Cephalaspis lleyli was a prehistoric jawless fish known for its bony head shield, which served as armor against predators. It thrived in the Devonian period's freshwater environments, using its flattened body to sift through sediment for food. Its distinct crescent-shaped head allowed for effective navigation along riverbeds.

Cephalaspis lleyli is an extinct species of jawless fish that lived during the Early Devonian period, approximately 419 to 393 million years ago. Fossils of Cephalaspis have been discovered in Europe, particularly in the United Kingdom and parts of continental Europe, indicating its native region in the ancient freshwater and estuarine environments of these areas.

Cephalaspis inhabited freshwater and estuarine environments, which were common during the Early Devonian period. These environments were characterized by shallow waters with slow currents, abundant vegetation, and muddy or sandy substrates. The climate during this period was generally warm and stable, providing an ideal habitat for early vertebrates.

Cephalaspis occupied a benthic ecological niche, living near the bottom of rivers and estuaries. It was adapted to life in these environments with its flattened body and broad head shield, which helped it navigate and search for food along the substrate. As a detritivore and filter feeder, Cephalaspis likely consumed organic matter and small organisms found in the sediment, playing a role in the nutrient cycle of its ecosystem.