Hemichordata Fossils, including Graptolites
the first dawn of life, all organic beings are found to resemble each other in
descending degrees, so they can be classed in groups under groups.
Darwin, Origin of Species, Chapter 13
people are unfamiliar with the animals within Phylum Hemichordata,
which is small and contains but a few hundred species. They turn
out to be, nonetheless, very important in the study of the evolutionary
origins of vertebrates. The name Hemichordate, meaning half chordate,
derives from their having only some characteristics of Chordates,
while lacking others. They are deuterostomes characterized by
embryonic development where the first opening (the blastopore)
becomes the anus, in contrast to protostomes where it becomes
the mouth. As deuterostomes, the hemichordates are a sister group
to both the true Chordates and the Echinoderms.
most distinguishing characteristic of the Hemichordates is a more
primitive form of notochord, called a stomochord that contained
a nerve system. This tube provides the basis for placing the Hemichordates
as a sister group to true Chordates. They also have branchial
openings, or gill slits, that open into the pharynx.
are three classes of Hemichordates. The Enteropneusta are the
acorn worms that remain extant. The Pterobranchia are also extant
and form a small and obscure group that form colonies in which
the individuals are interconnected by stems, or stolons made from
collagen protein. Members of Class Graptolithina (the Graptolites)
are common fossils with a worldwide distribution making them important
as index fossils, particularly for dating Palaeozoic sediments.
Graptolite fossils were once colony of tiny animals that either
floated in the sea or was attached to the sea floor like a crinoid.
The graptolite colony consisted of one or many branches that were
straight, curved or spiral. The individual animals lived within
structures along the length of a community exoskeleton formed
from collagen. Graptolite fossils appear from the Cambrian and
dissapear in the Carboniferous. Yannanozoon
lividum from the early Cambrian Chengjiang
Biota is a putative, primitive hemichordate, though some scientists
have assigned it to the enigmatic phylum
most prevalent Hemichordate fossils are the Graptolites which
were colonial animals that are primarily known from the upper
Cambrian through the Lower Carboniferous (Mississippian) of the
Paleozoic. A graptolite colony is called a rhabdosome and will
have a varying number of branches called stipes that come from
the initial animal individual called a sicula. Each subsequent
individual (zooid) was housed within a tubular or cup-like structure
(called a theca).