Creationism in Crisis - A 50 Million-Year-Old Bat Skull - And Another Gap Closes

Most bats use echolocation - using sound emitted from their mouths or noses - to sense their surroundings.

Photo: Getty Images

Holy bat skull! Fossil adds vital piece to bat evolution puzzle | UNSW Newsroom

Earlier this week I predicted that there would soon be more science papers which casually refute creationism - an easy prediction to make, since just about every serious science paper does exactly that. This is a prediction I'm beginning to regret as they are coming so thick and fast that it's difficult to keep up with them!

This time, it's a paper describing a near-perfect fossil bat skull from 50 million years ago that closes one of creationism’s beloved gaps in the fossil record. Of course, like almost all archaeology, geology and palaeontology, it deals with the 99.97% of the history of Earth that occurred before creationist dogma says the universe was created out of nothing by magic.

The evolutionary history of bats suffers from a lack of fossil evidence, probably because bat skeletons are generally small and thin and bat lifestyle doesn't predispose them to formation of fossils when they die. They mostly roost in caves where dead bats are rapidly consumed by a whole ecosystem of creatures that live on their droppings on the cave floor.

However, this latest find is of a near perfect skull and shows evidence that echolocation may have evolved earlier than previously thought, soon after or parallel with the evolution of powered flight.

First, some AI background:

Tell me about the evolution of bats and the fossil evidence for it. Bats are remarkable creatures that have undergone a long and complex evolutionary history. They belong to the order Chiroptera and are the only mammals capable of sustained flight. The evolution of bats is a fascinating subject, and while there are gaps in our understanding, scientists have made significant discoveries through the study of both modern and fossil evidence.

1. Early Bat Evolution:
   • The oldest known bat fossils date back to the Eocene epoch, approximately 50 million years ago. These early bat species were small, insectivorous animals that resembled some of today's microbats.
2. Flight Adaptations:
   • Bats evolved powered flight, a unique feature among mammals. Their wings are elongated finger bones covered in a thin membrane, which allows them to generate lift and maneuver effectively.
   • The adaptation of powered flight was a key milestone in bat evolution and enabled them to exploit a diverse range of ecological niches.
3. Suborders:
   • Bats are divided into two suborders: the Megachiroptera (large fruit bats or flying foxes) and the Microchiroptera (small insect-eating bats). These suborders are believed to have diverged around 50 million years ago.
4. Fossil Evidence:
   • The fossil record has provided valuable insights into the evolution of bats. Some notable fossils and findings include:
     1. Icaronycteris index: This is one of the earliest known bat fossils, dating to about 52 million years ago. It had many bat-like features but still retained some primitive characteristics.
     2. Onychonycteris finneyi: A 50-million-year-old fossil found in Wyoming, this bat had features transitional between tree-dwelling gliders and fully developed flying bats, shedding light on the evolution of flight.
     3. Fossils of various intermediate forms show the gradual elongation of the finger bones and the development of the wing membrane.
5. Diverse Adaptations:
   • Bats have diversified into more than 1,400 species, adapted to various ecological niches and diets. They range from nectar-feeding bats to those that prey on fish, birds, or even blood-feeding vampire bats.
6. Molecular Evidence:
   • Genetic studies have also provided insights into bat evolution. DNA analysis has helped clarify relationships among different bat species and their place within the mammalian tree of life.
7. Coevolution with Echolocation:
   • Many microbats have evolved the ability to echolocate, emitting high-frequency sounds to locate prey and obstacles. This remarkable adaptation has been essential to their success as insectivores.

In summary, the evolution of bats is a complex and ongoing field of study. Fossil evidence, molecular analysis, and ecological observations have helped scientists piece together their evolutionary history. Bats are a remarkable example of how natural selection and adaptation can lead to the development of unique and highly specialized traits in a group of organisms.

The present paper is the work of a team of palaeontologist led by Emeritus Professor Suzanne J. Hand of ESSRC, School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney, NSW, Australia, together with Dr. Jacob Maugoust and Professor Maeva J. Orliac of the Institut des Sciences de l’Evolution, Université de Montpellier, Montpellier, France and Professor Robin M.D. Beck of the School of Science, Engineering and Environment, University of Salford, Manchester, UK.

