Acanthostega

A very primitive tetrapod from the Late Devonian with eight fingers and toes.

A 'transitional form' between 'fishes' and 'tetrapods'.

Fish-like features:

  • Gill-bars like a fish
  • No true elbows, knees, wrists or ankles (ie not weight-bearing)
  • Long fin rays around the tail

Tetrapod-like features:

  • Head not joined to shoulder girdle
  • Ear-bone (stapes) fits into fenestra (ovalis/ vestibuli) in braincase
  • Large pelvic girdle, sacral rib
  • Femur as large as humerus
  • Tibia and fibula attached to 'ankle' bones
  • Digits on each limb

Acanthostega and the 'fish-tetrapod' transition

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. Acanthostega 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 does possess.

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 site, site, there have been many more popular publications in books and journals, and the origin of tetrapods has been covered in several television programmes.

Preparation of the Acanthostega material

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 or information. 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.

No single 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 rock.

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.

Further reading:

Gould, S. J. 1993. Eight little piggies. London, Jonathan Cape. (Essays: 'Eight little piggies' and 'An earful of jaw'.

Zimmer, C. 1998. At the water's edge. New York, Free Press (Simon and Schuster Inc). 290 pp.

Zimmer, C. 1995. Coming onto the land. Discover 16: 118-127.

McCleod, M. 2000. One small step for fish, one giant leap for us. New Scientist 167 (19 August): 28-32.

Television programmes:

BBC Horizon, 1st Feb, 2001

BBC4 Beautiful Minds, 11th April, 2012

Last updated 11th June, 2012 by Rob Clack