Partial Ammonite Fossil
Devonian Period (400 million years ago)
Excavation Ireland

I believe this is a section of a Giant Ammonite shell, a cephalopod animal related to modern squid and octopus. The animals went extinct during the end-Cretaceous mass extinction event. They had large eyes for finding prey and many were built for speed, with hydrodynamic shells with tapered edges. Their shells have beautiful patterns made by the very complex sutures or joins between the body chambers in the shell. During growth, these animals compensated for the heavier shell by allowing gas to diffuse into the central body chambers through a tube called the siphuncle.

A partial ammonite fossil is missing a portion of its shell. This can be due to a number of factors, including:

Erosion: Ammonites are often found in sedimentary rocks, which can be eroded over time. This can cause the shell to break or be worn away, resulting in a partial fossil.

Predation: The animal was preyed upon by other marine animals, such as fish and reptiles.

Taphonomy, or, how organisms are preserved as fossils. This example may have been resting in mud or sand, or transport by currents.

Though it is only a small section; it remains valuable to scientists and collectors. It can provide information about the ammonite's species, size, and morphology. I would suggest a custom metal display stand to adorn a shelf, console, or desk.

From images, this is what "jumps-out" at me. I would suggest an in-person assessment; perhaps with a regional Geology University department or organization. It is possible that the section is petrified wood. That can also have developed the crystalized positions and the apparent rings. I believe the FMV given here would be about the same as that for the Ammonite.