Saccorhytus

Saccorhytus (from Latin saccus "bag" and Ancient Greek ῥύτις rhytis "wrinkle") is an extinct genus of animal possibly belonging to the superphylum Ecdysozoa, and it is represented by a single species, Saccorhytus coronarius (from Latin attributive coronarius "[of a] crown"). The organism lived approximately 540 million years ago in the beginning of the Cambrian period. Initially proposed as a deuterostome, which would make it the oldest known species of this superphylum, it has since been determined to belong to a protostome group called the ecdysozoans.

Fossils of the species were first discovered in the Kuanchuanpu Formation of Shaanxi province of China by a team of scientists from the United Kingdom, China and Germany, and the findings were first published in January 2017.

Description


Saccorhytus was only about a millimetre (1.3 mm ) in size and is characterised by its globular or hemispherical body with a prominent mouth. Its body was covered by a thick but flexible cuticle. Surrounding its mouth, itself lined with integumental folds, is a ring of tridentate protuberances. Above this ring is another set of tridentate spines, which vary in number between one and five, possibly reflecting ontogenetic or intraspecific differences. It had three nodulate ridges above its mouth. Below its mouth were three pairs of sclerotized spines, termed "body cones", with the distal pair being the largest. A further five pairs of these spines are found elsewhere on the body, with a cluster of three on each lateral side, and two more pairs on the back - these final two pairs are not present in all specimens, suggesting some ontogenetic or intraspecific variation. These body cone spines may have served a defensive or sensory function. On the back of the animal were many smaller spines, distinct from the body cones, with apparently random organization.

There is no evident anus, which means that the animal must have consumed its food and excreted it from the same orifice. However, the strong folding found in the fossils makes this conclusion tentative, with Simon Conway Morris, one of the British scientists involved in its discovery, admitting the possibility that the team simply has not spotted it.

Phylogeny
Saccorhytus was initially classified as a deuterostome due to its possession of body openings in the form of its body cones, which appear similar to apparently equivalent structures in vetulicolians and vetulocystids, and thus was thought to be closely related to those two clades by Han et al. Below is a simplified phylogenetic tree based on that constructed by Han et al.

Since the earliest deuterostomes had a one-way through gut, the evident lack of an anus may either be a secondary loss (as seen in ophiurioids) or a plesiomorphic trait inherited from more primitive bilaterian ancestors, which may be linked to acoels and xenoturbellids.

In 2020, this classification was challenged, and it might have been an Ecdysozoan instead, possibly a stem group scalidophoran, due to its sclerites and posterial spines (which, when the tip is broken off look like openings) similar to that of other scalidophorans, and the lack of cillia. Below is a simplified phylogenetic tree based on this classification.

Phylogenetic analyses published in 2022 indicate that the organism belongs to Ecdysozoa and not Deuterostomia. What were thought to be pharyngeal openings used to support the deuterostome hypothesis were found to be taphonomic artefacts.

Palaeoecology
Saccorhytus most likely lived a meiofaunal lifestyle, with its body plan suited for an interstitial habitat, such as its thick but flexible cuticle providing protection and allowing it to wriggle through grains of sand. Additionally, it is possible that muscles could have attached to the body cone spines, allowing them to be used for locomotion.