YouTube video supports newest bat origin hypothesis

Figure 1. The false vampire bat hovering before attacking a mouse in dry fallen leaves, listening to locate is prey.

Figure 1. The false vampire bat hovering before attacking a mouse in dry fallen leaves, listening to locate is prey in accord with a hypothesis of bat origins first presented here. The first pre-bats were not as adept at falling on prey, but refinements followed.

Earlier we looked at a new hypothesis for bat origins
that separated the distance gliding origins of small-hand colugos from the accurate falling, flapping origin of big-hand bats. Today readers get to see a video (below, Figs. 1, 2) showing that ancient and original behavior – still retained by the carnivorous wooly false vampire bat (genus: Chrotopterus). This may not be the most primitive extant bat, but this video demonstrates the predatory behavior that led to the origin of bats:

  1. inverted hanging >
  2. falling on prey while flapping to brake its descent >
  3. covering the prey item with ankle-to-hand membranes >
  4. capturing the prey item with its mouth >
  5. leaving the scene of the attack with prey in tow to feed later.
Figure 2. Scenes from the video showing the stages in the bat attack on the mouse in the leaf litter.

Figure 2. Scenes from the video showing the stages in the bat attack on the mouse in the leaf litter. Note how the former nursery membrane, now a flight membrane, covers the prey, preventing its escape.

Click the video to view it.

Before bats had sonar
bats relied on rustling sounds in the leaf litter to find their rodent and insect prey. Gradually refining this ability is what led to sonar in micro bats.

Before bats could fly
inverted pre-bats fell from tree limbs, flapping their small hands to slow their inevitable descent. Gradually refining this ability, while gradually enlarging those big membraned bat hands is what led to slowing the decent, hovering prior to the attack and ultimately flying and chasing flying insect prey.

This bat origin hypothesis
solves the problem of bat flapping without display (as in theropods and fenestrasaurs) and without WAIR (wing-assisted inclined running, as in theropods and fenestrasaurs). Remember bats have very weak and rotated backwards hind feet. Bipeds they were, but inverted and non-cursorial, distinct from pterosaurs and birds.

Remember
colugos, bats and basal pangolins, like Zhangheotherium, were members of the clade Volitantia. This placental clade is close to metatherian stem placentals, like Monodelphis, that have ventrally open pouches. These pouches were originally to protect nursing underdeveloped newborns, then expanded to form nursery membranes, then further expanded and co-opted for gliding in colugos and flying in bats.

How wonderful
that some bats retain their original and ancient method of hunting, as shown in the video. So many times in paleo, the answer has been staring at us, out in the open, waiting for recognition. On that note, I have sent emails to several leading bat experts, referring them to the earlier blogpost on bat origins, asking for their feedback. None, so far, have responded.

References
photographer: Anand Varma

wiki/Chrotopterus

Advertisements

The origin and evolution of bats part 4: distance vs. accuracy

Earlier
we looked at bat origins here, here and here from several perspectives. Some of these are now invalid given the following scenario.

Today we’ll take a fresh look at
the behavior and traits of the closest bat relatives in the large reptile tree (LRT, 1233 taxa, subset Fig. 1) and see what they can tell us about bat origins. This is called ‘phylogenetic bracketing‘. In such a thought experiment we can put forth an educated guess regarding an unknown behavior or trait for a unknown taxa (e.g. pre-bats) if all related specimens share similar behaviors and traits inherited from a known or unknown last common ancestor.

We start off with a cladogram
focusing on bat relationships (Fig. 1) and take things one logical step at a time.

Figure 1. Subset of the LRT focusing on basal placentals, including bats.

Figure 1. Subset of the LRT focusing on basal placentals, including bats.

One. Living sister taxa.
The closest tested sister taxa to bats here (Fig. 1) are pangolins and colugos (flying lemurs) in order of increasing distance. The origin of bats and pangolins has remained a traditional enigma. Like the origin of pterosaurs and Longisquama, the surprise is, they are most closely related to each other, despite their current differences.

Two. Ancestral taxa
Th bat/colugo/pangolin clade had its genesis near the original dichotomy of placental mammals, when Carnivora split off from all others. At the next dichotomy the bat/colugo/pangolin clade split off from all others. So this clade is not far from an ancestral clades with living genera. Monodelphis, the short-tailed opossum today restricted to South America, nests just outside of all mammals with a placenta. Nandinia, the African palm civet, is a basal member of the Carnivora, somewhat larger than its Mesozoic forebearers.

Three. Timing for clade origins
The bat/colugo/pangolin clade had its origin in the Early Jurassic based on the more primitive egg-layers, Megazostrodon, Brasilitherium and Kuehneotherium in the Late Triassic and the much more derived arboreal multituberculate/rodent, Megaconus, in the Middle Jurassic. As you can see, Jurassic mammals remain extremely rare, currently represented only by the likes of Megaconus. Others will, no doubt, be discovered in time.

Four. Arboreality (tree niche)
Some bats, colugos and pangolins live in trees, and so do their last common ancestors, short-tailed opossums and African palm civets.

Five. Climbing trees
Bats no longer have to climb trees because they can fly. Colugos and pangolins both climb trees in a series of symmetrical short hops/extended reaches (colugo video, pangolin video), distinct from palm civets and short-tailed opossums, which put forth one hand at a time, like primates do.

Six. Descending trees.
Bats fly between trees. Colugos glide between trees. Pangolins use their prehensile tail to ease themselves down. The African palm civet drops out of trees in play. It also descends tree trunks like a squirrel, head first.

Seven. Nocturnal
Most bats, colugos, pangolins, palm civets and short-tailed opossums prefer to be active at night.

