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(3) and decreased agility (referring to the rate of movement).
Unlike extant anguilliform swimmers and suggestive, in part, of subcarangiform locomotion
(LINDSEY, 1978) was the inflexibility of the anterior third of the body along with a concentration
of mass anteriorly (LINDSEY 1978, table 2, characterizes subcarangiform mode as: similar to
anguilliform with reduced anterior undulations; fusiform body; "body tends to be heavier and
more rounded anteriorly"; deep caudal peduncle; low aspect ratio caudal fin; flexible caudal fin
with straight posterior margin or indented margin and intrinsic musculature). Effects of these
two parameters (flexibility, mass), concentrated undulations to the posterior two thirds of the
body and limited anterior yaw. Extant subcarangiform fishes often reduce the effect of yaw
through an enhancement of the lateral profiIe, which additionally shifts the center of gravi ty
forward (similar lateral compression is seen in Brachydeiridae). To go beyond these simple observations
is difficult since the hydrodynamics and range of responses in extant anguilliform
swimmers are still poorly understood. Associated with changes in arthrodiran caudal locomotion
were modifications in the pectoral fins improving maneuverability.
Pectoral fins are difficult to evaluate in placoderms, but there are some basic principles
that can be assessed (for a review of hydrodynamics see BONE and MARSHALL, 1982). In fishes,
fins can be oriented in any position from horizontal to vertical with the latter case operating in
a drag regime, analogous to using a boat oar. Horizontal fins can be either passive or active
lift structures with the amount of lift varied by shifting the angle of attack. An active lift system
is one in which lift is used to generate forward thrust, e.g. as seen in extant holocephalans
(LINDSEY, 1978) and possibly among ptyctodonts and rhenanids (MILES, 1969, argued that the
heavily scaled and narrow based fins of gemuendinids were incapable of generating forward
thrust, but were used in burying the animal in sediment; however, these two functions are not
mutually exclusive). In pachyosteomorph arthrodires, the fin was a nearly horizontal lift structure
in which the fin base was lengthened with a concurrent increase in the number of fin basals
(cr.art, Fig. 13; WESTaLL, 1958; STENSIO, 1959). Outgroups had a narrow based fin with fewer
basals (Fig. 13A; STENSIO, 1959; GOUJET, 1984a, YOUNG & ZHANG, 1992). Fin basals articulated
A
FIG. 13. - A comparison of narrow and broad based pectoral fins. A, Pholidosteus sp.; S, Rhinosteus parvulus, redrawn from
STENSI6, 1959.
Comparaison entre des nageoires pectorales etroites et larges. A, PhoJidosteus sp. ; B, Rhinosteus parvulus, d'apres STENSID,
1959.