
Hyperlioceras

Hyperlioceras is an extinct genus of cephalopod included in the ammonitid family Graphoceratidae that lived during the Bajocian stage of the Middle Jurassic. The type species is "Hyperlioceras discites" (Waagen, 18567)
Morphology.
The shell of "Hyperlioceras" is very compressed and involute, with a tall persistent keel and a deep umbilicus.
Biostratigraphic significance.
The International Commission on Stratigraphy (ICS) has defined the base of the Bajocian Stage of the Jurassic as the lowest occurrence of the genus "Hyperlioceras", at 170.3 ± 1.4 million years ago, which defines base of the "Hyperlioceras discites" Zone. This point also marks the end of the preceding Aalenian Stage. The upper boundary of the Bajocian is indicated by the lowest occurrence of the ammonite "Parkinsonia (G.) convergens", at about 167.7 Ma, which defines base of "Zigzagiceras zigzag" Zone.
Distribution.
Luxembourg, Saudi Arabia, Spain and the United Kingdom.