The Ammonoids of Mutriku: An Oceanic Heritage from 100 Million Years Ago.

This activity is part of the course program: 'Universities Looking at the Sea: Navigating Ocean Sustainability on the Training Ship Saltillo.'

Ammonoids are extinct cephalopod mollusks that formed one of the most diverse groups of invertebrates we know. They were abundant in the seas of our planet from 350 million years ago to approximately 65 million years ago, becoming extinct around the same time as the well-known dinosaurs.

The size of adult ammonoids ranges from 1 to 15 cm in diameter, but some groups deviate widely from these sizes, exhibiting shapes that can reach over 50 cm and even up to a meter in diameter.

The ammonoid fossils in Mutriku exhibit a wide range of morphologies and exceptional size compared to other sites of the same age (Albian, Middle Cretaceous; approximately 100 million years ago). The collection kept in the Nautilus museum contains more than 60% of specimens ranging from 35 to 50 cm in size, many of them being true giants when considering the average size of the represented families. Interestingly, these fossils had gone largely unnoticed by science until Jesús Narbaez and Esperanza Azkarraga from Arrasate, driven by their love for nature and innate curiosity, rescued them from coastal erosion and donated them for scientific research and the enjoyment of all.

To explain the large size of these fossil ammonoids, several lines of hypotheses have been proposed, which still require further study. In the marine environment, a large remains have a high chance of breaking and their fragments can disperse either through currents or biological activity, making their preservation in the fossil record challenging. The life cycle of the living being is an important factor. For example, a larger size could result from delayed sexual maturity, advantageous in certain situations (e.g., reproduction; larger females). The presence of large forms could explain reproductive behavior in a favorable environment. Environmental factors, such as episodes of high sea levels, allow the expansion of continental shelves with great potential for new ecological niches, or underwater volcanic episodes that contribute nutrients to the oceanic food chain.