The Virtual Petrified Wood Museum.  Dedicated to the Exhibition and Educational Study of Permineralized Plant Material
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Division (Phylum) Magnoliophyta
Flowering plants or angiosperms (Magnoliophyta) make their first unmistakable appearance during the Early Cretaceous (140 MYA) (Kenrick & Davis 2004, p. 195). Thus, angiosperms appear 300 million years after the first vascular plants and 220 million years after the first seed plants (Willis & McEwain, 2002, p 156). Angiosperms underwent a rapid adaptive radiation soon after their first appearance. In fact, angiosperms became the dominant flora across the globe by the Paleogene a mere 70 million years after their first appearance. Flowering plants continue to dominate the world’s flora today; extant pteridophytes species number 10,000, gymnosperms 750, and angiosperms up to 300,000 species. These new seed plants possessed a number of important characteristics that separate them from other seed plants.

A New Kind of Seed Plant

Flowering plants evolved distinctive characteristics that help to define this plant division. Angiosperms possess flowers, develop fruits, contain specialized conducting cells in their vascular tissues, develop a double-layered seed coat, exhibit a distinctive column-like structure in their pollen grain walls, and undergo double fertilization during their life cycle.

New Reproductive Stategies

New reproductive strategies helped angiosperms become a great success and diversify into the forms we know today. In angiosperms male and female structures develop within flowers. The pistil is the central, female organ of the flower and typically consists of an ovary with ovules, a style and stigma. The stamen is the male part of a flower and typically consists of a filament or stalk topped with pollen producing anthers.

When pollen comes into contact with a flower's stigma the growth of a pollen tube is activated. Each pollen grain carries two sperm. One sperm fertilizes an egg in the ovule; the other sperm unites with two haploid cells in the same ovule. This process is known as double fertilization and is an important adaptation found in angiosperms.

The fertilized egg will undergo cell division to become a zygote and then an embryo. The second fertilization results not in offspring, but rather the development of endosperm, which acts as a nutrient for the embryo. Cells in the endosperm have three sets of chromosomes. Endosperm not only serves as an important food source for the embryos of flowering plants it also is important to other animals. Humans depend upon the endosperm of rice, wheat, and corn. Recent research indicates the endosperm may also act as a fertilization sensor helping to abort embryos of incompatible crosses (Juniper & Mabberley 2006, p.27).

A seed is formed when the endosperm and the embryo become enveloped in a part of the ovule that hardens into the seed coat. The ovary or other parts of the flower in angiosperms develop into a fleshy fruit surrounding the seeds. Many organisms such as birds, bats, and insects have coevolved to help pollinate angiosperms. The fleshy fruits of angiosperms are an adaptation for seed dispersal. Many animals use the fruit as a food source, which results in the dispersal of seeds encapsulated within a natural fertilizer!

Monocots & Dicots

Traditionally angiosperms are divided into the monocotyledons and dicotyledons. Today angiosperms are divided into the monocots, eudicots, and magnoliids. Monocots and eudicots are monophyletic groups. Eudicots contain most of the dicots. It is useful to known the major differences between monocots and dicots (eudicots & magnoliids) when studying both extinct and extant plants.

Monocots have one cotyledon (seed leaf) at germination. Monocots usually have flower parts in threes, one aperture or furrow on their pollen, parallel leaf venation, a scattered arrangement of vascular bundles, and usually no secondary woody growth. Grasses and palms are well known examples of monocots. Petrified plam 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 is Palmoxylon.

Dicots have two cotyledons when they germinate. Today there are six times as many dicots as monocots. Dicots usually have flower parts in fours or fives, possess three apertures on their pollen (except the magnoliids, which have one), netlike leaf venation, vascular bundles arranged in rings, and commonly have secondary woody growth (Willis & McElwain, 2002, pp. 156-157). Woody dicots possess eustele stems; a central pith surrounded by secondary wood and bark. Woody deciduous trees such as oak, elm, and maple are good examples of dicots. When looking at permineralized wood in cross-section one can quickly distinguish between gymnosperms and angiosperms with a 10x loupe.

New Vascular Tissue

Most angiosperms have two cell types that are distinctly different in size. The large, water conducting cells, are called vessels; the smaller diameter, more abundant cells are fibers. Gymnosperm wood is made of small diameter tracheids. Tracheids are more easily seen with a 20x loupe. Angiosperms also have tracheids for water conduction. Among the angiosperms we can also distinguish between dicots and monocots. Dicots have their vessels and fibers arranged in rings while monocots have their vascular bundles scattered throughout the stem giving a speckled appearance even to the naked eye (Kenrick & Davis, 2004, p. 74).

