Which features of ancient agnathans were usually preserved as fossils?

Ancient agnathans, or jawless fishes, are primarily known to the fossil record through their hard, mineralized parts, which were much more likely to be preserved than soft tissues. The most commonly preserved features were fragments of heavy dermal armour, which served to protect the head and gill regions of these early vertebrates.12

While modern agnathans known as cyclostomes—which include lampreys and hagfishes—lack these hard parts and are consequently extremely rare in the fossil record, their extinct counterparts were characterized by these protective bony or cartilaginous structures. These armoured headcases are the primary evidence paleontologists use to study the early history of these jawless vertebrates.13