The quest for chordate origins

Lecture © Jennifer Duffy
BIOL 606 Session, University of Alberta, March 15, 2000

The Phylum Chordata is united by the presence of characters such as a notochord, a dorsal hollow nerve cord, pharyngeal gill slits, a post-anal tail, and segmented muscle blocks called myomeres. Modern members of the phylum include the subphyla Urochordata, Cephalochordata, and Craniata, but because of the major preservational bias for bone and its precursors, only the craniates possess a substantial fossil record. This lack of potentially basal chordate fossil material has proven a major obstacle in the search for chordate origins. Indeed, Garstang (1928) based his theory of a paedomorphic origin solely on the embryology of the extant chordates, and postulated that an organism similar to a urochordate or cephalochordate larva could have acquired sexually maturity without metamorphosing, thus providing a spring-board for the evolution of chordates and vertebrates. Chordate origin theories based on fossil evidence also exist, such as that of R.P.S. Jefferies (1986), who purports that a fossil group called the calcichordates are basal chordates. However, Jefferies' interpretations of the characters in this group are subject to criticism from the majority of the scientific community, who believe the calcichordates should be allied with the echinoderms. For any origin theory to be deemed viable, it seems as though an amalgamation of fossil and living studies would be most prudent.

Fortunately, the discovery of exceptionally preserved soft-bodied biotas, called konservat lagerstätten ("conservation mother-lodes") has provided opportunities to examine and describe fossil chordates. However, the vague and enigmatic nature of these fossils has allowed for widely varying interpretations of their relationships to living groups. Emmonaspis cambrensis, from the Lower Cambrian of Vermont, has been allied with the graptolites, chordates, arthropods, and frond-like organisms since its initial description in 1886 (Conway Morris, 1993). Even the most widely accepted earliest chordate, Pikaia gracilens, from the Middle Cambrian Burgess Shale, was originally interpreted as a polychaete annelid (Walcott, 1911), but has since been allied with the cephalochordates based on synapomorphies such as chevron shaped myomeres and an anteriorly extending notochord (Conway Morris, 1998).

With the general acceptance of Pikaia as a fossil chordate, the search for earlier material led to finds such as Yunnanozoon lividum (Chen et al., 1995) and Cathaymyrus diadexus (Shu, Conway Morris and Zhang, 1996) from the Lower Cambrian Chengjiang lagerstätte of China. Based on the presence of characteristics such as a notochord, muscle blocks, and gill slits, both of these organisms were originally interpreted as chordates. However, various interpretations of similar material have treated Y. lividum as a hemichordate (Shu, Zhang and Chen, 1996), or as the most basal of chordates (Chen, Huang, and Li, 1999), and have considered C. diadexus as only a junior synonym of Y. lividum (Chen and Li, 1997). With such debate, it is clear that the search for chordate ancestors is anything but resolved. In addition, with possible early craniates, such as Haikouella lanceolata (Chen, Huang and Li, 1999), being described from the same locality, the timing of chordate origins actually may be earlier than the Lower Cambrian, so a Precambrian search might be necessary.

With various interpretations and reinterpretations, it is possible that a general concensus for chordate origins may never be reached, but it is certain that the continued search will provide fuel for much future debate (and papers in Nature!).

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Chen, J-Y., Dzik, J., Edgecombe, G.D., Ramskold, L. and Zhou, G.Q. 1995. A possible Early Cambrian chordate. Nature, 377:720-722.

Chen, J-Y. and Li, C-W. 1997. Early Cambrian chordate from Chengjiang, China. Bulletin National Museum of Natural Sciences Taiwan, 10:257-273.

Chen, J-Y., Huang, D-Y. and Li, C-W. 1999. An Early Cambrian craniate-like chordate. Nature, 402:518-522.

Conway Morris, S. 1993. Ediacaran-like fossils in the Cambrian Burgess Shale-type faunas of North America. Palaeontology, 36:593-635.

--- 1998. The Crucible of Creation. Cambridge University Press, Cambridge. 242p.

Garstang, W. 1928. The morphology of the Tunicata. Quarterly Journal of Microscopical Science, 72:51-189.

Jefferies, R. P. S. 1986. The Ancestry of the Vertebrates. British Museum (Natural History), London. 376p.

Shu, D-G., Zhang, X. and Chen, L. 1996. Reinterpretation of Yunnanozoon as the earliest known hemichordate. Nature, 380:428-430.

Conway Morris, S. and Zhang, X-L. 1996. A Pikaia-like chordate from the Lower Cambrian of China. Nature, 384:157-158.

Walcott, C.D. 1911. Middle Cambrian annelids. Smithsonian Miscellaneous Collections, 57:109-144.


Discussion

Rapporteur: Sean Graham

A central problem with this kind of study is "how confident can we be that our interpretation of structures in long-extinct organisms is accurate?" In some studies we have little more to go on than "smudges in the rock," and so some early Cambrian specimens (Emmonsaspsis) have been interpreted variously as arthropods, basal chordates and "frond-like" fossils. Although the preservation of the Yunnanozoon specimen reported in the focal paper is exceptional, at various points it has been interpreted as a segmented worm, a hemichordate and (here) as a cephalochordate.

It seems clear that the latter hypothesis is open to interpretation unless and until more examples of this specimen are found. The recent discovery of multiple representatives of the putatively related craniate, Haikouichthys, out of this 525 Myr-old Chengjiang fauna, lends credence to the possibility that Yannanozoon is also a craniate.

This and other evidence suggests that the "Cambrian explosion" reflects multiple and nearly simultaneous origins of "skeletonization" of long-preexisting lineages, rather than rapid origins of the lineages themselves. This highly unparsimonious hypothesis of skeleton origin may have been favoured by a major shift in sea chemistry.

Animated discussion centred around the issue of "what is a phylum and why do we not see new ones in recent geological time?" In part this may be a function of the perception that there have been no recent origins of massively new body plans. That seems to be a false perception &endash; several audience members gave counter examples of highly divergent but recent lineages of animals and plants. The naming and circumscription of phyla (or "divisions" in botany) also seems to be highly tradition-bound.

Ranks such as phyla do have a utilitarian function, although perhaps this is of more use in some groups than others. There was some disagreement about whether the lack of equivalency of taxa at the same rank was of much import to biologists in general.

The discussion ended with general agreement on the importance of considering multiple specimens, providing clear illustrations and descriptions of material, making material readily available to competing workers, and the need to set up potentially falsifiable hypotheses of fossil relationships. The dangers of a lack of competition in scientific research were also mentioned. Some participants also questioned the high ratio of recent Nature and Science papers on this general topic, relative to the overall quality of the science reported.