What are the Agnatha?
Q: What are the Agnatha?
A: The Agnatha are a superclass of jawless fish.
Q: Why are the Agnatha considered paraphyletic?
A: The Agnatha are considered paraphyletic because it is a convenient term that does not follow the rules of cladistics, and most extinct agnathans belong to the stem group of gnathostomes.
Q: What are the living Agnatha called, and how many species are there?
A: The living Agnatha are called cyclostomes, and there are about 100 species.
Q: Are the living agnathans monophyletic?
A: Yes, recent molecular data shows that the living Agnatha (lampreys and hagfish) are monophyletic.
Q: Do hagfish have vertebrae?
A: No, hagfish are vertebrates but do not have vertebrae. It is believed that they lost their vertebrae during their lifestyle adaptations.
Q: How do the lifestyles of lampreys and hagfish differ from those of fossil groups?
A: The lifestyle of the lamprey (an ectoparasite on other fish) and hagfish (a scavenger) means that they are not typical of the fossil groups, which were free-swimming and often armoured.
Q: Should one sister group contain ancestors of another sister group?
A: No, according to the rules of cladistics, one sister group should not contain ancestors of another sister group.
