Evolution of Agnathan Vertebrates

Evolution of Agnathan Vertebrates

Anatolepis heintzi (Agnathans) are the earliest, most primitive jawless fish known in vertebrate evolution1,4,12. They belong to the superclass Agnatha, phylum Chordata, and subphylum Vertebrate9. Agnathans were known primarily by cephlapods (Osteostraci) present in Silurian to late Devonian period (425- 359 Ma), pterapsids (Heterostraci) in Ordovician (485 Ma) to Devonian, and accompanied with anaspids (Aanspida) and thelodonts4, 15. Fossils showed that the first agnathans had evolved in early Cambrian8 and became prominent in early Devonian/late Silurian period. They were a major component of Early Paleozoic aquatic bio systems and had fully adapted to aerial, aquatic, and terrestrial units1. Agnathans have unique features: no jaws or paired appendages, and a persistent notochord4,5,9. Ancestral forms of this class were Ostracoderm3, 4; they had ossified sclerotic bones, (which arose in relation to cartilage), and had the potential to develop bone in the lower jaws3. These bony-skinned fish gave rise to the gnathostomes 4, 14 vertebrates with jaws who emerged in early Paleozoic (Early Devonian period: 418–412 Ma) to recent vertebrates10. Agnathans give a level of morphological organization and showed the formation of diverging lineages in vertebrate evolution9, 14. Extant agnathans include two groups, called cyclostomes5,11 , because of their circular mouths: Lampreys (late Devonian: 358 Ma) 11 and hagfishes (Late Carboniferous: 300Ma) 4,6,11. They split from other agnathans before the formation of dentine and bone (which are present in fossil agnathans) 7, 15. These are the only jawless vertebrates to survive today2 and the least changed from the first vertebrates1. However, they are not direct survivors of the class Agnatha, but are instead distance cousins of the cartilaginous fish4. By the late Silurian period, the jawless vertebrates reached the max of their evolution and became extinct by the end of the Devonian period1,...