Size−shape relationships in the Mesozoic planispiral ammonites

H. PARENT, A. GRECO, AND M. BEJAS

Acta Palaeontol. Pol.

Institute of Paleobiology, Polish Academy of Sciences

Año: 2010 vol. 55 p. 85 - 98

0567-7920

Ammonites are of outstanding importance in dating events of the Mesozoic and in the study of mechanisms, modes andtiming of evolutionary processes. These applications rely on a detailed understanding of their morphology and the modesof variation. It has been known for a long time that their shape is composed of a number of highly correlated features.A new model, called the ADA−model, is introduced for the study of shell morphology (size and shape). The new model isbased on classic dimensions which are stable parameters throughout ontogeny, giving very close agreement between pre−dictions and actual observations. It was applied in the exploration of the morphospace occupied by the planispirally coiledand the regularly uncoiled Mesozoic Ammonoidea, based on two new reduced morphospaces introduced for the analysis.Results obtained expose close relationships between size and shape, and general patterns in the ammonite shell morphol−ogy and morphogenesis. (i) The relative apertural height of the whorl section relative to the diameter of the shell (H2/D) isinvolved in definition of size and shape. (ii) This same dimension shows a strong tendency to beH2/D= 0.3. (iii) There aresome geometrically possible shell shapes (or morphotypes) which seem to have not been developed since they are notknown in the current record. Assuming the known ranges of protoconch size and whorl number as constraints, theADA−model strongly suggests that these morphotypes have not been developed for the too large or too small sizes theshells would have attained, well outside of the actual size range of the planispirally coiled Ammonoidea. (iv) The law ofcovariation is shown to be a general pattern within the planispiral ammonites which describes structured variation of theshell shape. (v) A large fraction of the non−structured variation seems originate in the lack of correlation between the rela−tive umbilical diameter and width of the whorl section.