Short answers, pulled from the story.
Acanthodii possessed tiny rhomboid platelets that formed a bulbous base and flat diamond-shaped crown covering their skin. Their skeletons remained cartilage while the exterior wore armor resembling bony fish.
Fanjingshania and Qianodus from China date back approximately 439 million years ago. These earliest remains mark the beginning of acanthodian-grade chondrichthyans in geological history.
Scientists found robust bony shoulder girdles on some specimens while others displayed flexible joints typical of sharks. Researchers now classify them as stem-sharks rather than direct ancestors to living species due to this complex relationship with cartilaginous fish.
Their evolutionary journey spanned nearly two hundred million years of Earth's history. Marine populations dominated early eras before freshwater species became predominant during the Devonian period.
Distinctive ornamentation on each scale helps determine relative ages of sedimentary rock layers. Geologists use these tiny diamond-shaped crowns as biostratigraphic markers worldwide.