This post was updated January 11, 2023 with the addition of taxa
like Microcebus, the gray mouse lemur (Fig 1), now nesting basal to bats. Chriacus now nests with Chironectes, the extant water opossum in the large reptile tree (LRT, 2202 taxa).
Figure 1. A tiny basal bat from the Green River formation with reconstructed skull compared to the skull of Microcebus, the gray mouse lemur.
Scientists have long wondered about
the origin and evolution of bats. Bats seem to have appeared ready to fly at their first appearance in the fossil record. Even so, it is possible to determine their ancestors with cladistic analysis and a sufficient number of taxa.
Figure 2. Subset of the LRT focusing on bats and their ancestor, Microcebus, a mouse lemur.
The most primitive known bats
include Onychonycteris and Icaronycteris. Modern bats, like Myotis, are either small insectivores (with some nectar-, blood- and fish-eating thrown in) or large fruit-eaters, like Pteropus. [Extant Tadarida is the most primitive tested bat in the LRT.]
Current Views
Gunnell and Simmons (2005) reported, “The phylogenetic and geographic origins of bats (Chiroptera) remain unknown.”
Wikipedia reports, “Little fossil evidence is available to help map the evolution of bats, since their small, delicate skeletons do not fossilize very well. Bats were formerly grouped in the superorder Archonta along with the treeshrews, colugos, and the primates, because of the apparent similarities between Megachiroptera and such mammals. Genetic studies have now placed bats in the superorder Laurasiatheria along with carnivorans, pangolins, odd-toed ungulates, even-toed ungulates, and cetaceans.”
That’s a big list. Way too general. Most workers nest bats between Insectivores and Carnivores. Again, way too general. Let’s get specific, shall we?
Phylogenetic Analysis
Here in the LRT (subset Fig. 2) bats nest with the gray mouse lemur, Microcebus. Bats and primates are sister clades.
An updated subset of the family tree of bats in the LRT
is here (Fig 2). Fossil and extant mammals are rarely used in phylogenetic analyses of bat origins. Most workers prefer molecule analysis. Others have mixed bat and mice genes to get mice with longer limbs.
Chriacus was considered close to the ancestor of the Artiodactylia (hooved mammals and whales). Perhaps that is why the long list of mammals (see above) came into play.
Figure 3. An earlier view of bat origins starting with Ptilocercus here are several hypothetical transitional taxa leading to Onychonycteris, a basal bat. This has been updated with the addition of taxa. See figure 4 for an updated origin of bats.
The hands of bats
Baby bats have short fingers (Fig 3). The “hands” of adult bats have become so transformed that they can no longer be used to support the body in a typical mammalian manner. In the only other flying vertebrates, pterosaurs and birds, a bipedal phase enabled their “hands” to rise off the substrate and in time, become wings. The same is hard to imagine with bats because nothing about their anatomy suggests that bat ancestors were ever traditional bipeds. However, all bats hang by their feet, so they may be considered inverted bipeds — leaving their hands free to develop into something else.
Figure 4. Updated origin of bats graphic. Microcebus, the basalmost bat in the LRT, compared to fossil bats.
Like birds and pterosaurs,
bat hand/wings fold up for compact storage between deployments. The bat wrist folds and rotates to a much greater extent than in any other mammal and the metacarpals spread much more widely. As bat embryos develop, their metacarpals are widely abducted. Finger bones develop within the round buds that all tetrapod embryos have, but in bats there is no cell death between the digits to free them from one another. Thus the fingers remain webbed.
The hind limbs of bats
In similar fashion, the hind limbs of bats no longer operate like those of typical mammals. The pelvic openings and femora permanently splay the limbs in a lizard-like configuration. Together with a loose ankle joint, bats use this configuration to hang inverted with soles oriented ventrally. The question is: did the hind limbs lose their traditional abilities before or after the arrival of wings?
Comparisons to birds and pterosaurs
Pterosaurs and birds have similar pectoral girdles. Their scapulae are braced by immobile coracoids and anchored by close bony connections to their ribs and vertebrae. They flap their arms/wings principally with huge pectoral muscles anchored on huge sternal plates and keels.
In bats, however, there is no huge sternum and no coracoid to lock the scapula in place. Instead bats essentially flap their shoulder blades from spine to side, pivoting them on the proximal clavicles articulating with the narrow shallow sternum. Giant back muscles anchored on low wide vertebrae and broad flat ribs provide the power. Yes, the pectoral muscles are massive, but in essence bat arms/wings ‘go along for the ride’ as the scapulae swing back and forth through huge arcs.
Muscle attachments aside, broad ribs increase stability and decrease mobility in the thorax and vertebral column. Decreased thoracic mobility appears to be a preadaptation for flight, as demonstrated by birds and pterosaurs.
Comparisons to Ptilocercus (pen-tailed tree shrew)
Like bats, the carpals (wrist bones) of Ptilocercus (Fig 3) are able to rotate laterally much more so than is typical for other mammals. This facilitates hanging from and climbing down tree trunks head first, as in bats. Some civets also do this, but colugos never do. Ptilocercus has been observed climbing inverted on horizontal branches, as in colugos and bats. Like bats, Ptilocercus can spread its metacarpals, to such an extent that finger #1 opposes #5. This permits branch grasping in a fashion more typical of primates than carnivores. With such hands, Ptilocercus stalks and pounces on its insect prey, then shoves the meal into its mouth. At times Ptilocercus sits on its haunches to feed at leisure while holding prey. Nandinia, the palm civet, has similar habits. Bats no longer capture prey in this manner in trees, but continue to do so in the air.
