Sponge-Like Animals May Have Lived in Oceans 890 Million Years Ago
Paleontologists have found possible sponge body fossils in 890-million-year-old microbial reefs in northwestern Canada. If verified, they may pre-date the next-oldest undisputed sponge fossils by around 350 million years.
Genetic evidence from modern sponges suggests that sponges emerged during the Neoproterozoic era, between one billion and 541 million years ago.
However, fossilized sponge bodies from this period have been lacking.
“The search for definitive physical evidence of pre-Cryogenian multicellular animals is confounded by uncertainty about what to look for, but preserved physical evidence should be small, subtle and possibly altogether unfamiliar,” said Professor Elizabeth Turner, a paleontologist in the Harquail School of Earth Sciences at Laurentian University.
“Given that sponges are the most basic of known animals, physical evidence of Neoproterozoic sponges could be sought, but effort focused on the characteristics of mineralized sponge skeletons overlooks sponges with only spongin or keratin skeletons.”
“Early evidence of multicellular animals might instead resemble preservational products of sponge soft tissue rather than mineralized sponge skeletal components.”
“Although molecular clock data suggest that sponges emerged in the early Neoproterozoic, the oldest undisputed sponge body fossils are from the Cambrian period.”
In the new research, Professor Turner examined rock samples extracted from Little Dal reefs in northwestern Canada.
The reefs, which are part of the Stone Knife Formation, were built by calcifying cyanobacteria 890 million years ago.
Within the samples, the researcher identified branching networks of millimetric-to-centimetric tube-shaped structures that contained, and were surrounded by, crystals of calcite.
These structures closely resemble the fibrous skeleton found within demosponges and structures previously identified in calcium carbonate rocks that are thought to have been created by the decay of demosponge bodies.
Professor Turner proposes that the Little Dal structures may be the fossilized remains of demosponges that lived on, in and beside calcium carbonate reefs approximately 90 million years before Earth’s oxygen levels increased to levels thought to be necessary to support animal life.
If the structures are accepted as sponge body fossils, the findings could imply that the evolution of early animals occurred independently of this oxygenation event and that early animal life survived severe ice ages that occurred between 720 and 635 million years ago.
“If the vermiform-microstructured masses in the Little Dal reefs are accepted as early sponge body fossils, their approximately 890 million-year age would imply that the evolutionary emergence of metazoans was decoupled from the Neoproterozoic oxygenation event and early animal life was not catastrophically affected by the Neoproterozoic glacial episodes,” Professor Turner said.
“If the Little Dal objects are truly sponge body fossils, they are older than the next-youngest undisputed sponge body fossils by approximately 350 million years.”
The findings were published in the journal Nature.
E.C. Turner. Possible poriferan body fossils in early Neoproterozoic microbial reefs. Nature, published July 28, 2021; doi: 10.1038/s41586-021-03773-z