Definitions:

1. An order of marine arthropods that includes horseshoe crabs. NOUN
   • Xiphosura includes horseshoe crabs, some of the oldest known species still alive today.
2. Plural name that refers to an order of arthropods that includes horseshoe crabs and extinct related forms. NOUN
   • As members of Xiphosura, horseshoe crabs have changed little over millions of years, eraning them the title 'living fossils'.

Synonyms:

• arthropod
• horseshoe crab