In one of previous post “Biological classification of animals” dealing the classification of all the animals on our planet Earth we have seen that animals with three germ layers (triploblastic animals) can be divided in protostomes and Deuterostomes based on the fate of first opening of the development. Further we have seen that Deuterostomes can be classified into : Chordata, Echinodermata, Hemichordata and Xenoturbella. Today we shall try to gain some insights into an important group of animals among chordates belonging to “Craniata”
Craniates comprises of all vertebrates (fishes, amphibians, reptiles, birds, mammals), jawless lampreys and hagfishes. The important feature to be included into craniata is presence of Skull or cranium, (reason for their name) – It could be either bony or cartilaginous.The skull of craniates serve as a protection gear for the brain, olfactory organs, eyes, and internal ear. To put it into simple words, craniates are chordates with skulls and virtue of this features it excludes other members of chordate subphyla Urochordata (tunicates) and Cephalochordata (lancelets), but including hagfishes, which have cartilaginous skulls.As chordates, all craniates develop a notochord, which is primitively large (hagfishes, lampreys), but becomes transitory in most vertebrates and is replaced by elements of the vertebral column, the centra and arcualia.
The are many questions still remain related to phylogenetic relationships among craniates but widely followed classification divides craniates into two clades : the Hyperotreti, or hagfishes, and the Vertebrata. The vertebrata or vertebrates, is a very diverse group, ranging from lampreys to Man. It includes all craniates, except hagfishes, and are characterized chiefly by a vertebral column. Most of the vertebrates are jawed animals (Gnathostomata) except Lampreys,which lacks jaws.
Earlier lampreys and hagfishes were grouped together into “clade Cyclostomi” , but later it was known that Lampreys share more features with jawed vertebrates which do not found in hagfishes or non-craniate chordates (cephalochordates and tunicates). So now general consensus is that Hagfishes are sister group to vertebrata ( which inculdes Lampreys and Vertebrates) unlike the previous view of Hagfishes as sister group to Lampreys.
Hagfish are marine craniates of the class Myxini, also known as Hyperotreti and are the only animals with skull but without vertebral column.
Lampreys are eel-shaped jawless fishes. They can be readily recognized by the large, rounded sucker which surrounds their mouth and by their single “nostril” on the top of their head.