Paleoecology of heliolitids

Paleoecology of heliolitid corals from Wellin bioherm (Eifelian, Dinant Synclinorium, preliminary results)

13th International Symposium on Fossil Cnidaria and Porifera Modena 2019

Jan J. Król, Julien Denayer

Abstract:

Heliolitid tabulate corals from Wellin have been studied in respect to their growth patterns and colony development. The Wellin bioherm occurs within the uppermost Eifelian sediments, cropping out in southern parts of the Dinant Synclinorium in Belgium. It is located c.a. 40 km SE from Dinant. It is a part of the Wellin member of the Upper Eifelian – Lower Givetian Hanonet Formation, known for its abundant fauna of corals, stromatoporoids, brachiopods, and bryozoans.

Heliolitid corals are very abundant. They are small to medium sized (ranging between 3.4 cm and 17.2 cm in diameter). They mostly represent bulbous, domal, and irregular growth forms. Rejuvenations and following radical changes of the growth direction often occur. Growth interruption surfaces are very common within the coralla. They constitute dark layers that in cross-section show how the shape of the colony changed as it grew. These surfaces can be clean, but are often accompanied by sediment or encrusting organisms – mainly stromatoporoid sponges and alveolitid tabulates. Stromatoporoid encrusters commonly co-occur with syringoporid tabulate symbionts, growing inside the sponge body. In some cases the encrusters were later overgrown by the heliolitid, following its recovery after the growth interruption.

The abundant growth interruption surfaces consisting of sediment, as well as the common rejuvenations and changes of the growth direction, point to episodically increased sediment input. According to previous studies, it is very likely that at least some representatives of the genus Heliolites could be able to survive temporary sediment cover. The higher sediment input could be caused by the increased water energy e.g. during storms. The abundant growth interruption surfaces were also observed in the Devonian heliolitids from Morocco, Poland, and other localities in Belgium.

The interactions between heliolitids and their encrusters, settling on the surface of the colony during growth interruption, could suggest a competition for space between benthic organisms. The capability of heliolitids to survive sediment-derived growth interruption and to overgrow their epibionts, imply that these corals were very resistant. Heliolitids competing with stromatoporoids and overgrowing each other are also known from the Givetian of Morocco.

The continuation of the study of heliolitids from Wellin will include detailed taxonomy and further paleoecological analysis. The paleoecology of this group of tabulates is not very well understood, as the research of heliolitids in the past was often more focused on their taxonomy.

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Skeletons of Palaeozoic Heliolites (Tabulata) and modern Heliopora (Octocorallia): a case of ecological convergence?

14th International Coral Reef Symposium, Bremen 2021

Jan J. Król, Nina Yasuda

Abstract:

Heliolites was a widespread genus of heliolitid corals, a group belonging to Tabulata. They were unique among tabulates - unlike the others, they developed septa (in hexameric symmetry) and formed highly integrated, plocoid colonies, rare in the Palaeozoic. Their ecology remains, however, poorly understood.

Heliolites featured a skeletal structure almost identical to that of modern octocorals of the genus Heliopora. However, the possibility of these genera being related has been ruled out based on different symmetry, mineralogy, and microstructure of their skeletons. Therefore, in this study, we test the hypothesis of ecological convergence as the reason for the close similarity between Palaeozoic heliolitids and Recent helioporids.

Our analyses of morphology and growth patterns of Middle Devonian Heliolites from Morocco, Belgium, and Poland show that this genus was very resilient. It was able to survive under the conditions of increased sediment influx, high energy events, scarcity of hard substrate for settlement, and strong competition for space on the reef. Additionally, it thrived under the greenhouse climate of the Middle Devonian, commonly considered to be one of the warmest periods in all of the Phanerozoic.

The observations on modern Heliopora coerulea from the Shiraho reef (offshore Ishigaki Island, Japan) show that it occurs abundantly in places characterized by increased temperatures and sediment input in the shallow, protected, low-energy setting of the backreef, but also in the forereef, under increased hydrodynamics and lower temperatures. It shows great skeletal plasticity, which allows it to adapt to different environmental conditions. Interactions with other benthic organisms are also common, particularly with other corals and with Polychaeta.

The resilience and adaptability of both Heliolites and Heliopora seem to be the common link. The simple skeletons provided their plocoid colonies with increased flexibility and versatility. In both genera the gross morphology and the corallite spacing can vary, apparently depending on external factors. More in-depth analysis of the convergence patterns between Heliolites and Heliopora will help us to better understand the adaptive capabilities of such resilient corals, which could resist high temperatures and generally unfavourable conditions. This should prove especially useful during present-day climate change resulting in the retreat of the scleractinians as the main reef builders.

Heliolitids - the best Paleozoic analogues of present-day reef corals?

Mediterranean Geosciences Union Annual Meeting, Marrakech 2022

Jan J. Król

Abstract:

Similarly to present-day reefs, the coral communities of Middle Paleozoic had to live through major climate shifts. Understanding how they dealt with such crises could help us predict the future of coral reefs endangered by current climate change. Unfortunately, most of the Paleozoic corals were not all that similar to their modern counterparts. Highly-integrated colonies of heliolitids however, might share the most similarities with modern reef corals, both in terms of structure and ecology.

Heliolitid coralla were collected from Silurian and Devonian outcrops of Gotland, Morocco, Belgium and Poland. Polished slabs and thin sections of the specimens were studied in respect to their growth patterns, taphonomy, inter-species interactions, associated biota and surrounding microfacies.

Studied specimens exhibit colony-wide responses to unfavorable conditions, e.g. sediment influxes or infestations. These are recorded in abundant growth-interruption surfaces in the coralla. Their high abundance suggests that the colonies were easily affected by environmental stressors, but the corals were able to regenerate from such events. Some species (e.g. Steliporella parvistella, Heliolites interstinctus, Propora tubulata) were also capable of skeletal plasticity – like the ability to form branching colonies under increased sediment input or platy colonies in low-light environments. Additionally, cyclic growth banding is present in some of the specimens, which together with the platy growth forms, suggests the presence of algal symbionts.

These characteristics of heliolitids are very similar to some of the modern hermatypic corals, especially Heliopora coerulea, which also shares with them an uncanny resemblance in the skeletal structure.

Heliolitids present themselves as an evolutionarily advanced group of corals for their time. They exhibited high colony integration, remarkable regenerative capabilities and most likely were photosymbiotic. On the other hand, much like modern corals, they were susceptible to shifts in environmental conditions.

These characteristics of heliolitids as a group living in the time periods of extreme climate change, makes them great analogues of modern corals. Studying their development provides us with insights into how hermatypic corals might react to increasingly more extreme climate change in the future.