February 13, 2008

Bat Transitional Fossil: Intermediate Form Dated to 52 mya

Researchers have found and analyzed an apparent ancestor to modern bats, or at least a close relative of a bat ancestor, in 52 million year old shale from Wyoming. Dubbed Onychonycteris finneyi, it had some traits that we see in bats that currently exist and some that are clearly remnants from a species that was not a bat at all. It represents the oldest known specimen in the fossil record of bats.

The major importance of the well-preserved fossil is that it presents evidence that flight evolved in the ancestors of bats before echolocation. Onychonycteris was capable of powered flight but lacked the specialized bones in its skull to echolocate as modern bats do.

Onychonycteris finneyi, ancestor to modern batsSome other interesting points about this ancient animal:

  • Onychonycteris has claws on all five of its wing digits; modern bats have two at most.
  • Onychonycteris has hind limbs that are long in proportion to its forelimbs, unlike modern bats in which the situation is reversed. This is a trait the fossil species shares with climbing animals.
  • While Onychonycteris has a keeled breast bone, similar to those that allow modern bats and birds to fly rather than glide, its wings were broad and short. This combination of traits is likely evidence that this bat ancestor alternated to some degree between gliding and flapping. That in itself is a pretty good qualifier to call this a "transitional form" (or missing link, if you prefer).
  • Onychonycteris's teeth are very much like those of modern bats; it was almost certainly an insectivore.
  • This fossil organism already possesses a specialized spur-like bone that, in modern bats, supports a tail membrane that modern bats use to help them capture insects and which helps to support them in the air, whether flying or gliding.
According to one of the authors of the article about Onychonycteris, appearing in tomorrow's Nature,
There has been a longstanding debate about how bats evolved, centering around the development of flight and the development of the sonar system they use to navigate and hunt for prey... The three main theories have been that they developed the two abilities together, that flight came first, or that sonar came first. Based on the specimen described in this paper, we were able to determine that this particular animal was not capable of echolocating, which then suggests that bats flew before they developed their echolocation ability.

— Dr. Gregg Gunnell
University of Michigan Museum of Paleontology

It seems to me that this fossil also indicates that insectivory was the primitive condition of the bats' ancestors. Insectivory, then flight, then echolocation.

I haven't read the article in Nature yet, but my information above comes from Science Daily:
Bats Flew First, Developed Echolocation Later, Fossilized Missing Link Shows

The discovery of a remarkably well-preserved fossil representing the most primitive bat species known to date demonstrates that the animals evolved the ability to fly before they could echolocate. The new species, named Onychonycteris finneyi, was unearthed in 2003 in southwestern Wyoming. Bats represent one of the largest and most diverse orders of mammals, accounting for one-fifth of all living mammal species. The well-preserved condition of the new fossil permitted the scientists to take an unprecedented look at the most primitive known member of the order Chiroptera.
I'm waiting, of course, for the inevitable statement of denial from a Creationist group. Something like, "It's not an intermediate form because it had wings, so it's just another bat!" Someone out there has gotta do it, after all. Must keep those gaps open to give the creator/intelligent designer someplace to live.

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