Myoscolex

Myoscolex is an early animal known from the Cambrian Emu Bay Shale in South Australia. It is of unknown affinity but has been interpreted as an annelid and as an arthropod close to Opabinia. Myoscolex is the earliest known example of phosphotized muscle tissue, and as to which shows distinct annulation.

Myoscolex ateles was named by Glaessner in 1979.

Etymology
Myoscolex ateles derives its name from Greek, "Myo-" meaning muscle, "Scolex" meaning worm, and "Ateles" meaning incomplete. This definition of "Incomplete muscle worm" is due to its initial identification as a "muscular" annelid worm with indistinct features after fossilization.

As an Opabiniid
When described as an Opabiniid, Myoscolex is said to have at least 3 eyes, a thin proboscis jutting from under the proposed eyes, lateral lobes on the trunk, a tail fan on the posterior segments, and an upwardly curving trunk. It would have lived as a fast nektonic carnivore. However in 2022, new opabiniid Utaurora was described and Myoscolex is considered as animal with unknown affinity.

As a Polychaete
Myoscolex as a polychaete worm was unconventional in appearance being laterally flattened body with "rods" protruding from the ventral side. Movement would have been an undulation similar to that of Pikaia, and without the use of chaetae for propulsion, unlike other polychaetes.

Preservation
Myoscolex was preserved laterally compressed and in 4 layers. The outer two layers are composed of calcium carbonate and represents the skin, rods, lateral lobes, and possibly eyes and proboscis. The inner two layers were the internal muscles mineralized in apatite in resounding detail.