Belemnoteuthis antiquus is technically not a squid, but a belemnite. Belemnites were the first cephalopods (as far as we know) to develop the ingeniously internalized shell that led to the radiation of nearly all modern cephalopods--octopus, squid, and cuttlefish. External shells like those of the ammonites provided predator protection, but they were heavy and hard to maneuver. Presumably, internalizing the shell led to improvements in swimming, freeing these proto-squid so they could compete effectively with fish. However, belemnites did not evolve directly into modern cephalopods--they went extinct. It was their sister clade, the neocoleoids, which gave rise to modern cephs (according to the current evolutionary paradigm).

Anyway, a beautifully preserved specimen of B. antiquus recently made headlines after researchers actually reconstituted the ink and used it to illustrate the squid. Awesome! It is, as Dr. Phil Wilby says, "the ultimate self-portrait." For the record, this is not the first time that squid ink has been fossilized, just the first time that it's been reconstituted (by adding ammonia--like adding water to powdered milk!) and used in a drawing.

Why did this specimen get preserved so well? Apparently, sometimes animals get turned into stone right after death, instead of fossilizing slowly--the "Medusa effect." Why does it happen? In the case of the British belemnite and its fellow fossils, it sounds like they're not quite sure yet. But in a 2008 article about the Medusa effect in Geology Today, David Martill writes:

Firstly, an abundant fauna with preservation potential had to be present; secondly, mass-mortality events were required to concentrate the fossils into layers; thirdly, quiet water conditions were required so that current activity did not disturb the carcases; fourthly, scavenging had to be kept to a minimum to reduce disruption to carcases; fifthly, and most significantly, phosphatization had to take place during decomposition of the carcases; sixthly, burial had to take place fairly quickly to hide the newly phosphatized carcases from disruption; and finally, concretions had to form around the phosphatized carcases before overburden pressures squashed the carcases flat.

What I want to know: have any medusae ever succumbed to the Medusa effect?