Problematic Cambrian Fossil, Possibly a Primitive Chordate Fossil

The study of Precambrian and Cambrian soft-bodied fossils presents a key problem - how can we ever be confident of interpretations based on what are often little more than smudges in a rock? It is likely that postulates and conjectures will persist, but perhaps it is their persistence and continued mystery that endows Cambrian fossils with their allure.

Paleontologists who examined this specific fossil believe there is about a 50/50 chance this it is a chordate. But there is also a chance that it is a worm. This specimen has some similarities to a Nectocaris, an extremely rare animal of unassigned phylum found in the Burgess Shale. The description of Nectocaris is based on a single specimen and thoughts as to its affinity vary widely from hemichordate, chordate, arthropod, or less likely a crustacean, or possibly something completely different. It is postulated to have been a free swimming animal with no hard parts; a pair of short straight appendages on the front of the head appear to be unjointed.

Chordate origins are not fully understood though there are several theories. Burgess Shale has a small little organism called Pikaia that has all the ancestral chordate characteristics. The Chordates are distinguished most notably by a notochord , a semi-flexible rod running along the length of the animal. All chordates have a notochord at some stage in their lives, but in some the notochord is lost in the adult, whereas in others such as the vertebrates, the notochord is present in the embryo but is later largely replaced and surrounded by the vertebra, or backbone.

The primitive Chordates remain enigmatic. Emmonaspis cambrensis, from the Lower Cambrian of Vermont, has been variously associated with graptolites, chordates, arthropods, and frond-like organisms since they were first described more than 100 years ago. The most widely accepted earliest chordate, Pikaia gracilens, from the Middle Cambrian Burgess Shale, was originally interpreted as a polychaete annelid (Walcott, 1911). A number of Chordates are described from the Lower Cambrian Chengjiang lagerstätte of China.

Some references:

• Chen, J-Y., et. al. 1995. A possible Early Cambrian chordate. Nature, 377:720-722.
  
• Chen, J-Y., et. al. 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.
  
• Shu, D-G., et. al. 1996. Reinterpretation of Yunnanozoon as the earliest known hemichordate. Nature, 380:428-430.
  
• Conway Morris, et. al. 1996. A Pikaia-like chordate from the Lower Cambrian of China. Nature, 384:157-158.