The Morphological Evolution and Diversity of Fern Spores

Walker, Carrie (2022). The Morphological Evolution and Diversity of Fern Spores. PhD thesis The Open University.



Studies investigating biodiversity through time using fossils involve quantifying patterns of taxonomic diversity and morphological diversity (disparity). For animals, the comparative history of taxonomic and morphological diversity through the Phanerozoic is well established, and these two diversity measures are often decoupled. However, there are comparatively few studies on the patterns of plant morphological disparity.

To address this, the disparity of extant and extinct fern and lycopod (pteridophyte) spores has been investigated. Spores have been used as they are found in high abundance temporally and globally and are well-preserved in the fossil record. One hundred and fifty extant spores from 42 families were collected from herbarium sheets at the Royal Botanic Gardens, Kew and scored for discrete characters. An illustrated atlas detailing this spore morphology was constructed. The extant spores were used as a present-day baseline for Triassic and earliest Jurassic fossil spore data, and to determine any functional or ecological purpose to spore morphology.

Despite enormous variation in surface ornamentation, no direct ecological or functional purpose for morphology in extant pteridophyte spores was found. Results indicate that the morphospace occupied by spores has decreased from the Early Triassic, and that living pteridophyte spores represent a limited subset of the morphologies encountered in the Triassic–Jurassic fossil record. Pairwise Hamming distances were measured between 67 fossil spores. This showed that diversity and disparity are decoupled during two episodes in the Triassic, coinciding with the Early Triassic recovery from the Permian-Triassic mass extinction event and the Carnian Pluvial Episode. Additionally, similar levels of morphological disparity are reported for Early Triassic and extant spores. This finding supports a conceptual model of morphological evolution in which a clade’s recent morphological disparity is comparable to its early disparity and reflects similar patterns that have been found across animal macroevolution.

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