Is this the origin of bat flapping?

Updated Sept 30, 2016 with the shifting of Protictis to the Carnivora. 

Here on this YouTube video
is a bat hanging upside-down on a branch (Fig. 1). And then it takes off and flies. At first it doesn’t go anywhere, but it is flying/hovering after the feet finally let go of the branch.

Note, like roterosaurs and birds,
the bat is also bipedal — only upside-down, so balancing on hind limbs is not an issue for bats. Hanging upside down is nearly universal in bats.

Figure 1. Is this the origin of bat flapping. From an inverted position, this bat rises to horizontal by flapping, still clinging to its perch until release and flight. Click to open video.

Figure 1. Is this the origin of bat flapping. From an inverted position, this bat rises to horizontal by flapping, still clinging to its perch until release and flight. Click to open video.

With bipedal birds and pterosaurs,
wings appeared to develop first as decorative ornaments and flapping appears to have developed as a secondary sexual behavior to enhance feathers, plumes, hairs, and membranes already in place.

Bats were never glitzy.
They were always dull and brown. They never developed elaborate pre-sex mating rituals. They were never visual creatures. So elaborate frills and colors never appeared on them.

Since bats hang bipedally inverted,
their forelimbs and especially their hands were free to develop into something else. In this case foldable wings. But to get there, everyone knows, there had to be transitional phases in bat evolution, each phase fully functioning and conferring a competitive or survival advantage. Those transitional phases have not been discovered yet in the fossil record, nor have they been visualized. One problem has been a lack of phylogenetic bracketing, which only reptileevolution.com has provided (Fig. 2, as far as I know). Even so this is a fairly broad bracket.

Figure 2. Chriacus and Onychonycteris nest as a sister to the undiscovered bat ancestor and a basal bat. Miniaturization was part of the transition. So was enlargement of the manus. It is still a mystery why the transitional form decided to start flapping.

Figure 2. Chriacus and Onychonycteris nest as a sister to the undiscovered bat ancestor and a basal bat. Miniaturization was part of the transition. So was enlargement of the manus. It is still a mystery why the transitional form decided to start flapping.

Taxa in our phylogenetic bracket:
At present we have Chriacus without wings and Onychonycteris with wings. 

Figure 4. Ptilocercus, Icaronycteris and a hypothetical transitional taxon based on the ontogenetically immature wing of the embryo Myotis. If you're going to evolve wings it looks like you have to stop using them as hands early on. Note in the bat embryo there is little indication of inter-metacarpal muscle. That area looks identical to the web.

Figure 4. Ptilocercus, Icaronycteris and a hypothetical transitional taxon based on the ontogenetically immature wing of the embryo Myotis. If you’re going to evolve wings it looks like you have to stop using them as hands early on. Note in the bat embryo there is little indication of inter-metacarpal muscle. That area looks identical to the web.

Chriacus did not flap. Onychonycteris did.
Both were arboreal, long-limbed mammals. The big question everyone has been wondering is the how and why for transitional taxon flapping.

In the above video
we can see the inverted bat rising to a horizontal configuration while flapping and maintaining its perch with its hind limbs. This same action could be done with smaller wings in incremental steps, with advancements originating with little to no membrane and improving to a full-fledged membrane and extended fingers.

Note,
in pre bats this would not be considered flying. Instead it might be considered rowing through the air. And, except for grounded bats, there is little to no contribution from the hind limbs in becoming airborne. Instead, they simply let go. This is in direct contrast to colugos, Ptilocercus, primates, and arboreal pre-rabbits, all of which had leaping hind limbs. 

The next question is,
why would an inverted bat want to rise to a horizontal configuration when it has no intention of flying?

To get to the other branch
when a pre-bat detected an insect on another branch. (Before it could attack flying insects, a prebat had to attack crawling insects, some of which were resting after a flight or were just crawling out of a pupae or hive.)

The above video was taken in a lab environment
with a single branch and a single bat. In the natural world of the Paleocene there would have been more branches available (perhaps a tangle of vines and branches). Rising to the horizontal by vigorous flapping would have put other branches within reach. Evidently, once bats had traded grasping hands for flapping hands with grasping thumbs, there was no turning back. A horizontal branch works better than a tree trunk in providing clearance for flapping arms/wings.

Bats are not like slow-moving colugos.
When not hibernating or resting, bats are active little engines of flight. We can imagine their unknown ancestors were also active little predators, with an unusual method for getting around. From vines and branches to open air flying, they took risks, experienced failure as they parachuted into the leaf litter, then climbed another tree to start all over again. Over time some bats found they could reach further branches by flapping faster, with larger strokes with longer fingers provided with broader membranes.

