The origins of complex, multicellular animal life remain somewhat obscure. It appears to have accompanied the rise of oxygen levels over 600 million years ago, but the first animals generally had soft bodies, and left little behind other than impressions in stone. It appears that some of the animal groups that exist today got their start then, but the evidence isn't clear.
The arrival of the Cambrian era brought with it an "explosion" of novel life forms, albeit an explosion that took place gradually over tens of millions of years. In the Cambrian, the development of hard shells and mineralized forms allowed far more types of life to be fossilized. As a result, most of the major groups of animals alive today were preserved, along with a host of what appear to be evolutionary dead ends. Piecing together relationships among these creatures and their modern equivalents has been a challenge, one that creationists of various sorts have latched on to in their attempts to portray evolution as a failed theory.
But this portrayal relies on gaps in our understanding, and gaps in science have a habit of getting filled. One more will be filled by the description of the fossil on the right, which will appear in tomorrow's edition of Science. The newly described creature, termed Orthrozanclus reburrus, shares features with the ancestors of three major groups that are alive today: molluscs (such as clams and squid), annelids (segmented worms), and brachiopods, a type of shellfish that is only distantly related to molluscs. That distant relative appears to have been something very much like this new species.
More specifically, this creature has a long, thick plate with a segmented, convex anterior shell, much like the Halkieria, which gave rise to the brachiopods. But the lateral edges of that plate contain a set of spines with features similar to those found found in Wiwaxiids, which produced the modern annelids. The authors propose a new clade, the Halwaxiids, which encompasses these two previously separate phylogenetic groups. In addition to this pivotal location, the new fossil shares common features with the ancestors of molluscs, which suggests that this creature may be on the branch that led to that lineage.
This new fossil greatly clarifies the phylogenetic tree that describes the origin of these three modern groups, as it suggests the features that a common ancestor of all three should share. Those features appear in a group of fossils called Kimberella-Odontogriphus which previously had been difficult to place relative to other lineages; thanks to the new fossil, they appear to be at their base of all three branches.
If you find the blizzard of Latin words confusing, there's an easy take-home message: this new fossil has clarified the origins of three major groups of animals, revealing the path of their common ancestry and identified previously known fossils as representatives of that common ancestor. It shows that, despite its portrayal as a confused explosion of novel forms, the origins of animals in the Cambrian followed the same patterns of evolution seen today.