Ankylosaurids carried the same body armor of embedded bony plates seen across the wider armored-dinosaur lineage, but the family's defining feature is at the tail's end: a heavy knob of fused vertebrae and bone, swung on a stiffened tail handle, capable of delivering real force to an attacker's legs. Some genera also had armored eyelids and complex, sinus-riddled nasal passages whose exact function -- vocalization, temperature regulation, or both -- is still debated.
Ankylosaurids were concentrated mostly in the Cretaceous, with strong representation across North America and Asia. Ankylosaurus, the genus that names the family, is known from relatively fragmentary remains despite its fame; better-preserved relatives like Euoplocephalus and Pinacosaurus have filled in much of what's understood about how the family's armor and tail club actually worked in life.