Your Daily Fossil

Acanthostega

Mounted specimen on display at the American Museum of Natural History, NYC 

When: Late Devonian (~365 million years ago)

Where:  The ancient swamps of Greenland, near the Devonian equator. 

What: Acanthostega is a basal tetrapod - one of the first vertebrates to develop limbs instead of fins, though it was not fully able to maneuver on land. Its pectoral girdle was constructed much more like that of a fish than a later tetrapod and could not bear the weight of the animal on land. However, it is the first taxon known that did have a pelvic girdle capable of supporting weight and propelling it forwards, this was accomplished by the fusion of either sides of the pelvis to one another ventrally, and a firm contact established between either pelvis and at least two vertebrae - precursors of the fused vertebra that would become the sacrum in later tetrapods. What is the use if only one half of the body is able to be supported by limbs? This type of pelvic girdle most likely did not at first develop for support. This structure also marks a shift from a primarily forelimb driven locomotion mode to one propelled by activity in the hind-limbs. Acanthostega could move in extremely shallow waters by ‘walking’ on the sediment with its hind-limbs, with its forelimbs steering instead of providing the primary movement source.

This interpretation is supported by its jaw structure, which lacks features relating to suction feeding. Rather it is hypothesized this primitive tetrapod would feed by directly biting onto insects and other terrestrial invertebrates that it could reach from the water’s edge. Thus, the predatory mode that characterizes the first terrestrial vertebrates was first developed by an aquatic animal. Later tetrapods were able to emerge from the waters, at least for short periods of time, to hunt prey that were beyond the reach of their ancestors. Another interesting note is that Acanthostega had 8 digits on its forelimb - it took a while in the evolution of tetrapods for five digits on the hands and feet to become established.