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 hasnít 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 alís 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 alís 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 Ė 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.