Eight. Omnivorous diet
Some bats eat insects, others prefer nectar or hanging fruit. Colugos prefer leaves, shoots, flowers, sap, and fruit. Pangolins eat ants. Palm civets and short-tailed opossums are omnivorous. African palm civets feed by holding their prey in their hand-like front paws, biting it repeatedly and then once dead, swallowing it whole.

Nine. Extradermal membranes
Colugos and bats both have extradermal membranes to their unguals that extend their glides in the former and enable flapping flying in the latter. Such membranes are lost in living pangolins, but the Early Cretaceous pangolin, Zhangheotherium appears to have scale-lined membranes between the elbows and knees. These were overlooked in the original description. The gliding membrane in colugos is fur-covered and camouflaged dorsally, naked underneath. In bats the flying membrane is naked, translucent and never camouflaged.

Ten. Mobile clavicle, interclavicle and scapula
The basal pangolin, Zhangheotherium, has a mobile clavicle-interclavicle and the large scapula rises above the  dorsal vertebrae, as in bats, but not colugos.

11. Sprawling femora
Zhangeotherium and bats share sprawling hind limbs, distinct from the more erect hind limbs of most limbed mammals.

12. Silent vs. noisy
African palm civets are noisy. Colugos and pangolins are largely silent. Bats are constantly chirping to one another and (micro-bats only) as part of their sonar attack system.

13. Enemies
All current enemies of bats (e.g. birds, snakes) evolved during or after the Late Cretaceous. Jurassic trees might have been a refuge for small early climbing mammals, like colugo, bat and pangolin ancestors. However…the minimally feathered, small theropod dinosaur, Sinosauropteryx, contained the jaws of Zhangheotherium, perhaps caught after descending from the trees or plucked out of lower branches. Certain pterosaurs (e.g. giant anurognathids) might have preyed on arboreal  mammals in the Jurassic, but no evidence of this is yet known.

FIgure x. Calcaneal spur in Zhangheotherium. Not venomous, but perhaps to anchor a uropatagium.

FIgure 2. Calcaneal spur in Zhangheotherium. Not venomous, but perhaps to anchor a uropatagium as in bats.

14. Calcaneal spurs
Hurum et al. 2006 originally considered the small spurs found on the calcaneum of Zhangheotherium (Fig. 2) similar to venom spurs found on the platypus, Ornithorhynchus. Phylogenetic bracketing indicates the closer homolog is with the basal bat, Onychonycteris, which has longer calcaneal spurs framing a trailing uropatagium.

Figure x. Monodelphis babies in an open pouch. This is how placentals began, slowly evolving from the less open pouch.

Figure 3 Monodelphis babies in an open pouch. This is how placentals began, slowly evolving from the less open pouch.

15. Newborns and mothers
All basal placental mammals give birth to helpless newborns that ride with the mother until mature enough to go out on its own. Monodelphis demonstrates a primitive version of this, protecting its ten young with lateral flaps of skin (Fig. 3). Carnivore mothers make nests for newborns (2-4 for African palm civets), but colugo, bat and pangolin mothers take their one or two babies everywhere they go, like marsupial mothers do. Zhangheotherium might have been fossilized with several newborns. (Fig. 4) and extradermal membranes between elbows and knees, as in bats and colugos. As we know from colugos, these extradermal membranes in basal pangolins (and Chriacus?) likely formed a playpen or nursery for developing young riding beneath their mother during the earliest stages of development.

Figure x. Zhangheotherium showing possible extradermal membranes (green) with keratinous scales (red) and several newborns scattered in the abdominal area, similar to Monodelphis in figure x.

Figure 4. Zhangheotherium showing possible extradermal membranes (light blue and green) with keratinous scales (red) and several newborns scattered in the abdominal area, similar to Monodelphis in figure x. These amorphous blobs with tiny tail bones need further inspection. Some may just be stains and shapes.

16. Curling (flexing the spine)
Mother opossums, palm civets, colugos, bats and pangolins are able to curl their spines so much that the mother’s mouth is able to assist wiggling newborns climb to the abdominal nipples. This curling ability is co-opted by pangolins as they defend themselves by rolling into a tight ball and by bats that catch prey in their tail before curling up to bite the victim as it is brought close to the jaws. Higher mammals lose the ability to curl ventrally in this manner. Humans and other primates have a limited ability to do this. Instead they use their hands. More derived mammals with stiffer backs have more developed newborns.

17. Upside-down vs. right-side up nursery for the young
Colugos may rest right-side up (preferring to hang from below a slightly leaning tree trunk) or upside down hanging by all fours beneath a horizontal branch. When doing so the mother’s extradermal membranes form walls making a protective nursery for the young ones.

By contrast, bats rest up-side down, sometimes hanging by only one locked foot. To fly bats simply release this foot lock, then plummet and start flapping. Bat membranes also provide a protective nursery for their young as they cling to their mothers’ chest and her wings fold over them.

Nowadays pangolins roll into a ball while nursing their young. Later in life, babies ride on the mother’s back and tail when able to do so. Zhangheotherium (Fig. 4) appears to have provided a colugo-like, but scale-lined membrane nursery for several growing babies. The late-surviving pre-bat, Chriacus (Fig. 5), likely did the same, based on phylogenetic bracketing.

18. Claws
Short-tailed opossums and African palm civets use their claws to climb trees and grab prey and fruit, bringing it to the mouth. So do basal primates. Colugos, bats and pangolins use their larger, curved claws principally to hang from trees, though living pangolins have co-opted their large claws to dig out ant and termite nests from trees and underground.

19. Distance vs. accuracy
Colugos leap and turn away from their tree trunk base in order to launch themselves into a glide. Can they do this while hanging beneath a branch? I don’t know. With their long limbs, colugos can just leap (without gliding) across gaps of 5m or more. With limbs extended, they can glide for 136m at 10m/second. Gliding is good for a quick escape from predators, and access to patches of food that are otherwise inaccessible. It does not save them energy to glide, let along climb back to a gliding height.

