If a non-palaeontologist has heard of a Devonian tetrapod, it will probably be
, the 'four-legged fish'. Our 1987
some excellent material from Stensiö Bjerg of this animal as well as Acanthostega
which revealed its curious complement of seven toes on the hind limb.
However, many questions remain unanswered
about its anatomy, body form, and environment. One of the main objectives of the
was to collect material that would try to answer some of these
questions. For example, three papers have placed Ichthyostega
in their stratigraphical
and sedimentological context (Blom et al. 2005,
Larsen et al. 2009). Others have concentrated on the anatomy of Ichthyostega
Although several reconstructions had been made of whole animal, of both its skeleton and
the 'living' animal, in fact, there were many problems with them. For one thing, most of
the articulated fossils showed either just a head and thorax or a tail, pelvis and hind
limb. None really showed how those two halves meshed together. The 1998 material included
some that did. In 2000, then, we received the first of our grants from the Natural Environment
Research Council to work up this material alongside the previously collected specimens.
Major discoveries resulted from these.
That study concentrated on the skeletal anatomy of Ichthyostega
. Using mainly the original
material, we found that the axial skeleton showed many differences from previous reconstructions:
the tail was shorter, the shoulder girdle was larger and there were fewer vertebrae in front of
the sacrum. Most surprisingly, the pre-sacral column showed distinct regionization along its length,
including neural spines whose orientation varied, and in the immediate presacral region, they
alternated in size and shape between broad and narrow as seen in side view. We made an new
reconstruction, published in 2005 (Ahlberg et al. 2005
However, this was only a two-dimensional
reconstruction: it remained to be tested in three-dimensions.
In 2009, a new study also funded by the NERC, aimed to resolve some of the puzzles set by the
early tetrapods, especially Ichthyostega
. Using the latest computer techniques, in collaboration
with the Royal Veterinary College
, London, (see
Stephanie Pearce and John Hutchinson
Collaborators section), we have built a realistic virtual 3-D model of the skeleton of Ichthyostega
to see how the bits really did fit together. Thus we corroborated most of the findings of the 2-D
model from 2005, with some subtle differences.
We were then able to study the potential mobility of the axial skeleton and limbs of both Acanthostega
, and compared these with analagous modern tetrapods. Our work on the range of motion
of the limbs of Ichthyostega
was published in 2012. We showed that Ichthyostega
did not 'walk' in a
conventional sense, but probably used its forelimbs in a mud-skipper-like manner, with the hind limbs
contributing stability but not power to the movement. The hind limbs were more likely to have been
used in swimming.
We have also discovered that its vertebrae are not formed in the way expected of an early tetrapod,
but the centra were made of open-topped rings composed of a ventral intercentrum, combined with
anterordorsally sutured or fused pleurocentra. This is unusual, and is the reverse positioning of
the inter-and pleuro-centra from what was considered normal for tetrapods. We also found that the
ribs were not bicipital, contrary to expectations, but had elongate articulations with the transverse
processes. Such a structure rules out their use in breathing motions. See Professor John Hutchinson's RVC
Jenny and Stephanie were interviewed by Richard Hollingham for the NERC website
Planet Earth Online and the result broadcast recently in their podcast at
We have also looked at Pederpes
in our study, and our colleague Julia Molnar
has been working on this.
Clack et al. (2003) showed that its braincase and ear region were unique,
and were probably adapted for underwater audition. Those parts of its anatomy had
previously been difficult to interpret, and earlier studies by Jarvik had effectively
opted out of trying to do so.
We used microCT scanning of our new material as well as fresh
preparation of original material to come to our conclusions. We also discovered that,
contrary to previous observations, Ichthyostega possessed ossified and grooved gill-bars,