An Investigation Into Ichthyosaur Ontogeny, Sexual Dimorphism And Body Size Evolution

Samuel Bennett

Research output: ThesisDoctoral Thesis

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Ichthyosaurs are an extinct group of diapsid marine reptiles that existed from the Olenekian (251Ma) to end Cenomanian (93.9Ma). Morphometric data (length measurements) and meristic data (counts of repeated elements) were collected for Lower and Middle Jurassic taxa from several museums in England and one in Germany. Additional morphometric data were collected from the published record. Principal Components Analysis (PCA), Principal Coordinate Analysis (PCO) and Reduced Major Axis Regression (RMA) were used to analyse morphometric data relating to ontogeny. Linear Regression, also known as Ordinary Least Squares (OLS) was used to analyse meristic data relating to ontogeny as well as body size evolution. Sexual dimorphism was analysed using the Mann-Whitney test as well as Discriminant Analysis. The analysis of ichthyosaur ontogeny showed that neonate and juvenile ichthyosaurs had significantly larger skulls and eyes compared to body length. Once maturity is reached growth becomes isometric, and no other features varied with relative age. The numbers of repeated elements in ichthyosaurs remain stable throughout life, with the exception of postflexural caudal vertebrae, where the numbers increase with the size of the tail, and therefore, age. Investigation of sexual dimorphism indicated that inferred males are isometrically larger than pregnant females. However, the age at which maturity is reached cannot currently be identified in individual specimens. Furthermore, the gender of an individual cannot currently be determined, with the exception of pregnant females. The study of body size evolution was not conclusive and no statistically significant trends were identified. Due to the nature of the fossil record, only 53% of the taxa examined could be used in the analysis. More taxa, particularly from the Triassic and Cretaceous, need to be included to improve the understanding of ichthyosaur body size evolution.
Original languageEnglish
Awarding Institution
  • Royal Holloway, University of London
  • Collinson, Margaret, Supervisor
  • Barrett, Paul, Supervisor, External person
  • Goswami, Anjali, Supervisor, External person
Award date1 Jun 2015
Publication statusUnpublished - 2015

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