Allometry between Suture Line Length and Phragmocone Volume in Some Cretaceous Ammonoids

Abstract. The allometric relationship between suture line length and phragmocone volume was examined in 28 specimens belonging to six ammonoid species recovered from the Upper Cretaceous Yezo Group of Hokkaido, Japan. The suture perimeter was calculated by summing the distances between digitized points along the suture line in a photograph. The phragmocone volume was calculated using a modified Raup's model. The relationship between suture perimeter and phragmocone volume in each species was expressed as an allometric equation on a logarithmic scale. The exponent of ontogenetic allometry estimated for each species ranged from 0.462 to 0.626. The exponents estimated were much greater than the exponent in the case of isometry (0.333) and the value previously obtained through interspecific comparison. Calculation using a theoretical morphologic model indicates that allometric exponents for the suture perimeter of approximately 0.5 reflect positive allometry of the septal surface area with respect to body size. The present results suggest that strong positive allometry of the suture perimeter leads to a considerable positive allometry of the septal surface area resulting in satisfying Kleiber's scaling law, although its functional significance remains unclear.