Reconstruction of Monograptus exiguus

Artist’s impression of the graptolite Monograptus exiguus. This graptolite was a colonial animal, with multiple individual organisms, known as zooids, living within an organic tubular structure known as a thecae. Each colony originates from one zooid, known as a sicular zooid, from which the subsequent zooids will develop. On the left of this figure is an extreme close-up of a M. exiguus colony, showing the zooid arms and tentacles protruding from the theca. These colonies were tiny, with the overall length of the whole colony (rhabdosome) measuring only approximately 10 to 20 mm long. Despite their small size, this species is biostratigraphically important, as it allows dating of the State Circle Shale to the early Silurian period at about 434 million years ago.