Osteostraci

Osteostraci is an extinct group of jawless fish that lived during the Silurian and Devonian periods. Their name, meaning "bone-shield," refers to the prominent bony plates that covered their heads and anterior bodies, forming a protective carapace. These ancient vertebrates are characterized by a flattened body, a terminal mouth, and paired pectoral fins, which were among the earliest known appendages in vertebrates. Their eyes were positioned dorsolaterally, meaning they faced upwards and outwards.

Fossils of Osteostraci are found primarily in marine and estuarine environments, suggesting they inhabited shallow coastal waters. Their diet likely consisted of detritus or small invertebrates that they sifted from the seabed using their ventral mouths. Osteostraci are considered to be among the most advanced of the early jawless fish, possessing a more complex sensory system than their predecessors, including lateral line canals and distinctive paired sensory fields on their head shields, the function of which is still debated.

The evolutionary significance of Osteostraci lies in their place within vertebrate phylogeny. They are a sister group to the Gnathostomes (jawed vertebrates), providing crucial insights into the transition from jawless to jawed fish. Their internal anatomy, particularly the structure of their paired fins and braincase, has been extensively studied to understand the early evolution of vertebrate anatomy and the development of complex sensory and motor systems. The group became extinct by the end of the Devonian period, likely outcompeted by the diversification of jawed fish.