Haul of Fossil Fish Pushes Back the Origin of Teeth and Jaws

Our teeth and jaws are incredibly ancient. They’re older than dinosaurs, older than arms and legs, older than trees–adaptations that paleontologists have tracked to our distant, fishy relatives that thrived in the seas around 425 million years ago. But a trove of delicately-preserved vertebrates found in China has set a new date for the earliest record of jaws and teeth. The aquarium’s-worth of early vertebrates include some of the earliest fish with paired appendages, one of the earliest relatives of sharks, and, at about 436 million years old, the most ancient fossils yet found of fish with teeth and jaws.

Researchers have disagreed about exactly when the earliest bites evolved. Molecular studies based upon estimations of genetic changes have proposed that the first jaws and teeth evolved about 450 million years ago. Until now, the oldest fossils of jawed vertebrates–or gnathostomes –dated to about 425 million years ago, a significant gap. As Institute of Vertebrate Paleontology and Paleoanthropology researcher Min Zhu and colleagues describe in a series of Nature papers published today, the new finds from South China sit in the middle of this gap and offer a startling look at a critical moment in evolutionary history.

“My first impression is ‘wow’,” says University of Chicago paleontologist Yara Haridy, who was not involved in the new study. The new fossils represent what paleontologists call a “Konservat-Lagerstätte,” or a fossil site that records intricate details of ancient creatures that are often lost in other settings. In this case, Zhu and coauthors note in their research, the fish they uncovered were small and had delicate bones. That means these fish were generally unlikely to become fossils except in truly outstanding circumstances where they could be buried quickly and shielded from scavengers that would otherwise eat them.

That an exceptional stroke of luck was required to preserve such fish likely explains the difference between the molecular estimate and fossil record. And having such complete fossils lets paleontologists know what all the strange bits and pieces from such ancient waters really belonged to. “The skeletal debris that dominates the pre-Devonian record of jawed vertebrates provides few characters to help with identification,” writes University of Michigan paleontologist Matt Friedman in a comment on the new papers. Whereas previous finds were like finding individual puzzle pieces, having complete fossil fish is like getting a look at the picture on the box to know what goes where.

The assortment described by Zhu and coauthors represent various forms of very early fish, not all of which had jaws. One of the best-preserved fossils is of a jawless fish called a galeaspid, more or less a biological Roomba with a tail. The bones of this animal’s tough head shield have been known for a long time, but the new South China fossils reveal the form of the soft body behind those bones. Another form of armored fish–named Xiushanosteus–is the most common in the collection, an armored fish with jaws that was only about an inch long. Even more surprising is a new, ancient relative of sharks and cartilaginous fish the researchers have named Fanjingshania renovata, a creature that complicates how paleontologists understand how the ancestors of sharks, bony fish and extinct groups like placoderms evolved.

The big evolutionary picture can sometimes get murky when paleontologists trace lineages–such as sharks or bony fish–all the way back to their origins. The first members of a group probably don’t look very distinct from their ancestors and close relatives, meaning that categorizing these creatures can be very complex. Fanjingshania is proof of this concept. While being a chondrichthyan–or an early member of the fish group that contains sharks, rays and ratfish today–the fish shows some characteristics only seen in bony fish. For example, Fanjingshania has hard bone tissue with living cells that could re-absorb and remodel bone, a characteristic previously associated with bony fish. Knowing that traits were shared among different early fish lineages will help paleontologists better identify shark precursors, bony fish and placoderms. “This helps us flesh out the trajectory of evolution of these lineages,” Haridy says, calling the fossil of the early shark relative an “exceptional datapoint” in understanding the nature of these fish.

And then there are the teeth. In its own paper, the researchers describe some isolated teeth from an early shark relative they call Qianodus duplicis. While the whole body of the animal is as-yet-unknown, Zhu and coauthors propose that these fossils are the earliest definitive evidence known so far about when fish with teeth started to munch on prey.

So far, the new fish from the South China site push back the oldest occurrences of teeth and jaws. But that doesn’t mean that these were the first creatures to grin. “This assemblage puts teeth behind what many have postulated for a long time,” Haridy says, “which is to say that it is very likely that many of these lineages have Ordovician origins” dating back more than 443 million years. The new finds from South China are a hint at what paleontologists should be looking out for as they dig deeper. The fossils indicate that luck is sometimes on the side of paleontologists and other such fossil hot spots could be out there. Little by little, the earliest days of amazing oral adaptations are starting to swim into focus.

Riley Black | | READ MORE

Riley Black is a freelance science writer specializing in evolution, paleontology and natural history who blogs regularly for Scientific American.