Steve wrote said:
My rambling has a point (I hope). When did seaweeds first appear?? If you had rafts of these things (weeds) might it not be an appropriate habitat in which to look (or photoshop) for juvenile ammonites, belemnites and nautiloids ....
It's hard to imagine life in the sea without seaweed, isn't it? Having had a quick search around for fossilised seaweed references, it seems that it has been around in some form at least since the mid-Cambrian, and possibly earlier. I've found an interesting reference to fossils of tiny Cambrian agnostid trilobites (four or five millimetres long) that have been found arranged in long strips, the strand of seaweed they presumably were living on long since rotted away and unpreserved. One of the theories about the lifestyle of
Plectronoceras and
Palaeoceras, the first recognised upper Cambrian cephalopods, is that they may have been tiny drifters, perhaps they were attached to rafts of seaweed feeding on such herbivorous trilobites or their planktonic larvae(?) (Another theory holds they were more snail like and crawled along the substrate). So perhaps, just possibly, seaweed may have provided the right environment for the earliest cephalopod evolution?
Many forms of graptolite, though not all, were believed to have been attached to seaweed as were some forms of ancient crinoids. I totally agree that rafts of seaweed may have supported a wealth of fauna, perhaps nautiloids would have drifted with them feeding off associated trilobites? Their conches remained bouyant after death and did not sink down, held aloft by remnants of gasses in the chambers. One can imagine mats of these shells intertwined with seaweed drifting for months; afterall, if they had all sunk to implosion depth ammonite fossils would be rare fossils indeed!
Such ammonoid and nautiloid shells must have occasionally drifted for years on the ocean currents as Nautilus shells do today. One would think specimens must have been published with growths of barnacle-type animals attached as the conch of the dead animal formed a host, much as Kevins specimen may be an example of.