Eigenshape analysis of ammonoid sutures

A morphometric method based on eigenshape analysis is proposed for characterizing the morphospace of widely varied ammonoid suture shapes. The analysis requires initially the placement of a suture line in an x- y coordinate system, with the ventral and umbilical extremes located on the x-axis. Series of x- and y-coordinates along the suture line are used as descriptors. The coordinate data are summarized into the two largest principal components using eigenshape analysis. A total of 115 species belonging to six ammonoid orders, spanning from the Devonian to the Cretaceous, was utilized in the present analysis. The analysis, using y-coordinate data, revealed differences in morphological variation in suture shape among taxa within the Mesozoic ammonitids: the Lytoceratina and Ammonitina were characterized by small negative values of the first eigenshape scores, whereas the Phylloceratina (the sister group of the Lytoceratina plus Ammonitina), as well as the Triassic ceratitids and Palaeozoic ammonoids, have a wide range of the first eigenshape scores. The pattern of data obtained from many different ammonoid species, as plotted on eigenshape axes in the morphospace constructed based on y-coordinate data, reveals a plesiomorphic aspect of suture shape in some phylloceratine species with respect to other ammonitids. Ammonoids, eigenshape analysis, morphometrics, suture line.