From Logic Law
- Superclass Agnatha ("jawless" - generally the lampreys and possibly/probably the hagfishes, eel-like animals with cartilaginous, endoskeleton spines that first appeared about 510 million years ago).
- Infraphylum Gnathostomata ("jawed-openings" - vertebrates possessing jaws, first evolved about 440 million years ago).
- ↑ The classification of hagfish as outside of the grouping of vertebrates is controversial. The issue is whether the hagfish is itself a degenerate type of vertebrate-fish (most closely related to lampreys), or else may represent a stage which precedes the evolution of the vertebral column (as do lancelets). The original scheme groups hagfish and lampreys together as cyclostomes (or historically, Agnatha), as the oldest surviving class of vertebrates alongside gnathostomes (the now-ubiquitous jawed-vertebrates). An alternative scheme has been proposed where jawed-vertebrates are more closely related to lampreys than to hagfish (i.e., that vertebrates include lampreys but exclude hagfish), and introduces the category craniata as a super-group of the vertebrates. Although recent DNA evidence has supported the original scheme, this outline follows the more recently proposed scheme for morphological reasons.
- ↑ True eels possess bones.