stemtetrapods

Stem tetrapods represent a paraphyletic group of extinct sarcopterygian (lobe-finned) fishes that lived from the Devonian period onwards. They are considered ancestral to tetrapods, the group that includes all four-limbed vertebrates, but they themselves did not possess true limbs in the tetrapod sense. Instead, they had pectoral and pelvic fins that were more robust and muscular than those of their fish ancestors, featuring internal skeletal structures that foreshadowed the limb bones of later tetrapods.

These fins were likely used for maneuvering in shallow water, pushing off the substrate, and possibly for rudimentary locomotion in terrestrial or semi-terrestrial environments, though their exact capabilities are a subject of ongoing research. Stem tetrapods typically possessed lungs in addition to gills, allowing them to breathe air and survive in oxygen-poor aquatic environments. Their skeletal anatomy, particularly in the skull and vertebral column, shows a mosaic of fish-like and tetrapod-like features.

Key fossils that represent stem tetrapods include species like Tiktaalik, which exhibits a unique combination of fish traits (scales, fins) and tetrapod traits (flat head, mobile neck, robust fin skeleton). Other important taxa like Acanthostega and Ichthyostega, while sometimes considered early tetrapods due to their digits, are often placed within or very close to the stem tetrapod lineage due to their retaining of significant fin-like structures and other primitive features. The study of stem tetrapods is crucial for understanding the evolutionary transition from aquatic life to terrestrial existence.