A recent sponge, Pleroma aotea Kelly ("Order” Lithistida: Family Pleromidae), in the late Eocene Ototara Limestone of Otago, New Zealand

Abstract

Numerous remarkably well‐preserved lithistid sponges recovered from the late Eocene‐early Oligocene Ototara Limestone at Kakanui, North Otago, represent the first sponge body fossils to be described from the New Zealand Cainozoic. The sponges are scattered throughout a 1–3‐m‐thick volcaniclastic limestone horizon immediately overlying the Kakanui Mineral Breccia. The fossils are now solid calcite, the former siliceous skeleton having been replaced during diagenesis. The Kakanui sponges are the only known body fossils remaining from an extremely diverse sponge fauna which formed a major component of the benthos in the Kakanui‐Oamaru region in late Eocene‐early Oligocene times (c. 35–33 Ma). The sponge body fossils are described and compared with living sponges that have a similar flattened globose morphology. The fossils are morphologically indistinguishable from the living lithistid sponge Pleroma aotea Kelly ("Order” Lithistida: Family Pleromidae) from deep‐water seamounts and banks off north‐eastern New Zealand, and are significant in that they represent a stratigraphic range for the species of more than 35 million years. The present‐day distribution of P. aotea, limited to silica‐rich deeper waters, is in marked contrast to the relatively shallow warm water volcanic environments occupied during the Palaeogene. This restriction, and that of related lithistid sponges to silica‐rich deeper waters off northern New Zealand, is paralleled in other demosponges and several calcareous invertebrate groups such as barnacles, bryozoans, and crinoids.