Oldest digestive tracts/guts

The world's oldest known digestive tracts (i.e. guts) are a series of 550-million-year-old fossils, belonging to an extremely early group of marine animals known as cloudinids. Dating back to the late Ediacaran Period, about 10 million years before the Cambrian Period began, these small but highly-significant cylindrical anatomical remains were detected inside the tubular shells of cloudinid fossils found in the Wood Canyon Formation, Nevada, USA, and formally documented in January 2020. With a total length not exceeding 15 cm, cloudinids are among the earliest shelly life forms known to have existed on Earth.

These fossilised guts are the first recognisable soft tissues recorded for cloudinids, which are otherwise known only from their external shells. These resemble a vertically-stacked tubular series of calcareous cones, each cone with its apex inserted inside the cone directly below it, and are very abundant in the fossil record. The taxonomic affinities of cloudinids to other animal groups are unclear, although some researchers have proposed that they may constitute stem-group polychaete worms, and there are no modern-day representatives.