April 2019

When most of us think of fossils, we probably picture bones — dinosaur bones pieced together into a skeleton, leaving observers’ imaginations to fill in the blanks of the organs, muscles, and skin that once hung on that frame. The drawers of paleontology museums are also crammed full of teeth, shells, and other hard bits of living things that don’t easily rot away, leaving them available for fossilization. But every so often, when circumstances on ancient Earth were just right, far more than hard parts were preserved. Last month, scientists announced the discovery of a fossil bed in China laid down in one of those unusual circumstances. It provides a fascinatingly detailed glimpse of life in the ocean 518 million years ago, long before any animal had ever heaved its hard bones onto dry land.

Where's the evolution?

The recently described Qingjiang fossils date to the Cambrian Period, a time in Earth history when major branches of the Tree of Life had recently evolved and were just becoming established. The fossils represent organisms both familiar (sea anemones that could easily be mistaken for their modern counterparts) and strange (worm-like creatures with fleshy appendages known as lobopodans). In many cases, a remarkably faithful record of the living animal was preserved. In these fossils, we can easily see the multiple layers of a jellyfish’s boneless body, the once squishy internal guts of an ancient crustacean relative, and the gills of an early chordate.

While fossils this detailed are unusual in the geologic history of Earth as a whole, they are less surprising coming from the early to middle Cambrian Period. A little more than a century ago, the Burgess Shale Cambrian fossils were discovered in Canada. These fossils are about 10 million years younger than the Qingjiang fossils but preserve a similar level of detail. Through the Burgess Shale, the world was introduced to the oddities of early animal life: Anomalocaris, a fierce predator with a disc-like mouth and barbed feeding appendages, and Hallucigenia, a walking worm with spikes running the length of its back, and Opabinia, a soft-bodied animal with a ribbed proboscis like a combination vacuum cleaner hose and elephant’s trunk with a venus fly trap at the end. One might guess that such spectacular organisms would be a one-off in the fossil record. But in fact, in the last decade, paleontologists have made careful studies of Cambrian rocks and pinpointed many new locations on multiple continents that preserve such animals and their ancient brethren, including the Qingjiang fauna, often in exquisite detail. The Cambrian Period predates the golden age of dinosaurs by hundreds of millions of years, but we have a more exact picture of the bodies of these ancient misfits than we do of T. rex‘s.

The picture of early life provided by the Qingjiang, Burgess, and other Cambrian fossil beds makes it clear that evolution is not a coherent march up a ladder of progress, but a contingent, often unpredictable process. Some lineages seem entirely unphased by the passing of eons. Jellyfish and sea anemones are abundant in the Qingjiang fauna, just as they are in modern oceans, and appear to have changed little. What worked for them hundreds of millions of years ago does just as good a job today. Other lineages boasting a suite of complex adaptations have disappeared entirely. Anomalocaris seems so alien simply because nothing remotely similar has survived to today. Other lineages have survived, but their modern representatives seem very different from those ancient fossils. Hallucigenia and other lobopodans are common Cambrian fossils from marine environments, but their modern relatives, onychophorans are relatively rare and inhabit tropical terrestrial ecosystems. Some things stay the same, some disappear, and some change, depending on the vagaries of history, chance, and evolution.

The Qingjiang fossils also illustrate that for all the genetic data now available, there are still biological questions that only fossils can answer. For example, scientists have long been interested in which modern animal was the first to branch off of the Tree of Life. Once we developed the technology to cheaply sequence DNA, answering this question seemed like a slam dunk. But the DNA was equivocal: perhaps the earliest split produced sponges — or perhaps it led to ctenophores, jellyfish-like tentacled animals also known as comb jellies. Further study of the Qingjiang biota may soon clarify this conundrum, as the fossils include both well-preserved ctenophores and sponges.

As scientists further study the Qingjiang site and other recently identified Cambrian sites, we can expect more exciting discoveries.  Despite the diversity of organisms preserved in the Burgess Shale and another productive Cambrian site in China (the Chengjiang site), 53% of the species that scientists have so far found in Qingjiang were previously unknown. Even more surprisingly, only 8% of the species from Qingjiang were also found in Changjiang, even though these deposits are the same age and likely represent different locales in the same ancient ocean. When it comes to the Cambrian, more fossils does not mean more of the same! Stay tuned to see what new, weird, and wonderful creatures come out of these rocks next.