A lower jurassic ammonite image database and its applications
Creators: name: Liang, Bo
Publisher: name: University of British Columbia
Year: 2009

description: Amnion is the first computerized image database for ammonites that allows automated measurement of morphological parameters from images. The database contains 7790 specimens representing 15 families, 179 genera and 1319 species. Each specimen has 102 descriptors covering taxonomy, quantitative morphology, qualitative morphology, stratigraphy, locality information and general comments. Species density diagrams are plotted with respect to basic shell geometry. The data show strong correlations among basic geometric parameters which may be related to restrictions imposed by whorl shape and size. Buckman's Law of Covariation is shown to be applicable to Lower Jurassic ammonite stocks and new covariations are demonstrated among tuberculation, ribbing, whorl shape and ventral features, all of which have functional implications. Morphological and stratigraphical data from Amnion suggest a possible correlation between basic shell geometry, morphological variability and taxa duration. Low within-population morphological variabilities and short durations are associated with taxa that inhabit the most densely populated geometric region. Away from this peak, there is a steady increase in both morphological variability and taxa longevity implying a decrease in selective pressure. The taxonomic diversity of Lower Jurassic ammonites peaks first at the family level in the Ibex Zone, followed by generic data one zone later, and finally species in the Margaritatus Zone implying a delay in radiation for lower levels of ammonite taxa. The Late Sinemurian and Early Pliensbachian have high family origination rates and relatively low species diversities; species reach their highest diversity level in the Late Pliensbachian which also has the highest family extinction rate and an almost zero origination rate. Two morphological diversity indexes are used to study changes through time. The range index describes the range of the morphospace occupied. The Shannon index measures both the range and the evenness of the occupation. Both indexes indicate a similar pattern. There is a rapid rise to an Early Sinemurian maximum through morphological radiation, especially in terms of the basic shell geometry, followed by a pronounced fall in the middle Sinemurian with diversity remaining relatively high and stable thereafter. It is evident by comparing taxonomic and morphological diversity changes through time that initial morphological diversification in Lower Jurassic ammonites exceeded the proliferation of lower-level taxa. For example, the morphological diversity of the Bucklandi Zone with 46 species is slightly higher than that of the Margaritatus Zone with 177 species. It demonstrates that morphological diversity and taxonomic diversity are not simply correlated but that taxa initially occupy discrete positions within a broad morphospace followed by the occupation of intervening areas. It is observed that all Lower Jurassic ammonite families radiating into numerous species in the first zone of their occurrence have durations of less than 4 zones; in other words, rapid origination is associated with short duration.