Tournaisian (earliest Carboniferous) lungfishes were much more diverse than we thought. A paper from the TW:eed Project has described seven new taxa, with a range of tooth plate types not previously encountered in such early lungfishes(Smithson et al. 2015). It suggests that lungfishes underwent rapid evolution and developed dental novelties in the post-extinction world, exploiting newly available niches. One particular bed described in Clack et al. 2019 yielded elements of the largest Carboniferous lungfish ever found, and possibly of the largest lungfish ever. Curiously, no large tooth plates have been found, although several very small ones have been. This bed also includes elements of very large rhizodonts, and we suspect that at least two taxa are present. A peculiar rhizodont humerus (upper arm bone) has been found in the bed described in Otoo et al. (2018). For more details see below under ‘The rhizodont humerus project’.
Our team has described a new, relatively large, lungfish from low down in the Ballagan at Burnmouth ( Challands et al. 2019), and we're preparing a paper on a new Devonian lungfish from Greenland, alongside a review of other lungfishes from similar-aged localities in Greenland (Clack et al. 2018b). This paper shows that there was more diversity among Late Devonian lungfishes than previously appreciated, and our new taxon, named after Per Ahlberg, fits more closely anatomically among Carboniferous forms than Devonian ones.
Until our project, only two securely dated Tournaisian chondrichthyans had been described from the UK, and indeed those recorded from the rest of the world were questionably dated. Our project has found literally hundreds of chondrichthyan teeth in the Ballagan Formation, and many are now described and include two new taxa (Richards et al. 2018). Most of the teeth are complex plates that indicate durophagous (crushing) dentitions, and are more closely related to modern holocephalans (chimaeras) than to elasmobranchs (sharks).
Please see the TW:eed Project website for more details. Layman's summaries of some papers can also be found there, under Outreach->Publication Summaries.
Esther Sharp's (see Students) 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.
The lungfish Sagenodus is the most commonly found genus among Carboniferous lungfishes in the UK, and probably worldwide. Two species are being described by the UMZC team. One species is found in the Early Carboniferous, S. quinquecostatus, and one from the Late Carboniferous, S. inaequalis. The latter is more frequently found. Although its fossils are well-known, it has not been described in detail since 1923, when it was studied by Watson and Gill. The Early Carboniferous species has only recently been recognized and is being described by our team.
The work was initially carried out by PhD student Esther Sharp (now Beeby), who studied under Jenny, and Esther, Jenny and Tim Smithson are the co-authors of the study. A manuscript will shortly be submitted to EESTRSE. During this work, Esther realised that the holotype of Sagenodus was not actually diagnostic of the genus, so the team has made an application to the International Committee on Zoological Nomenclature (ICZN) to erect a neotype that consists of a diagnostic skull. This constitutes Case 3800 submitted to the ICZN.
During the study, we noticed that there was a unique arrangement of the tooth plates in Sagenodus. It meant that in order for the teeth on the tooth plates to occlude fully, they had to move on their long axes so that at the beginning of the move, the most anterior ridges of the upper and lower tooth plates could occlude, but the more posterior tooth ridges could not. To make them meet, the tooth plates had to be rotated along their axes in the other direction. This observation was paired with the fact that neither the quadrates nor articulars (the bones forming the jaw-joint) were ossified and must have remained as cartilage, and that the symphysis (where the two halves of the jaw meet) was also unossified.
One of the factors that the TW:eed team noticed is that at Burnmouth, where rhizodont fishes were found, there were usually two types found. Among the material was a rhizodont humerus found in one particular bed about 340 m above the Devonian/Carboniferous boundary (for a laymans' summary see the TW:eed project web site (www.tetrapods.org under 'Outreach: Publication summaries: A fish and tetrapod fauna from Romer's Gap)'.
This humerus was found initially by micro-CT scanning a composite block from the bed that contained a set of articulated fin spines, shoulder girdle elements and a row of large teeth that were clearly rhizodont. However the humerus itself had some unique features that made it different from others that had been found. This stimulated a research paper on all the rhizodont humeri from the UK (about 6 or 7 in four genera) for comparative anatomy and functional implications. One of the first snags we hit was that they had been named on the basis of size or where they had been found but with no reference to holotype specimens. In three cases (Rhizodus, Strepsodus, Archichthys), the holotype specimen was an isolated tooth, and the fourth (Screbinodus) was based on scale morphology. The latter are highly variable across the fish's bodies, and the former, being isolated teeth, could not inform us about the rest of the dentition and whether all the teeth in the jaw had the same characters.
We therefore looked at the humeral shape and grouped them into similar morphological types, but we have to leave the resolution of the identity problem to further study, probably using micro-CT scans of higher resolution than we currently have available.
We will have more information on the histology of these bones, and colleagues will provide a functional analysis of the humerus to study the use of the humerus in locomotion.
The study is being under taken by Jenny with colleagues Zerina Johanson from the Natural History Museum, Jonathan Jeffery from Bristol University, and Stephanie Pierce from Harvard, USA.
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. In 2006, we 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.
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 come mainly from two sites: Bleaklow Quarry (= Backdale Mine) and a mine dump from an 18th century
lead mine, the Shaw Engine mine. Fossils from the former site 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. Other sites
nearby in Derbyshire have also yielded more chondrichthan teeth.
These specimens formed the material for the doctoral thesis of Kelly R. Richards, whose thesis was accepted in 2013.
Among the collections we made in 1987 were examples of three lungfishes. Two, Soederberghia and Jarvikia were already known, but from limited material. Our new finds of Soederberghia in particular yielded new information on skulls, postcrania and even braincases. These were worked on by Matt Friedman who completed a Masters thesis on them and subsequently published several papers from it. (He’s now a Professor at the University of Michigan.)
The third lungfish was only recognized for what it was as I was packing up the Greenland material for return to Copenhagen in 2017. It was a very small specimen on a block only about 6 cm across featuring some scattered white bones. We didn’t know what it was at the time. On close examination, it turned out to a small lungfish skull, which was quite different from any then known Devonian lungfish, and I published it under the name Celsiodon ahlbergi in the journal Papers in Palaeontology. Sadly, it's not Open Access, but anyone can read the Abstract. "Celsiodon" because it came from Celsius Bjerg, and "ahlbergi" for my then student Per Ahlberg, who picked up the specimen in 1987.
We examined its anatomy and relationships, and found that in several respects, it was more similar to Early Carboniferous taxa than to Late Devonian ones. A small number of other Devonian taxa clustered with it in our analysis, crossing the morphological and phylogenetic divide between these forms that had been recognised up till then.
Alongside this, to make certain that this animal did not fall into any of the previously known taxa, I examined specimens described in the past, stored in the Natural History Museum of Denmark. Indeed our specimen was new, but also several of the other specimens there represented a much greater diversity than the earlier descriptions had suggested. Clearly, much is still missing from the story of lungfish evolution during the end-Devonian into the Early Carboniferous.