The discovery of excellent specimens of this Devonian tetrapod revitalised study of the
so-called 'fish-tetrapod' transition, and changed many perceptions about this major
evolutionary event. For example, rather than envisaging a 'fish' crawling out of the water,
to evolve feet to walk on land, we now think that animals with feet - 'tetrapods' - evolved
their feet for uses in water, and only later became land-going. Rather than gaining all
their distinctive features early on in their history, tetrapods seem to have evolved fully
terrestrial adaptations only gradually over the 30 million years between the end of the
Devonian and the middle of the succeeding Carboniferous period.
Features such as a terrestrially adapted ear, an occiput with a mobile joint (a condyle), a
sacral joint for attachment of the pelvic girdle, loss of gill-breathing and acquisition of
the structures associated with air-breathing using ribs for aspiration all seemed to have
been acquired bit by bit after the end of the Devonian.
appears to be in many ways the most primitive tetrapod described so far,
in that it retained internal gills for breathing, a tail-fin with fin rays above and below
the vertebral column, a braincase into which the notochord passed as it did in the tetrapods'
fish relatives, and a radius that was longer and more substantial than the ulna. Although it
had limbs with digits, its ankle, knee, wrist and elbow joints were not developed into
weight-bearing joints with the characteristic degrees of movement found in terrestrial
tetrapods. Despite appearances, however, it could still be the case that Ichthyostega
is more primitive: it lacks features shared by later tetrapods that Acanthostega
and its influence on our understanding of the fish-tetrapod transition
has been covered in a variety of books and papers. As well as the formal publications
listed in my publications list and on the web page for Acanthostega
in the University
of Arizona Tree of Life
there have been many more popular publications in books and journals, and the origin of
tetrapods has been covered in several television programmes.
One of the first major discoveries from the Acanthostega
project was that of the braincase
and stapes (ear bone). This was the earliest tetrapod stapes ever discovered, but it showed resemblances
to those to two other, unrelated, tetrapods, my PhD thesis animal, the embolomere Pholiderpeton
(see under Other Early Tetrapods
) and the colosteid
. They showed no evidence of
adaptations for high-frequency terrestrial hearing in air, rather, they gave us clues to the primitive
condition for tetrapods. They were quite unlike some hypothetical suggestions in the literature for what
an early tetrapod stapes might be like. These ears were probably adapted for low-frequency sound reception,
probably in water, but perhaps also in air.
The braincase of Acanthostega
could be compared quite closely with that of the Devonian fish
, so it too could be showing us the primitive condition for tetrapod braincases.
The contrast with that of Ichthyostega
(see the Ichthyostega
page) was total, the description of that of Ichthyostega
coming rather later in our studies, but
giving us pause for thought.
The material of Acanthostega
from the Stensiö Bjerg site is preserved in a hard
micaceous silty sandstone, of heterogeneous mineral content and with a wide range of particle
size. It is partly soluble in acid, though the minerals filling the internal cavities of the
bone are preferentially dissolved so that the bone cannot be extracted from the matrix by
acid solution without it being destroyed itself. Therefore almost all of the specimens from
this site have been prepared by mechanical means, with pneumatic pen, dental mallet and mounted
needle (see Sarah Finney in the Collaborators section
A few were sliced or their blocks
reduced using a diamond-wire saw. The wire used in this case has a diameter of only 0.3mm so
that, for example, a skull can be sliced into 1.5mm thick sections without much loss of material
In a few other cases, the bone has been dissolved to reveal the surface of
the bones, where they have been split by weathering and damaged by erosion. This reveals a
natural mould of the fresh surface that is not easily available by other means.
specimen is worked on continuously for long - each is done bit by bit in rotation, in response
to scientific needs. However, for the skull known as 'Grace', about two years was needed to
prepare the whole thing. This specimen was initially only visible as a small part of the lower
jaw, and cross-sections across a broken block, indicating that the whole skull was present in
The specimen known as 'Boris' was prepared during a period of about three years. To
begin with, only the eroded jaw articulation and snout were visible of the skull, and two
processes of the humerus were exposed of the postcranial skeleton, though a series of bumps
in the matrix suggested the presence of the vertebral column. The skulls that were found by
Nicholson were completed by the discovery of the block containing the bodies of two of these
animals by the 1987 expedition. The rate of erosion in East Greenland is surprisingly low, and
there have been several instances of finding parts of specimens collected by earlier expeditions.