Oesia disjuncta (USNM 57630) – Lectotype, part and counterpart. Complete specimen. Specimen length = 85 mm. Specimen wet – direct light (top row), wet – polarized light (bottom row). Walcott Quarry.
© SMITHSONIAN INSTITUTION – NATIONAL MUSEUM OF NATURAL HISTORY. PHOTO: JEAN-BERNARD CARON
The position of Oesia is uncertain. Originally described as an annelid worm by Walcott (1911), a recent reinterpretation as a chaetognath (Szaniawski, 2005, 2009) has been vigorously rejected, and a position closer to the hemichordates proposed instead (Conway Morris, 2009).
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 appearance of this species.
Burgess Shale and vicinity: none.
Other deposits: none.
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). However, a recent interpretation as a chaetognath (Szaniawski, 2005, 2009) has spurred new debate (Conway Morris, 2009, Szaniawski, 2009) with a view that a hemichordate relationship might be possible (Conway Morris, 2009). A thorough restudy of other hemichordate-like organisms from the Burgess Shale will be required to confirm this new hypothesis.
The body of Oesia is elongate with an anterior section which appears to be swollen. There is no evidence of grasping spines. The presence of fine transverse striations on the body has been interpreted as transverse muscle bands, but other features, such as fins and internal organs, are contentious.
Walcott recognized nine specimens of this species, but Oesia is probably more abundant than previously thought. Oesia and another similar hemichordate-like form (“Ottoia tenuis”) represent about 2.2% of the Walcott Quarry community (Caron and Jackson, 2008).
The vermiform aspect of this fossil and apparent lack of fins suggest a benthic lifestyle, but its mode of feeding is unknown.
BRIGGS, D. E. G. AND S. CONWAY MORRIS. 1986. Problematica from the Middle Cambrian Burgess Shale of British Columbia, p. 167-183. In A. Hoffman and M. H. Nitecki (eds.), Problematic fossil taxa (Oxford Monographs on Geology and Geophysics No. 5). Oxford University Press & Clarendon Press, New York.
CARON, J.-B. AND D. A. JACKSON. 2008. Paleoecology of the Greater Phyllopod Bed community, Burgess Shale. Palaeogeography, Palaeoclimatology, Palaeoecology, 258: 222-256.
CONWAY MORRIS, S. 1979. The Burgess Shale (Middle Cambrian) fauna. Annual Review of Ecology and Systematics, 10(1): 327-349.
CONWAY MORRIS, S. 2009. The Burgess Shale animal Oesia is not a chaetognath: A reply to Szaniawski (2005). Acta Palaeontologica Polonica, 54(1): 175-179.
LOHMANN, H. 1920. Oesia disjuncta Walcott, eine Appendicularie aus dem Kambrium. Mitteilungen aus dem Zoologischen Staatsinstitut und Zoologischen Museum in Hamburg, 38: 69-75.
WALCOTT, C. 1911. Cambrian Geology and Paleontology II. Middle Cambrian annelids. Smithsonian Miscellaneous Collections, 57(5): 109-145.
SZANIAWSKI, H. 2005. Cambrian chaetognaths recognized in Burgess Shale fossils. Acta Palaeontologica Polonica, 50(1): 1-8.
SZANIAWSKI, H. 2009. Fossil Chaetognaths from the Burgess Shale: A Reply to Conway Morris (2009). Acta Palaeontologica Polonica, 54(2): 361-364.
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