Carr, R. K., 1995a. - Biological Sciences
Carr, R. K., 1995a. - Biological Sciences
Carr, R. K., 1995a. - Biological Sciences
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(3) and decreased agility (referring to the rate of movement).<br />
Unlike extant anguilliform swimmers and suggestive, in part, of subcarangiform locomotion<br />
(LINDSEY, 1978) was the inflexibility of the anterior third of the body along with a concentration<br />
of mass anteriorly (LINDSEY 1978, table 2, characterizes subcarangiform mode as: similar to<br />
anguilliform with reduced anterior undulations; fusiform body; "body tends to be heavier and<br />
more rounded anteriorly"; deep caudal peduncle; low aspect ratio caudal fin; flexible caudal fin<br />
with straight posterior margin or indented margin and intrinsic musculature). Effects of these<br />
two parameters (flexibility, mass), concentrated undulations to the posterior two thirds of the<br />
body and limited anterior yaw. Extant subcarangiform fishes often reduce the effect of yaw<br />
through an enhancement of the lateral profiIe, which additionally shifts the center of gravi ty<br />
forward (similar lateral compression is seen in Brachydeiridae). To go beyond these simple observations<br />
is difficult since the hydrodynamics and range of responses in extant anguilliform<br />
swimmers are still poorly understood. Associated with changes in arthrodiran caudal locomotion<br />
were modifications in the pectoral fins improving maneuverability.<br />
Pectoral fins are difficult to evaluate in placoderms, but there are some basic principles<br />
that can be assessed (for a review of hydrodynamics see BONE and MARSHALL, 1982). In fishes,<br />
fins can be oriented in any position from horizontal to vertical with the latter case operating in<br />
a drag regime, analogous to using a boat oar. Horizontal fins can be either passive or active<br />
lift structures with the amount of lift varied by shifting the angle of attack. An active lift system<br />
is one in which lift is used to generate forward thrust, e.g. as seen in extant holocephalans<br />
(LINDSEY, 1978) and possibly among ptyctodonts and rhenanids (MILES, 1969, argued that the<br />
heavily scaled and narrow based fins of gemuendinids were incapable of generating forward<br />
thrust, but were used in burying the animal in sediment; however, these two functions are not<br />
mutually exclusive). In pachyosteomorph arthrodires, the fin was a nearly horizontal lift structure<br />
in which the fin base was lengthened with a concurrent increase in the number of fin basals<br />
(cr.art, Fig. 13; WESTaLL, 1958; STENSIO, 1959). Outgroups had a narrow based fin with fewer<br />
basals (Fig. 13A; STENSIO, 1959; GOUJET, 1984a, YOUNG & ZHANG, 1992). Fin basals articulated<br />
A<br />
FIG. 13. - A comparison of narrow and broad based pectoral fins. A, Pholidosteus sp.; S, Rhinosteus parvulus, redrawn from<br />
STENSI6, 1959.<br />
Comparaison entre des nageoires pectorales etroites et larges. A, PhoJidosteus sp. ; B, Rhinosteus parvulus, d'apres STENSID,<br />
1959.