Sponges are curious creatures. They are considered to be animals as they are multicellular and some of them have fixed body shapes, however they show no cell differentiation, and can be broken down into individual cells (by, for example, forcing them through a sieve) and they will re-assemble themselves without apparent ill-effect. In some ways they are more like colonial protists than true animals. Biologists have long regarded them as the most primitive animal group, something that genetic studies have now confirmed; sponges are more closely related to animals than to anything else, but all other animals are more closely related to one-another than they are to sponges. Had the reverse happened, and genetics had revealed that sponges were not closely related to other animals, then it is unlikely that anyone would have lost any sleep over it. As the most primitive extant animal group, sponges are also expected to be the first, so palaeontologists looking for the earliest animals are often looking for sponges.
In a paper published in the South African Journal of Science on 18 January 2012, a team of scientists lead by Bob Brian of the Ditsong Museum in Pretoria describe what appears to be a primitive sponge from rocks up to 760 million years old in Namibia. Named as Otavia antiqua (the ancient one from Otavi), the fossils range from 0.3 mm to 5.0 mm in length, and are interpreted as calcium carbonate tests with a phosphatic lining, inside which a sponge, or sponge-like-organism, would have lived. The fossils were found in finely laminated limestones ranging from 760 to 550 million years old, suggesting the organism lived in fine, calcareous, muds (that would have been regularly disturbed by larger animals in more recent deposits), filtering the waters above for food. Some reworked tests have also been found in courser sediments, suggesting they were reasonably robust.
Scanning electron microscope images of Otavia antiquita. (a & b) from the ~635 million year old Auros Formation in the Etosha National Park, northern Namibia. (c) 760 million year old specimen from the upper part of the Ombombo Subgroup near Opuwa, north-western Namibia. (d) 548 million year old specimen from the Kuibis Subgroup near Kliphoek, southern Namibia. Scale bars are all 100 μm (0.1 mm). From Brian et al. (2012)
The oldest specimens of Otavia date from the middle of the Kaigas glaciation during the Cryogenian Period (850-635 million years ago), that predates the Ediacaran. Otavia apparently survived this and two subsequent glaciations during the Cryogenian, the Sturtian and the Marinoan. These Cryogenian Ice Ages are thought to have been near-global in extent, and were previously thought to have inhibited the development of the first animals, which were thought to have evolved in the subsequent Ediacaran. The Cryogenian is sometimes referred to as the Neoproterozoic Snowball Earth.
The geological periods of the Phanerozoic (Cambrian onwards) are reasonably familiar, but only cover the last 55o million years of the Earth's 4.5 billion year history. The earlier history of the Earth is also divided into (less familiar) geological periods, the last two of which are the Cryogenian and the Ediacaran. Image from Svenolov Lindgren's Botanical Micropaleontology Blog.
The subsequent Ediacaran Period (635-544 million years ago) gets its name from the Ediacaran Fauna, although these organisms are in fact restricted to the upper (later) part of the period. The exact nature of the Ediacaran Fauna is unclear; they may-or-may-not have been animals, but do not appear to be closely related to any living animal group. The Ediacaran Fauna are named for the Edicara Formation of South Australia, although they are global in distribution, and were documented from other sites before the discovery of the Australian fossils. Putative Sponges have been described from the Ediacaran before. The embryo fossils from the early Ediacaran Doushantuo Formation of southern China have been interpreted as Sponges (and just about everything else), the reef building Namapoikia rietoogensis of Namibia has been variously described as a Sponge or Cnidarian (the group of animals that includes Jellyfish, Corals and Sea-Anemones), and some organic chemicals found in the Huqf Supergroup of Oman are thought to be indicative of the presence of sponges.
The team did discuss the possibility that Otavia antiqua might be an amoebiod foraminiferan (single celled organisms not closely related to animals), which have previously been documented from the Cryogenian. However Otavia antiqua would be exceptionally large for a foraminiferan (giant foraminiferans are known from the Mesozoic, but there is no reason to expect them in the Cryogenian) and it is morphologically and compositionally unlike any known foraminiferan. This does not rule out a foraminiferan origin for for Otavia antiqua, but makes it a less likely explanation than it being a sponge. Neither are the team able to rule out the possibility that Otavia antiqua belongs to an extinct group of which we know nothing, but there is no more evidence to suggest this than to rule it out.
Reconstruction of Otavia antiqua as a sponge. From Brian et al. (2012)
Brian et al. suggest that because Otavia antiqua was so long lived a species (over 200 million years) then it is highly unlikely to have been endemic to Namibia, and predict that the fossil (or similar ones) could be found in other parts the world in future.