- First episode of diversification
- The best known primitive shark
- It arose some 400 million years ago in the Late Devonian
- The average length ~ 1 meter
- It had long slender jaws with a cylindrical body shape
- No anal fin, two dorsal fins
- ***Terminal mouth*** (like a fish)
- Jaws fused to its braincase
- Fins were stiff and simplistic
- no claspers (external fertilization?)
- Symmetrical tail, similar to the modern mackeral shark in the order Lamniformes
- Decent agility and speed
- It had knifelike spines projecting from its dorsal fins [as seen in drawing] (Fenton and Fenton)
