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Life habits

Most of the Asaphida, indeed most trilobites, were benthic (bottom dwellers).  This is widely agreed upon as they are generally fairly flat in lateral profile and although trilobite limbs are rarely preserved, what is known of them suggests that they walked rather than swam. Also, in most trilobites, the eyes consist of a fairly narrow strip positioned so that the animals could see around them but not above or below. Additionally they are found to be closely related to type of substrate and palaeoenvironment as well as palaeogeographic distribution (Fortey 1985) - being bottom dwellers they are adapted to particular conditions whereas swimming trilobites are independent of bottom conditions and can swim across oceans.

However, some asaphids are considered to be pelagic. Among the asaphids this swimming habit arose in the Cyclopygidae and the Bohemillidae, as well as in the unrelated Telephinidae and Opipeuteridae. According to Fortey (1981) it also appeared on a fifth separate occasion in Girvanopyge (= Cremastoglottos) but according to Jell and Adrain (2003) this genus is a cyclopygid which would mean the habit only appeared four times.

Evidence for a pelagic habit comes from the overall long fusiform shape of the body which was longitudinally very flexible and had strong vaulting which would have allowed attachment for powerful muscles for swimming. These forms had greatly expanded eyes which could see all around including backwards and downwards and the animals had downward-projecting spines indicating that they did not rest on the sea floor. Also, while bottom-living trilobites are associated with particular palaeoenvironments, pelagic trilobites have a much wider distribution (Fortey 1985).

Author: Laurence Dale
Last updated: 18/11/06
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