I wanted very much to discuss the basal skate/ray Heliobatis this week, unfortunately the information that exists for the little fossils is not very good. The fossils themselves are plentiful and beautiful, so we may venture back to the topic, but this week we are going to discuss what is, on television, considered one of the more important fish in the history of evolution. All fish are actually very important to our history. The reasons that all fish are important is that a lot of what life has become on this planet can be traced back to fish, including fish without mouths. How does a fish not have a mouth one might ask. This is certainly one of the more interesting questions of science. As with any evolutionary question, the line of evidence and the existence or absence of traits is sometimes shocking to us as we look at living animals to try to understand fossil animals. Animals have, through their history, gone through forms without heads to forms with heads, then they gained distinct mouths, and eventually teeth. That very cursory history of the development of jaws and teeth is lacking, but the general path is well known. The small fish Haikouichthys fits into the very early part of that development. Cladistics argues against Haikouichthys placement on the trees as one of the earliest fishes, but does agree that it could be either a basal craniate, animals having clearly defined heads and skulls, or a basal chordate, animals with spinal cords, often referred to as notochords in the primitive conditions. Since we have spinal cords (the more "advanced" version of the dorsal nerve cord or notochord) and clearly defined skulls, somewhere along the line we are actually related to this small jawless fish.
What is a jawless fish? How can a fish be jawless? Since we are accustomed to extant animals having jaws (for the most part), it can be difficult to entertain the idea of a fish without a recognizable skeletal jaw. Appearing somewhat similar to a hagfish, Haikouichthys had an opening where a skeletal jaw could have been, if it had existed, but was instead little more than an open vent through which the tiny Cambrian fish could filter food. Lacking jaws even made sucking in food impossible for agnathan ("no jaws" from the Greek) fish. As we shall see, this kind of life was difficult, but even more things in the Cambrian oceans made life more difficult for this small fish.