They hammered away, hammer after hammer, and made a surprising discovery

The ecological restoration diagram of the spiral grid sponge. Drawing by Yang Dinghua

In the long process of evolution, sponges have luckily survived five mass extinctions on Earth and survived to this day. Research suggests that sponges originated about 700 million years ago, but sponge fossils before the Cambrian period, about 540 million years ago, are extremely rare, so it is controversial.

Why did sponges "disappear" for about 160 million years? An international team of early life researchers led by Yuan Xunlai, a researcher at the Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences (hereinafter referred to as Nanjing Institute of Paleontology), discovered a crown-group sponge from the late Ediacaran period in the Shibantan Biota, about 550 million years ago in Yichang, Hubei, filling an important link in the early evolution of sponges. The results were published online in Nature on June 5.

Fossils missing for about 160 million years

"Sponges are generally considered to be the most basic and primitive metazoans. The search and study of early sponge fossils on Earth can provide key evidence for our exploration of the origin of animals and early evolutionary patterns." Wan Bin, the paper's corresponding author and associate researcher at the Nanjing Institute of Geology and Paleontology, told China Science Daily.

Existing studies have shown that sponges originated about 700 million years ago. However, actual research has found that sponge fossil records only appeared in large numbers in the Cambrian period, with a gap of about 160 million years. This confusing period of history is called the "lost years" in the study of early sponge evolution.

Why is the Precambrian sponge fossil record missing?

Wan Bin explained, based on past research, that, first, since most living sponges have siliceous or calcareous spicules, the researchers assumed that the common ancestor of sponges had mineralized spicules, and the lack of Ediacaran sponge fossils, which existed between 635 million and 539 million years before the Cambrian period, may be due to the fact that the environment at that time was not conducive to the preservation of spicules; second, the common ancestor of sponges did not have mineralized spicules, and only after the main groups differentiated did each group independently evolve mineralized spicules. Therefore, the early sponges in the Precambrian period did not have spicules, had relatively small preservation potential, and were difficult to identify from the fossil record.

Early sponges did not have spicules

In recent years, Yuan Xunlai's team has conducted large-scale fossil excavations in the Shibantan Biota and discovered a type of large sponge fossil 5 years ago. Because the fossil surface has a multi-level grid structure, the grid at the bottom of the specimen is connected to the disc-shaped fixed structure at the bottom in a spiral arrangement, and it lives upright on the sediment surface, so it was named " spiral grid sponge (Helicolocellus)".

Wan Bin introduced that the surface of the newly discovered spiral grid sponge is composed of regular squares, which can be subdivided into 4 secondary squares of the same shape, and the secondary squares can be further subdivided. This special square structure of the spiral grid sponge is similar to the morphological characteristics of the Paleozoic hexapod sponge. The difference between them is that the grid of the spiral grid sponge is composed of organic matter, while the grid of the Paleozoic sponge fossil is composed of mineralized bone needles.

Yuan Xunlai's team, together with an international collaborative team consisting of researchers from the University of Cambridge in the UK and Virginia Tech in the United States, believe that the spiral grid sponge fossils may represent an early sponge animal that did not have mineralized spicules.

To prove this explanation, the research team constructed a morphological data matrix containing various living animals and Paleozoic fossil animals, and conducted a rigorous phylogenetic analysis. The results showed that the spiral grid sponge fossils belong to the crown group sponge and are closely related to the six-release sponge.

"The discovery of the spiral grid sponge fossils shows that non-biomineralized sponges did exist in the Precambrian period," said Wan Bin. " The discovery of this fossil also suggests that we cannot completely use modern sponges as a blueprint to find Precambrian sponge fossils, because early sponges may not have biomineralized spicules and may not have all the characteristics of modern sponges. "

The study also speculates that in the early evolution of hexapod sponges, there may have been a stage in which organic matter was used to construct a network skeleton. It was not until the Cambrian period that they acquired the ability of biomineralization, adding minerals to the existing organic skeleton to form a skeleton composed of complex mineralized bone needles.

Controversy drives discipline development

"You never know what new fossils will be found with the next hammer strike!" This is a phrase Yuan Xunlai often says. Fossils are the evidence of the evolution of life on Earth, and fossils of key evolutionary stages are even more rare. These fossils need to be hammered out of the stone by paleontologists one hammer at a time.

Yuan Xunlai's team conducted long and arduous excavations on the Shibantan Biota and discovered that the thin layers of limestone not only preserved rich and diverse Ediacaran fossils, but also a large number of animal trace fossils and physical fossils of other animals.

In 2018, Science Advances reported the earliest animal footprint fossils discovered by the team; in 2019, Nature reported the earliest bilaterally symmetrical animal with locomotion discovered by the team, Yilingworm. The discovery of the spiral grid sponge fossil is another important achievement of the team.

"This paper was submitted in July 2023 and was published after four rounds of review by five reviewers," Wan Bin told China Science Daily. "Our discovery is a challenge to the traditional theory of the early evolution of sponges. The process was difficult, but the result was relatively ideal. We finally convinced the reviewers with exquisite fossil specimens and rigorous arguments."

The academic community once believed that there were huge differences in the appearance of biological communities before and after the Precambrian-Cambrian boundary.

This is because the Ediacaran period was dominated by the Ediacaran biota, which had strange shapes and unknown affinities. In the Cambrian period, various living animal phyla emerged one after another, and the modern marine ecosystem familiar to people was established.

The discovery of the spiral grid sponge connects the Ediacaran biota and the Cambrian biota, and also suggests that some types of the Ediacaran biota may have some direct evolutionary relationship with modern animals.

This study found that sponges without mineralized spicules lived in the ocean 10 million years before the Cambrian period, which means that sponges without mineralized spicules had undergone a certain scale of radiation before the Cambrian period, that is, a species acquired a certain key feature, which led to the emergence of a large number of new features and species.

So, did other types of animals also radiate before the Cambrian period? What caused sponges to produce mineralized spicules? "Our research may trigger new controversies, and the resolution of these controversial issues is expected to promote the further development of the discipline," said Wan Bin.

Planning and production

Source: China Science Daily

Author: Shen Chunlei, China Science Daily reporter

Editor: Yang Yaping

Proofread by Xu Lai and Lin Lin