Zoology - Theses
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ItemEvolution of the Proboscidea and the Sirenia( 2000)Sixteen African elephant (Loxodonta africana), two dugong (Dugong dugon) and four manatee (Trichecus manateus) fetuses were collected for this study. The fetuses were measured, dissected and serially sectioned. Aspects of development were described and discussed in the context of the evolution of the elephant, dugong and manatee and the possible aquatic ancestry of the elephant. The consequences of an aquatic ancestry of the elephant are reflected in numerous ways during its development: the specialisation of the lungs and respiratory system and the trunk, the presence of nephrostomes in the fetal kidney and the permanent intra-abdominal location of the testes. The trunk is obviously an ancient feature of the elephant and this is reflected in its development and is present in even the earliest stages. The lungs of the elephant develop in a pleural cavity and then adhesions develop, obliterating the cavity, at birth. The adhesions are thought to provide support for the lungs as the elephant sucks water up its trunk and to support the lungs whilst using the trunk as a snorkel when swimming. The adhesions could be a more recent adaptation as the elephant evolved from an aquatic to a terrestrial mammal. Nephrostomes were observed in the mesonephric kidney of the elephant. Nephrostomes are a feature of primitive vertebrates and the presence of nephrostomes, are a clear example of ancient structures persisting in development. The manatee does not have nephrostomes in the mesonephros late in gestation although it is possible that nephrostomes were present in early gestation. The elephant, dugong and manatee are primary testiconda, a feature also consistent with an aquatic origin. The position of the testes in the elephant, dugong and manatee clearly represents a pleisomorphic condition. Elephants do not have a sexually dimorphic ano-genital distance unlike their closest relatives the dugong and manatee. This could be due to androgens produced by the fetal adrenal that are causing the masculinisation of the female genitalia and perhaps also masculinise the brain, and ultimately behaviour. In summary, the study of the development of the fetuses of these three closely related groups has furthered our knowledge about the development and evolution of testicond species. The finding of nephrostomes in the mesonephros of the elephant has reinforced the idea that ontogeny can provide clues about phylogeny. The development of the elephant has strengthened the evidence for an aquatic ancestry of the elephant. Additionally, the similar ana-genital distances in male and female fetal elephants, has provided a step to a fascinating area of future research.