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The Strange Saccorhytus

October 17, 2022

Although its recent offering have been disappointing, Star Trek was a cultural phenomenon for many decades starting with the original series in 1966. Among its many plot innovations was the transporter device that was introduced as a means to eliminate the expense of always needing to land a spacecraft on a planet.

The transporter quickly evolved into a deus ex machina plot element to beam people to and from covert and secluded locations and away from trouble. One Star Trek anecdote I remember involves the design of the outdoor scenery on alien planets. One director, after seeing that the vegetation seemed too normal, plucked a small tree out of the ground, placed it upside-down with the root system on top, and explained that alien plants should look more like that.

There's a fanciful variety of alien creatures depicted in science fiction films and television, but there are many creatures on Earth which have an alien appearance. The reason for this is that evolution has adapted life's form and function to Earth's many ecological niches. Examples include the Remipedia, colorless and blind aquatic crustaceans that live in dark caves, and the giant tube worms (Riftia pachyptila) that live in the hydrogen sulfide environment of deep sea hydrothermal vents. One especially alien creature I first encountered in one of my childhood Golden Books[1] is the humpback anglerfish (Melanocetus johnsonii), as shown below.

Humpback anglerfish (Melanocetus johnsonii)

A female humpback anglerfish (Melanocetus johnsonii).

This fish is a species of the black seadevils. It exists at ocean depths of about a kilometer, at which depth there is very little light. Females of this species are 5-10 times larger than males, and they possess the bulbous bioluminescent lure to attract prey in its low-light environment.

Both males and female have sharp teeth. The largest females are only six inches in length; so, they're not as dangerous as they look.

Wikimedia Commons image by August Brauer (1863–1917) from Ref. 2.[2]

A huge international team of palaeontologists, paleobiologists, and scientists from other specialties have just published an extremely detailed study on fossils of an ancient sea creature, the Saccorhytus coronarius, a creature as strange as any imagined alien organism (see figure).[3-4] Team members were from Chang'an University (Xi'an, China), the University of Bristol (Bristol, UK), the Chinese Academy of Sciences (Nanjing, China), the Chinese Academy of Geological Sciences (Beijing, China), the Shandong University of Science and Technology (Qingdao, China), the Freie Universität Berlin (Berlin, Germany), the Ministry of Natural Resource (Qingdao, China), the Paul Scherrer Institut (Villigen, Switzerland), and Virginia Tech (Blacksburg, Virginia). The preponderance of Chinese team members is a consequence of the 500 million year old fossils having been found in China.[3-4]

Saccorhytus coronarius (University of Bristol)

Saccorhytus coronarius. Left (anterior), middle (side), and right (dorsal) images from the University of Bristol and used with permission. (Click for larger image.)

The Saccorhytus, a microscopic animal from the early Cambrian, was thought to have been an early member of the deuterostomes, which are animals with the anus forming in the embryo before the mouth.[3] This classification was derived principally because of the low quality of Saccorhytus fossils.[3] This would have placed these animals on the same portion of the evolutionary family tree as humans.[4] This new fossil analysis shows that Saccorhytus had a terminal mouth, but no anus, and spiny armor.[3] Saccorhytus coronarius is now classified as a member of the Ecdysozoa, such as insects, nematodes, and crustaceans.[3]

Holes around its large mouth were interpreted as pores for gills, but this new analysis shows that the holes around the mouth were actually the bases of spines that broke away from the fossils.[4] The new fossil specimens are much more representative. As Yunhuan Liu, a professor of paleobiology at Chang'an University, remarks, "Some of the fossils are so perfectly preserved that they look almost alive... Saccorhytus was a curious beast, with a mouth but no anus, and rings of complex spines around its mouth."[4]

The analysis of the internal and external features of these microscopic fossils was done using a synchrotron Xray source to take detailed images.[4] Images at various angles were processed by a supercomputer to create a three-dimensional digital model of the fossils.[4] The digital model showed that the pores around the mouth were actually spines that might have helped the Saccorhytus to capture and process its prey.[4]

Says palaeontologist Philip Donoghue, co-principal investigator of the study and a professor of paleobiology at the University of Bristol,
"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... 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."[4]
The final question on every reader's mind is, "How can an animal function without an anus?" The simple answer is that the mouth serves as an intake orifice for food, and as an exit orifice for digestive waste.[4] As Shuhai Xiao, a professor of geobiology at Virginia Tech and the other co-principal investigator of the study, remarks,
"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... "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."[4]


  1. Bertha Morris Parker, "The Golden Book of Science for Boys and Girls (A Giant Golden Book)," Simon and Schuster, January 1, 1956, 97 pp. (via Amazon).
  2. August Brauer, "Die Tiefsee-Fische. I. Systematischer Teil," Wissenschaftliche Ergebnisse der Deutschen Tiefsee-Expedition auf dem Dampfer "Valdivia" 1898-1899, 1908.
  3. Yunhuan Liu, Emily Carlisle, Huaqiao Zhang, Ben Yang, Michael Steiner, Tiequan Shao, Baichuan Duan, Federica Marone, Shuhai Xiao, and Philip C. J. Donoghue, "Saccorhytus is an early ecdysozoan and not the earliest deuterostome," Nature (August 17, 2022), https://doi.org/10.1038/s41586-022-05107-z.
  4. Scientists relieved to discover ‘curious’ creature with no anus is not earliest human ancestor, Bristol University Press Release, August 17, 2022.
  5. Supplementary Video no. 2 for ref. 3, Three-dimensional animation showing the general morphology of S. coronarius, Nature Website.
  6. Tomographic data at the University of Bristol data repository.

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