Ammonitida. - Ammonshörner. Book illustration from Library of Congress

Ernst Heinrich Philipp August Haeckel (1834 – 1919), German zoologist, naturalist, philosopher, physician, professor, marine biologist, and artist who promoted and popularized Charles Darwin's work, was born on 16 February 1834, in Potsdam (then part of the Kingdom of Prussia). The published artwork of Haeckel includes over 100 detailed, multi-color illustrations of animals and sea creatures, collected in his Kunstformen der Natur ("Art Forms of Nature").

Ammonoidea, commonly known as ammonites, were a diverse group of marine mollusks that existed from the Devonian period to the end of the Cretaceous period, spanning a time frame of approximately 400 million years. They were part of the class Cephalopoda, which also includes modern-day octopuses, squids, and nautiluses. Ammonites were characterized by their coiled shells, which could range from a few centimeters to over two meters in diameter in some species. These shells often displayed intricate patterns of ridges, spines, and chambers. The chambers were interconnected by a tube called a siphuncle, which the animal used to regulate its buoyancy. These creatures were highly successful in their evolutionary history and occupied a variety of ecological niches in oceans worldwide. They were active predators, using their tentacles to capture prey, much like modern-day squids. Despite their success, ammonites, along with many other marine organisms, became extinct at the end of the Cretaceous period, likely due to the catastrophic event associated with the Chicxulub asteroid impact and subsequent environmental changes.