The Cabrières Biota Reveals a Polar Marine Ecosystem of the Early Ordovician

Reconstruction of Cabrières Biota（Image source：Christian McCall ， CC BY 4.0 ）

Lagerstätten are sedimentary deposits that preserve unusually rich fossil assemblages, often including delicate soft tissues that rarely survive the fossilization process. Because these deposits capture such a wide array of organisms, they provide an exceptional window into the ecological structure of ancient environments. Most Lagerstätten known from the Cambrian and Ordovician periods come from regions that were located in tropical or temperate climates. Fossil deposits from within the polar circle during the early Paleozoic are extremely rare, which means that our understanding of Cambrian–Ordovician ecosystems in polar regions has long remained limited.

A newly discovered fossil deposit in the Montagne Noire of southern France now sheds light on this poorly known environment. The site preserves an Early Ordovician Lagerstätte that, during the Ordovician period, lay within the Antarctic Circle. This assemblage is known as the Cabrières Biota. At that time, the region formed part of a southern polar sea adjacent to the supercontinent Gondwana. The fossils occur in strata corresponding to the Landeyran Formation, dated to the Floian stage of the Early Ordovician. Geological evidence indicates that these sediments accumulated in an offshore marine setting.

The Cabrières Biota contains numerous sponges belonging to Demospongiae, some reaching lengths of up to 10 cm, along with a variety of algae. In addition to these organisms, the fossil assemblage includes several major animal groups. Based on fossil abundance, the community is composed of bivalved arthropods (16%), molluscs (14%), trilobites (12%), brachiopods (9%), hyoliths (7%), and cnidarians (6%), whereas echinoderms are remarkably rare.

Among the trilobites present in the assemblage are representatives of the genera Ampyx, Asaphellus, and Calymene. One specimen shows an unidentified gastropod attached to the outer surface of the tube-like organism Sphenothallus, a conulariid cnidarian. Some fossils also reveal brachiopods of the order Orthida attached to the sponge Leptomitus, while others occur near bivalved arthropods. A few specimens contain worm-like organisms whose morphology suggests that some may represent armoured lobopodians. These animals were previously thought to be largely confined to the Cambrian period, and their presence here indicates that they survived into the Ordovician.

（Image source：Farid Saleh, et al. ， CC BY 4.0 ）

The fossils of the Cabrières Biota are preserved mainly as dense, irregular crystals of iron oxide embedded in a matrix rich in aluminium-bearing phyllosilicates. Their coloration typically appears brown, red, or orange. Many specimens are not fully mineralized, retaining traces of original skeletal and cellular structures. This mode of preservation confirms that the site qualifies as a Lagerstätte.

In terms of species composition and diversity, the Cabrières Biota resembles other Cambrian–Ordovician fossil deposits such as the Liexi and Fezouata biotas. However, one striking difference is the extreme rarity of echinoderms. This pattern closely resembles the Fenxiang Biota. Compared with Fenxiang, the Cabrières assemblage contains a much higher diversity of arthropods and abundant sponges and algae, yet lacks nematodes, scalidophoran animals, corals, and bryozoans. The discovery of this assemblage demonstrates that many animal groups characteristic of Cambrian ecosystems continued to survive well into the Ordovician, offering new insights into the relationship between the Cambrian Explosion and the later Great Ordovician Biodiversification Event.

The unusually high diversity found in this high-latitude ecosystem may be explained by the extremely warm global climate of the Early Ordovician. At that time, tropical oceans at lower latitudes may have reached temperatures exceeding 45°C. Under such conditions, cooler polar waters would have offered comparatively less stressful habitats. Many marine organisms may therefore have migrated southward through ocean circulation toward Antarctic regions, producing a distinctive and diverse ecosystem.

Later in the Ordovician, global temperatures declined. As climates cooled, ecosystems similar to the Cabrières Biota began to appear in lower-latitude regions as well.

The Cabrières Biota therefore provides a rare glimpse into marine life near the Ordovician South Pole. Its rich and varied fossil assemblage offers valuable evidence for understanding biodiversity patterns in high-latitude environments and contributes important data for studying the ecological and evolutionary processes that shaped the Ordovician radiation of life.

Author: Shui-Ye You

Reference:

Saleh, F. et al. (2024). The Cabrières Biota (France) provides insights into Ordovician polar ecosystems. Nature ecology & evolution.

（Paid content. Unauthorized reproduction or use is prohibited.）