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Chordata |
The Vertebrates | Haikouella |
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Abbreviated Dendrogram
Chordata |--Urochordata | |--Larvacea | | |--Oikopleuridae | | `-+--Kowalevskiidae | | `--Fritillaridae | `--+--Ascidiacea | `--Thaliacea `--+--Cephalochordata `--+--Haikouella lanceolata `--Vertebrata |
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Overview |
Haikouella:
H. lanceolata Chen et al. 1999.
Range: Early Cambrian of South China.
Phylogeny: Chordata::: Craniata + *.
Characters: Body 25-30 mm, broadly triangular anteriorly, narrowly
triangular posteriorly [C+99]; brain present and bi- or tri-partite, but degree of
integration with cranial sensory structures unclear [C+99] (contra
[SM03]: brain is absent); rostral "skirt" similar to larval lamprey
& also present in Haikouichthyes [M+03]; lateral eyes or eyespots
[M+03] (contra [SM03]: eyes are rare artifacts); ventral buccal (?) cavity with tentacular
structure [C+99]; probable mineralized
pharyngeal teeth [C+99]; 6 pairs of branchial arches [C+99] (compare
[SM03]: gills are external); gill slits between arches,
covered by folds of body [C+99]; unclear if gills open to exterior or atrial
cavity [C+99];
notochord extending well into
anterior [C+99]; dorsal nerve cord present [C+99] [SM03]; body segmented
dorsally with segmental blocks of muscle fibers (hence myomeres) [M+03] (contra
[SM03]: no evidence of "cone-in-cone" structure); muscle fibers &
segmentation extend well dorsal to notochord [M+03]; about 24 myomeres
not in V- or W-shape [C+99]; postanal tail present [M+03] (compare
[SM03]: tail is quite variable and is probably an artifact); heart, plus branchial, ventral &
dorsal aortae present [C+99]; gut differentiated into esophagus, spiral mid-gut and
straight intestine [C+99].
Links: Haikouella and Myllokunmingia; An early Cambrian craniate-like chordate; Titulo (Spanish); Das Kambrium (German); m-hydrate-nov99; ScienceNow; Taipei Times - archives (newspaper article on recent work); Haikouella (Wikipedia article); Chinese Science Bulletin ISSN-1001-6538 2003 Vol.48 No.8 725-735 ... (abstract of article arguing that it's a stem deuterostome).
Reference: Chen et al. (1999) [C+99]; Mallatt et al. (2003) [M+03]; Shu & Morris (2003) [SM03].
Note: The description given above is only slightly more conservative than given in the reference. Haikouella, unlike most other early Paleozoic species, is known from over 300 well-preserved specimens, and Chen's description therefore has very high credibility, despite the great age and small size of the organism. Chen suggests that Haikouella may actually be a craniate, and this may well be correct in the sense that it has of the features typically listed as synapomorphies of Craniata. However, it seems unlikely that it is a member of the crown group subtended by hagfish and hogs. In any case, with the addition of conodonts to the chordates and the wealth of new information from South China, it is no longer very certain exactly what the synapomorphies of Craniata are going to be. ATW040314.
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