horsetails (phylum Sphenophyta or Equisetophyta) range from
the Devonian to recent times. Equisetum is the only
extant (living) genus (Willis & McElvain, 2002, p. 104).
The species representing this genus are all small herbaceous
ribbing. All the branches, leaves and cones are borne on
whorls (Kenrick & Davis, 2004, p. 89). Like the lycopsids
the sphenopsids are vascular plants with true roots,
plants reproduce with spores. Horsetails (sphenopsids) are
closely related to ferns. Some taxonomists place the sphenopsids
in the fern division Pteridophyta, as is the case with the 2015 Science Olympiad Fossil List. When placed within the fern division, horsetails are grouped in the class Equisetopsida or Sphenopsida.
The first major coal deposits were formed during the Carboniferous.
Within the coal measures are found thin marine sediment layers,
which may represent interglacial periods (Kenrick and Davis,
2004, p. 81) or the periodic deposition and erosion of delta
lobes (Selden and Nudds, 2004, p. 59). The large coal deposits
in the eastern U.S. and Western Europe formed between 295
and 320 mya. These coal-forming forests grew in humid, tropical
environments. Lycopsids (clubmosses) and sphenopsids (horsetails)
would reach their greatest diversity in these Carboniferous
forests, which have no analogues today (Kenrick and Davis,
2004, p. 35). Lycopods (Lycophyta) represented the dominant
tree form. However, arborescent forms of sphenopsids also formed
a significant portion of the organic matter in late Carboniferous
coal measures (Willis & McElwain, 2002, p. 107).
Wet, humid environments are needed to complete the life cycles
of both nonvascular plants and spore producing vascular plants.
In the life cycle of plants a gametophyte (haploid) generation
vascular plants such as lycopods, sphenopsids, and ferns possess
structures that produce spores dispersed by wind. These spores
are more drought-resistant than the non-vascular plants like mosses
and liverwarts. The spores grow into small, inconspicuous gametophyte
plants that possess one set of genetic instructions (haploid).
Gametophyte plants have structures that produce sperm or eggs.
Water is required for the sperm to
to and fertilize
eggs, forming zygotes. Zygotes with two sets of genetic instructions grow
into the large sporophyte plants.
Ancient Horsetail Trees
Several arborescent sphenopsids are known from the Carboniferous
to the Permian. Calamites was the largest horsetail tree. Calamites grew
up to 18 m in height (Willis & McElwain, 2002, p. 104). Calamites stems
were segmented. The joints of the segmented stem are referred to
as nodes. Arborescent horsetail trunks had a ring of woody tissue
surrounding a hollow interior, which gave the effect of a reinforced
This design was fast growing and used minimal material for strength.
Trunks of this type are prone
to buckling at branching points, so branches are constructed
at small angles. Overall, branching is sparse
in the tree design (Kenrick & Davis, 2004, p. 70).
Branches and whorled leaves sprouted at
the joints or nodes. Leaves were
In our article we use Calamites to describe the entire plant. However, the genus Calamites was intially used only for pith casts of the stem. Different organs of Calamites, like many other Carboniferous plants, were given seperate names before it was known that they represented one plant. The whorled
leaves are known as Annularia and Asterophyllites.
The trees grew from a creeping rhizome. Roots and root hairs also
Roots are known as Astromyelon (Rich, 1996, p. 385). Spores
grew in sacs organized into cones known as Calamostrachys and Paleostachya.
Casts of the pith cavity (Calamites) are a common fossil. The most common calamitean petrified stem is known as Arthropitys. Today, the genus Calamites is often used to encompass the stem, pith, and central canal (Taylor, Taylor, and Krings, 2009, p. 345). It is believed that Calamites did not
undestory to the lycopsids, but grew in clearings or on waterside
edges of forests (Willis, & McElwain, 2002, p. 107).
Science Olympiad Fossil Event
The 2016 Science Olympiad Fossil List includes the genus Calamites (Horsetail) and the plant leaf Annularia within the phylum Pteridophyta (Ferns).
Calamitean stem Arthropitys
Bieland, Maranhao Province, Brazil
Pedra de Fogo Formation
10.5 cm x 8 cm
Center & Upper
Right (maybe related to horestails)
Plate 6.5 cm x 5 cm
Upper Carboniferous, Westfalien
Plate 7 cm x 4.7 cm
Upper Carboniferous, Westfalien
Plate 6 cm x 5.2 cm
Graissessac, Herault, France
Plate 10.5 cm x 5.5 cm
Calamites Pith Cast
16 cm long x 6.5 cm diameter
P. and Davis, P. (2004). Fossil Plants. Smithsonian
P. & Nudds, J. (2004). Evolution of Fossil Ecosystems.
Chicago: The University of Chicago Press.
P.V., Rich T. H., Fenton, M.A., & Fenton, C.L. (1996). The
Fossil Book: A Record of Prehistoric Life. Mineola, NY: Dover
K.J. & McElwain, J.C. (2002). The Evolution of Plants.
New York: Oxford University Press.