Hexactinellid Sponges from the Early Ordovician Cabrières Biota, France: Insights into Early Paleozoic Polar Ecosystems

The Cabrières Biota of the Landeyran Formation in southern Montagne Noire, France is one of the key Early Ordovician faunas, as it represents a Lagerstätte close to the Ordovician South Pole. Sponges are a major component of this biota, despite their relatively lower abundance in comparison to algae and arthropods. Four distinct morphotypes of hexactinellids are recognized, including two types of reticulosan sponges (hintzespongiids and protospongiids) along with root tufts attributable to two distinct hexactinellid morphotypes. These hexactinellids appear to combine both Cambrian and late Palaeozoic sponge characteristics, providing a link between these two evolutionary lineages. From an ecological perspective, the long and flexible root tufts in the Cabrières Biota indicate a high-energy hydrodynamic environment, where hexactinellid sponges could regulate their osculum and canal systems to optimize nutrient filtration. Additionally, these root tufts likely allowed the sponges to anchor to soft substrates lacking hard surfaces, consistent with sedimentological evidence placing the biota in a distal mid-shelf soft-bottom environment. This distinctive sponge fauna differs markedly from its coeval tropical and temperate counterparts, demonstrating that water depth and local environmental conditions strongly shaped sponge faunal composition, and highlighting a previously unrecognized ecological complexity. It thereby provides a unique opportunity to investigate the Ordovician polar ecosystem through sponges.