Die evolution der altesten Ammonoidea

The theories of the origin of coiled ammonoids are discussed by examining former Statements in favour or against a derivation from coiled nautiloids (Spath) or from bactritids (Schindewolf), respectivcly. Results of additional studies ndicate an ancestorship of the Bactritidae. Evidence is represented by homologies of morphological elements of the shell, by the similarity of the early ontogentical development, by a progressive evolutionary succession of specics and gencra connecting primitive goniatites with the Bactritidae, and by iterative as well as regressive tendencies indicating relationship.

The evolution of early Ammonoidea (including the Bactritidae) is studied. The main evolutionary succession leading from bactritids to coiled ammonoids seems to be: Bactrites - Lobobactrites - early Cyrtobactrites - Anetoceras hunsrueckianum - A. aff. hmsrueckianum - A.(Erbenoceras)sp. A. - Teicherticeras (?) n. sp. B - (?early Convoluticeras) - ^«,/,-group of Mimagoniatites - «Wgroup of Mimagoniatites - Anarcestes - Anarcestidae without umbilical opening - other groups of Ammonoidea.

In this succession some of the most outstanding features are a.o. the increase in involubility of the shell, in complexity of the suturc, and in complication of the growth lines. Almost from each of the a.-m. members originate separate branchs in which identical evolutionary trends may become manifested independently from the main succession. In late phylogenctic stages of some of these separate branches has been observed a regressive tendency to secondary uncoiling, to a secondary simplification of the growth lines, etc.