During the cambrian explosion, some of the body plans that appeared for the first time are hard external skeletons.
What is Cambrian explosion?
This is also referred to as biological big bang and a period where all major animal phyla started appearing in the fossil record.
The major characteristics of the animals in this period include the presence of spines,shells etc.
Read more about Cambrian explosion here https://brainly.com/question/1345078
#SPJ4
