Oesia is considered a stem-group enteropneust (acorn worms) and has the characteristic three-part anatomy of the group, consisting of a proboscis, collar and trunk (Nanglu et al. 2016; Nanglu et al. 2020).
Oesia — from Lake Oesa, a small lake located a few kilometres southeast of the Burgess Shale.
disjuncta — from the Latin prefix dis, to signify a negation, and junctus, “joined.” The name is probably in reference to the crooked or bent shape of the early discovered specimens of Oesia.
Burgess Shale and vicinity: none.
Other deposits: none.
Marble Canyon (Kootenay National Park), the Walcott Quarry on Fossil Ridge.
Walcott (1911) described this species as a polychaete worm, but this view was challenged by Lohman (1920) who suggested a tunicate (chordate) affinity instead. Conway Morris (1979) rejected both interpretations, and this animal was later regarded as a problematic organism of unknown affinity (Briggs and Conway Morris, 1986). Szaniawski suggested a chaetognath affinity in 2005, which was argued against by Conway Morris in 2009. Nanglu et al. formally redescribed Oesia as an enteropneust (hemichordate) in 2016.
Oesia ranged in length from 2.4mm – 120mm, with an average length of 53mm. The anteriormost region is an oval or “acorn” shaped proboscis, which gives the acorn worms (Enteropneusta) their common name. The proboscis is frequently preserved with a darker, more dense carbon content than surrounding tissues, suggesting that it was highly muscular as the proboscis is in modern acorn worms. Behind the proboscis was a short cylindrical region called the collar, which enclosed the mouth. Behind the collar was a long, cylindrical region called the trunk, which maintain a roughly even width throughout. Unlike in modern acorn worms or its contemporary Spartobranchus, the trunk of Oesia was not divided into a pharynx and a posterior trunk. Instead, the collagenous gill bars that define the pharynx continue throughout the entire length. This gives Oesia a relatively inflexible appearance. At the posterior end of Oesia was a bilobed shaped attachment structure. The interior of this structure also preserved highly concentrated carbon relative to surrounding tissue which, along with its shape, suggests that this appendage was for grasping. Oesia is often found inside another fossil previously described as the alga Margaretia dorus, but which is now recognized as the secreted dwelling of Oesia, which was likely used for feeding as a pre-filtration device. This tube was typically twice the width of Oesia and could reach nearly 50 cm in total length. The tube contained a series of spirally arranged pores and could bifurcate into branches as many as 5 or 6 times (although 1 or 2 bifurcations is more common).
Oesia is relatively rare at the Walcott Quarry, but is abundant at Marble Canyon where it represents the third most abundant species with 3,373 specimens (Nanglu et al. 2020).
Oesia was likely a suspension feeder, owing to its extended pharynx laden will gill bars. These gill bars would have been covered in small hair-like structures called cilia which would move to create a flow of water towards the mouth and into the body. Excess water would then be expelled through pores, while food was passed through the gut. The large tubes of Oesia would have projected from the muddy seafloor into the water, with the pores allowing for water to move in and out of the tube. This would allow for fresh water for Oesia to feed on and refresh the tube with oxygenated water.