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The early evolutionary history of sharks and shark-like fishes

Andreev, Plamen Stanislavov (2014)
Ph.D. thesis, University of Birmingham.

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Abstract

The Middle Ordovician to Late Silurian represents an interval of approximately 50 million years, which has been recognised as the initial, cryptic, period in the evolutionary history of chondrichthyan fish. The fossil remains attributed to early chondrichthyans are dominated by isolated dermal scales that predate the appearance of undisputed chondrichthyan teeth and articulated skeletons in the Lower Devonian. Investigation of the inter-relationships of these scale taxa and their systematic position relative to high- ranked chondrichthyan clades has been hampered by the lack of developed scale-based classification schemes for jawed gnathostomes, coupled with the limited use of scale characters in phylogenetic studies of Palaeozoic Chondrichthyes. Here, all previously documented scale types of alleged Lower Palaeozoic chondrichthyans were examined using a combination of X-ray microtomography, SEM and Nomarski DIC optics. These were found to exhibit a set of characteristics (symmetrical trunk scales, areal crown growth and lack of hard-tissue resorption, cancellous bone and enamel) recognised as specific to the dermal skeleton of chondrichthyans among derived gnathostomes. The collected data permitted the establishment of a hierarchy of scale characters for separate taxonomic ranks, leading to the recognition of three Orders (Mongolepidida, Elegestolepida ordo nov. and Altholepida ordo nov.) of early chondrichthyans, differentiated by distinct types of scale-crown morphogenesis.

A scale-based cladistic analysis of jawed gnathostomes corroborated these results by recovering a chondrichthyan clade that incorporates all examined taxa and ‘acanthodians’ with non-superpositional crown growth patterns. It is thus proposed that chondrichthyan dermoskeletal characters carry a phylogenetic signal, allowing to interpret the documented diverse types of scale morphogenesis as evidence for a major radiation of chondrichthyan lineages in the Lower Palaeozoic.

Type of Work:Ph.D. thesis.
Supervisor(s):Sansom, Ivan and Cooper, Paul and Shelton, Richard M.
School/Faculty:Colleges (2008 onwards) > College of Life & Environmental Sciences
Department:School of Geography, Earth and Environmental Science
Subjects:GC Oceanography
QH Natural history
Institution:University of Birmingham
ID Code:5491
This unpublished thesis/dissertation is copyright of the author and/or third parties. The intellectual property rights of the author or third parties in respect of this work are as defined by The Copyright Designs and Patents Act 1988 or as modified by any successor legislation. Any use made of information contained in this thesis/dissertation must be in accordance with that legislation and must be properly acknowledged. Further distribution or reproduction in any format is prohibited without the permission of the copyright holder.
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