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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