Bats drop from trees, then fly wherever they please, typically landing upside down on another high branch or cavern roof. The origin of bat flight enabled by flapping hyper-elongated webbed fingers is the key question here, and it is answered by combining all of the above numbered traits.

Before bats could fly Jurassic pre-bats had to climb trees, probably like colugos and pangolins do (see #5 above), before standing bipedally, but upside-down, on a horizontal branch. Why would they do that? To prepare to dive bomb insects on and in the leaf litter below. Here is where sonar became valuable, detecting insects in the leaf litter at night. Here is where the leaf litter became valuable, cushioning the early awkward landings of small dive-bombing pre-bats. Here is where flapping, even with small hands around colugo-like dermal membranes became valuable, at first in panic, then in gradually learning how to better direct the fall to cover the prey below.  (By analogy birds flap their wings vigorously while dropping to slow their descent.)

Upon landing the extended pre-bat nursery membranes ‘put a lid’ on the prey. Then, curling the tooth-line jaws toward the tail and the tail toward the jaws (see #16 above) spelled doom for the captured food item. Over time, larger fingers made better flapping parachutes. Ultimately flapping bats  learned how to hover before diving bombing their prey, like owls do. Later, after further development, bats gained the power and morphology to enable flight, slowly at first, then better and better to escape ground-dwelling predators and avoid having to climb a tree for the next attack. Only later did bats learn to use their sonar and flying skills on flying insects.

So what began as a small pouch, then a larger nursery membrane for bat and colugo infants became a killing zone for bat prey on the ground, another example of co-opting an old trait for a new behavior in derived taxa. Distinct from birds and pterosaurs, which used their nascent flapping behavior to ascend tree trunks to escape predators, create threat displays and slow their descents from branches, bats used their nascent flapping ability only to slow and direct their descent from branches. Distinct from colugos, which glided for distance, bats dropped for accuracy. Distance came later, after flight developed.

Remember the fall need not be far at first. Conifers can have very low branches and leaf litter can be a soft cushion for a mouse-sized mammal. Graduating slowly to higher branches provides bats a wider ‘field-of-view’ for their slowly developing sonar, and more time to develop flapping. Bat hind limbs are not long or heavily muscled. They are not good at leaping, like colugos.

Fruit eating bats could not have developed until flowering and fruit-bearing trees developed, later in the Cretaceous. The LRT and the fossil record indicates that fruit-eating bats are derived relative to smaller insect-eating bats. So sonar-emitting apparently was lost in fruit-eating bats, rather than never a part of their lineage. The great variation now seen in sonar-emitting bat morphology was likely developed during and after the Cretaceous, based on the current fossil record. I think we’ll find fully volant fossil bats in the Cretaceous someday.

I happened upon this idea while watching a pigeon descend from a roofline to a balcony beneath it and wondered if accuracy was more important for bats, while distance was more important for colugos. That distinction seems to be the key driver in both clades. In any case, it is important that any proposed scenario be viable at every point during the gradual evolution of new traits and behaviors. In this case, developing flapping forelimbs had to originate with a bipedal configuration, even it inverted. Developing sonar had to originate from simply listening to nocturnal insects and other small prey rustling in the leaf litter, not far below, gradually getting better in those families that randomly had slightly better skills once dive-bombing and trapping became the method for predation.

20. Bat ontogeny
Recapitulates this phylogenetic scenario. The fingers elongate last. 

21. Solitary vs. communal
Colugos and pangolins are solitary. So are African palm civets except when food is plentiful. Bats are communal, whether nesting in trees or caves. According to Kerth 2008, “Variable dispersal patterns, complex olfactory and acoustic communication, flexible context-related interactions, striking cooperative behaviors, and cryptic colony structures in the form of fission-fusion systems have been documented. tropical bats often form groups year-round, whereas sociality in temperate-zone species is sometimes restricted to certain times of the year. In most species, females form so-called maternity colonies to rear their young communally, whereas males are solitary, form groups of their own, or join female groups. In only a few species are both sexes solitary, meeting only to mate.”

Kerth concludes, “None of the three factors that I identify as important for the evolution of sociality in bats (ecological constraints, physiological demands, and demographic traits) can fully explain the frequency and diversity of group living in bats.”

Figure 1. Basal placentals at two scales, all arising from a Middle Jurassic sister to Monodelphis, based on the Earliest Cretaceous appearance of Zhangheotherium, in the lineage of pangolins.

Figure 5. Basal placentals at two scales, all arising from a Middle Jurassic sister to Monodelphis, based on the Earliest Cretaceous appearance of Zhangheotherium, in the lineage of pangolins..

22. Soles of the feet oriented opposite to those of most mammals
Distinct from most mammals, the knees of bats are splayed laterally, which should extend the toes laterally. However, the ankle is rotated another 90º producing a foot in which the soles are ventral during flight and while hanging. In the case of long-legged fish-eating bats, the feet help bring captured fish back to the mouth.

FIgure 1. Wondering if Chriacus had an inverted stance and dermopteran membranes? Comparisons to Onychonycteris and Pteropus.

FIgure 6. Wondering if Chriacus had an inverted stance and dermopteran membranes? Comparisons to Onychonycteris and Pteropus are shown. Yes, the knees are straight in derived fruit bats, bent in Onychonycteris and micro bats. The uropatagia are spread while inverted and while flying. Chriacus appears to be a much larger and much later-surviving version of much smaller Jurassic pre-bats. The membranes are conjectural and may have been lost in this large specimen, but it illustrates the possibility of a dive bombing taxon that covered prey like a casserole lid.

