The Only Flying Mammal: Why Bats Are One of Evolution's Greatest Mysteries

Bats are the only mammals that have the ability for true flight. They have been seen as early as 50 million years ago (m.y.a.) in the fossil record, represented by teeth, jaw parts, and full skeletons, from India and Australia to Paris and Wyoming.

The anatomy of the ears of primitive bats shows that they are not capable of echolocation. Early bats relied on their eyes, nose, and tactile sense to look for food. In addition, modern bats only have one claw on the digit corresponding to the thumb, while prehistoric bats had more finger claws. The 52-million-year-old fossil Onychonycteris finneryi found in 2008, possessed claws on all five fingers.

Scientists are struggling to answer questions about bats' evolutionary origins. The earliest fossil bats already look like bats. Where did they originate? How did they obtain flight? Bats undoubtedly came from terrestrial ancestors, but where can these fossils be found?

Emily Brown, a paleontologist at the University of Birmingham, says that they do not know the reason for the ten-million-year gap in the fossil record. She and a team of colleagues said in a publication last year that many factors may be involved.

Brown says that the environment of early bats may have caused the poor fossil record. She theorized that this is due to their forested habitats, which have poor potential for preserving fossils.

Well-preserved bat fossils include the Icanonycteris index,  which is among the earliest bats known. It had thin bones that are no thicker than a human hair. Their fossils were preserved because their habitat was around lakes with oxygen-depleted water and fine sediment, which let fossils become quickly buried out of reach of scavengers and decomposers.

Brown adds that tooth fossils comprise most of the bats' known fossils. Scientists can only glean a good picture of prehistoric bats by studying deposits which preserved articulated and complete skeletons, from which researchers may understand how bats evolved and when they first appeared.

Kevin Seymour, a paleontologist from the Royal Ontario Museum, says that information can be gained from studying extant mouse-tailed bats, which combine gliding and fluttering when flying. Based on fossils like Onychonycteris, Seymour says that bats may have undergone a gliding stage before powered flight. Other than that, scientists can only speculate. Seymour says that more information comes from more complete material.

While the search for relevant fossils continues, other mammals could offer useful clues. While bats are the only mammals that have powered flight, mammals such as colugos and flying squirrels are capable of gliding with the use of expanded membranes. Prehistoric bats could have evolved similarly, using extra skin to glide.

New information gathered from existing fossils supports the theory that early bats were tree climbers. Research presented by paleontologists Kenneth Dial and Kevin Padian at the Society of Vertebrate Paleontology discussed how some early bats possessed hindlimbs flexing to their side instead of directly aligning beneath their body. Such configuration is favorable to climbing trees and rock faces instead of walking. The discovery of more tree-living animal fossils will widen our understanding in this area.

Some of the earliest known bat fossils are communities with various species, which means they were already diversified as early as 50 m.y.a. Ancestors of bats will be seen in rocks from the dinosaur age 66 million years ago when they first started to fly.