Like bats, the femora of Ptilocercus are able to spread widely. Pen-tailed tree shrews are better adapted to belly-crawling and tree-clinging than to running and leaping. The ankles are similarly loose and permit rotation of the feet, soles down, but not to the same extent seen in bats. While the toes in civets and Ptilocercus are able to oppose one another for branch grasping, this ability is not as developed as in primates. In bats this ability is lost. Ptilocercus and some civets are plantigrade or flat-footed, as in bats and other primitive mammals.
Like bats, the long tail of Ptilocercus is not fur-covered (except at the tip). Like bats, Ptilocercus gives birth to one pup (rarely two) at a time. Like bats and Nandinia, Ptilocercus is nocturnal. Like bats, Ptilocercus changes its body temperature to fit climatic conditions, but not to the same degree. Civets are generally solitary. Ptilocercus sometimes nests in groups. Bats are typcially communal.
Hypotheses for the Development of Wings in Bats.
Post-dusk and pre-dawn basal placentals like Nandinia, Microcebus (Figs 3, 4) and Ptilocercus (Fig 2) feed by creeping up on resting prey, whether birds, eggs or grasshoppers. With stealth and speed, they grab their prey with their “hands” before shoving their meals into their mouths.
Given these phylogenetic starting points,
we should expect a hypothetical pre-bat to do the same, but in a more specialized manner. If this pre-bat had proportions midway between Myotis and Ptilocercus (Fig 2), it would have a larger scapula than Ptilocercus, double the arm length, four times the hand length, a thirty-percent longer leg, half the length of tail and an overall increase in claw size. At this point the pre-bat would cease using its fore and hind limbs in traditional locomotion to become a sit-and-wait predator. Inverted it might stand almost motionless, locked onto rough tree bark by feet in which the metatarsals are reduced and the toes lengthened so as to conform more closely to the irregular substrate, like those of bats. This configuration is also used by nursing bats to attach themselves to their mother. After waiting for an insect to come within range, the pre-bat would extend elongated fingers to cage the prey item before attacking with its teeth.
The ability of bats to enter torpor,
and thus to remain motionless for long periods of time, as well as their general inability to walk in a traditional fashion supports this “sit-and-wait” hypothesis. If valid, the legs lost their traditional abilities to leap and run before the onset of flight.
Finger 2 in bats is much shorter than 3-5,
which supports the “finger cage” hypothesis. As in the hands of Ptilocercus, bats and humans, as fingertips 3-5 touch a flat surface, fingertip 2 remains elevated. Thus in the wings of Pteropus and Icaronycteris only digits 1 and 2 retain claws and they are much shorter. Essentially bats fly with only digits 3-5.
At some point in the genesis of bats the skin between the pre-bat’s fingers
was not diminished during embryogenesis and the enlarged hand snare became complete. Of course, the fingers would have to be kept together during the sweep forward. Otherwise they would act like twin parachutes, slowing the adduction of the hands and betraying their imminent arrival by the advancing gust they would produce – unless they moved very slowly.
Flight as a Means to Escape Predators
Provided with such hands, a pre-bat would not only have sufficient membrane to drop and glide, but the distal development of those membranes could provide thrust if flapped. Flapping is not an option for the colugo, Cynocephalus, with its extended proximal membranes and smallish hands. It can only glide and does so very well. Nandinia has no gliding membranes whatsoever, but it has been observed free-falling from trees over and over in a spread-eagle configuration, apparently in play. This technique might also be used to avoid aerial and arboreal predators, such as birds, snakes and army ants. Ptilocercus has not been observed falling from trees, but its diminutive size would preclude damage if falling into leaf litter. If a predator approached our hypothetical pre-bat, and traditional forms of escape (i.e. running and leaping) were no longer in its forte, survival would depend on dropping and finding another safer perch. Flapping and the continuous development of the ability to fly, of course, would open up grand new vistas of unoccupied niches. The Big Bang of Eocene bat evolution that followed the origin of bats is a testament to that.
References
Cope ED 1882. Synopsis of the Vertebrata of the Puerco epoch. Proceedings of the American Philosophical Society 20:461-471.
Gunnell, GF and Simmons NB 2005. Fossil evidence and the origin of bats. Journal of Mammalian Evolution 12: 209-246 (2005).
Mac Intyre GT 1962. Simpsonictis, a new genus of viverravine miacid (Mammalia, Carnivora). American Museum Novitates 2118:1-4..
Matthew WD 1937. Paleocene faunas of the San Juan basin, New Mexico. Transactions of the American Philosophical Society, new series 30: 1-510.
Simmon NB, Seymour KL, Habersetzer J, Gunnell GF 2008. Primitive Early Eocene bat from Wyoming and the evolution of flight and echolocation. Nature 451 (7180): 818–21. doi:10.1038/nature06549. PMID 18270539.
Simpson GG 1935. New Paleocene mammals from the Fort Union of Montana. Proceedings of the U. S. National Musem 83: 221-244.
wiki/Onychonycteris
The Pterosaur Heresies 