ECOLOGICAL IMPLICATIONS OF THEORETICAL MORPHOLOGIC DISTRIBUTIONS OF ARCHIMEDES COLONY MORPHOLOGIES

At least six species of Archimedes occurred in the mid-Carboniferous (Viséan – Namurian) shallow seas of North America, but the distribution of a sample of 116 colonies within theoretical morphospace forms three clouds of points, two of which are composed of specimens from two different paleoenvironmental settings (basinal versus back-shoal). The third is constituted by the species A. laxus , which occurred in both paleoenvironments. The distribution within the morphospace of the two Archimedes ecomorphologic colony types is interpreted as a function of the vertical extent of quiet-water zones above the sea floor in back-shoal versus basinal environments rather than to differences in feeding efficiency of the two colony types. Overall, back-shoal Archimedes had short colonies with greater variation in the angle between the colony axis and the lateral whorls of branches than did basinal colonies. Basinal colonies varied from short to tall, with a greater range in scaled distance per complete revolution about the helical axis than did back-shoal colonies. A. laxus differed from all other Archimedes studied in having consistently low angles between the colony axis and the lateral whorls of branches, consistently greater scaled distance per complete revolution about the helical axis, and very light axial calcification. It is interpreted to have had a relatively rapid, weed-like growth.