Time to call in the media coaches, because two news stories at Science Daily show evolutionary biologists discussing difficulties posed by the Cambrian explosion. One article, "Marine Worms Reveal the Deepest Evolutionary Patterns," offers University of Bath evolutionary biologist Matthew Wills explaining the "real headache" that the Cambrian explosion causes him:
The fossils from the Cambrian period can cause a real headache for evolutionary biologists. Instinct tells us to expect simple organisms evolving over time to become increasingly more complex. However during the Cambrian period there was an apparent explosion of different major groups of animals, all appearing simultaneously in the fossil record. We looked at priapulid worms, which were among the first ever predators. What's remarkable is that they had already evolved into a diverse array of forms -- comparable to the morphological variety of their living cousins -- when we first encounter them in the Cambrian fossil record. It's precisely this apparent explosion of anatomical diversity that vexed Darwin and famously attracted the attention of Harvard biologist Stephen Jay Gould.
In the same article, biologist Marcello Ruta of the University of Lincoln confirms that one can't appeal to the incompleteness of the fossil record to explain the abrupt appearance of these worms in the Cambrian explosion:
Our work has shown that despite many new fossil finds, including many from China in the last decade, the picture remains largely unchanged. This is really important because the fossil record is notoriously incomplete. It is often difficult to know whether a pattern is just an artifact of this incompleteness, or biologically meaningful. Our study resolutely confirms the latter. Priapulids are fascinating animals with much potential in evolutionary studies. They have a long history, with the earliest known species being 505 million years old, and with some of their extinct relatives being even older. They were important components of ancient bottom-dwelling marine invertebrate communities, and their predatory habits are well documented in the fossil record. However, for all their abundance and diversity, priapulids are a remarkable and often cited example of a morphologically conservative group, their overall shape and proportions having changed relatively little during their history.
To see how little priapulids have changed since the Cambrian, compare this living priapulid worm with a couple of photographs of fossil priapulid worms from the Cambrian explosion: