Reconciling ichthyology and palaeontology with exceptionally preserved fossils

Half of all animals with backbones are fishes. Remarkably, most evolutionary studies of living fishes ignore palaeontology, despite a rich fish fossil record. Similarly, even though fishes represent a key group for studying functional anatomy, our understanding of functional diversity in extinct species remains unconstrained. The disconnect between ichthyology and palaeontology reflects very real concerns about the quality of data that can be drawn from imperfect fossil specimens; most fossil fishes are poorly preserved or flattened to the degree that important anatomical information cannot be recovered. However, many basic questions relating to sequences of evolutionary change and the absolute timing of major events cannot be tested without palaeontological data, leaving our picture of the evolution of a major fraction of vertebrate diversity incomplete. How did striking anatomical innovations evolve? When did modern morphologies and functional strategies appear? What can morphological diversity tell us about the functional repertoires of ancient species?

Thankfully, not all fossil fishes are poorly preserved, and palaeontology can help us to answer these questions. Some exceptional localities yield three-dimensionally preserved fossil remains that bear detail comparable to that found in modern fish skeletons. There are two such deposits in the UK: the Late Cretaceous (ca. 100-70 million years old) and early Eocene (ca. 56-49 million years old) London Clay. Three-dimensional fish skulls from these horizons are well represented in museum collections, but have been little studied. As with other palaeontological specimens, a major obstacle to studying the anatomy of these fish fossils lies with the rock that conceals both external and internal details. Computed tomography (better known as CT scanning) has sparked a revolution in palaeobiology, allowing researchers to virtually dissect ancient fossil specimens that remain encased in rock. This technique provides especially good results when applied to the three-dimensional fishes of the English Chalk and London Clay, providing the data necessary to address our questions related to evolutionary relationships and biomechanical function.

Duplexdens high res jpg

53-million-year-old skull of the tuna relative Duplexdens from the London Clay. Top image shows the fossil specimen, with many internal bones obscured by rock. Lower image is a virtual model of the skull generated using computed tomography (CT), and from which the surrounding rock has been removed digitally. This unusual three-dimensional preservation enables detailed anatomical and biomechanical study generally not possible for fossil fishes.

Beyond offering exceptional preservation, these two British deposits capture snapshots of key phases in a critical episode in vertebrate history: the evolutionary radiation of acanthomorph (spiny-finned) fishes. Today, this anatomically diverse group numbers nearly one in three living vertebrate species, and includes important targets of biological research (e.g., cichlids) and economically significant members (e.g., tuna). Our team of palaeontologists, morphometricians, and ichthyologists combines the expertise necessary to harness these materials in an integrative framework, allowing us to illuminate the evolutionary history of an enormously successful and exceptionally diverse vertebrate radiation while simultaneously addressing major issues concerning the relationship between morphological and functional diversity in the fossil record more broadly.

Dr Matt Friedman
University of Oxford

Matt was awarded a Research Project Grant in June 2012.