An ?Equation of state? for the morphology of Jurassic ammonites : a

HORACIO PARENT AND ANDRÉS F. GRECO

REVUE DE PALéOBIOLOGIE

Museum d´Histoire Naturalle. Ville de Geneve. Suisse.

Año: 2007 vol. 26 p. 41 - 56

0253-6730

Characterization of the morphology of the ammonite shell is an old, relevant and open problem in ammonitology. Since many yearsago the morphology and growth of ammonoids has alternatively been described in terms of the logarithmic law. However a descriptionin terms of the variables or dimensions most commonly accessible and used (D : diameter or size ; U : umbilical width ; H1 : whorlheight ; H2 : whorl ventral or apertural height ; and W : width of whorl section) is still missing due to the lack of relationship betweenthese variables with those of the logarithmic law. In this paper we have found the relationship between the spiral logarithmic lawand a multidimensional law which we have called the equation of state of the post-embryonic shell of Jurassic ammonites. This lawis written in terms of D, U, H1 and H2 and has been tested on 25 species of Middle and Late Jurassic ammonites of the superfamiliesPerisphinctoidea, Stephanoceratoidea and Haploceratoidea. Results obtained show an agreement with only 1-22% of uncertaintybetween observations and predictions, thus describing, at least, the 80-90% of the morphology of the studied ammonites. It was alsoconcluded that H1/D and H2/H1 are fundamental parameters for describing the basic form and growth of ammonites in the equatorialplane.