Bat flight is different than bird and pterosaur flight.
According to Tian et al. 2006, “The kinematic data reveals that, at the relatively slow flight speeds considered, that the wing motion is quite complex, including a sharp retraction of the wing during the upstroke and a broad sweep of the fully extended wing during the downstroke. In some respects it is almost like the animal is “rowing” rather than flying!”

In other words,
bats don’t rely so much on

Bat carpus (wrist) issues

The bat wrist (carpus)
is interesting. And I’d like to know more about it than I do.

Figure 1. Bat carpus compared to that of the basal bat Onychonycteris, and two other mammals, Home and Ptilocercus.

Figure 1. Bat carpus compared to that of the basal bat Onychonycteris, and two other mammals, Home and Ptilocercus. The pink bone is the pisiform. Some carpals appear to fuse in certain taxa. Note that in Homo and Ptilocercus the carpals are locked together, but rotate as a unit proximally. That is not the case with bats, which appear to have mobility within the carpals, not at the proximal articulation with the radius and ulna. On Onychonycteris digit 1 is preserved below the other digits and so is hidden in this graphic.

The wrist bones 
of Homo and Ptilocercus are pretty well locked in as a unit, but a sliding joint permits movement (rotation in two axes) at the radius and ulna.

Bats appear to be different,
perhaps due to their ability of completely fold their hands (wings) against their antebrachia (radius + ulna or forearm), in the manner of birds and pterosaurs, in the plane of the wing.  Above (Fig. 1) I have attempted to identify homologous bones. Please advise if any mistakes were made.

In the basal bat,
Onychonycteris, the carpal bones were not locked into place. This could be a taphonomic issue with elements drifting. Or perhaps it represents an early stage in bat carpus evolution. Or both.

In the only bat carpus illustration
I could find (genus unknown), the proximal carpals appear to be fixed to the ulna and radius. The distal carpal elements appear to be loose. The metacarpals appear to overlap like Japanese fan struts. I don’t see the same proximal metacarpal expansion in the basal bat Onychonycteris.

Since bat wing claws are still present
on all of the fingers of Onychonycteris and with wrist rotation in the plane of the wing, the claws would still have been oriented toward the palmar side and thus could be used. Or if not used, this orientation of the fingers and claws provides clues to those of ancestral bats with smaller hands not yet used for flying that might have been likewise rotated 180 degrees while oriented ventrally on tree trunks and other vertical substrates.

I have asked for,
but have not yet received, the pdf for Greene 2005 listed below. I’m sure there will be more to say on this subject when that pdf comes in.

References
Greene WE Jr. 2005. The development of the carpal bones in the bat. Journal of Morphology 89(3):409-422. Article first published online: 6 FEB 2005 DOI: 10.1002/jmor.1050890303

 

Origin of bats 2

Updated Sept 20, 2016, with better data on Protictis and more taxa added to the mammal clade.  See Part 4 here. It solves many of the problems attending the origin of bats.

One of the most popular blogposts here,
year after year, has been the post on bat origins back in 2011. Nothing has changed since then except for the fact that I have added a few bats and kin to the large reptile tree (Fig. 1, subset) and Protictis has moved to the Carnivora following better data.

Figure 2. Bat origins cladogram. Here Onychonycteris and Pteropus represent bats.

Figure 2. Bat origins cladogram. Here Onychonycteris and Pteropus represent bats.

And here (Fig. 2), for good measure are Chriacus and Onychonycteris, a bat ancestor candidate and a basal bat respectively, according to the large reptile tree.

Figure 2. Chriacus and Onychonycteris nest as a sister to the undiscovered bat ancestor and a basal bat. Miniaturization was part of the transition. So was enlargement of the manus. It is still a mystery why the transitional form decided to start flapping.

Figure 2. Chriacus nests as a sister to the undiscovered bat ancestor. Onychonycteris was a basal bat. Miniaturization was part of the transition. So was enlargement of the manus. It is still a mystery why the transitional form decided to start flapping. Click to enlarge.

 

Phylogenetic miniaturization contributed to bat origins. The teeth became better adapted to insect eating. The larger scapulae and clavicles anchored larger muscles. The ulna became reduced relative to the radius and fused to it. The hands became enlarged. Membranes spanned the forelimbs and hind limbs. This is the only flapper that did not have an obvious bipedal phase.

