Final answer:
During the Devonian Period, fish developed lobed fins that enabled the transition towards land dwelling, as well as the evolution of gnathostomes with jaws and paired fins for improved mobility and predation.
Step-by-step explanation:
Development of Fish During the Devonian Period
During the Devonian Period (410-360 million years ago), remarkable evolutionary advances in fish occurred. One of the significant developments was the evolution of lobed-finned fish, which laid the groundwork for the eventual colonization of land by vertebrates. These fish had fins that allowed them to maneuver through vegetation in shallow waters, and importantly, they developed the ability to breathe air by lifting their heads above water. This adaptation was crucial for life on land. Aquatic tetrapods such as Ichthyostega and Acanthostega also originated in this period. They possessed both gills and lungs and had four limbs with a skeletal structure similar to those of today's tetrapods. While their limbs were not yet capable of supporting their bodies well out of water, their existence marks significant progress towards terrestrial life.
Fish also developed other key features during the Devonian, including gnathostomes, or jawed fishes, which expanded the range of food sources available through active predation. Important evolutionary advancements such as jaws and two sets of paired fins (pectoral and pelvic) evolved, enabling fish to become increasingly dominant in aquatic environments.