Carboniferous Chondrichthyans, Rhizodonts and Lungfishes

The Carboniferous lungfish genus Ctenodus

Esther Sharp's thesis on British Carboniferous lungfish included a review of the genus Ctenodus. This has now been worked up into a monograph for publication. Shortly after she had finished her thesis, a new specimen of Ctenodus interruptus was discovered and will be included in the final monograph. It is a remarkable specimen, being an almost complete articulated individual displaying not only a beautifully preserved skull roof with all six tooth plates visible, but also much of the postcranial skeleton. The tail region in particular is of interest for the primitive features it retains.

Tournaisian rhizodonts and lungfish

New specimens of rhizodonts have been collected from the locality of Burnmouth, including a cleithrum and a ptergyoid of what is almost certainly a new genus. These, and isolated lungfish bones await description and we anticipate new finds as collections made in the Tournasian of the Borders Region continue.

Early Carboniferous chondrichthyan faunas from the Peak District of Derbyshire: skeletal anatomy, systematics, phylogeny and environments.

The phylogenetic relationships of early chondrichthyans is a topic currently engendering wide debate and controversy, but has at its heart the major question of the of relationships between all of the groups of vertebrates with jaws (gnathostomes). Without a clear understanding of basal gnathostome relationships, the problem of the origin of jaws themselves will remain one of evolutionary biology's most tantalizing questions.

Part of the reason why early chondrichthyan relationships are so hard to work out is the lack of good skeletal remains that give anatomical contexts for the many different 'genera' of teeth that have been named. For example the genus 'Cladodus' has long been a polyphyletic assemblage of similarly shaped teeth deriving from many different taxa. Only with some recently described skeletal material is its status becoming clarified (Ginter, Ivanov and Lebedev 2005, Ginter and Maisey 2007).

The Carboniferous Limestone of Derbyshire is well known for isolated chondrichthyan teeth and scales (Ford, 1964), however anatomically preserved material has so far been very rare. Recently we have discovered articulated skeletal material including lower jaw, shoulder girdle, cranial and branchial arch elements associated with teeth and dermal denticles representing at least three genera. The material comes from two localities near Eyam: Bleaklow Quarry and a spoil heap consisting of material from the 18th century Shaw Engine lead mine.

Stratigraphically these derive from the Eyam Limestone, either the Brigantian Visean P2 zone or the Namurian Pendleian E1a zone (Korn and Tilsley 2002, Tilsley and Owens 2003) which makes them contemporaneous with the two or three other sites worldwide that have yielded good anatomical remains of chondrichthyans. These include Bearsden, near Glasgow (Wood 1982, Coates and Sequeira 1998, Coates and Sequeira 2002), and Bear Gulch , Montana , USA. The Early Carboniferous age makes these faunas particularly important for understanding chondrichthyan phylogeny and ecology.

A range of prismatic cartilage structures have been found including specimens probably attributable to Akmonistion and Denaea.

Specimens from Bleaklow Quarry (= Backdale Mine) are preserved in both a carbonaceous marine limestone with brachiopods, nautiloids, bivalves, trilobites and crinoids (Korn and Tilsley 2002, Tilsley and Owens 2003), and from nodular sandy carbonaceous limestones containing few other fossils. Material from the Shaw Engine mine found so far consists of a complex assemblage of prismatic cartilage skeletal elements associated with chondrichthyan and actinopterygian teeth and scales, coprolites, fusain and other organic remains. It clearly represents a different, probably nearer-shore, environment from that at Bleaklow quarry.

The specimens collected so far require preparation by mechanical and chemical means, and one series of specimens from the Shaw Engine Mine that appears to represent a braincase have been micro-CT scanned.

Potentially, the new specimens offer a broader view of early chondrichthyan palaeoecology and distribution as well as potentially new anatomical insights. Comparison of the environments between the two Derbyshire sites and contemporary sites elsewhere should throw valuable light on early chondrichthyan ecology and diversity.

Graduate student Kelly Richards has been working on this material and is currently writing up parts of the project.