Today, April 17, is Bat Appreciation Day . These fascinating animals are the only mammals with the ability to fly . They do this thanks to wings formed by the entire forelimb, with four extraordinarily elongated fingers —one of the fingers of the hand, the first, remains free—, and a membrane, called chiropatagium , that joins the end of these to the flank. of the body and to the tibias. Additionally, it has a secondary membrane , called the uropatagium , which joins the hind legs to the tail for its entire length.
As far as we know, only five groups of animals have evolved to acquire wings and the ability to fly with them: insects, pterosaurs, two groups of dinosaurs, scansoriopterygids and eumaniraptors —including birds— and chiroptera or bats . And whenever the origin of the wings of any of these groups has been attempted , there have been difficulties .
Ever since Darwin and Wallace formulated the theory of evolution by natural selection, birds have been a great unknown . The discovery of Archeopteryx, a fossil with reptilian and bird features, was crucial to unraveling the mystery. But with bats, there was no such luck . Perhaps because of its small size, or perhaps because of the fragile and delicate nature of its skeleton , or simply because it did not live in suitable areas for fossilization, there are no fossil remains of animals that can be classified as proto-bats , that is, mammals that have primitive features that, evolutionarily, could lead to the anatomy of bats.
the fossil record
We know that the fossil record is extraordinarily incomplete , and that we cannot take it as an exclusive model, especially when we talk about groups that still exist today. The knowledge obtained from the fossil record must be completed with other tools such as genetics. But in the case of bats, the situation is extraordinary. There is a complete absence of Bats or animals approximating Bats, and almost suddenly, in the Eocene, we find Icaronycteris and Onychonycteris .
These species, of which we have several fossils, are exceptional. Not only are we seeing the oldest bats in them —about 52 million years old, Onychonycteris half a million years older than Icaronycteris —but they are fully functional bats .
The only thing that can be described as primitive in them is their proportions, especially with respect to the length of the phalanges that hold the chiropatagium —shorter than those of current chiropterans—. However, between both fossils there is a difference. Icaronycteris , the more recent of the two , retains all the cranial structures present in current chiropterans, so it follows that it had the ability to echolocate —the sonic radar of bats— that Onyconycteris did not , which indicates that the flight was prior to echolocation.
The morphology of these primitive bats shows a full capacity for flight through flapping wings . All the muscular and skeletal structure required for flapping wings they already had. Which implies that the transition to flight had to happen earlier.
From the ground up, or falling from the trees?
Traditionally, when talking about animals capable of taking flight , two possible hypotheses arise to explain their origin. The one that in English is called ground-up , that is, from the ground upwards, and the one called tree-down , that is, from the trees downwards.
In the absence of information, both hypotheses are plausible , both have had their defenders. On the one hand, there are scientists who support a terrestrial origin ; animals with webbed hands with which to help each other to propel themselves and capture insects, which evolutionarily has led to a behavior in which they jump longer and longer , supported by rudimentary wings, which finally led to true flight .
Other scientists defend an arboreal origin , according to which the hands would act together with an increasingly developed patagio , to glide from tree to tree, like the current flying squirrels, colugos or gliders . In fact, the presence of these skin membranes in three evolutionarily separate groups is striking: flying squirrels are rodents like common squirrels or rats; colugos are dermopterans, closer to primates; and gliders are marsupials like kangaroos. Three types of animals that have the same structure, but have evolved independently , so it is likely that it has happened more times in the past, and the ancestors of bats had this type of structure. This makes it the most probable hypothesis.
Some authors, during the 80s and 90s of the last century, came to propose that dermoptera and bats must belong to the same phylogenetic group, which they called Volitantia. However, much more recent studies show that, in reality, the animals closest in evolutionary terms to bats are, oddly enough, the artiodactyls —camels, wild boars, ruminants…— and the perissodactyls —horses, tapirs, rhinoceroses… —. And any resemblance to colugos or flying squirrels is due solely to evolutionary convergence .
Los unknown proto-murciélagos
It is evident that, although there are no fossils, this evolutionary process had to happen in some way . And given that the fossil record and its incompleteness do not solve the dilemma for us, we can at least carry out a theoretical prediction exercise to find out what those mammals that later evolved into bats would be like .
The evolutionary convergence with other gliding mammals —flying squirrels, dermopterans…— can serve as an analogy to infer what they would have been like and where they would have lived .
They were probably small, nocturnal , insectivorous, arboreal mammals . Primitive bats are, and the largest bats appear later on the evolutionary tree. In addition, lacking the capacity for real active flight —we are talking about gliding animals— they would probably feed on surface insects , little or not flying at all. Chasing a flying insect requires great maneuverability that these animals would not have, so they would use gliding only to move from tree to tree . It is probable that they were excellent climbers and would rest , like the current dermoptera, hanging from branches using all four limbs.
REFERENCES:
Gunnell, GF et al. 2005. Fossil Evidence and the Origin of Bats. Journal of Mammalian Evolution , 12 (1-2), 209-246. DOI: 10.1007/s10914-005-6945-2
Simmons, NB et al. 2008. Primitive Early Eocene bat from Wyoming and the evolution of flight and echolocation. Nature , 451 (7180), 818-821. DOI: 10.1038/nature06549
