Nautiloids
Nautiloids are cephalopods, a type of mollusk. They are related to the modern squid, octopus, cuttlefish, and nautilus. Ammonites, a popular fossil for collectors, are also cephalopods but they lived later, from 240 million years ago to 66 million years ago.
All nautiloids have internal chambers and a siphuncle which is a tube connecting the chambers. Using this tube, they can change their buoyancy to move up and down and act like little water pistols to move through the water horizontally. When Tyndall Stone was formed, they were the top predator in the oceans. Some nautiloids were over 5 meters long!
Nautiloid shells came in lots of shapes: some were straight, some were curved, and some were coiled. They were made of aragonite which dissolved or re-crystalized over time. These molds were then preferentially filled by mud that became dolomite, just like the Tyndall Stone burrows, and that’s why the fossils we see are brown.