Huge Mouth, No Anus: Meet One of Our Earliest Ancestors

SCIENCE

You won’t find it in your family album, but a tiny prehistoric creature with a bag-like body, a huge mouth and no anus has become the best candidate yet for our earliest known relative. (The Guardian and Christian Science Monitor)

Where does this mouthy mystery fit in the circle of life?

Teachers, scroll down for a quick list of key resources in our Teachers Toolkit.

Meet Saccorhytus coronarius—literally, “wrinkled sack.” Don’t worry, it’s only four millimeters long and lived 540 million years ago. Yes, that gaping maw is a mouth, but those pores aren’t eyes and the wrinkles around its mouth are not teeth. Illustration courtesy Jian Han, Simon Conway Morris, Qiang Ou, Degan Shu, and Hai Huang. “Meiofaunal deuterostomes from the basal Cambrian of Shaanxi (China),” Nature (2017) doi:10.1038/nature21072

Meet Saccorhytus coronarius—literally, “wrinkled sack.” Don’t worry, it’s only four millimeters long and lived 540 million years ago. Yes, that gaping maw is a mouth, but those pores aren’t eyes and the wrinkles around its mouth are not teeth.
Illustration courtesy Jian Han, Simon Conway Morris, Qiang Ou, Degan Shu, and Hai Huang. “Meiofaunal deuterostomes from the basal Cambrian of Shaanxi (China),” Nature (2017) doi:10.1038/nature21072

Discussion Ideas

  • Saccorhytus coronarius, the microfossil discovered in China, is a deuterostome. What is a deuterostome?
    • Deuterostomes are a “superphylum” of animals. Deuterostome means “mouth second” and refers to a major division (ha!) of embryonic development. “In the earliest stages of embryo development, when there are only a few cells and the embryo resembles a tiny globe of cells, a small pucker develops on one side of the embryo. This grows into a pocket, and allows some cells to migrate inside to form an additional layer of cells within the outer layer … In the Deuterostomia … the pocket edge develops into the anus, and the mouth is formed later.” (Among protostomes, the other major “superphylum”, the embryonic pucker forms the mouth first.)
      • Yes, deuterostomes are defined by the development of an anus, and this animal does not (we think) have an anus. Because science. (And embryonic development does not necessarily mirror mature specimens.)
    • Deuterostomes include two great big phyla of animals, the chordates and the echinoderms, and many smaller groups.
      • Chordates include all vertebrates—like us!
      • Echinoderms include sea stars and sea urchins. In fact, one of the early questions scientists asked about Saccorhytus was: “was this a very early echinoderm, or something even more primitive? The latter now seems to be the correct answer.” (Exciting!)

 

  • Where did the little Saccorhytus live?
    • Saccorhytus was part of the meiofauna of the ancient, shallow sea of what is now central China (the province of Shaanxi). Meiofauna are small invertebrates that live at the bottom of seawater or freshwater habitats.

 

  • Saccorhytus doesn’t look a lot like ancestors we’re familiar with, like our pal Homo naledi or the other hominins. In fact, it doesn’t even look like our deuterostome cousins, the sea stars. What characteristics do we share with this possible common relative?
    • All deuterostomes have bilateral body symmetry. This means the left and right sides of the animal’s embryo mirror each other.
    • Saccorhytus has “radial folds” around its mouth. Scientists think these folds might have allowed Saccorhytus to swallow prey. (And, yes, probably expel waste through the same orifice.)
    • Saccorhytus was covered in a thin, flexible skin. This indicates a sort of musculature beneath, and the ability to move, probably by wiggling.
    • Saccorhytus has weird conical structures on its body that may, possibly, be precursors to gill slits and gills. (Fish, like all vertebrates, are deuterostomes.)
      • No, humans do not have gills. But human embryos, like all chordate embryos, “go through a stage where they have slits and arches in their necks like the gill slits and gill arches of fish. These structures are not gills and do not develop into gills … but the fact that they are so similar to gill structures in fish at this point in development supports the idea that humans share a common ancestor with fish.”

 

  • How did scientists find Saccorhytus?
    • Microfossils like Saccorhytus are generally not preserved very well, so it took a lot of work! To find the 45 tiny specimens, paleontologists had to sift through three tons of limestone.
      • First, scientists treated the limestone with an acetic acid solution, which dissolved the stone but not the fossils.
      • Then, scientists looked at the fossils with regular binocular microscopes.
      • Then, they coated the fossils with gold (yes, gold) and imaged them under a scanning electron microscope. “For a single microfossil, [scientists] acquired 990 successive pictures each with a resolution of 1,013 × 1,013 pixels, and each pixel was about 1 μm.”
      • The internal anatomy of Saccorhytus was examined using microcomputer tomography.

 

  • So Saccorhytus is about 540 million years old, and may be our oldest deuterostome fossil relative. Is Saccorhytus the oldest animal fossil ever found?

 

TEACHERS TOOLKIT

The Guardian: A huge mouth and no anus – this could be our earliest known ancestor

Christian Science Monitor: This wrinkled old bag might be humans’ earliest known ancestor

Nat Geo: Circle of Life study guide

(extra credit—and this is a toughie!) Nature: Meiofaunal deuterostomes from the basal Cambrian of Shaanxi (China)

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