Why do bats hang upside down?
Without a phylogenetic or deep-time perspective, the following video is the best answer current bat workers can provide:

Bats are not using their wings to cool off.
A recent heat wave killed many fruit bats. They fell dead out of the trees (see below). None were creating a cooling breeze with their wings or extending their wings in a cooling fashion, like elephants sometimes do. Microbats that live in caves never have this problem.

Bat wings notes:

  1. Finger flexibility during flight varies greatly in bats.
  2. The flight stroke is otherwise bird-like with elbows raised above the back, nearly meeting at the midline, for maximum power at low airspeed, or less so for cruising at higher airspeeds.
  3. The large fingers do nothing else but push air for thrust and lift. They are not extended to cool the bat, nor do they extend or flash during courtship.
  4. Bat fingers hyper flex at the wrist to tuck away the flight membrane and reduce its surface area when not in use, as in pterosaurs and birds. When flexed they do little but envelope the bat and its clinging young.

Miscellaneous notes:

  1. Zhangheotherium was originally considered a symmetrodont mammal, but its teeth seems to converge with archaeocete whales in this regard. The reappearance of a more primitive symmetrodont molar shape is here considered an atavism in the evolution of toothlessness in both certain odontocetes and pangolins by convergence.
  2. The uncoiled cochlea of highly derived Zhangheotherium and multituberculates, has been traditionally considered a trait that nests these taxa in more basal branches of the mammal family tree. Here, in the LRT, these traits appear to be neotonous or atavistic developments that, taken alone, tend to confuse systematics. No traits should ever be taken alone to determine systematics. That would be ‘pulling a Larry Martin.’
  3. The initial splitting up of Pangaea in the Early Jurassic gave the previously dry climate a more lush, subtropical parade of cycads, conifers, ginkgoes and tree ferns. So there were plenty of standing and fallen trees for early mammals to gambol upon, learning how to climb and leap. The forest floor was likely cushioned with a carpet of leaves and fronds to absorb accidental falls and hunger-driven dive bombs mediated by fluttering pre-wings and large membranes co-opted for eventual flight.

Addendum
Video showing a bat descending on a mouse in leaf litter appears here.

References
Byrnes, Libby, Lim & Spence. 2011. Gliding saves time but not energy in Malayan colugos. Journal of Experimental Biology http://dx.doi.org/10.1242/jeb.052993
Hurum JH, Luo Z-X and Kielan-Jaworowska Z 2006. Were mammals originally venomous? Acta Palaeontologica Polonica 51(1): 1–11.
Kerth G 2008. Causes and Consequences of Sociality in Bats. BioScience, Volume 58, Issue 8, 1 September 2008, Pages 737–746, https://doi.org/10.1641/B580810
Online here.

Much more interesting than pterosaurs: BATS!

Evidently,
(Fig. 1) interest in the origin and evolution of bats blog post (September 21, 2011) far exceeds that of any other subject here at PterosaurHeresies.Wordpress.com. Every day of every week this single page has several to ten times the views of any other page. Curious about the numbers, I finally looked up the viewing history of this blogpost:

Figure 1. WordPress stats for the evolution and origin of bats page here at PterosaurHeresies.

Figure 1. WordPress stats for the evolution and origin of bats page here at PterosaurHeresies. 2018 could exceed 2017 at this rate.

Bats origins are fascinating
and in need of more precise data and hypotheses. I hope the present data spurs further discovery in this small corner of the reptile family tree. Parts 2 and 3 of this subject were posted here and here.

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

Figure 2. Hypothetical bat ancestors arising from a sister to Chriacus, which may be a large late survivor of a smaller common ancestor.

Phylogenetic (trait-based) analysis
is a powerful tool that can answer our most baffling traditional enigmas. In many cases this tool is only a blunt instrument, but as more pertinent taxa are added, it becomes a finer and sharper needle and scalpel.

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 3. 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 the fleshy inter-metacarpal muscles. That area looks identical to the web, even in the Myotis embryo.

No matter what you like to read about here at PH
thank you for your continued interest.

The origin of flight in bats: what we knew in 1992.

Famous for his whale studies,
JGM (Hans) Thewissen turned his attention to bats as a postdoctoral fellow in 1984. His co-author, SK Babcock, was a graduate student at the time.

Their introduction
includes their intention of reviewing then current controversies despite “extremely sparse” fossil evidence. They mentioned the hundreds of Eocene bat skeletons known from the Messel quarry near Darmstadt, Germany, but note that even late Paleocene bats were “nearly as specialized as their modern relatives.”  Their report preceded by several years the publication of Onychonycteris (Simmons, Seymour, Habersetze and Gunnell 2008), the most primitive bat known at present.

Two kinds of bats were noted, Megachiroptera and Microchiroptera.
“Megabats have a simple shoulder joint and a clawlike nail on thumb and index finger, whereas mi-crobats have a complicated shoulder joint and a claw only on the thumb.” Microbats use echolocation to eat insects with their sharp crested teeth. Megabats generally do not, but a few do. They are herbivores with blunt molars.

Earlier we looked at
the dual origins of turtles,  whales, seals and the four origins of the “pterodactyloid”-grade pterosaurs. Workers have wondered if mega bats and micro bats also had dual origins.  This was the main theme of the Thewissen and Babcock paper, penned before the widespread advent and adoption of computer-based phylogenetic analysis. Instead, everyone looked at a few to many traits and pulled a Larry Martin. Sometimes they were right. Othertimes, they were wrong to slightly wrong. Smith and Madkour 1980 first proposed a dual origin for bats by looking at the penis.

Thewissen and Bacock renege on their headline promise when they report,
“If the problem of bat origins is ever solved, it will be after a careful anal-ysis of all characterso f interesti n the bats and their potential relatives.” Of course this was shortly  before PAUP and MacClade came on the scene the same year.