Their research is explained in a University of New South Wales press release by Lachlan Gilbert and published, open access, in the Cell Press online journal, Current Biology:

Bats may have lived in caves and used soundwaves to navigate much earlier than first thought.

Of all the mammals, bats have one of the poorest fossil records, with palaeontologists estimating that about 80 per cent of it is missing.

This has made it difficult to pinpoint exactly when they first began to fly, or began roosting in caves, or developed their unique way of ‘seeing’ their surroundings in the dark using sound – called echolocation.

But a near-perfectly preserved bat’s skull discovered by French palaeontologists in a cave that dates back about 50 million years has shed new light on what we thought we knew about this ancient, hypothetical creature.

Emeritus Professor Sue Hand from UNSW Sydney’s School of Biological Earth and Environmental Sciences is a leading palaeontologist with expertise in bat evolution. She led an analysis of the skull, published today in the journal Current Biology, that involved Dr Jacob Maugoust and Professor Maeva Orliac from University of Montpellier in France, and Professor Robin Beck from the University of Salford, UK.

Prof. Hand says prior to the discovery of this skull – which was among 23 separate fossilised individuals found in the cave belonging to the extinct species Vielasia sigei – only fragments or completely flattened skeletons of early bats had existed in the fossil record.

“We don’t know very much about the beginnings of bats because we don’t have the missing links like we do, say, between dinosaurs and modern birds,” she says.

“The oldest bat fossil is about 57 million years old, and it's a single tooth from a site in Portugal – that's all we know about it. The first bats are all just known from fragmentary fossils, mostly teeth. When bats appear in the fossil record a little later, about 52 million years ago, some are wonderfully complete bats, but they’re flattened.”

While these flattened specimens are, in Prof. Hand’s words, “beautifully preserved”, the fact that they’ve been flattened by layers of rock deposited over millions of years makes it difficult to decide with conviction, the exact positioning of bones in their three-dimensional anatomy. And when it comes to determining whether a fossil is from a species of bat that is already using echolocation, detailed and precise anatomy of the skull is crucial.

“In modern bats, between the voice box and the ear, there are some bones called the hyoid bones. In all modern bats that echolocate, one of these bones directly contacts the middle ear bones and appears to be involved in transmitting high frequency sound.

“But in the flattened fossils, while we can see these various bones, there is a question about their precise relationships to each other. This has led to a lot of debate among scientists about whether or not a species used echolocation.”

A table showing the various scanned bones against their place in the bat skeleton

Image: Hand, Maugoust, Beck & Orliac

Uncrushed skull

But in the case of Vielasia sigei, not only is the skull almost entirely intact, but it has been preserved in limestone in its original three-dimensional shape which the scientists describe as ‘uncrushed’.

“In this particular bat, we can see more directly what's going on deeper, in the inner ear,” Prof. Hand says.

“We took fine measurements of that inner ear bone and compared it with that in the bats that do echolocate today and bats that don’t, and it sits in the middle of the ones that echolocate.”

Not all bats echolocate, Prof. Hand explains. Flying foxes regularly seen in the Sydney night sky around the Botanical Gardens, Centennial Park and the Royal National Park rely on their very good eyesight to navigate and find fruit, without echolocation. Meanwhile, Sydney microbats such as the Eastern Bentwing Bat, Gould’s Wattled Bat and the Chocolate Wattled Bat, are well known for navigating and catching insects using feedback from the high frequency sound they emit.

While Prof. Hand stops short of concluding that Vielasia sigei used echolocation with 100 per cent certainty, she says the new evidence is compelling.

“It’s very convincing that the type of echolocation some of these early bats used was indistinguishable from what many echolocating bats use today, and at 50 million years ago, this is well ahead of whales developing this ability.

“Prior to this find, we were only really certain that echolocation developed in the modern families of bats.”

The Vielasia bat was closely related to a common ancestor of extant - or modern bats. The Eocene stem bats are now all extinct.

Image: Maeva Orliac

Back to the bat cave

In all, there were 400 fossil bones and teeth discovered by the French team in the cave in south-western France, which represented 23 individuals. Vielasia – which is not a direct ancestor of today’s bats but may have been closely related to it – was only a small bat, with the uncrushed skull measuring only 1.8 cm long.

