Ammonites

Ammonites possessed coiled shells divided into multiple chambers, separated by walls called septa. These septa form complex patterns at the junction with the outer shell, known as suture lines, which vary among species and serve as key features for species identification and stratigraphic dating.

They primarily lived in shallow seas and coastal regions. Some species were nektonic (free-swimming), others nektobenthic (living near the seabed), and some inhabited soft or hard substrates directly. Isotopic studies indicate that certain species occupied different water depths, which correlated with variations in shell morphology and suture line complexity.

Over 10,000 ammonite species have been identified. Based on shell shape and suture patterns, ammonites are classified into several major groups:

• Ammonitida: tightly coiled shells with complex suture lines


• Ceratitida: simpler suture lines, mainly in the Triassic


• Goniatitida: simple suture lines, primarily in the Paleozoic

Ammonites became extinct at the end of the Cretaceous, at the Cretaceous–Paleogene boundary (around 66 million years ago). Probable causes of their extinction include drastic climatic changes, extensive volcanic activity, and large meteorite impacts. Fossil evidence shows that some species remained widespread until the very end of the Cretaceous.

References

- Klug, C., Kröger, B., Vinther, J., & Fuchs, D. (2015). Ammonoid Paleobiology: From macroevolution to paleogeography. Springer.

- Wikipedia contributors. (2023, August 15). Ammonoidea. In Wikipedia, The Free Encyclopedia. Retrieved from https://en.wikipedia.org/wiki/Ammonoidea

- Suture pattern formation in ammonites and the unknown rear mantle. (2016). Scientific Reports, 6, 33689. https://doi.org/10.1038/srep33689

- Ammonite habitat revealed via isotopic composition and comparisons with co-occurring benthic and planktonic organisms. (2016). Proceedings of the National Academy of Sciences, 113(43), 12174–12179. https://doi.org/10.1073/pnas.1507554112

- Isotopically reconstructed temperatures of the Western Interior Seaway during the Late Cretaceous. (2023). Geochimica et Cosmochimica Acta, 373, 1–17. https://doi.org/10.1016/j.gca.2020.12.023