500-million-year-old creature with 'no anus' is not the earliest human ancestor, research finds

Saccorhytus is not our grandpa anymore.
Nergis Firtina

Philip Donoghue et al. 

Scientists from Bristol University have solved a mystery of a 500 million-year-old microscopic creature with a mouth but no anus. The study reveals that the spiny creature is not the earliest human ancestor, after all.

This creature - called Saccorhytus - was first discovered in 2017. The study found that a wrinkly sack with a vast mouth entwined by spines and holes is a primitive feature of the deuterostome group from which our ancestors emerged.

“Some of the fossils are so perfectly preserved that they look almost alive,” shares Yunhuan Liu, professor in Palaeobiology at Chang’an University, Xi’an, China. “Saccorhytus was a curious beast, with a mouth but no anus and rings of complex spines around its mouth.”

However, the new study suggests that Saccorhytus should be put into an entirely different group of animals.

500-million-year-old creature with 'no anus' is not the earliest human ancestor, research finds
Saccorhytus dorsal.

What is the true story of Saccorhytus?

The true story of Saccorhytus’ ancestry lies in this tiny fossil's microscopic internal and external features. With the help of powerful computers, a detailed 3D digital fossil model could be reconstructed by taking hundreds of X-ray images at slightly different angles.

“Fossils can be quite difficult to interpret, and Saccorhytus is no exception. We had to use a synchrotron, a type of particle accelerator, as the basis for our analysis of the fossils. The synchrotron provides very intense X-Rays that can be used to take detailed images of the fossils. We took hundreds of X-Ray images at slightly different angles and used a supercomputer to create a 3D digital model of the fossils, which reveals the tiny features of its internal and external structures," explains researcher Emily Carlisle from the University of Bristol’s School of Earth Sciences.

In digital models, the study revealed another layer enlarged by the mouth. "We believe these will help Saccorhytus capture and process its prey," said Huaqiao Zhang of the Nanjing Institute of Geology and Paleontology.

Saccorhytus is an ecdysozoan

The researchers believe that Saccorhytus is an ecdysoszoan, a group that includes arthropods and nematodes.

“We considered lots of alternative groups that Saccorhytus might be related to, including the corals, anemones, and jellyfish, which also have a mouth but no anus,” said Prof Philip Donoghue of the University of Bristol’s School of Earth Sciences, who co-led the study.

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“To resolve the problem, our computational analysis compared the anatomy of Saccorhytus with all other living groups of animals, concluding a relationship with the arthropods and their kin, the group to which insects, crabs and roundworms belong.”

In addition to all the statements made by scientists, "It's a bit confusing - [most] ecdysozoans have an anus, so why didn't this one?" also said researcher Emily Carlisle to BBC Radio 4's Inside Science.

What is Saccorhytus?

Saccorhytus is an ancient creature. It is believed that it lived approximately 540 million years ago in the Fortunian stage of the Cambrian Period. If confirmed as a deuterostome, the saccorhytus would represent the oldest known species of this superphylum. Saccorhytus’ lack of anus is also an intriguing feature.

Although there is a question mark on minds about how the digestive system works, Saccorhytus is very important for evolutionary biology.

“This is a really unexpected result because the arthropod group have a through-gut, extending from mouth to anus. Saccorhytus’s membership of the group indicates that it has regressed in evolutionary terms, dispensing with the anus its ancestors would have inherited,” says Shuhai Xiao from Virgina Tech, USA, who co-led the study. “We still don’t know the precise position of Saccorhytus within the tree of life, but it may reflect the ancestral condition from which all members of this diverse group evolved.”

The international team included researchers from the University of Bristol, Chang’an University (Xi’an, China), Nanjing Institute of Geology and Palaeontology (China), Chinese Academy of Sciences (Nanjing, China), Chinese Academy of Geological Sciences (Beijing, China), Shandong University (Qingdao, China), Swiss Light Source, Virginia Tech (USA) and First Institute of Oceanography, Ministry of Natural Resource (Qingdao, China).

Study abstract

The early history of deuterostomes, the group composed of the chordates, echinoderms and hemichordates, is still controversial, not least because of a paucity of stem representatives of these clades. The early Cambrian microscopic animal Saccorhytus coronarius was interpreted as an early deuterostome on the basis of purported pharyngeal openings, providing evidence for a meiofaunal ancestry and an explanation for the temporal mismatch between palaeontological and molecular clock timescales of animal evolution. Here we report new material of S. coronarius, which is reconstructed as a millimetric and ellipsoidal meiobenthic animal with spinose armour and a terminal mouth but no anus. Purported pharyngeal openings in support of the deuterostome hypothesis are shown to be taphonomic artefacts. Phylogenetic analyses indicate that S. coronarius belongs to total-group Ecdysozoa, expanding the morphological disparity and ecological diversity of early Cambrian ecdysozoans.