“There were 23 of these wonderful little bats living in a cave, which also makes it the oldest cave-dwelling bat in the world that we know of,” says Prof. Hand.

“We didn't think that these early bats actually lived in caves. The information had been that they lived in trees around lakes and in forests which stretched right up to both poles because the Earth was very warm at this time.”

But when these greenhouse conditions started to deteriorate later in the early Eocene period – around 50 million years ago and about the same time that this bat was living – there were much more wildly fluctuating changes in temperature.

“So it could be that this bat lived in a cave because this is much more stable environment.”

Passing the baton

Whether or not the analysis of the uncrushed Vielasia skull has settled the echolocation debate about early bats, Prof. Hand hopes that it will inspire further exploration of the fossil record.

“We think some of the characteristics of this bat would have also characterised the last common ancestor for modern bats. So it's exciting, and it is actually going to be an important specimen that people will get a lot of information from and use in their own analyses.”

Technical details and the estimated phylogeny of this ancient species of bat is provided in the team's open access paper in Current Biology:

Highlights

A new bat species is described from a 50-million-year-old cave deposit in France.

Its well-preserved fossils include the oldest uncrushed skull of a bat yet known.

This stem bat appears to have been capable of advanced (laryngeal) echolocation.

The fossils suggest that advanced echolocation predates the crown bat radiation.

Summary

Bats are among the most recognizable, numerous, and widespread of all mammals. But much of their fossil record is missing, and bat origins remain poorly understood, as do the relationships of early to modern bats. Here, we describe a new early Eocene bat that helps bridge the gap between archaic stem bats and the hyperdiverse modern bat radiation of more than 1,460 living species. Recovered from ∼50 million-year-old cave sediments in the Quercy Phosphorites of southwestern France, Vielasia sigei’s remains include a near-complete, three-dimensionally preserved skull—the oldest uncrushed bat cranium yet found. Phylogenetic analyses of a 2,665 craniodental character matrix, with and without 36.8 kb of DNA sequence data, place Vielasia outside modern bats, with total evidence tip-dating placing it sister to the crown clade. Vielasia retains the archaic dentition and skeletal features typical of early Eocene bats, but its inner ear shows specializations found in modern echolocating bats. These features, which include a petrosal only loosely attached to the basicranium, an expanded cochlea representing ∼25% basicranial width, and a long basilar membrane, collectively suggest that the kind of laryngeal echolocation used by most modern bats predates the crown radiation. At least 23 individuals of V. sigei are preserved together in a limestone cave deposit, indicating that cave roosting behavior had evolved in bats by the end of the early Eocene; this period saw the beginning of significant global climate cooling that may have been an evolutionary driver for bats to first congregate in caves.

Figure 2. Phylogenetic relationships of Vielasia sigei among bats

Consensus tree of post-burn-in trees from Bayesian-dated total evidence analysis of 2,665 craniodental characters plus 36,860 base pairs of DNA sequence data from 27 nuclear loci from O’Leary et al.⁶¹ Values at nodes represent Bayesian posterior probabilities. 3D model of Vielasia sigei cranium (compiled from UM-VIE-250, 251, and 268) and mandible (from UM-VIE-254 and 255), see STAR Methods.

Abbreviations: EECO, early Eocene climate optimum (53.26–49.14 mya⁶²); LE, laryngeally echolocating; mya, million years ago.

Scale bar, 5 mm.
See also Figure S2 and Data S1 and S2.

Background photo, Causses du Quercy, P. Lasvenes.

Hand, Suzanne J.; Maugoust, Jacob; Beck, Robin M.D.; Orliac, Maeva J.
A 50-million-year-old, three-dimensionally preserved bat skull supports an early origin for modern echolocation
Current Biology. doi: 10.1016/j.cub.2023.09.043

Copyright: © 2023 The authors.
Published by Elsevier Inc. Open access.
Reprinted under a Creative Commons Attribution 4.0 International license (CC BY 4.0)

What we have here then, is one of those supposedly non-existent transitional forms showing features of both echolocating and non-echolocating bats which lived about 50 million years before creationists think the Universe was created out of nothing by magic.

All in all, it would be another terrible day for creationists, if only they had the moral courage and intellectual integrity to read about it.

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