An Echo from the Past

18th Feb 2008

The current edition of New Scientist (16th February) carries a small report entitled “Earliest Bats had no use for echos”.  The article describes the work of Nancy Simmons of the American Museum of Natural History (New York) and her team who found the fossil of an ancient bat (Onychonycteris finneiyi) in the Green River Formation in Wyoming. Their work has been published in detail in the journal Nature [Nature 451818-821 (14 February 2008)]. In the words of the authors of this paper:

The Green River Formation of Wyoming has produced many spectacular fossils, includingIcaronycteris index, widely regarded as the oldest and most primitive known bat. Icaronycteriscomes from the Fossil Butte Member of the Green River Formation, late early Eocene epoch, about 52.5 Myr ago. Recently a new bat was discovered in the Fossil Butte Member that differs from Icaronycteris and other Eocene bats in being larger, having more primitive limb proportions and basicranium, and possessing well-developed claws on all wing digits. Because it cannot be assigned to any existing taxon, we here describe it as a new family, genus and species.

Bats are unique among mammals as they possess the ability to fly and, in many cases, the ability to catch their prey using echolocation. Technically, bats are included in the order Chiroptera (meaning hand-wing) which is the second largest mammalian order with 966 species identified to date. There are two suborders, the Megachiroptera (megabats) and the Microchiroptera (microbats). All of the megabats belong to the same family (Pteropodidae) which includes the Old World fruit bats or flying foxes. The microbats are distributed across four superfamilies (17 families in total).

Richard Dawkins has devoted an entire chapter on echolocation in bats in “The Blind Watchmaker”. This is a useful overview of this remarkable phenomenon. In the second chapter, entitled “Good Design”, he writes: "Not all bats use echolocation. The Old World tropical fruit bats have good vision and only use their eyes for finding their way around." [The Blind Watchmaker(1991) London: Penguin p.24]

In fact, megabats have large eyes and exceptionally good colour eyesight, similar to the vision of an owl at night. All bats are mainly nocturnal, microbats rely on echolocation to catch their prey whereas most megabats rely on their eyesight and other senses.  As Professor Dawkins rightly points out, there are a few exceptions. An example is the Egyptian Fruit bat (Rousettus aegyptiacus) which also uses low pitched sounds and echo location in addition to its eyesight.

Characteristic of echo-locating bats is the increased size of the inner ear (cochlea) together with the associated complex neuroanatomy of the brain stem.  According to the Nature article, O. finneyi possessed a relatively small cochlea similar to that seen in modern non-echolocating bats (Pteropodidae). In addition, the New Scientist suggests:

... it lacked the special ear-bone modifications needed for echolocation - the ability to use reflected sound to find and identify objects while flying - and probably flew "deaf".

This of course is total nonsense as the article goes on to include the following quote from Nancy Simmons:

Without echolocation, Onychonycteris would probably not have been good at catching insects in flight, though it might have used vision or smell to help find insect prey. Like some modern bats, it might also have used ‘passive audition’, such as listening out for sounds that insects make when they crash into vegetation.

If the bat is listening out for sounds of “crashing insects” it can hardly be flying “deaf”. 

The feature that seems to mark out O. finneyi as different is the fact that it possessed teeth characteristic of an insectivore. Modern megabats, however, can and do eat insects as an extra source of protein [see SE Courts Dietary Strategies of Old World Fruit Bats: how do they obtain sufficient protein? Mammal Review (1998) 28:185].

And so the Green River formation continues to yield up its treasures. Within the same strata have been found fossil fish, insects, snails, turtles, crocodiles, birds, and plant remains. The new discovered species (Onychonycteris finneiyi) is now added to the list of mammals that also includes another ancient bat (Icaronycteris index).

Without any justification, New Scientist concludes as follows:

but it was not long before echolocating bats replaced O. finneyi. Fossils of another bat, Icaronycteris indexfound in the same rock, have skull bones consistent with echolocation [emphasis added].

The fact of the matter is that both echolocating and non-echolocating bats exist today and this seems to have been the case in ancient times as illustrated by the Fossil Butte sediments in Wyoming. The evidence clearly suggests that these ancient bats co-existed. They are an echo from the past.