Stromatolite, brain coral, or ?

[heatherhjorth]

This is probably an exercise in pareidolia - but could it be a brain endocast?

 

This was self collected July 9, 2011 on San Juan island in Washington State. It was found on private property near the shoreline on the SW corner of the island. The local geology is a complex mixture of Orcas chert and formations of sedimentary and volcanic layers contorted together, peppered with glacial deposits. There are other fossils in the vicinity, including various plants and what I believe are devonian era fish fossils.

 

At first, I thought this was a thunder-egg or nodule that turned to chert/agate. Then, it struck me that it looked like a brain. Perhaps a brain coral of some sort? But it doesn’t look like the photos of brain corals. It did look more like the stromatolite fossils, but no clear match. Something from my biology studies years ago noticed what looked like features of the mammalian brain. 

 

But brains don’t fossilize. 

 

It also looked too big to be a brain, especially a single lobe without a cerebellum. Was it part of a marine mammal brain? Some sort of brain endocast that turned to agate? 

 

It weighs 77g and has not been thoroughly cleaned. One of the photos included is 'as found' and the others show the fossil after it was rinsed off. I haven't given it a thorough cleaning to avoid harm to the sandstone-like coating.

 

I’d love to hear what people think.

[Tetradium]

Quartz nodule similar to thunder eggs but more messy. 

[abyssunder]

When you write " paradoleia" are you referring to pareidolia? 

[DPS Ammonite]

It is a broken rounded chert nodule. 
 

An endocast is a confusing name for an internal mold of a brain case. It preserves the features of bone and not all the exterior crenellations and lobes of a brain.