It is still a mystery what evolutionary events spanned these two taxa. The rest has to be imagined.

Figure 2. Known bat ancestors to scale. Click to enlarge.

Figure 2. Known bat ancestors to scale. Click to enlarge.

The earliest bats – svp abstracts 2013

From the abstract
Padian and Dial (2003) wrote: “Reconstructing ancestral character states in the evolution of chiropteran flight is challenging because, whereas the earliest bats are clearly capable of flight, their immediate laurasiatherian outgroups provide no useful information about intermediate states between fully volant bats and their terrestrial ancestors. To date, the postcrania of early bats have received less attention than the skull and teeth, and the principal problem addressed has been whether flight preceded echolocation. New preparation of the postcrania of the most basal bat Onychonycteris and comparative character analysis of other basal fossil bats (Icaronycteris, Palaeochiropteryx) reveal a variety of unusual morphological features that shed light on the functional and ecological origin of bats. Newly revealed anatomical features of Onychonycteris and comparative re-examination of other basal bat postcrania demonstrates that the trochanters of the proximal ends of the femur in Onychonycteris were not typically mammalian but subequal, and the head was offset laterally, as in crown-group bats, indicating a relatively “modern” hip flexure and a lateral orientation of the hindlimb. However, details of the tarsus and the claws, both manual and pedal, in these basal bats suggest a different ecological context than would be evident from the basal conditions of living crown-group bats. The functional morphology of the forelimb and hindlimb indicate scansorial habits that imply climbing tree trunks and rock faces. Phylogenetic analysis, integrated with analysis of functional traits, suggests a sequence of simplification and reduction of claws, coupled with increase in wing size, and an eventual shift from generalized scansorial habits to inverted perching on substrates that led to the unusual pedal claw morphology seen in most living bats today.”

Added Jan. 23, 2019:
We looked at the origin of bats
in four part series here, here, here and the latest here. While lacking obvious transitional taxa, the large reptile tree (LRT, subset Fig. 1) provides bat ancestors back to Devonian tetrapods while testing 1380+ candidate outgroup taxa.

Notes
There’s a DNA vs morphology battle out there with regard to bats and their kin. Volitantia is the morphological clade uniting bats and dermopterans. Here’s how bat expert Eric Sargis reported the battle with documentation.

Sargis 2002 reported, “Based on morphological evidence, one must conclude that the sister taxon of Dermoptera is Chiroptera, not Primates (Szalay, 1977; Novacek, 1982, 1986, 1989, 1990, 1992, 1993, 1994; Novacek and Wyss, 1986; Wible and Covert, 1987; Novacek et al., 1988; Wible and Novacek, 1988; Thewissen and Babcock, 1991, 1992, 1993; Johnson and Kirsch, 1993; Szalay and Lucas, 1993, 1996; Wible, 1993; Simmons and Quinn, 1994; Simmons, 1995; Shoshani and McKenna, 1998; Kriz and Hamrick, 2001; Sargis, 2001b, in prep.; Silcox, 2001a,b, 2002; contra Beard, 1989, 1993a,b; McKenna and Bell, 1997).

Sargis 2002 also reported, “Alternatively, molecular evidence has continually rejected Volitantia (Cronin and Sarich, 1980; Adkins and Honeycutt, 1991, 1993; Honeycutt and Adkins, 1993; Allard et al., 1996; Porter et al., 1996; Liu and Miyamoto, 1999; Waddell et al., 1999; Liu et al., 2001; Murphy et al., 2001a,b), and it is certainly possible that  dermopterans and chiropterans evolved their similarities independently in relation to gliding and flying, respectively. If this is the case, then Volitantia is also an unnatural grouping based on convergent, rather than homologous, characters.”

Figure 4. Mesozoic euthrerians (placentals, in black). Later taxa in light gray. All taxa more primitive than Mesozoic taxa were likely also present in the Jurassic and Cretaceous. None appear after Onychodectes. Madagascar separated from Africa 165-135 mya, deep into the Cretaceous with a population of tenrecs attached. No rafting was necessary. 

Figure 1. Mesozoic euthrerians (placentals, in black). Later taxa in light gray. All taxa more primitive than Mesozoic taxa were likely also present in the Jurassic and Cretaceous. None appear after Onychodectes. Madagascar separated from Africa 165-135 mya, deep into the Cretaceous with a population of tenrecs attached. No rafting was necessary.

