Newly discovered "sponge" evokes a "lost era" in early evolution

Sponges are generally considered to be the most primitive group in the animal kingdom. Molecular clock studies speculate that they originated about 700 million years ago. However, sponge fossils before the Cambrian period are extremely rare and controversial. Recently, an international collaborative team of early life researchers led by Yuan Xunlai, a researcher at the Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, 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 the British journal Nature on June 5, 2024.

There is a 160-million-year gap in the fossil record

Sponges are often considered 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. Modern molecular biology research and comprehensive molecular clock speculation indicate that the origin and differentiation of sponges should be about 700 million years ago. However, the exact sponge fossil record did not appear in large numbers until the Cambrian period, which began about 539 million years ago. The sponge fossil record before the Cambrian period is very scarce and mostly controversial. There is a gap of 160 million years in the sponge fossil record. This confusing period of history is called the "lost years" in the early evolution of sponges.

Wan Bin, an associate researcher at the Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, introduced that there are two main hypotheses to explain the lack of Precambrian sponge fossil records: first, because most living sponges have siliceous or calcareous spicules, the researchers assume that the common ancestor of sponges had mineralized spicules, and the lack of sponge fossils in the Ediacaran period before the Cambrian period (about 635 to 539 million years ago) may be because 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.

A new type of sponge fossil discovered

In recent years, Yuan Xunlai's team has conducted large-scale fossil excavations of the Shibantan Biota and discovered a new type of large sponge fossils, named "spiral grid sponges". The discovery of spiral grid sponges fills the gap in the early evolution of sponges and provides key fossil evidence of early sponges. This achievement was jointly completed by an international cooperation team composed of scientists from the Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, the University of Cambridge, and Virginia Tech.

The discovery of the spiral grid sponge fossils is another important achievement of the research team's long-term arduous excavation and systematic research on the Shibantan Biota. The thin limestone in the Shibantan Biota production section not only preserves a rich variety of Ediacaran fossils, but also a large number of animal trace fossils and solid fossils of other animals. For example, the earliest animal footprint fossils were published in the journal Science Advances in 2018; the earliest bilaterally symmetrical animal with mobility, Yiling worm, was published in the journal Nature in 2019.

Early sponges may not have biomineralized spicules

Yuan Xunlai introduced that the newly discovered spiral grid sponge has morphological characteristics similar to those of the six-release sponge: such as radially symmetrical conical body, disc-shaped fixed structure at the bottom, possible central cavity and inferred water outlet. In addition, the surface of the spiral grid sponge is composed of regular squares, which can be subdivided into four secondary squares with the same morphology, and the secondary squares can be further subdivided. This special square structure also exists in some typical Paleozoic six-release sponges. Their morphology and structure are very similar, the difference is that the grid of the spiral grid sponge is composed of organic matter, while the Paleozoic sponge fossils are composed of mineralized spicules. This study believes that the spiral grid sponge fossil may represent an early sponge animal without mineralized spicules. To further test this explanation, the research team constructed a morphological data matrix containing various living animals and fossil animals, and conducted a rigorous phylogenetic analysis. The results showed that the spiral grid sponge fossil belongs to the crown group sponge and is closely related to the six-release sponge.

The discovery of spiral mesh sponge fossils indicates that non-biomineralized sponges did exist in the Precambrian. This discovery suggests that researchers 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. This discovery also shows that in the early evolution of hexapod sponges, there may have been a stage in which organic matter was used to construct 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.

The appearance of the biomes before and after the Precambrian-Cambrian boundary was once thought to be very different. The Ediacaran period was dominated by the Ediacaran biota, which had strange morphology and unknown affinities; and 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 shows that the Ediacaran biota may have some direct evolutionary relationship with modern animals. An industry expert commented: The spiral grid sponge may be the "Rosetta Stone" for understanding animal evolution (a famous stone tablet of the Ptolemaic dynasty in ancient Egypt, which interprets the meaning and structure of Egyptian hieroglyphs that have been lost for more than a thousand years, and has become an important milestone in the study of ancient Egyptian history).

Reporter/Tao Tao Correspondent/Sheng Jie