Thewissen and Babcock report:
“Both microbats and megabats have a propatagial muscle complex, but it is surprisingly different in the two groups.” In mega bats this complex has four proximal origins,

  1. the back of the skull
  2. the side of the face
  3. the ventral side of the neck and
  4. the midline of the chest

compared to only two origins in micro bats (1 and 4). There is also variation within micro bats and within mega bats. As readers know, there is no way to understand this unless outgroups have one or the other pattern and they don’t (at present). Thewissen and Babcock report, “gliding flight has evolved six times in mammals.” But gliders don’t make good flyers. To fly one needs thrust provided by flapping. How and why bats started flapping has really been the key underlying, unanswered question, which we looked at earlier here and here.

Back in 1910
WK Gregory concluded after careful study that bats, flying lemurs, tree shrews, elephant shrews and primates were closely related and called that group (clade) Archonta. According to the large reptile tree (LRT, 1043 taxa) many of these taxa are indeed related. Elephant shrews are not, which Thewissen and Babcock later note. Elephant shrews are also the only ones from that list that are not arboreal climbers. Thewissen and Babcock add the clade Plesiadapiformes, which were thought to be rodent-like primates, but turn out to be primate-like rodents nesting close to multituberculates in the LRT.

Figure 1. Bat cladogram. Here pangolins are the nearest living relatives of bats.

Figure 1. Bat cladogram. Here pangolins are the nearest living relatives of bats.

Flying lemurs,
like Cynocephalus, also have a propatagium that originates from the side of the face and midline of the neck, but the nerves within them terminate in different places in bats. The LRT recovers flying lemurs close relatives to bats, but pangolins, like Manis, are closer.

Thewissen and Babcock conclude: 
“We believe that the evidence from the propatagial muscle complex of bats supports the idea that all bats share a single ancestor with wings. This idea is consistent with bats going through a flying lemur-like stage before acquiring active flight.”

Unfortunately
the LRT recovers a topology in which the last common ancestor of flying lemurs and bats was likely arboreal, but not a leaping glider. That means membranes developed in parallel (close convergence). Remember, gliders don’t become flappers. And flappers usually develop flapping for reasons other than flight, then co-opt flapping traits for flight.

The ancestors of bats and pangolins
have had a long time to diverge. Likely that was in the Late Jurassic because we have the pangolin ancestor, Zhangheotherium, appearing in the Early Cretaceous. That puts the last common ancestor of flying lemurs, pangolins and bats, Ptilocercus, back in the Middle Jurassic, several tens of millions of years after the likely first appearances of therian mammals, like the living and very late surviving Didelphis and Monodelphis sometime in the Early Jurassic. Earlier we looked at the origin of bats here, here and here.

Figure 2. Select basal cynodonts and mammals set chronologically. The divergence times for placentals (Eutheria), marsupials (Metatheria) and monotremes (Mammalia) are estimated here.

Figure 2. Select basal cynodonts and mammals set chronologically. The divergence times for placentals (Eutheria), marsupials (Metatheria) and monotremes (Mammalia) are estimated here.

References
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.
Smith, J. D., and G. Madkour. 1980. Penial morphology and the question of chiropteran phylogeny. Pages 347-365 in D. E. Wilson and A. L. Gardner, eds. Proceedings of the 5th International Bat ResearchC onference. Texas Tech Press, Lubbock.
Thewissen JGM and Babcock SK 1992. The origin of flight in bats. BioScience 42(5):340–345.

wiki/Onychonycteris

 

The origin and evolution of bats, part 4, an inverted thought experiment

There are no fossils
that currently document the origin of bats from non-volant carnivores or omnivores. Birds have a long fossil history. So do pterosaurs. For bats we have to conduct thought experiments in order to get from points we know: 1) a skilled arboreal omnivore like Ptilocercus, to 2) an Eocene fossil bat, like Icaronycteris (Fig. 1). It won’t help to have a Paleocene tooth, or skull. Those don’t change much in bat origins. We need to see, or visualize, the post-cranial body. Earlier forays into bat origins can be seen here, here and here.

Figure 1. GIF animation thought experiment on the origin and evolution of bats from a Ptilocercus-like omnivore.

Figure 1. GIF animation thought experiment on the origin and evolution of bats from an inverted Ptilocercus-like omnivore. Click to enlarge. Perhaps long fingers originally pulled maggots out of fruit and excellent hearing helped probate find where to dig.

We start with what we know

  1. All or most bats hang inverted
  2. The basal phylogenetic split is between Megachiroptera (fruit eaters) and Microchiroptera (insect eaters)
  3. Bat embryos probably recapitulate the development of those unknown phylogenetic predecessors, And they have big webbed hands early on.
  4. Bats don’t fly until their wings are nearly full size.
  5. What separates Ptilocercus from Icaronycteris is chiefly the size of the hands.
  6. There is no evidence that bats find their wings or wing size sexually attractive
  7. Caves are derived roosting spots. You have to fly in those to get a spot.
  8. Likewise, catching insects on the wing and echolocation follows the advent of flying, but listening to maggots munching fruit might have been a precursor skill.

The big question has always been
how do you get a flight stroke out of quadruped? Pterosaur and bird ancestors were both bipeds with strong hind limbs and they evolved wings as 1) gaudy secondary sexual traits; and 2) to aid in locomotion, especially up steep inclines (Heers et al. 2016 and references therein). The only way that bats were bipeds was inverted with weak hind limbs, which is a whole different story, or, in this case, a whole different thought experiment.

Figure 2. Pteropus, a fruit bat.

Figure 2. Pteropus, a fruit bat, has relatively shored clavicles and larger scapulae extending over most of the rib cage. The extremely long toes are derived. Parallel interphalangeal joints present on bat wings show the phalanges flex in sets.

