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The functional significance of nectridean tabular horns amphibia lepospondyli






Proceedings of the Royal Society of London Series B Biological Sciences 209(1177): 513-538

The functional significance of nectridean tabular horns amphibia lepospondyli

The order Nectridea of the subclass Lepospondyli is a Paleozoic age. Within this order, later members of the Keraterpetontidae developed hyper-extended tabular horns, so that in plan view the skull if boomerang-shaped. Many unsuccessful attempts have been made to explain this shape in functional terms. The genera showing the greatest development of these horns, Diplocaulus Cope and Diploceraspis Romer, are analyzed experimentally in a low-speed wind-tunnel. The former is about twice the size of the latter genus and it has a prominent, ventrally directed flange on the quadratojugal-squamosal region not seen to the same extent in Diploceraspis. In Diplocaulus the otic notch, which lies on the ventral side of the skull, is large and extends proportionally further towards the tip of the horn than in Diploceraspis. A full-scale model of the Diplocaulus skull was made. It was mounted in the wind-tunnel so that angles of incidence varying from -10 to +25.degree. were possible, and a fixed body was modeled to account for interference effects. The Diploceraspis condition was simulated by removing the prominent quadratojugal flange. Four conditions were investigated: the Diplocaulus model; the Diploceraspis model; the investigation of the effect of roughness of the surface of the model, to simulate the labyrinthodont condition of the dermal bones; and an investigation of the effect of mouth-opening on the behavior of the model. A flow-visualization test also was made. All the experiments were carried out at a speed corresponding to the animal moving at 1.65 m/s in water. Coefficients of lift, drag and pitching moment were measured, over the range -10 to +25.degree., at 2 degree intervals. The significance of the results lies in the behavior of the lift and pitching moment coefficient curves. The position of the center of pressure does not move with the change of these 2 parameters and the point of action of the center of lift is fixed with respect to the occipital condyles. Forces exerted on the head are proportional to the deflexion of the head. Small but significant differences are seen when the curves for the 2 genera are compared. In Diplocaulus the lift and pitching moment curves cross the zero-line very close to the origin, but in the other genus they cross considerably to the left of the origin. Roughening of the surface in 2 stages diminishes lift coefficients and increases drag. The surface of the living animals was smooth or almost so. With the smooth model, when the mouth was opened, neither lift nor drag coefficient was significantly altered. When taking prey these animals must have suffered little deceleration. The flow visualization tests show that 2 flow regimes operated on the upper surface of the Diplocaulus model. To begin with, the flow was streamlined and parallel to the midline over all the head area, but after a critical angle of incidence the central region of the head stalled and the flow over the horns became stabilized as laterally directed vortices. The Keraterpetontidae probably were active midwater feeders preying on small fish, larval amphibia, aquatic arthropods and gastropods. They used the unique physical properties of the head to effect steep climbing ascents from the lake or stream bed to attack their prey, before returning to the bottom. The differences between the 2 genera are related to their contrasting environments and the possible course of evolution giving rise to these extreme adaptations is considered. Dorso-ventral flattening aids an active mode of life in the 2 genera and the same possibly applies to the Labyrinthodontia as a whole.

Accession: 006680769

DOI: 10.1098/rspb.1980.0110

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