Acanthostega

Acanthostega was an early tetrapod, an extinct genus of stem-tetrapod, among the first vertebrate animals to possess recognizable limbs. It lived in what is now Greenland during the Late Devonian period, approximately 365 million years ago.

Acanthostega is significant in evolutionary biology because it showcases a transitional form between lobe-finned fishes and fully terrestrial tetrapods. While it possessed limbs and a tetrapod-like body plan, its limbs were not well-suited for walking on land. They were more likely used for navigating shallow, swampy environments.

Key features of Acanthostega include:

• Eight digits on each hand and foot: Unlike modern tetrapods with five digits (pentadactyly), Acanthostega had a higher number, illustrating early experimentation with limb structure.
• Internal gills: Indicating that it still relied on aquatic respiration, even with the development of limbs.
• Fish-like tail: Possessing a caudal fin for swimming.
• Weak vertebrae: Suggesting limited weight-bearing capability on land.
• A relatively flat skull: Indicative of an aquatic lifestyle.

The discovery of Acanthostega, along with other contemporaneous tetrapods like Ichthyostega, revolutionized our understanding of the transition from water to land. It demonstrated that the development of limbs likely occurred before the need for fully terrestrial locomotion, suggesting that these early tetrapods may have used their limbs for maneuvering in shallow water or for hauling themselves onto riverbanks rather than for sustained walking. Acanthostega remains a crucial fossil for understanding the evolutionary history of tetrapods and the emergence of terrestrial vertebrates.