Hypothetical stages in bat development

  1. Start with an agile arboreal omnivore like Ptilocercus, derived from long-legged arboreal carnivores in the Cretaceous/Paleocene, like Chriacus.
  2. Hanging fruit and the maggots therein can be attacked by likewise hanging on the supporting branch.
  3. The tiny hands of Ptilocercus could hold the fruit more steadily if the f fingers were longer. Maybe digging out maggots was aided by longer, thinner fingers.
  4. Webbing on even longer fingers would help trap juices, pieces, maggots from dropping out, and (see #6).
  5. At this stage the inverted biped no longer uses those hyper-elongate fingers for climging, so they are capable of being folded, not from the metatarsophalangeal joint, but at the wrist.
  6. In tropical forests bats use their wings as fans to cool themselves off (see video here), often after salivating on themselves for evaporative cooling. This is one of two pre-flight-stroke actions I have found.
  7. To rise from an inverted position on a branch, bats will flap vigorously (Fig. 3), which is the other pre-flight-stroke action.
  8. Mother bats wrap developing infants in their folded wings, but that doesn’t get them into the air.
  9. At a certain point, the pro-bat has wings that are capable of fanning the air, but incapable of flying. This is when the first branch-to-branch and tree-to-tree flapping leaps took place. If the pro-bat falls to the ground, it dies. Only successful arboreal flapping ‘acro-bats’ survive and improvements increase those odds.
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 3. 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.

In summary,
hanging pro-bats first developed long fingers to hold hanging fruit and perhaps remove maggots. Fanning for cooling could only develop with large webbed hands. Vigorous flapping from an inverted configuration is one solution to elevating the head and body. Letting go with the feet during this activity is the first awkward and potentially lethal stage to ultimately perfecting the flight stroke over many generations. The origin of flapping in bats is only a thought experiment at present with no other evidence currently available.

References
Heers AM, Baier DB, Jackson BE & Dial  KP 2016. Flapping before Flight: High Resolution, Three-Dimensional Skeletal Kinematics of Wings and Legs during Avian Development. PLoS ONE 11(4): e0153446. doi:10.1371/journal.pone.0153446
http: // journals.plos.org/plosone/article?id=10.1371/journal.pone.0153446

New fossil bats video from the Royal Tyrrell Museum: Dr. Gregg Gunnell

The origin of bats
has been THE hottest topic here at the PterosaurHeresies.Wordpress.com blogsite. See earlier posts here, here and here.

“Fossils of the Night – The History of Bats Through Time” is a new YouTube video (53 minutes) brought to you by Dr. Gregg Gunnell from Duke University, speaking in the Royal Tyrrell Museum series on prehistoric topics.

Dr. Gunnell reports:

  1. only one extinct genus of fruit bat/flying fox
  2. 40+ extinct microbats (all echo-locators)
  3. Bats not close to primates, but with carnivores, hooved mammals, etc. (pretty broad!)
  4. Origin to 65 mya according to molecular clock
  5. Appear at 52 mya. We lack bat fossils from the Paleocene
  6. 11 extinct families of bats
  7. Icaraonycteris and Onychonycteris are two of the oldest known fossil bats. (Eocene, 52 mya) complete
  8. Messel bats (48 mya) more or less complete.
  9. More recent bats are bits and pieces, mostly dental taxa
  10. None of these are directly related to living families
  11. By the Pliocene nearly all modern taxa are known from fossils.
  12. Brachial index (forelimb/hindlimb ratio) midway between non-volant and flying mammals.
  13. CT scans of the teeth were made. All the inner halves of the teeth are crushed into small pieces.
  14. Certain lacewings, both extinct and extant, have a auditory organ on the wings that enables them to detect bat sonar. They stop flying when bats are detected.
  15. Bats have a low metabolism for their size. They live for up to 40 years.
  16. Smaller size increases wingbeat and sonar frequencies
  17. ‘Phyletic nanism’ describes body size decrease, island dwarfism. Onychonycteris was 38-40g. Microbats run about 14g.
  18. Gunnell reports on Yi qi, accepting the patagium/extra wrist bone hypothesis, which was falsified here.
  19. The origin of bats — Dr. Gunnell reports we don’t know what came before Onychonycteris.
  20. Nice morph video (5 seconds) of an inverted mammal on a tree trunk turning into a bat at the very end of the presentation.

This origin agrees with the large reptile tree,
which pulls both bats and primates out of carnivores. Here (Fig. 1) the extant Ptilocercus is employed as a model bat ancestor morphotype.

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 1. 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.

 

 

The Origin and Evolution of Bats part 3

Earlier here and here we looked at the origin and evolution of bats. See Part 4 here. It solves many of the problems attending the origin of bats.

Now let’s add a few more pieces to the puzzle to make the various transitions more complete, more logical and more gradual.

The bat flight stroke
(Fig. 1) is similar to that of birds during takeoff, when the airspeed is minimal. So, there is less reliance here on Bernoulli’s principle and more Newton’s third law as the bat wing scoops up and pushes down a mass of air with each downstroke. The incredible ability of bats to ‘turn on a dime’ is due to their ability to greatly modify their wings in flight, push a volume of air in any direction, and to play the mass of their own wings against the remaining 80% to succeed at maneuvers birds would never try. This includes the ability of bats to invert themselves at low airspeed in order to cling to ceilings and perform an upside-down, two-point landing.

Figure 1. Bat flight stroke in anterior view. This is a different stroke than birds or pterosaurs used, depending more on elbow and finger  flexure and extension.

Figure 1. Bat flight stroke in anterior view. Note how the wing tips gather toward the torso during part of the flight stroke.  Click to view YouTube video. Note the elbows don’t move a great deal here, but the wrist rises above the back, unlike other mammals.

