Name Meaning: Finger Horn
Geologic Era: Lower Jurassic (180-172 million years ago)
Location Discovered: England
Estimated Range: Worldwide
Size: 6.5 cm in diameter (average)
Extinction Date: 172 million years ago
Dactylioceras was a type of ammonite, or spiral shelled marine mollusk. Dactylioceras is a relatively common fossil found on all continents. It is so common that is is used as an index fossil to help provide a way to date the sediments it is found in. Dactylioceras earned its name from the distinct, finger like grooves on its shell. Other species of ammonites have smooth shells that sometimes become iridescent when fossilized.
Dactylioceras are often found in mass mortality aggregations. I am not sure if our specimen was found in this manner, though it is a possibility. As you can see, it is incompletely prepared and cleaned, though what has been cleaned is very well done. It is possible that the reason for these mass mortality finds was the reproductive strategy used by this group of ammonites. It is likely that Dactylioceras laid all of its gametes at the end of its lifespan, and died shortly afterward. The dead animals could then either have been buried on the seafloor or washed up on shore.
This is a reproductive strategy still in use today by a few of the ammonites’ living relatives. It is a strategy that is very energy efficient, and has minimal stress on the adult animal. Also, if a large number of gametes are released at once, the chances of successful fertilization are increased (especially if other individuals of the same species are doing the same thing at the same time).
Due to their common nature, this species of ammonite has a charming legend attached to it. Pictured below is a “serpent stone,” which is a Dactylioceras ammonite with a stylized snake head carved into the front. The legend states that it was St. Hilda of England who was the reason for these stones. Supposedly, in the 6th century, St. Hilda successfully removed a plague of snakes from England through prayer. These snakes were turned to rock by her prayer and fell into the sea. If these ammonites washed up on shore, people said they were some of the snakes that St. Hilda petrified.
Later on, entrepreneurial types took to carving heads into the Dactylioceras ammonites to make them look more like snakes. They would then sell these “serpent stones” to tourists, a tradition that continues to this day. You can sometimes find these “serpent stones” for sale on the internet. The best and most authentic (for this legend, anyway) come from the Whitby area in England.
Life Reconstruction: By NobuTamura email:firstname.lastname@example.org www.palaeocritti.com – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=19459145
Snake Stone: http://fossil-ammonites.blogspot.com/