Argillaceous: sedimentary rocks having a very small grain size and high clay content. Some also contain a high percentage of organic matter.
Benthic: mode of life whereby organisms attach themselves, or rest on the bottom sediments of marine and/or freshwater environments.
Biostratigraphy: a branch of stratigraphy involving the use of fossils to date and correlate rock units in space and time.
Black Shale: a sedimentary rock with a very high concentration of organic material, which produces the characteristic black colour. These rocks are deposited in very low oxygen environments, typically in very deep, quiet water.
Cuticle: an outer layer of some organisms secreted by the epidermis (outer skin cell layer). It can be very complex in some animals, as well as very resistant to the fossilisation process.
Facies: total features of a rock that describe the environment in which it was deposited.
Filter-feed: mechanism of feeding whereby particles of food are collected from the water column by tentacles and directed to the digestive system of the organism.
Geologic Time Scale: the rock record is divided into units of time. Each of these time units has a name which refers to the amount of time represented in that unit of rock. The Palaeozoic represents the first (542-251 Ma) of three subdivisions within the Phanerozoic (542 Ma-Present). The Palaeozoic is further subdivided into time units called periods; Cambrian (542-488 Ma), Ordovician (488-443 Ma), Silurian (443-416 Ma), Devonian (416-359 Ma), Carboniferous (359-299 Ma), Permian (299-251 Ma).
Graptolite Fauna: unique assemblages or 'provinces' of graptolites found distributed worldwide during specific times (specifically the Ordovician). Specimens collected from any part of these provinces are thought to share similarities with each other at a generic level. It has been suggested that specific graptolite faunas were confined to particular water masses.
Hemichordata: phylum of animals related to the Chordata (vertebrates). They are commonly called the acorn worms.
Holdfast: biological structure in some marine and freshwater organisms which allows attechment to solid structures found on the sea-floor or floating in the water column.
Isotonic: a cellular state within tissues where there is an equal amount of pressure within the cell and the surrounding environment. In the case of graptolites it was thought to allow stability within the water column.
Lophophore: tentacles of soft-tissue used in filter-feeding.
Periderm: thin, sheet-like material that composes the skeleton of graptolites. Once thought to be chitin, it is now thought to be a type of collagen (fibrous-type of connective tissue).
Planktic: mode of life of some marine and freshwater organisms where they float within the water column.
Pterobranchs: class of small, colonial, deep-sea organisms that secrete an external cuticle. Within the cuticle are individual zooids. Pterobranchs have been suggested to be the closest living relative to the graptolites. They use a lophophore system for filter-feeding.
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