Ammonites

Ammonites (or ammonoids) were the most important of all the cephlapods. They lived from the Devonian (345 million years ago) until the end of the Cretaceous (65 million years ago). Their closest living relatives are Nautilus, but they are also related to Squid and Octopii. Their shell is coiled planispirally, and chambered. The suture line is folded or fluted into forward-directed saddles and backward-directed lobes*.

Ammonites became extinct at the end of the Cretaceous as part of a mass-extinction event due to global climate change, as a result of increased greenhouse gasses released into the atmosphere by the eruption of millions of cubic kilometers of lava in India (the Deccan Traps). The impact of a large meteorite in Chixelhub on the Yucatan peninisular was merely co-incidental to this event.

The frankly, rather dull exterior of an Ammonite
A rather fine polished Ammonite, clearly showing the suture patterns
In this section through the same specimen as shown at the top, you can clearly see the internal chambers and septae

*Practical Guide to Fossils, Richard Moody Phd, pub 1994, Chancellor Press