Bat Evolution
Simmons
et al Nature 451 2008 818
and commentary Speakman p774 same volume
Which came
first the flight or the echolocation?
Until the
eighties the answer was echolocation. This was because some terrestrial animals
such as some shrews show rudimentary echolocation and because extant primitive
bats tend to hunt using echolocation from a perch rather than actively on the
wing (aerial hawking), plus if bats started out nocturnal surely finding their
way came before flight? These primitive
bats and ancient bat fossils also lack a calcar which
is a piece of cartilage that forms a spur from the hind limb towards the tail
which helps support the wing membrane around the
tail.
During the
eighties evidence emerged that generating the sound pulses necessary for
echolocation was energetically costly unless making those pulses is coupled to
wing beats when the fact that those muscles are already in use makes
echolocation virtually cost free. And flight first gained supporters even if
this meant that early bats may have to have been diurnal.
Fossil
study hasn’t really helped because the earliest bat fossils from the Eocene
around 50mya are similar to extant bats, and because of their enlarged cochleae suggest that they are already echolocators.
Simmons et al’s two fossils dating
from 52.5mya change things.
Named Onychonycteris finneyi it is a
medium sized bat but larger than other Eocene fossils with claws still present
on each of the digits. The previous most primitive fossil Icaronycteris only has a claw on
the second digit.
Onychonycteris has a very low
aspect ratio (wingspan squared divided by wing area) of 1.74 which compares
with modern small
bats with aspect ratios of 5 or 6. Simmons
et al believe that the robust clavicle, keeled sternum and modified scapula
indicate powered flight and connect the low aspect ratio to extant mouse tailed bats that flutter and
glide more than fly.
The
scatter diagram of intermembral (arm length divided
by leg) index against brachial (radius length/ humerus)
index places Onychonycteris
closer to the flying lemur or even sloth than to extant or previous fossil
bats.
The calcar is elongated in Onychonycteris but absent from Icaronycteris and
other similarly aged fossils. Simmons suggests this absence may be an artefact
of preservation.
The
authors present a scatter diagram of cochlea width vs
basicranial width for extant and fossil bats.
Generally these fall onto three trend lines, large cochlea extant echolocators, intermediate sized cochlea extant echolocators and fossil bats and non echolocating
extant fruit bats. It is onto the non echolocating
line that Onychonycteris
falls, but there is some overlap in the scatter in the relevant area.
Simmons et al’s bat with claws and
low aspect ratio appears more primitive than other bat fossils, but has the
modifications to the thorax associated with powered flight in bats and has a calcar, most importantly the cochleae suggest that it was
not echolocating – unfortunately the eye orbits were
crushed and so no conclusions can be drawn as to whether Onychonycteris had the enlarged
eyes associated with non echolocating nocturnal
mammals.