Chordata ├─Urochordata │ ├─Larvacea │ │ ├─Oikopleuridae │ │ └─┬─Kowalevskiidae │ │ └─Fritillaridae │ └─┬─Ascidiacea │ └─Thaliacea └─┬─Cephalochordata └─┬─Haikouella lanceolata └─Vertebrata
Geologic Time: Early Cambrian (~525 million years ago)
Size: 16-22 mm long on a 55 mm by 50 mm matrix
Fossil Site: Chengjiang Maotianshan Shales,: Quiongzhusi Section, Yu’anshan Member, Heilinpu Formation, Anning, Yunnan Province, China.
This exceptional specimen is a multiple example of Haikouella lanceolata, thought by its describers to be the earliest craniate-like chordate. This fish-like animal has many similarities to the contemporaneous Yunnanozoon lividum, but differs in several aspects: it has a discernible heart, dorsal and ventral aorta, gill filaments (see the closeup), and a neural chord. For all these reasons, it was identified by Chen, Huang, and Li in the seminal Nature paper (Nature 402, 518-522, 02 December 1999) as a chordate.
The debate rages on, but whatever the outcome, this creature was much like the ancestor of all the vertebrates. It derives its generic name from its resemblance to the modern day lancet Amphioxus.
Also see: Chengjiang Biota, Chengjiang Fossils, Cambrian Explosion
Haikouella: H. lanceolata Chen et al. 1999.
Early Cambrian of South China.
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.