The only other fossils I can see in the beds are small gastropods less than 2mm high. Other fossils found in other strata of the thaynes formation in the study area are gastropods, bivalves, scaphopods, echinoids, ophiuroids, crinoids, and sponges.
Most of the grains in the Meekoceras beds and other limestone beds in the area are small broken pieces of mollusc shells and echinoderms, the matrix is a sparite, making a Bio-sparite.
The grains in the anasibirites beds are alot smaller, the matrix is a micrite, making a Bio-micrite.
Ammonoids are usually dimorphic, a small microconch (male) and a larger macroconch (female), sometimes ornament on the shell differentiates male and female, a robust, strongly ornamented (male), and a gracile, smooth (female) shell, both about the same size. But as Phil mentioned the adults would all be about the same relative size.
But what if you had a polymorphic specie? Just a thought! I have to do a lot more cutting and sectioning to see if all the fossil ammonoids in the beds are adult, or a family grouping (theres another thought, ammonoids rearing their young).
It seems that any kind of toxic environment would not only kill the ammonoids, but every other critter swimming in the sea at that time and that place. Why only ammonoids? Why only ammonoids fossilized? Why only one basic shape of ammonoid? Some of the ammonoids in the lower beds occur in beds above the anasibirites beds, so they would have been around at that time, maybe just not that place.
