The functional morphology and population structure ofRastrites maximusfrom the Southern Uplands

Abstract

Synopsis Rastrites maximus is a species that shows relatively great intraspecific variation in the length, spacing and orientation of its thecae. The thecae are characterized by a long pair of spines made from the distal ends of a hood-like thecal hook. The metathecae are very thin and enclosed in a wide cowl, which is always preserved flattened. The rhabdosome rarely exceeds 12 thecae in length and was of a wide spiral form whose thecae showed torsion through about 11° per theca. It is never found complete beyond theca six because stresses transmitted to the stipe by these angled thecae caused the specimen to break once it was on the sea floor. This form, of a spiral graptolite with persistent ‘sails’ along each theca, would have been very stable and would have rotated through calm water in a sicula-down position with the thecal apertures widely separated for food gathering. The large rhabdosome and small zooid number and size suggest that this species was adapted for a predictable environment where populations were maintained close to their carrying capacity. Populations of R. maximus show a convex survivorship curve, which is typical of species living in stable environments where death is mediated by ageing.