Sargis 2002 also reported, “Molecular evidence, on the other hand, has repeatedly supported a Scandentia-Dermoptera clade (Liu and Miyamoto, 1999; Liu et al., 2001; Madsen et al., 2001; Murphy et al., 2001a,b) with Primates as the sister taxon to this clade (Liu and Miyamoto, 1999; Liu et al., 2001; Murphy et al., 2001a,b).

It keeps getting less traditional, more heretical.
Ptilocercus and Tupaia are basal members of Glires in the large reptile tree (Fig. 1).

Phylogenetic analysis (the bat tree) nests Chriacus (this taxon as noted by Halliday et al. 2013), and the Cretaceous pangolin ancestor  Zhangheotherium and the long-legged Cynocephalus as the closest sister groups to bats. We looked at bat origins earlier here based on morphological, not DNA evidence.

As noted earlier,
pterosaurs and birds had bipedal ancestors. Bats did too, inverted as they configure themselves nowadays. That alone differentiated them from quadrupedal ptilocercids and dermopterans.

The hypothetical origin of bats likely resembled the taxa here (Fig. 2). I have no answer for why DNA doesn’t match morphology in bats, basal lizards and turtles, among other clades. But if you look up Cynocephalus on ReptileEvolution.com, you’ll be amazed how bat-like it is. Ptilocercus is informative in an analogous way.

The evolution of bats beginning with Nandinia.

Figure 2. The evolution of bats beginning with a sister to Nandinia. Click to enlarge.

References
Halliday T, Upchurch P and Goswami A 2013. A phylogenetic analysis of palaeocene mammals. Journal of Vertebrate Paleontology abstracts 2013.
Padian K and Dial KP 2013. New morphological data illuminate hindlimnb function and the ecological context of flight in the earliest bats. Journal of Vertebrate Paleontology abstracts.
Sargis EJ 2002. The Postcranial Morphology of Ptilocercus lowii (Scandentia, Tupaiidae); An Analysis of Primatomorphan and Volitantian Characters. Journal of Mammlian Evolutin 9(12):137-159.

Bat Origins and DNA

Updated September 30, 2016 with the addition of taxa to the LRT and new data on Protictis.

Where DO Bats Nest? The Question Returns.
A renowned (unnamed) professor interested in the origin of bats questioned my morphological nesting of bats with Ptilocercus and Nandinia (among living taxa) and Palaechthon (among fossil taxa). The professor sent me a pdf of Meredith et al. (2011), the most recent DNA tree to lump and split living mammals, as his best hypothesis on bat origins.

Bats and their sisters

Figure 1. Bats and their sisters according to Meredith et al. 2011.

Mammal Diversification
Meredith et al. (2011) sought diversification patterns and times in mammals. They constructed a molecular supermatrix for mammalian families and analyzed these data with likelihood-based methods and relaxed molecular clocks. Their results came in traditionally, with Monotremata, Marsupialia and Placentalia at the base. The latter was divided into Xenartha + Afrotheria and all other placentals, which were divided into Laurasiatheria and Euarchontoglires.

DNA Results for Bats
Lots of bats were tested and they all lumped together in a single clade subdivided into three with fruit bats (Fig. 1 in orange) separating two microbat clades (in blue and green). Bats appeared as the unresolved sisters to Carnivora and Artiodactylia. Basal insectivores (not shown hre) nested as outgroup taxa to this super clade.

That’s an overly general nesting for bats that doesn’t provide much insight. On the other hand, I wasn’t surprised to see bats nesting so close to basal carnivores, like Nandinia and the vivverids, because the morphological results recovered the same relationship. I was surprised to bats nesting close to rhinos and camels.  :-) Pangolins are indeed close to bats, so we agree here (Fig. 2).

Figure 2. Bat origins cladogram. Here Onychonycteris and Pteropus represent bats.

Figure 2. Bat origins cladogram. Here Onychonycteris and Pteropus represent bats.

DNA Results for Flying Lemurs
The base of the Euarchontoglires (Meredith et al. 2011, not shown in Fig. 1) included tree shrews and demopterans. I wasn’t surprised to see rabbits nesting close to Tupaia, the common tree shrew, because the morphological results recovered the same relationship. I also wasn’t surprised to see Ptilocercus, the pen-tailed tree shrew, nesting close to the flying lemurs, because the morphological results recovered the same relationship. Note these taxa didn’t nest with bats in the DNA study, but they did all nest at or near their unresolved common base.

Figure 2. Known bat ancestors to scale. Click to enlarge.

Figure 2. Known bat ancestors to scale. Click to enlarge.

