The fossil record is biased against bats. The flying mammals are small, making their fossilized remains very hard to find. And their light skeletons—ideal for flying around—mean it takes special circumstances for their bodies to be preserved. And yet, against these odds, paleontologists recently uncovered the exceptionally complete skeleton of what now stands as the earliest known bat.
To date, the most complete early bat fossils have come from an area paleontologists call Fossil Lake in Wyoming. The rock layers are world-famous for containing beautifully preserved fish, birds, mammals and other organisms that lived in the area about 52 million years ago. Among the stunning fossils recovered from these rocks, Naturalis Biodiversity Center paleontologist Tim Rietbergen and colleagues report Wednesday in PLOS One, are fossils of a new bat species the researchers have named Icaronycteris gunnelli. By comparing this new species with other early bats, paleontologists are beginning to develop a deeper understanding of how bats spread around the world in that period.
Rietbergen didn’t find this specimen onsite at Fossil Lake, however. He first spotted Icaronycteris gunnelli online as a fossil for sale from a commercial dealer. “I looked at the pictures and got excited,” Rietbergen recalls. The cream-colored limestone and tan-shaded bones were an immediate giveaway that the bat had come from Fossil Lake, where several other early bat fossils had been uncovered over the years. The new bat fossil seemed to be smaller in size than the other two bat species found from similar, overlying levels in the lake limestone, and its bones seemed to be stouter. Rietbergen contacted bat expert Nancy Simmons at the American Museum of Natural History and arranged for the fossil to be purchased for the museum’s collections.
Upon closer study, the new fossil turned out to be a previously unknown species. But that was hardly all. Rietbergen traveled to different museum collections to study other fossil bats and, in the process, discovered a second specimen of Icaronycteris gunnelli in the collections of the Royal Ontario Museum. Paleontologists had thought the fossil was something different until Rietbergen took a second look.
While bats are diverse and numerous today, they’re relatively rare in the fossil record. “Bat skeletons are small, light and fragile,” Rietbergen notes. The habitats early bats lived in were often warm and humid, which favors faster decomposition. In the case of Icaronycteris gunnelli and the other bat species from Fossil Lake, the early bats either fell into the lake and sunk or were somehow washed from the shore into the lake, where they could be quickly preserved in calm bottom waters.
Even earlier bats have proved elusive, but that’s unsurprising given the nature of the fossil record. The unlikelihood of uncovering fossilized flying creatures can leave gaps between the earliest record of a group of organisms and the emergence of their modern ancestors. Mount Marty University paleontologist Alex Dececchi, who was not involved in the new study, notes other examples of flying animals with missing pieces to their backstory: Among flying squirrels, he notes, a 12-million-year gap exists between the first possible flying squirrel teeth and the earliest body fossils from the gliding rodents. Flight often requires small size and begins to evolve in habitats where preservation is more unlikely. Every bat fossil, then, is a fossil that formed against the odds.
The misfortune of the few fossilized ancient bats was good luck for paleontologists trying to reconstruct the mystery of how the first bats evolved. “The very first fossils of bats were bats that are already bats,” Rietbergen says. So far, no one has discovered what might be called a “transitional bat,” or a bat that shows features in-between a flying bat and its ground-dwelling ancestors. Icaronycteris gunnelli and two other, geologically younger species found in the Fossil Lake rocks stand out as the most archaic bats yet known, which shifts the emerging picture of how quickly the earliest bats began to spread around the world.
In addition to the Fossil Lake bats, paleontologists have also described early bats from India and France. These fossils were previously thought to represent two other Icaronycteris species, which would mean that one bat lineage quickly spread around the world early in the group’s history. But upon comparing the new Fossil Lake bat to the fossils from India, France and elsewhere, Rietbergen found something different. The three Fossil Lake bats form a group unto themselves, while the fossils from France and India are from different early bat lineages, a diversity of flying mammals rather than a single genus. Paleontologists know this as a radiation, or when many different lineages split from each other and evolve side by side over a relatively short period of time.
If bats were already spreading around the world by around 52 million years ago, then their origin was even further back in time. The challenge facing paleontologists is where to look for these protobats. The task is daunting, especially given the rarity of localities like Fossil Lake that have the potential to preserve entire skeletons rather than isolated teeth and bones. “Unless we get very lucky and more small mammals around this transition happened to fall into exactly the right spots,” Dececchi says, “we are unlikely to see much of what happened.” Nevertheless, he points out, the fact that fossils of early bats have turned up on three continents bodes well for paleontologists looking to find fossil bats and their forerunners—each place a dot on the map that may eventually lead to the sought-after protobats.