[1] The barrel-shaped main body extends apically into two long siphons of similar size, which have millimetric circular transverse muscle bands.

Leptocardii/Acrania (lancelets) Appendicularia Shankouclava † Megasiphon † Burykhia † Stolidobranchia Phlebobranchia* Aplousobranchia Thaliacea Vertebrata (fish and allies) Another possibility is that, as with Shankouclava, Megasiphon may represent a stem-tunicate, a position that would indicate that a sea squirt-like body plan is ancestral to the entire Tunicata group, and the tadpole-like appendicular motile is a derived state.

Supported by developmental biology and the report of massive gene loss in appendicularians,[5][6] this hypothesis would imply that the first non-appendicular tunicates were probably sedentary-pelagic ascidian-like organisms, with a two-phase life cycle that included larval metamorphosis.

[7] The unusual morphology of Shankouclava makes its relationship with the rest of the tunicates somewhat ambiguous because the oral siphon is not very defined and presents tentacle-like structures reminiscent of ambulacraria and lophophorates, it lacks an atrial siphon, and does not resemble phlebobranchs such as Megasiphon and Burykhia; Therefore it is possible that it is actually a more basal group of Olfactores.

Leptocardii/Acrania (lancelets) Shankouclava † Burykhia † Megasiphon † Larvacea Stolidobranchia Phlebobranchia* Aplousobranchia Thaliacea Vertebrata The Megasiphon authors postulate the organism as a stem tunicate, rather than a stem Acopa, as more plausible because the position of Megasiphon as a main group tunicate does not require drastically changing the estimate of the molecular clock nor the origin of Ascidiacea given by the same, would instead only set minimum divergence estimates for the origins of Tunicata at about 500 million years ago; second, an evolutionary scenario involving sedentary-pelagic sessile ancestry appears to be more likely for Olfactores and Tunicata when consider other lines of evidence.