Sponges are considered to be the
sister group to all other groups of Animals, which is to say all other animals
are more closely related to one-another than they are to Sponges and all
Sponges are more closely related to one-another than to other Animals, but
Sponges and other Animals are more closely related to one-another than they are
to anything else. Phylogenetic studies have suggested that the common ancestor
of Sponges and other Animals lived in the deep Cryogenian (the geological period
that lasted from 850 to 635 million years ago) and a number of putative Sponge
fossils have been found from the Cryogenian and Ediacaran (635-540 million
years ago), which tends to support this hypothesis. However the simple
organization of Sponge bodies, which lack specific tissues and can reform if
squeezed through a sieve, makes it very hard to determine if these putative
Sponges are true members of the group, or ‘Sponge-grade organisms’, which may
be ancestral to Sponges, other Animals, both or neither.
In a paper published in the
Proceedings of the National Academy of Sciences of the United States of America
on 9 March 2015, Zongjun Yin and Maoyan Zhu of the State Key Laboratory of Palaeobiology and Stratigraphy of the Nanjing Institute of Geology and Palaeontology of the Chinese Academy of Sciences, Eric Davidson of the Division of Biology at the California Institute of Technology, David Bottjer of the
Department of Earth Sciences at the University of Southern California, Fangchen Zhao, also of the State Key Laboratory of Palaeobiology and Stratigraphy of the
Nanjing Institute of Geology and Palaeontology of the Chinese Academy of
Sciences, and Paul Tafforeau of the European Synchrotron Radiation Facility describe
a Sponge-like fossil from the Doushantuo Formation of Guizhou Province, China.
The Doushantuo Formation outcrops
across much of South China, and has produced a large number of spectacular
microfossils showing a remarkable level of cellular preservation, including fossils
interpreted as possible embryos, dating from between 635 and 551 million years
ago. Unlike other fossil sites producing such remarkable levels of
preservation, it is interpreted as having formed in a high-energy, wave
dominated environment, which led to the preservation of small biological
particles in phosphatised granules, but not the preservation of larger
structures such as body fossils of non-microscopic Animals. The possible Sponge
comes from a gray oolitic dolomitic phosphorite layer exposed at the Badoushan
Phosphorite Mining Quarry in Weng’an County in Central Guizhou, which is
interpreted to be about 600 million years old.
The fossil is named Eocyathispongia qiania, where ‘Eocyathispongia’ means
‘Dawn-cup-shaped-Sponge’ and ‘qiania’
is a term for Guizhou Province. It is approximately 1.2 mm by 1.1 mm, and
comprises three cup-shaped tubes emerging from a common base. These tubes have
numerous pore-like openings, which if this organism was biologically similar to
a modern Sponge would be inflow channels through which the filter-feeding
Animal drew water prior to expelling it through the larger openings at the end
of each tube. The outer surface of the fossil is covered by flattened cells 8–12
μm in diameter; these are split into two classes, with oval cells being
consistently larger than circular ones, implying that cellular differentiation
was present (something found in all Animals, including Sponges, but not usually
in colonies of single-celled organisms).
Scanning electron micrograph of Eocyathispongia
qiania showing the main tubular chamber with a large opening and additional
chambers viewed from the exterior. Yin et
al. (2015).
Based upon the available data,
Yin et al. interpret Eocyathispongia qiania as being a genuine
Sponge, albeit one that lived prior to the differentiation of the group into
its modern orders. The cellular structure of the fossil resembles modern
Sponges, though it does not show preserved choanocyte cells (flagella bearing
cells which drive the movement of water through the Sponge) would be the best
indicator of a true Sponge, it does have a structure that would be difficult to
interpret in any other way, and dates from approximately the same time as the
earliest Doushantuo specimens interpreted as Cnidarians (the group that
includes modern Jellyfish, Corals and Sea Anemones) and Bilaterians (all
animals other than Sponges, Cnidarians and Ctenaphores – Comb Jellies).
See also…
An enigmatic animal from the Australian continental shelf, with possible similarities to some members of the Ediacaran Fauna.
The Ediacaran Fauna comprises a group of fossils from the Late Ediacaran Period, found at sites around the world and pre-dating the Cambrian Explosion, which is considered to indicate the origin of the majority of modern animal groups, and in particular those with mineralized skeletons. Some biologists have suggested that these organisms represent an entirely separate experiment...
The fossils of the Ediacaran Period record the first widespread macrofossils in the rock-record. Many of these fossils do not appear to belong to any modern group, but instead are thought to belong to an extinct taxa (sometimes known as ‘Vendobionts’), which may-or-may-not be related to modern Animals, though some fossils have been linked to Sponges (a group which also has...
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...
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