a near shore environment that
Gulf of Mexico (Berry, 1916, pp 227 & 228). In Louisiana the
Catahoula Formation forms a belt across the central part of the
palm groves, beaches and deltas that made up this environment lay
a further 200 kilometers inland than today's coastline (Daniels
& Dayvault, 2006, p. 398). This Oligocene environment had two
elements necessary for forming good permineralized specimens, a
chance for quick burial
and a volcanic silica source.
the most abundant plant fossil
from the Catahoula Formation and often exhibits excellent preservation.
In the past many weathered specimens could be found on the surface
or reworked in more recent deposits. Berry, in his 1916 paper,
describes 7 Palmoxylon species from the Catahoula Formation,
providing a key to their identification (p. 234). Differences in
used to key
out the species. The species described include: P. ovatum, P.
mississippense, P. texense, P. lacunosum, P.
cellulosum, P. remotum and P. microxylon.
Plates are included that provide illustrations showing cross-sections.
Louisiana Palmoxylon may be unmatched worldwide for its fine preservation
and color. The permineralization with silica is so fine that cell
structure is faithfully preserved. When tapped, specimens produce
a sound not unlike fine china. The red, yellow, white and lavender
invite our imaginations to dream of ancient sunrises and sunsets.
To borrow a phrase from Mary White, the finest of Louisiana Palmoxylon specimens
are truly semi-precious gemstones that serve as keys to the geologic
past. Click on the image below to enter our Oligocene Louisiana
Gallery. Click on Palmoxylon of the Catahoula Formation for a printable
version of our article.
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