Differential growth in three ichthyosaurs : Ichthyosaurus communis, I. breviceps, and Stenopterygius quadriscissus (Reptilia, Ichthyosauria) /

Abstract

Changes in morphological proportions during growth were investigated in the Lower Jurassic ichthyosau'rs, Ichthyosaurus communis Conybeare 1822, I. breviceps Owen 1881, and Stenopterygius quadriscissus (Quenstedt) 1858. The growth constants a and b from the allometric equation y = bxa were evaluated using Bartlett's Model 11 regression method, and Kermack and Haldane's reduced major axis method.Intracranial growth was mainly isometric in all three species. Growth of the orbit had a negative allometry in I. communis with some indication of positive allometry in I. breviceps. The external naris had a positive allometric growth in I. communis. There is some indication that maxillary growth had a negative allometry in I. breviceps, and a positive allometry in Stenop terygius quadriscissus. Independent evidence suggests that positive allometric growth in the maxilla was characteristic of the longipinnates.Growth of the head relative to the body was negatively allo metric in I. comnzunis and S. quadriscissus while growth of the forefins was largely isometric. In S. quadriscissus the area of the tail and dorsal fin increased exponentially, and the dorsal fin came to lie proportionally further forward. There was a trend towards increasing the angle of the tail bend. The possible significance of these changes is discussed, together with the functional morphology of the tail.