acanthostega skeleton

Acanthostega gunneri

The skull roof of Acanthostega gunneri was first recovered from Famennian deposits (360 million years ago) in eastern Greenland in 1933, and was described and named in 1952 by Erik Jarvik. Additional fossils were recovered during a 1970 geological expedition, but they languished in obscurity until rediscovered by Jennifer Clack. In a 1987 expedition led by Clack and Per Ahlberg recovered some exceptionally well preserved material from several individuals.

The abundance and quality of Acanthostega remains has made it the best known of the early tetrapods. From their investigations of these remains Clack and Michael Coates have reported a series of remarkable findings that have necessitated changes in our thinking on early tetrapod evolution.

Prior to these findings, most scientists assumed that the evolution of legs and feet was initiated and driven by the colonization of land. Here, however, was an early tetrapod that was ill-suited for life on land. It had well-defined digits (fingers and toes), but no wrists or ankles. It had relatively long limb bones, but they couldn't support much weight. Its hip also couldn't support much weight since it was weakly attached to the spine.

A firm attachment to the spine wouldn't help much anyway, since its spine was structurally based on the (ancestral) notochord rather than on a series of interlocking, yet flexible, vertebrae. The spine was well-suited for handling the mechanical stresses of swimming but was nearly useless for supporting weight. Moreover, its short and thin ribs were incapable of protecting vital organs. Acanthostega also had a deep tail which sported a large bony fin. In short, it had a tail suited for swimming, a fish's spine and paddle-like limbs.

A primarily, if not exclusively, aquatic lifestyle for Acanthostega is further indicated by the presence of internal, fish-like gills. (Evidence for internal gills include bony gill arches and post-branchial lamina on the leading edge of the shoulder girdle. In contrast, all Carboniferous-to-modern gill-breathing amphibians have external gills.) Other features that indicate an aquatic lifestyle include a fish-like stapes (a bone which will evolve into the middle ear of terrestrial tetrapods) and the retention of the sensory lateral line system found in fishes. Acanthostega's small, fish-like nares (nostrils) were probably used only for smelling under water; air may have been brought to the lungs by gulping.

Although Acanthostega had many fish-like characteristics it did have legs and feet rather than fins. These feet, however, also affected our thinking on the evolution of tetrapod limbs. It was practically an article of faith that the first tetrapods had five digits, but Acanthostega had eight digits on the front leg and at least eight digits on the hind. (Subsequent analyses have indicated that at least two other early tetrapods, Ichthyostega and Tulerpeton, also had more than five digits.)

With its combination of fish-like and tetrapod features Acanthostega has engendered a variety of speculation about the paleoecology and evolution of early tetrapods. Its feet may have been superior to fins in negotiating shallow waters filled with aquatic plants and woody debris. (These shallow water habitats could have been wetlands, stream margins or inundated floodplains.) Acanthostega also had a protective covering of elongated ovoid scutes on its belly, but no scales on the rest of its body. These scutes may have protected it from abrasion as it moved about.

Acanthostega has been recovered from river deposits, and the presence of well-articulated and minimally reworked remains indicate that it probably lived there. Fossils of lobe-fins (Holoptychius, Eusthenodon and lungfishes) and placoderms have been collected in localities yielding Acanthostega.

Acanthostega was about 60 cm (2 ft) in length.

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Web:
Jenny Clack's web page on Acanthostega:
www.palantir.fsnet.co.uk/acanthostega.htm
Discover Magazine Online: Coming Onto the Land.
enews.com/magazines/discover/magtxt/060195-1.html
G.R. Morton's Transitional Forms: Fish to Amphibian:
home.entouch.net/dmd/transit.htm
Richard Hammond's reconstruction of and information on Acanthostega.
www.personal.u-net.com/~paleomod/p97/aca-intr.htm
Tree of Life's (tolweb.org) web page on Acanthostega gunneri.
Books:
Clack, J.A. 2002. Gaining Ground: The Origin and Early Evolution of Tetrapods. Bloomington: Indiana Univ. Press.
Janvier, P. 1996. Early Vertebrates. Oxford: Claredon Press.
Long, J.A. 1995. The Rise of Fishes: 500 Million Years of Evolution. Baltimore and London: John Hopkins Univ. Press.
Maisey, J.G. 1996. Discovering Fossil Fishes. New York: Henry Holt & Co.
Zimmer, C.. 1998. At the Waters' Edge. New York: Touchstone Press.
Magazines:
Discover Magazine. June 1995: "Coming Onto the Land."
National Geographic. May 1999. "From Fins to Feet."
New Scientist. 19 August 2000. "One Small Step for Fish, One Giant Leap for Us."
Scientific American. December 2005. "Getting a Leg Up on Land."
Scientific Papers
Clack, J.A. 1989. "Discovery of the earliest-known tetrapod stapes". Nature (London),342:424-427.
Clack, J.A. 1994. "Acanthostega gunnari, a Devonian tetrapod from Greenland; the snout, palate and ventral parts of the braincase, with a discussion of their significance." Meddelelser om Grønland: Geoscience, 31: 1-24.
Clack, J.A. 1994. "Earliest known tetrapod braincase and the evolution of the stapes and fenestra ovalis". Nature (London),369: 392-394.
Clack, J.A. 1997. "Devonian tetrapod trackways and trackmakers: a review of the fossils and footprints". Paleogeography, Paleoclimatology, Paleoecology 130: 227-250.
Coates, M.I. 1996. "The Devonian tetrapod Acanthostega gunnari Jarvik: postcranial anatomy, basal tetrapod interrelationships and patterns of skeletal evolution". Trans. Royal Soc. Edinburgh; Earth Sciences, 87:363-421.
Coates, M.I. and J.A. Clack, 1990. "Polydactyly in the earliest known tetrapod limbs". Nature (London), 347: 66-67.
Coates, M.I. and J.A. Clack. 1991. "Fish-like gills and breathing in the earliest known tetrapod". Nature (London) 352: 234-236.
Coates, M.I., 1994. "The origin of vertebrate limbs". Development 1994. Supplement, 169-180, p. 174.
Daeschler, E.B. and N. Shubin. 1995. "Tetrapod Origins". Paleobiology 21(4): 404-409.

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