739. EARLY SHELL MORPHOLOGY IN SOME UPPER CRETACEOUS HETEROMORPH AMMONITES

Abstract

This paper describes the early shell morphology in some Upper Cretaceous heteromorph ammonites (five nostoceratids and three diplomoceratids) on the basis of well-preserved specimens from Hokkaido. The ontogeny of the species examined may be divided essentially into the planispiral ammonitella first stage and the uncoiled or abnormally-coiled post-ammonitella second stage, both of which are bounded by a nepionic constriction. The surface ornament first appears at some phase of the second stage. These lines of evidence support the interpretation that an ammonitella was formed within an egg capsule. The three diplomoceratids are clearly distinguished from the five nostoceratids in the presence of well-marked, prorsiradiate constrictions in the earliest post-ammonitella stage. The mode of coiling of the examined species is roughly classified into the three types (planispiral, helical and repeating U-turn coiling types) by means of RAUP'S (1966) parameters. We postulate that all of the examined species might have passed a planktonic life for a limited period of the earliest post-ammonitella (=post-embryonal) stage. However, a benthonic life can be inferred as to their middle to later growth-stages, because of the loss of bilateral symmetry of the shells and other various indirect evidence.