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From extinction to recovery : Late Triassic and Early-Middle Jurassic ammonoid morphology Zhang, Jin

Abstract

As one of the five largest mass extinctions occurring in the Phanerozoic, the extinction at the end of the Triassic dramatically affected the evolution of ammonoids, taxonomically and morphologically. The major aim of this thesis is to compare and contrast the ammonoid morphological evolution for the Upper Triassic Carnian Stage through the Middle Jurassic Bajocian Stage and explore the relationships among ammonoid morphospace, ornamentation, and suture types. "Buckman's law of covariation" (Westermann, 1966) indicates that, in the Bajocian, certain combinations of ammonoid morphological parameters occur more frequently and presumably conferred an evolutionary advantage. In brief, the law states that ammonoid whorl width, umbilical ratio, and strength of ornamentation are negatively correlated with whorl expansion rate. However, since "Buckman's law of covariation" was established for a group of Middle Jurassic Hildoceratids, a major question being addressed in this thesis is whether the "Buckman's law of covariation'" is more widely applicable to Upper Triassic and Lower-Middle Jurassic ammonoid data. Specimens were sampled at genus level. Data are mainly taken from AMMON database and other published literature. Raup's (1966) numerical model is used in this thesis to describe the basic shell geometry of planispiral ammonoids. The density of the occurrence of real shell geometries (whorl expansion W, and umbilical ratio U) for a set of ammonoid samples was contoured to create a density-contour map and threedimensional surface plot for each stage. The results prove that "Buckman's law of covariation" is apparently applicable to Jurassic ammonoids, but weakly to Late Triassic ammonoids. Combining the appearance and the morphology of heteromorph ammonoids, three intervals can be recognized: 1. Late Triassic (pre-extinction interval), 2. Early Jurassic (post-extinction interval), and 3. Middle Jurassic (recovery interval). The possible covariations between whorl shape WWWH (ratio of whorl width WW and whorl height WH) and the basic morphological features W and U are approached in this thesis because this is important part in "Buckman's law of covariation". The U-WWWH contoured density maps and the W-WWWH contoured density maps indicate positive correlations between U and WWWH, and negative correlations between W and WWWH. In consideration of taxonomic effects, I also explore these relationships by using taxonomic subsets (for the superfamilies Nathorstitaceae and Tropitaceae) of the Norian Stage. The positive correlations between U and WWWH and negative correlations between W and WWWH can also be seen. W-U density contoured maps of equidimensionally whorl shaped ammonoids (WWWH ≈ 1) are created for Upper Triassic (Carnian) through the Middle Jurassic (Bajocian) in order to explore whether ammonoids adopt shell shapes with maximal hydrodynamic efficiency. Low drag coefficient (C[sub D]) values determined from the data of shell models (Chamberlain, 1980) are plotted on these maps. Generally Later Triassic and Middle Jurassic ammonoids have broad distribution across W-U morphospace and relatively low hydrodynamic efficiency. Early Jurassic ammonoids have better hydrodynamic efficiency and linear W-U geometric patterns. Areas of high drag on the uncoiled side of the offlap line (W = 1/D) are only exploited in the Norian and Bajocian due to the existence of heteromorph ammonoids in these two stages. The accumulated morphological range diversity index (MRDI), which describes the range of the morphospace occupied, is used to study morphological changes through the stages. There are two declines in the MRDI: from the Norian to the Hettangian and from the Toarcian to the Aalenian. In this thesis, I explore the possible correlation between Late Triassic ammonoid shell geometry and suture type. In the Upper Triassic there are three types of suture and the shell geometries are quite diverse, while in the Hettangian Stage, there is only one type of suture and the shell geometry spectrum is greatly reduced. The research shows that involute Upper Triassic shells tend to have complex sutures, whereas evolute shells have simple sutures. Hettangian ammonoids are relatively evolute but their sutures are complex. Reasons for these differences are not yet clear.

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