Ammonoid early internal shell structure: its bearing on early life history

Abstract

Morphologic analysis was made on early whorls of two Paleozoic and 43 Mesozoic ammonoid species to obtain basic data on early life history. A positive linear relationship between protoconch and ammonitella sizes is recognized among the species examined. Total rotation angle of the ammonitella (ammonitella angle) seems to be independent of ammonitella size but has a significant negative linear relation with whorl expansion rate. This supports the model that the embryonic shell of ammonoids consists of gas-filled protoconch and the succeeding body whorl up to the primary varix. Growth analysis of siphuncle diameter in the early post-embryonic stage also indicates that the shell growth rates of the species hatched from large-sized eggs were smaller than those of species hatched from small-sized eggs.