In bats the wing stroke 
cycles from extension and maximum wing area (on the downstroke) to flexion with minimum wing area (on the upstroke). On some bats the wing tips ventrally touch one another during the flight stroke, but on this Myotis specimen (Fig. 1) the wing tips graze the torso. This sort of flight stroke appears to have evolved from a prey gathering stroke when the much smaller forelimbs were simply enlarged webbed hands (Fig. 5) gathering prey like a catcher’s mitt.

Bats were not always so incredibly gifted in flight.
Obviously there were millions of transitional generations that evolved from quadrupedal arboreal forms in the Late Cretaceous before bats became such supreme aerialists in the Early Eocene.

The question is:
what sort of behavior led to this sort of flight stroke on non-flying bat ancestors with small unwebbed hands? In other words, what can simple hands and small pre-wings do that presages and evolves into the flight stroke we see in bats with large wings? Fossils do not yet tell us all the details, but enough is known to create hypothetical transitional forms (Fig. 5).

Inverted bipeds
It should be obvious that pre-bats cannot begin to flap their wings unless their forelimbs are freed from typical support duties. Virtually all living bats are inverted bipeds (the rare exceptions are vampire bats and grounded bats). The legs and feet of bats have gone through such radical changes that ordinary quadrupedal locomotion is impossible for them now.

An evolutionary starting point: the pen-tailed ‘tree shrew’
Ptilocercus
(Fig. 2; actually a pygmy civet) is a small, extant, arboreal, quadruped at home both on tree trunks and narrow branches. It seems unlikely that the transition to bipedalism could have happened upside-down on a tree trunk because the hands would still have been used for support. Rather, the transition had to happen while inverted on a horizontal branch or vine (Fig. 2) with only the hind limbs hanging on and the forelimbs free to flap.

Figure 3. The pen-tailed tree shrew, Ptilocercus, the closest living non-flying relative of bats. Note the pose, perpendicular to the narrow branch. It arrived there by leaping from one branch to another, rather than walking along the length of the branch. When pen tails 

Figure 2. The pen-tailed tree shrew, Ptilocercus, the closest living non-flying relative of bats. Note the pose, perpendicular to the narrow branch. It arrived there by leaping from one branch to another, rather than walking along the length of the branch.

Figure 3. Bat and human scapula compared. Red arrows point to acromion process, reduced and moved away from the shoulder joint in bats to enable greater freedom of motion.

Figure 3. Bat and human scapula compared. Red arrows point to acromion process, reduced and moved away from the shoulder joint in bats to enable greater freedom of motion.

Ptilocercus
is the closet tested living sister to bats and dermopterans, like Cynocephalus. Notably, the pen-tail has no trace of any extradermal flight/gliding membranes anywhere on its body. Thus the gliding membrane of Cyncoephalus and the flying membrane of Pteropus are not homologous, but developed independently.

Inverted Flapping
Earlier I found a video of a full-fledged bat flapping on a horizontal branch, not releasing its feet until attaining a horizontal attitude. That’s the best data I’ve found so far for flapping while inverted before flying. (And I’m not forgetting that this bat had a fully evolved set of wings.) By comparison, smaller hands, like those in Ptilocercus, would have been nearly useless for pushing air around.

When an animal is hanging by its feet, no matter how much it ‘flaps’ its little hands and arms, it’s not going to go anywhere. It’s not going to find food and it’s not going to attract mates. I don’t see any evolutionary advantages to this sort of behavior.

Unlike quadrupedal mammals,
bats can raise their wrists and hands over their backs as part of their flight stroke. Brachiating primates (anthropoids including humans) can only lift their hands over their heads, but not over their backs. Bat shoulder flexibility is due both to their elongated clavicles, that extend above/behind the neck, and to the cranial shifting of the acromion process on the scapula (Fig. 3). That removes the scapular block that restricted dorsal humerus abduction. Only bats have this key trait.

Another question: Which key trait came first? The long clavicle? The dorsally open glenoid? The large webbed hand? At present, we just don’t know. We don’t have the fossils.

Which traits were lost in bats? 
As in birds and pterosaurs, the manus in bats is unable to supinate or pronate because the ulna becomes little more than a splint and the radius no longer axially rotates around it. Thus the bat manus is unable to operate in a normal fashion (palmar side in contact with the substrate, pointing in the direction of quadrupedal locomotion, flexing and extending in PIL sets). Rather in bats, birds and pterosaurs the palms face each other when adducted (like clapping), except at maximum extension (abduction, lateral forelimbs) when the palms face ventrally.

Figure 3. Fruit bat (Pteropus) skeleton with hypothetical muscles added).

Figure 4. Fruit bat (Pteropus) skeleton with hypothetical muscles added).

Ontogeny recapitulates phylogeny
We don’t have to rely solely on extant adults and fossils. We can also glean data from extant juvenile and embryo bats, which appear to replay the evolutionary journey of bats as they grow up. Their wings are much smaller than in adult bats. We also have behavioral clues to work with.

Newborns
of Ptilocercus are nest-bound. One, two, or three nestlings receive extremely sparse maternal care as the mother visits her young every other day for no more than ten minutes at a time. The parents themselves are solitary feeders. By contrast, single newborn bats are carried by their mothers everywhere they go and adult bats are communal.

Bat juveniles
do not fly shortly after birth, but do so only as subadults when their wing fingers reach adult length. By this evidence we can draw an analogy that bat ancestors did not achieve competent flight until they had adult-length fingers. The evidence also shows that less than competent flight was good enough to survive and flourish in that Paleocene environment devoid of predators and likely filled with prey and nearby landing sites. Picture a tangle of branches and vines with pre-bats leaping and flapping between them.

