Bennett, CV; (2014) The evolution of metatherian morphological and taxonomic diversity. Doctoral thesis , UCL (University College London).

Abstract

Extant therian mammals comprise marsupials and placentals. The lineages leading to these groups have been evolving separately since the Jurassic, resulting in substantially lower ecological and taxonomic diversity of marsupials than placentals. In order to elucidate the potential evolutionary mechanisms for this, the comparative cranial morphology between marsupials and placentals and patterns of taxonomic diversity through the fossil record were investigated. Cranial morphologies of a comprehensive range of extant placentals, marsupials and fossil metatherians were quantified using a geometric morphometrics. Variation in total skull morphology, and that of developmentally significant regions, of extant marsupials and placentals, was calculated and compared. Principal components analysis and delta variance tests were used to visualise and statistically investigate the data. Entire cranial shape was found to be significantly less varied in extant marsupials than in extant placentals, also true specifically in the viscerocranium. However, marsupial and placental neurocrania showed no significant difference in morphological varianc. Inclusion of fossil metatherians with the extant marsupial data did not significantly increase the variance of metatherian morphology. These findings are consistent with expectations based on the developmental constraint hypothesis. Metatherian fossil data were compiled from the literature and subjected to rigorous statistical adjustments, namely shareholder quorum subsampling (SQS) an d classic rarefaction, to ameliorate effects bias in the fossil record. Metatherian diversity was shown to fluctuate through time largely in accordance with major environmental changes. The diversity patterns revealed here contra st those based on raw data. Specifically, SQS analysis found North American Paleocene diversity to be higher than in the late Cretaceous, challenging the idea of an end Cretaceous decimation. Further, metatherian diversity in the late Miocene of Oceania was found to increase rather than decrease from the middle Miocene. This dual angle approach supports a combination of developmental and environmental effects on the evolution of metatherian biodiversity.

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