M.J. Melchin and M.E. DeMont (1995). Possible propulsion modes in
Graptoloidea: a new model for graptoloid locomotion. Paleobiology. 21(1):
110-120.
Summary
The motion of locomotion of any swimming animal is constrained
by its size, architecture, and phylogenetic history. Considering
these factors and the range of locomotory systems used by extant
zooplankton, the range of possible modes of locomotion for
graptoloids can be effectively limited. Three assumptions have
been made: (1) graptoloids did not use a mode of locomotion unknown
among modern organisms; (2) all graptoloids employed essentially
the same mode of locomotion except, possibly, in their early growth
stages; and (3) graptoloids did not rely entirely on passive
buoyancy - no extant zooplankton groups in the size range of the
graptoloids do. Structures that increase buoyancy or drag are
often found in actively swimming zooplankton. They enhance feeding
efficiency and reduce sinking rates during nonswimming periods.
The modes of locomotion utilized by extant zooplankton groups
are ciliary propulsion, elongate body undulation, jet propulsion,
rowing with skeletonized appendages, and rowing or undulation with
muscular appendages. Of these, all but the last can be rejected
for the graptoloids on the basis of scale or architecture. It is
concluded that graptoloids probably used a rowing or undulatory
motion with muscular appendages for swimming. Using a pterobranch
model for the graptoloid zooids, the lophophore is considered an
unlikely propulsive structure because the design requirements would
conflict with those of a ciliarly suspension-feeding organ.
Winglike, lateral extensions of the muscular cephalic shield, the
same structure used for creeping locomotion in the benthic
pterobranchs, is regarded as the most likely propulsive organ,
analogous to the pteropod swimming wings.
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Edwin DeMont, Associate Professor
Biology Department, St. Francis Xavier University
P.O. Box 5000, Antigonish, Nova Scotia, B2G 2W5 Canada
Voice 902-867-5116 FAX 902-867-2389
edemont@juliet.stfx.ca -
June 18, 1995