Question #53742
1 Answer
Feb 1, 2018
Answer is vertebral column.
Explanation:
- The illustration that comes with the question presents evolution of vertebrates in very simplified order.
- It begins with hagfish, a jaw less vertebrate (Agnatha) related to evolutionary ancestors of vertebrates with jaws.
- First jawed vertebrates (Gnathostomata) were members of aquatic extinct group Placodermi. The jawed vertebrates are primarily of two types: superclass Pisces and superclass Tetrapoda.(Relatives of lung fishes acted as connecting link between the two.)
- Four classes under Tetrapoda are Amphibia, Reptilia, Aves and Mammalia. Human and his ancestors are included in order Primata.
![()
Now when we place another node in the beginning, it will represent a branching towards early chordate. Here an organism with permanent notochord in its body has been selected: e.g. Lancelet.
Evolutionary picture suggests that, a line of early chordates must have given rise to early jaw less vertebrates like Ostracodermi, another extinct group of aquatic organisms.