UNVEILING THE WORLD OF SCLEROBIONTS IN MICRASTER ECHINOIDS

 

Cretaceous (~ 85 million years old) sea urchin Micraster coranguinum Leske, 1778 from Lleida (Catalonia) (Photo: Jordi Sánchez).

The study of sclerobionts—organisms that attach to hard surfaces—provides a unique window into ancient marine ecosystems. Recent research (*) focusing on Micraster and Gibbaster echinoids from the Upper Cretaceous of northern Spain has revealed a wealth of information about these interactions. Here are some expanded insights:

• Rich Biodiversity: The research highlights a diverse array of sclerobionts associated with Micraster echinoids. This includes various bivalves (such as Dimyidae, Anomiidae, and Plicatulidae), polychaete annelids, bryozoans, and lituolid foraminiferans. The presence of such a variety of organisms indicates a complex and thriving ecosystem in the ancient marine environment, showcasing the intricate relationships that existed among different species.

 

Sclerobiont traces: 1- Valve of Atreta sp. (bivalve mollusc): 2- Spirorbis sp. (carcareous tube-form), attached to the test of a cretaceous (~ 85 Myo) sea urchin Gibbaster brevis Desor, 1847 from Lleida (Catalonia) (Photo: Jordi Sánchez).

• Dominance of Encrusting Bivalves: Encrusting bivalves were found to be particularly prevalent, appearing in approximately 90% of the studied echinoid specimens. This suggests that these bivalves played a significant role in the ecological dynamics of the time, potentially influencing the survival and behavior of the echinoids they inhabited.
• Bioerosion and Epibiosis: The study emphasizes the significance of bioerosion—where organisms bore into the echinoid tests—and epibiosis, where organisms live on the surface. Structures such as Oichnus simplex and Trypanites solitarius were identified as bioerosion indicators. These interactions not only provide insights into the ecological relationships of the time but also inform us about the processes that lead to fossilization.
• Patterns of Preservation: The preservation of sclerobionts is often seen as calcified or pyritized internal molds. Their distribution tends to be concentrated on the apical side of the echinoid tests, particularly in the interambulacra regions. This pattern can reveal important information about the living conditions and ecological preferences of these organisms during their life.

 

Large Spiraserpula sp.Regenhardt, 1961 (Polychaete worm) encrusting in a cretaceous (~ 86 Myo) seaurchin Gibbaster brevis Desor, 1847 from Olazti (Nafarroa) (Photo: Jordi Sánchez).

• Taphonomic Pathways: The presence of sclerobionts on Micraster tests allows researchers to analyze taphonomic pathways, distinguishing between accumulated and non-accumulated fossils. This analysis is crucial for understanding fossil preservation processes and the ecological interactions that occurred during the life of these organisms. The study concludes that the taphonomic history observed is common among infaunal echinoid populations living in organic-rich, mixed sediments during the Mesozoic era.

In summary, the exploration of sclerobionts associated with Micraster and Gibbaster echinoids not only enhances our understanding of past marine ecosystems but also provides essential insights into the taphonomic processes that shape the fossil record. These findings contribute to a deeper appreciation of the intricate relationships that existed in ancient marine environments, revealing the complexity and richness of life in the Upper Cretaceous seas.

(*) Zamora, S., Mayoral, E., Vintaned, J. A. G., Bajo, S., & Espílez, E. (2008). The infaunal echinoid Micraster: taphonomic pathways indicated by sclerozoan trace and body fossils from the Upper Cretaceous of northern Spain. Geobios, 41(1), 15-29.