Intact stalked crinoids from the late Aptian of NE Spain offer insights into the Mesozoic Marine Revolution in the Tethys
Summarized by: Randolph Kruman. He is a biology major and a sophomore at Binghamton University. He plans to graduate from Binghamton and attend dental school. He loves plants and has many houseplants.
What Data Were Used: Researchers found fossils in the Oliete Formation in northeast Spain’s Maestrazgo Basin. The area contains Aptian-age rocks, an Early Cretaceous age from about 121–113 million years ago. Researchers discovered a bed of 20 intact crinoid fossils (Fig. 1), which is rare because crinoids are made up of thin and fragile parts that break off easily and are held together with soft tissue that often decay before fossilization occurs. The orientation of the crinoid fossils as they were found in the rocks was recorded. Researchers collected the crinoids and coated them in ammonium chloride, NH4Cl, which was used to highlight the detailed features of the crinoids for better study. They also collected oyster shells from the same layer to perform isotope analyses.
What was the hypothesis being tested: Scientists tested the hypothesis that crinoids shifted their habitat as a result of predation, and not salinity shift, during the Mesozoic Marine Revolution (MMR). The MMR was a huge shift in ecology and types of marine organisms, occurring over the Mesozoic Era, during which many new predators with adaptations like shell crushing emerged.
Methods: Researchers analyzed the position of the crinoid specimens that they were found in in order to determine information about their death and environment. Also, researchers analyzed crinoid specimens for evidence of predation (by determining if they had any regenerated body parts). They also analyzed the oyster shell remains to determine stratigraphic location and level of abrasion. as well as being analyzed using strontium isotopic analysis, a radioactive isotope, in order to determine information about the environment and piece together factors of why the crinoids stopped living at shallow depths. This is because strontium levels can indicate levels of salinity in paleoenvironments. The presence of well-preserved sponge spicules was noted. They also analyzed the data on water and environmental levels of the Oliete formation to determine the environment and piece together why the crinoids left after the beds were deposited.
Results: Scientists found numerous instances of regenerated or injured arms within the specimens, indicating they were being preyed on. The crinoids were preserved with their bodies in three different arrangements: “starburst-up” (mouth pointed up), “starburst-down” (mouth pointing down), and on the side. A sub position within on the side is the “shaving brush” position, where the arms fold together away from the calyx, which would typically form as a response to high-flow currents. Eleven are in the starburst-up position, two are in the starburst-down position, and eight are on their side (among them, one in the shaving brush position). Analyzing where and how the crinoids were found can say a lot about their environment. For example, the “shaving brush” position is similar to other crinoids, both living and fossil, responses to fast current and sediment-laden conditions, implying the eight on their side were buried alive in a storm and died. Starburst-up also implies they were buried alive and tried to crawl out because that’s how they crawl. These crinoids could crawl, an ability only seen in some fossil crinoid groups.
Other lines of evidence, besides the crinoids’ positions, point to a storm-dominated area. The crinoids and sponges are well-preserved, even the sponge spicules, implying being buried alive in either low energy muddy environment or high-energy waters. The oyster shells are all broken and abraded, implying high energy constant energy like waves, in contrast to the well-preserved crinoids and sponges. The shells probably came from nearby, meaning waterflow and direction was changing frequently, or storms transported them, again pointing to an unstable environment.
Strontium isotopic analysis on oyster shells indicated high levels of strontium, which is connected to lower salinity levels than was expected by the scientists, suggesting that salinity fluctuations and periods of low salinity was common. In the Oliete formation, sedimentological data from another study by the same researchers indicates that there was a shallow marine environment where crinoids lived in the form of a lagoon connected to the ocean. Notably, though, is that connection to the ocean was on the east side of the lagoon, where coral meadows prospered in the stable environment and low energy lagoon water, but on the other side, farthest from marine circulation, the lagoon was a less stable, high stress, peritidal habitat with frequent fluctuations in salinity, temperature, water flow, and waves, again evidenced by the broken oyster shells. It would not make sense that salinity change was the sole factor that caused the bathymetric change (marine depth change) in crinoids, because even though salinity levels steadily decreased during the Mesozoic, there is evidence that they were adapted to salinity fluctuations. This contradicts the commonly held hypothesis that crinoids shifted their habitat to the deeper ocean to escape the change in salinity. However, these crinoids from the Oliete formation have regenerated/injured arms, meaning they were preyed upon by predators. Other fossils found in other studies in the deeper ocean do not have the same predation evidence, the deeper sections of sea probably were a safer environment for crinoids to live in, meaning deep-water crinoids survived at higher rates than shallow-water crinoids, eliminating shallow-water crinoids entirely given enough time. It is plausible that predation drove the crinoids bathymetric shift, as a part of the Marine Mesozoic Revolution.
Why is this study important? This study is important because it is a piece of evidence that contradicts the common hypothesis that salinity change and stress from the changing environment is the reason why crinoids left shallow marine environments and are only found in deeper waters today.
Broader Implications Beyond Study: This study’s findings also contributes to a greater understanding of the Mesozoic Marine Revolution, including information on how widespread predator organisms were, how they affected ecosystems, and how organisms respond to change in general. While crinoids ended up abandoning shallow seas for deeper seas, likely, at least in part, because of predation, other species reacted differently during this time.
Citation: Álvaro García-Penas, Tomasz K. Baumiller, Marcos Aurell, Samuel Zamora; Intact stalked crinoids from the late Aptian of NE Spain offer insights into the Mesozoic Marine Revolution in the Tethys. Geology 2024;; 52 (8): 594–599. doi: https://doi-org.proxy.binghamton.edu/10.1130/G52179.1