In the Chinese fossil, scientists have discovered a primitive animal - the ancient ancestor of chordates, echinoderms and other deuterostomes.
A microscopic fossil discovered by Chinese and European paleontologists is dated to about 540 million years ago and identified as Saccorhytus coronarius, the most primitive Deuterostome. This multicellular animal is difficult to see with the naked eye, its dimensions do not exceed a millimeter. However, under the microscope, Hai Huang and his colleagues were exposed to many beautifully preserved and fine details.
It is worth explaining that today all chordates belong to the group of deuterostomes, including you and me: in the course of embryonic development, our primary mouth turns into the anus, and the oral one appears later and at the opposite end of the embryo. In the Protostomes, the mouth does not "move" anywhere; these include worms, molluscs, arthropods and other modern animals. The oldest deuterostomes and described by Hai Huang and his colleagues in an article published in the journal Nature.
The creature looked like a tiny bag with a huge mouth opening. Scientists speculate that he was covered with thin skin and already had a simple muscular system that allowed him to contract and move in the sea sand in search of food. At the same time, Saccorhytus did not yet have an anal opening, and he got rid of waste, apparently, expelling them through the same mouth.
Photo: Jian Han et al., 2017
"All deuterostomes have a common ancestor," said one of the authors of the find, Cambridge researcher Simon Morris, "and we think that this is what we have in front of us." The conical tubercles on the body could have been the primordia of the gills that would develop in fish many years later. Until this momentous moment, not much remained from the time of Saccorhytus: between 520 and 510 million years ago, the ancestors of the main modern groups of Deuterostomes, including sea stars and chordates, already appeared.
Sergey Vasiliev
Promotional video: