Gastropods
Gastropods are invertebrate mollusks and we know them today as snails and slugs. The ones in Tyndall Stone were marine animals and they grew to be quite large.
Gastropods are very diverse. There are tens of thousands of living species today and at least 15,000 fossil species. The first big gastropods appeared around 495 million years ago and so they have had a very long and rich history on Earth! They are often used in palaeontology and biology research to study evolution, ecology, and physiology.
Their shells 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.