Far beyond human imagination! More than 500 million years ago, "sponges" swam on the seabed?

□ Feng Weimin

Sponges are the most primitive multicellular metazoans in the animal kingdom, with no real tissues and organs, only cell differentiation. Recently, an international research team led by the Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, published a paper in the international scientific journal Nature, pointing out that they have discovered sponge fossils dating back about 550 million years ago, pushing the sponge fossil record that originally appeared in the Cambrian period (539 million to 485 million years ago) about 539 million years ago to the end of the Precambrian period (4.6 billion to 539 million years ago), providing important evidence for uncovering the mystery of the origin of sponges and the evolution of early sponges.

Sponges have a long evolution and abundant fossils, but they are mainly found in strata from the Phanerozoic (539 million years ago to the present) and are preserved in the form of spicules. Spicules are equivalent to skeletons, and there are glass fiber-like siliceous spicules and calcium spicules. Since the discovery of the Weng'an Biota in Guizhou, sponge spicule fossils from the late Precambrian have been reported from time to time, but they have all been denied by subsequent studies. So, did Precambrian sponges really exist? Molecular biology research and comprehensive molecular clock speculation show that the origin and differentiation of sponges should be about 700 million years ago. The fossil record of sponges before the Cambrian is very scarce and mostly controversial.

Organic chemistry research also reveals that sponges have survived longer than imagined. In 2018, the international scientific journal Nature Ecology & Evolution reported the discovery of 26-methylstigmasterane steroid compounds in rocks in Oman and other places, which are compounds that can only be synthesized by sponges. Based on this, biomarkers show that sponges existed on the seafloor as early as 660 million to 635 million years ago.

In 2021, a paper published in Nature showed that Canadian geologist Elizabeth Turner found traces of sponge fossils dating back 890 million years in the Mackenzie Mountains in the Northwest Territories of Canada. When observing dark stromatolites under a microscope, Elizabeth Turner unexpectedly found a large number of interlaced white dendritic lines. This is a special protein secreted by sponge cells, called spongin. It is still possible that it is gradually replaced by calcium minerals during the disappearance and burial process of sponge bones, and finally presents a hollow tubular structure.

In my country, a 2mm-sized fossil of the Guizhou Proto-cup sponge dating back about 600 million years was discovered. The discovery of a macroscopic sponge fossil dating back about 550 million years further proves the existence of sponges in the Precambrian period.
Biomineralization studies have shown that non-biological mineralization skeletons appeared in the Precambrian, which is the early stage of biomineralization. A large number of organic tubular casing fossils found in Ningqiang, Shaanxi, my country are considered to be the primary products of biological skeletonization. These casing fossils often have round holes on their surfaces left by other organisms, so one of the causes of skeletonization seems to be related to biological predation.

The newly discovered spiral grid sponges have similar morphological characteristics to hexaphyllous sponges. For example, the radially symmetrical conical body surface is composed of regular squares, which can be subdivided into secondary squares, which can be further subdivided into smaller squares. This special structure is extremely similar to the Paleozoic hexaphyllous sponges in morphology and structure, but the spiral grid of the former is organic matter, while the latter is composed of mineralized spicules. This reflects that sponges have different biomineralization characteristics in the two different geological stages of the Precambrian and the Phanerozoic.

The discovery of spiral mesh sponge fossils indicates that non-biomineralized sponges did exist in the Precambrian period, but they did not have all the characteristics of modern sponges. The discovery also shows that in the early evolution of hexapod sponges, there may have been a stage in which organic matter was used to build a mesh skeleton, and it was not until the Cambrian period that they acquired the ability to biomineralize, adding minerals to the existing organic skeleton to form a composite skeleton composed of mineralized spicules.

Sponges are very primitive. They are not sensitive to temperature changes and oxygen levels. They can filter water regularly and absorb nutrients from dead organisms. Therefore, after experiencing disasters on Earth, sponges can not only serve as the vanguard of biological recovery, but also take on the important task of post-disaster marine ecological restoration. At the same time, sponges were the first members to participate in the construction of metazoan reefs in the late Precambrian period, and they played a leading role in the construction of metazoan reefs in the Phanerozoic Eon.

The newly discovered sponge fossils further clarify its status and role in the Precambrian biological world, and show its evolutionary process in two stages from the perspective of biomineralization.

(The author is a researcher at the Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, and the honorary curator of the Nanjing Paleontological Museum)