The Paleocene insect fossil record
is currently underrepresented, but giant ants and beetles are known. Some of these would have been on Paleocene bat menus.

If you’re going to evolve wings like a bat,
you have to stop using them as hands. Whenever a digit extends or membranes develop, they are going to get damaged and become increasingly awkward with increasing size, unless folded, as pterosaurs and birds do. In adult and juvenile bats the carpus rotates the hand posteriorly for complete wing folding. However, the embryo bat manus (Fig. 5) does not appear to rotate and fold in the same fashion. Even so, in the bat embryo there is little indication of inter-metacarpal muscle. Instead that area looks indistinct from the inter-digital membrane. Note the unguals of manual digits 2-5 in all bats, including embryos, are vestiges and not in use.

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 5. Ptilocercus, Icaronycteris and two hypothetical transitional taxa 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.

Lessons from gliding squirrels, dermopterans and sugar gliders
These living mammal gliders are not related to each other and not related to bats. None develop large hands or a flapping behavior. None of these gliders produce thrust while gliding and therein lies the difference with large-handed bats. Furthermore none of these gliders develop a larger clavicle and scapula, which bats use to anchor large flapping muscles.

Why would a sister to Ptilocercus start to flap?
Elephants flap enlarged ears to cool off. Baby birds flap tiny wings to encourage their parents to feed them. Before they could fly, pre-pterosaurs and pre-birds likely flapped to threaten rivals and seduce mating partners. Bats don’t appear to use their wings except to fly, catch insects and perhaps to wrap themselves for insulation from the cold and rain.

Prey Acquisition > Locomotion 
Given the large gap between Ptilocercus and Eocene bats (Fig. 5), along with the present evidence from embryo/juvenile bats, we can only imagine transitional stages starting with an enlarged hand (HTTaxon 1) and continuing to enlarge that hand while developing extradermal membranes (HTTaxon 2). Here’s how the current data can be used to rebuild the missing portions in the evolutionary stages of bat evolution (Fig. 5).

Figure 5. Ptilocercus in vivo, holding prey with its small hands while eating it.

Figure 6. Ptilocercus in vivo, holding prey with its small hands while eating it.

  1. The Ptilocercus state Ptilocercus is an omnivore that pounces on insects that have landed on trees. Ptilocercus grabs its prey with its hands and kills with its mouth (Fig. 6). That’s pretty much what bats do, too.
  2. Hypothetical Transitional Taxon 1 stage – The five fingers are longer here. That increases the ability to grab prey, like a catcher’s mitt (Fig. 7) does with baseballs. The metacarpals were more loosely joined at their carpometacarpal joints. Conversely the fingers were bound by webbed membranes creating more flexible mitts. Perhaps lateral extradermal membranes, like those found in gliding squirrels, dermopterans and opossums, also had their genesis at this stage. Flight, such as it was, would have been restricted to close vines or branches. Frantic flapping would have supplemented each glide in a rudimentary fashion.
  3. Hypothetical Transitional Taxon 2 stage – Only fingers 3-5 are longer here and they form support structures for more extensive wing trailing membranes. At this stage the hand could no longer be used for traditional quadrupedal locomotion, so bats probably became inverted bipeds at this stage. When forced to, bats rested and walked on the medialventral rim of their folded hands with only their thumbs able to grab surfaces, as they do today. Flight duration continued to expand.
  4. Icaronycteris state – The lumbar vertebrae were longer. The tail was shorter. The clavicle, chest and scapula were enlarged at this stage. The ulna was a splint. The hand was full-sized and capable of both folding completely and flying competently. Flight duration had no practical limits.

Behavioral stages
The following hypothetical behavioral stages gradually grade from quadrupedal pygmy civets to aerial bats. As you’ll see, bat wings developed principally for prey gathering, whether that prey was on tree branches or in mid-air.

  1. Arboreal prey grabbing with small hands, then eating at leisure (Ptilocercus).
  2. Arboreal prey gathering with larger hands, making large sweeping motions toward the body and mouth (the genesis of the flight stroke, Fig. 1) during capture and transfer to the mouth (HTT1).
  3. Branch-to-branch or vine-to-vine leaping/flapping with even larger hands now transforming into flying organs. Loss of the tibial malleolus increases pedal rotation. (HTT2).
  4. Tree-to-tree leaping/flapping with even larger hands that could be folded during inverted hanging. At this stage the whole body starts to act like a catcher’s mitt as flying insects are added to the menu. After capture the tail or wing curls in to transfer the prey to a descending skull to bite and kill while on the wing (Icaronycteris).
  5. Open air flapping/flying with wings large enough that juveniles/babies could be carried by mothers. Shorter tail and expanded uropatagium adds to the surface area of the ‘catchers mitt’ as these posterior elements are increasingly used for prey acquisition (Myotis) and the wings less so.
  6. Radiation/Evolution into various species, some frugivorous, others with echolocation.
Figure 7. Everything evolves, even catcher's mitts. By analogy, bat's wings evolved to become better catcher's mitts.

Figure 7. Everything evolves, even catcher’s mitts. By analogy, bat’s wings evolved to become better catcher’s mitts. Click to enlarge.

All we have to do now
is find fossils of a Paleocene pygmy civet with large fingers, like those of an embryo bat.

References
Jepsen GL, MacPhee RDE 1966. Early Eocene bat from Wyoming. Science 154 (3754): 1333–1339. doi:10.1126/science.154.3754.1333. PMID 17770307.
Le Gros-Clark WE 1926. On the Anatomy of the Pen-tailed Tree-Shrew (Ptilocercus lowii.) Proceedings of the Zoological Society of London 96: 1179-1309.
DOI – 10.1111/j.1096-3642.1926.tb02241.x
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.

wiki/Icaronycteris
wiki/Onychonycteris
wiki/Ptilocercus