Dactylioceras

Dactylioceras, meaning ‘Finger Horn’ is a species of Ammonite which in habited the open seas during the Early Jurassic period 200-175 million years ago.

Dactylioceras is a common find in Jurassic bituminous shales. These shales formed when limited water circulation allowed stagnant (still, oxygen-poor) conditions to develop in dense sediments on the sea floor. This was favourable for preservation of ammonites and other shells in various ways. 
The impermeable nature of the sediment prevented the shell’s structure of aragonite material from dissolving away.
In addition, the stagnant conditions encountered by the shells when they sank to the bottom meant that burrowing animals or currents would not disturb them as the fossilisation process occurred.
Several individuals are preserved in the block shown here, discovered that Dactylioceras had gregarious (group-living) habits.
Possibly, like many modern cephalopods, such as squid, they congregated in large swarms or schools to breed.