Two years ago, "the most primitive bat known" was reported in Nature. It was not primitive in its wings and body, but "the morphology of the ear region suggests that it could not echolocate, making it a possible intermediate link between bats and their non-flying, non-echolocating mammalian ancestors". At the time, the find was suggested to settle the question as to which came first: flight or echolocation? The answer was a definite flight first.
"The problem of understanding bat evolution dates back at least to Charles Darwin, who in The Origin of Species enumerated a list of difficulties he saw with the theory of evolution by natural selection. The example often discussed is the origin of the eye. But Darwin also mentioned the vexed issue of how bats had arisen from terrestrial ancestors." Speakman 2008).
Onychonycteris finneyi was featured on the cover of Nature in February 2008 (Source here)
This assessment of the fossil must now be reappraised. New work on modern-day bats has revealed another mechanism of echolocation. One of the authors described the work thus:
"We borrowed 35 specimens from the Royal Ontario Museum in Toronto and performed micro-computed tomography on them. This imaging technique allowed us to see the fine details of the bats' ear and throat regions: the larynges, stylohyals and tympanic bones. Previous work had relied on dissecting these bones, a challenge in animals as small as bats. We found that the fusion or connection of two bone structures - the stylohyal bone in a bat's throat and the tympanic bone in the ear region of its skull - was a feature of all laryngeally echolocating bat species we studied."
This finding is new and unexpected. It means that previous conclusions need to be reappraised. This is exactly what is happening with the "most primitive fossil bat".
"The relatively small cochleae and lack of paddle-like expansions on the cranial tips of the stylohyal bones have been interpreted as evidence that O. finneyi lacked laryngeal echolocation, which supports the hypothesis that flight evolved before echolocation. However, we find that articulation between the stylohyal and tympanic bones is a better predictor of laryngeal echolocation ability than the shape of the stylohyal bone, at least among extant bats. If the stylohyal bones articulated with the tympanic bones in O. finneyi, then we propose that this species had the capacity for laryngeal echolocation. Our results thus reopen basic questions about the timing of the appearance of echolocation and flight in the evolution of bats."
There are several lessons to be learned here - and one of these is to always be prepared to hold judgment on the word "primitive" - even if it is said to be the "most primitive"!
A bony connection signals laryngeal echolocation in bats
Nina Veselka, David D. McErlain, David W. Holdsworth, Judith L. Eger, Rethy K. Chhem, Matthew J. Mason, Kirsty L. Brain, Paul A. Faure & M. Brock Fenton
Nature 463, 939-942 (18 February 2010) | doi:10.1038/nature08737
Echolocation is usually associated with bats. Many echolocating bats produce signals in the larynx, but a few species produce tongue clicks. Here, studies show that in all bats that use larynx-generated clicks, the stylohyal bone is connected to the tympanic bone. Study of the stylohyal and tympanic bones of a primitive fossil bat indicates that this species may have been able to echolocate, despite previous evidence to the contrary, raising the question of when and how echolocation evolved in bats.
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