Latest Oligocene–earliest Miocene Australasian freshwater sponges (Demospongiae: Spongillida) from a maar paleolake in southern New Zealand

Abstract

The evolutionary history of freshwater sponges (Porifera: Spongillida) in Australasia is poorly understood due to a paucity of fossils. A new genus and new species, Protooncosclera zealandiae n. gen. n. sp., family Potamolepidae, was discovered in southern New Zealand from lacustrine diatomites/spiculites of latest Oligocene–earliest Miocene of the Fossil-Lagerstätte at Foulden Maar. The fossil spicular complement is similar to that of the extant genus Oncosclera but differs from that and all other Spongillida genera by possessing a structured gemmular architecture armed by ornamented strongyles and strongyloxeas, with theca surrounded by a spicular cage of slender acanthoxeas, and a skeletal spicular complement of stout, smooth to spiny oxeas. This is the first fossil (pre-Quaternary) record of freshwater sponges from Australasia and fits into the Gondwana-like distribution of potamolepid freshwater sponges. Its discovery in a subtropical maar lake on the southwestern margin of Zealandia confirms a formerly wider geographic distribution of Potamolepidae in the Cenozoic, followed by range retractions related to post-Early Miocene climate cooling. The stratigraphic distribution of sponge remains at Foulden Maar demonstrates that sponges colonized the isolated maar lake soon after its formation, most likely by passive dispersal by water birds, and then thrived in the shallow water margins of the paleolake for ca. 130,000 years. Sponge remains, skeletal spicules and gemmules, frequently associated with coprolites indicate that sponges were consumed by one or more spongivorous taxa, presumably fish belonging to the Southern Hemisphere family Galaxiidae.