Graptolites, a fossil case-history of evolution from sessile, colonial animals to automobile superindividuals

Abstract

Abstract Graptolites were ciliary-feeding, colonial animals; their small (1‒3 mm) zooids were supported by tubular periderm. Bush- or tree-like colonies attached to the sea floor gave rise to planktonic colonies held in suspen­sion by their ciliary currents. Adaptation to automobility led to a reversal of the evolutionary trends that characterize most colonial organisms; the outreaching branching patterns of the sessile colonies became gradually simplified, and this led to the evolution of small, active, streamlined superindividuals. Constructed of ‘collagenous’ increments made up of three or four fibrillar fabrics, graptolite periderm appears to be unique and almost indestructible. Whether graptolites became extinct or returned neotenously to individuality, losing their characteristic periderm, is discussed in this paper.