Morphological Results
The Meredith et al. (2011) results do not match the morphological evidence, which derives both bats and flying lemurs from a sister to Ptilocercus, a Paleocene pro primate and Chriacus, all close to basal carnivorans like NandiniaNandinia is a living carnivore that sometimes drops from trees and has an omniovorous diet. Chriacus was a long-legged tree-dwelling omnivore. Phylogenetic bracketing indicates that post-cranial characters were something like Chriacus and/or PtilocercusPtilocercus is a flying lemur ancestor, but shares with bats several characters including flat ribs, a high floating scapula, wide cervicals, a rotating carpus and metatarsal + phalanx ratio similarities.

The question is…
why don’t the DNA results more closely match the morphological results, and vice versa?

DNA results cannot include fossil taxa. With bats evolving prior to the Eocene (52 mya), fossil taxa are necessary in any study on bat evolution.

The DNA of modern tree shrews and bats, etc. is not the same as the DNA of Paleocene tree shrews and bats, etc.

The Meredith et al. (2011) evidence indicates that DNA results for large clades of mammals  cannot resolve large clades. DNA and amino acid results do not agree with one another in the case of large reptile clades and the same is true in large mammal clades. DNA and amino acids apparently become more useful the more closely taxa are related. The resolution is very high, for instance, in human DNA, which is why it can be used in criminal investigations.

On the Other Hand
In fossil evidence you can point to a long list or suite of homologous morphologies, from tooth cusps to phalanx ratios. DNA results cannot provides these details. Morphology will always trump DNA, especially when bats nest with camels in DNA studies. DNA can only be verified with morphological evidence. DNA results can guide our efforts but the bottom line is morphology. The Meredith et al. (2011) study was unable to provide a specific sister taxon to bats. The morphological study provided Chriacus. When closer sisters are discovered, they will be reported.

Dermopterans and Bats
Flying lemurs nested close to bats and bat babys have short fingers like those of flying lemurs. Problem is: Ptilocercus, which comes between the two, has no extradermal membranes or webbed fingers and its limbs are not elongated. I have no answers for that other than both bats and flying lemurs are about 60 million years old and likely had a common long-limbed ancestor with extradermal membranes in a sister to Ptilocercus. Or bats and flying lemurs both developed extradermal membranes by convergence. Or Ptilocercus lost its ancestral long limbs and membranes.

Can we trust results?
In science we don’t trust anything. Not DNA. Not morphology. Everything is tentative and provisional.

 

References
Meredith RW et al. 2011. Impacts of the Cretaceous Terrestrial Revolution and KPg Extinction on Mammal Diversification. Science 334:521-524.

The Origin and Evolution of Bats

Parts 2 and 3 of this subject were posted here and here.
See Part 4 here. It solves many of the problems attending the origin of bats.

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.

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.

Current Views
Gunnell and Simmons (2005) reported, “The phylogenetic and geographic origins of bats (Chiroptera) remain unknown.” Wiki 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 treeshrewscolugos, 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 carnivoranspangolinsodd-toed ungulateseven-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?

Figure 1. Hypothetical bat ancestors arising from a sister to Chriacus, which may be a large late survivor of a smaller common ancestor.

Figure 1. Hypothetical bat ancestors arising from a sister to Chriacus, which may be a large late survivor of a smaller common ancestor.

Phylogenetic Analysis
Here (Fig. 2) bats nest with Panprimates, specifically: Chriacus, Palaechthon and Ptilocercus in order of increasing distance to bats. Essentially bats were derived from small, tropical arboreal mammals with an omnivorous diet.

Figure 2. Bat origins cladogram. Here Onychonycteris and Pteropus represent bats.

Figure 2. Bat origins cladogram. Here Onychonycteris and Pteropus represent bats.

The Family Tree of Bats
Here (Figure 2) Chriacus is the closest sister taxon to bats and Ptilcercus (Fig. 2) is a close second. Fossil 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.

Colugos are sisters to Ptilocercus, the extant pen-tailed tree shrew. Formerly tree shrews were associated with primates.

Arboreal Chriacus was considered close to the ancestor of the Artiodactylia (hooved mammals and whales). And that is why the long list of  Laurasiatherian mammals (see above) comes into play.

The Hands of Bats
Baby bats have short fingers, so they more closely resemble little colugos. 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.

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 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 Nandinia and Ptilocercus feed by creeping up on resting prey, whether birds, eggs or grasshoppers. With stealth, rather than 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 Pteropus and Ptilocercus, 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 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.
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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.

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