Old Earth Ministries Online Earth History Curriculum
Presented by Old Earth Ministries (We Believe in an Old Earth...and God!)
This curriculum is presented free of charge for use by homeschooling families.
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Chapter 5 - The Silurian Period
Lesson 27: Graptolites
(Graptolithina) are fossil colonial animals known chiefly from the Upper
Cambrian through the Lower Carboniferous (Mississippian). A possible early
graptolite, Chaunograptus, is known from the Middle Cambrian.
Graptolites are one of the more common fossils of the Silurian Period.
In fact, the Silurian Period is divided into four parts, with the beginning
of each part based on the appearance of a certain species of graptolite.
Chapter 5: The Silurian Period
|Artist representation of the basal planktonic graptolite Rhabdinopora|
originates from the genus Graptolithus, which was used by
in 1735 for inorganic
crustations which resembled actual fossils. In 1768, in the 12th volume of
Systema Naturae, he
included G. sagittarius and G. scalaris, respectively a possible
fossil and a possible graptolite. In his 1751 Skånska Resa, he included a
figure of a "fossil or graptolite of a strange kind" currently thought to be
a type of Climacograptus (a
of biserial graptolites). Later workers used the name to refer to a specific
group of organisms. Graptolithus was officially abandoned in 1954 by the
ICZN, partly because
of its original purpose as a grouping for inorganic mimicries of fossils.
(Bulman, 1970: V 6)
Graptolites as Index Fossils
Graptolites are common fossils and have a worldwide distribution. The
preservation, quantity and gradual change over a geologic time scale of
graptolites allows the fossils to be used to date strata of rocks throughout
the world. They are important
index fossils for dating
Palaeozoic rocks as they evolved rapidly
with time and formed many different species. British geologists can divide
the rocks of the Ordovician and Silurian periods into graptolite biozones;
these are generally less than one million years in duration. A worldwide ice
age at the end of the Ordovician eliminated the majority of the then-living
graptolite; species present during the Silurian period were the result of
diversification from only a one or two species that survived the Ordovician
graptolite colony is known as a
rhabdosome and has a
variable number of branches (called
originating from an initial individual (called a
Each subsequent individual (zooid)
is housed within a tubular or cup-like structure (called a
In some colonies, there are two sizes of theca, and it has been suggested
that this difference is due to
The number of branches and the arrangement of the thecae are important
features in the identification of graptolite fossils. Their general shape
has been compared with that of a hacksaw blade.
Graptolite fossils are often found in
and mud rocks where sea-bed fossils are rare, this type of rock having
formed from sediment deposited in relatively deep water that had poor bottom
circulation, was deficient in oxygen, and had no scavengers. The dead
planktonic graptolites, having sunk to the sea-floor, would eventually
become entombed in the sediment and are thus well preserved.
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