A New Age of Plants

The first angiosperms had small seeds, which may indicate they were small herbaceous weedy generalists (Willis & McElwain, 2002, p162). The lack of angiosperm wood in the early Cretaceous would also support the idea that the first flowering plants were small herbaceous plants. Fossil evidence from flowers, leaves and pollen suggests that dicots evolved before monocots. Cladistic analysis indicates a close relationship between Bennettitales, Gnetales and angiosperms (Willis & McElwain, 2002, p. 184).

The diversification of flowering plants during the Cretaceous helps to mark a significant change in the world's flora. Paleozoic flora was dominated by ferns and clubmosses (Paleophytic flora). The Paleophytic flora gave way to a Mesophytic flora during the Triassic period. Woody seed-bearing plants and their relatives dominated Mesophytic flora. Thus, the change from Paleophytic to Mesophytic represented a change in reproductive strategy; from spore producers to seed producers. Conifers, cycads, and ginkgoes diversified during this time and dominated the landscape. Flowering plants first emerge during the Early Cretaceous and undergo a great adaptive radiation during the Middle Cretaceous. Flowering plants quickly became a major constituent of species diversity and the world entered the third great age of plant life known as the Cenophytic by the Late Cretaceous (Kenrick & Davis, 2004, p. 143).

The transition from Mesophytic to Cenophytic represents a change in reproductive strategies. Gymnosperms and their relatives relied mostly on wind pollination and bore naked seeds clustered in cones or on the end of stocks. Flowering plants coevolved with animal pollinators, underwent double fertilization, and encased seeds in a fleshy ovary that encouraged seed dispersal. Our modern plant world is a continuation of the Cenophytic age of plants.

Science Olympiad Fossil Event

The 2016 Science Olympiad Fossil List includes three genera under the phylum Anthophyta (Flowering Plants): Acer, Populus, and Platanus. Acer is the maple genus. Today maples are an important part of the deciduous hardwood forests of North America. Maples are important economically as a source of hardwood and syrup. Maples are also used as ornamental shade trees (Elias, 1980, p. 775). Populus includes familiar plants such as poplar, cottonwood, and aspen trees. Platanus includes sycamore and plane trees. In the section titled "Other", petrified wood is listed, which does include flowering plants.


Fraxinus sp.
Grassy Mountain Basalt
Miocene
Grassy Mountain, Malheur Co, OR

12 cm x 10 cm

Schinoxylon sp.
Green River Formation
Eocene
Blue Forest, Wyoming

12 cm x 10 cm

Palmoxylon sp.
Catahoula Formation
Oligocene
Rapides Parish, Louisiana

11 cm x 9 cm

Palmoxylon sp.
Green River Formation
Eocene
Farson, Wyoming
4 cm diameter

Palm Fiber & Roots
Palmoxylon & Rhizopalmoxylon
Denver Formation
Paleocene
South Platte River Quarry
Denver, Colorado
19 cm x 17 cm

Palm Trunk
Palmoxylon
Denver Formation
Paleocene
South Platte River Quarry
Denver, Colorado
12 cm x 11 cm


Sycamore
Platanus sp.
Denver Formation
Paleocene
South Platte River Quarry
Denver, Colorado
20 cm x 15 cm

Maple
Acer sp.
Denver Formation
Paleocene
South Platte River Quarry
Denver, Colorado
12 cm diameter

Fig
Ficus sp.
Miocene
Swartz Canyon, Oregon
12 cm diameter

Oak
Quercus sp.
Columbia Group Basalts
Miocene
Vantage, Washington
16 cm x 12 cm

Colorado Palm Trunk Close-Up
Showing Vascular Bundles
20x

Vantage Oak Close-Up
Showing Vessels
10x

Balloon Vine
Cardiospermum coloradensis
Green River Formation
Eocene
Bonanza, Utah
7 cm long

Cedrelospermum sp.
Miocene
Randecker Maar Esslingen, Germany
Leaf 4 cm long, Seed 1 cm long


Sycamore Leaf
Platanus wyomingensis
Green River Formation
Eocene
Uintah County, Utah
11 cm wide x 10 cm tall


Oak Leaf
Quercus dayana
Sucker Creek Formation
Miocene
Succor Creek
Malheur County, Oregon
6 cm long x 2.2 cm wide


Poplar Leaf with Insect Damage
Populus wilmattae
Green River Formation
Eocene
Bonanza, Utah



Bibliography

Elias, T.S. (1980). The Complete Trees of North America. Van Nostrand Reinhold Company: New York.

Juniper, B.E. & Mabberley, D.J. (2006). The Story of the Apple. Timber Press, Oregon.

Kenrick, P. and Davis, P. (2004). Fossil Plants. Smithsonian Books: Washington.

Willis, K.J. & McElwain, J.C. (2002). The Evolution of Plants. New York: Oxford University Press.


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