"A Living Trilobite?"
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"What's Your Name"
Lynyard Skynyrd
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No,even though it may appear similar, a Horseshoe crab is not a trilobite,but it is the closest living relative of the trilobite.It has sometimes been called "a living fossil",since it has remained basically unchanged for millions of years.Horseshoe crab fossils have been found which date back to the Carboniferous period (around 350 million years ago).There are 4 species of Horseshoe crabs alive today,Limulus polyphemus,Tachypleus tridentatus,Tachypleus gigas,and Carcinoscorpinus rotundicauda.When Horseshoe crab eggs are hatched,the larvae look amazingly similar to the trilobite,which is why they are called "trilobite larvae".And even though it's called a crab,it's not a crab or a crustacean,it is an Arachnid,and is related to scorpions,ticks,and land spiders,and is classified along with the extinct Eurypterids in the Class Merostomata.
***UPDATE***
Two tiny horseshoe crab fossils have recently been discovered in Central and Northern Manitoba,Canada.They were unearthed in Ordovician rocks dating back 445 million years.The new species is named Lunataspis aurora,and are only 1.5 inches long.Scientists say they aren't sure whether these are just juvenile specimens,or if this was as large as Lunitaspis grew.
Class Merostomata
left photo-Eurypterid (Eurypterus remipes)
right photo-Horseshoe crab (Limulus polyphemus)
 
Fossil Horseshoe Crab
(Mesolimulus walchi)
Photo courtesy of
SculptedStone.Com
Below are photos of some trilobites and Horseshoe crabs,
in which you can see the similarities of the two:
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