Photographs of readers

June 7, 2020 • 2:30 pm

Today’s reader photos come from Bruce Thiel, whom I met at a talk I gave in Portland, Oregon. And there he gave me one of his fantastic preparations of fossil crabs, which I cherish and keep on my mantelpiece. You can see his preparations in his “reader’s photos” here. They are fantastic.  Bruce’s words are indented.

Here I am removing matrix from a Cretaceous Avitelmessus crab from North Carolina, using a pneumatic air chisel.  After 120 hours of work, the crab is still unfinished –including 40 hours work by two previous owners.  The two crabs on the right (#13) were given to the Smithsonian Natural History Museum and on display in the “Deep Time” Exhibit which reopened last summer after a five-year remodel.  A third crab also on display is not pictured.

What started out as a retirement hobby turned into an obsession to search for crab-bearing concretions in the 30-50 million year old ocean sediments in the Pacific NW.  Refining my technique led to the challenge of seeing if I could free the claws from the rock to create more sculptural poses.  However crabs are prepared exactly as they fossilize with no “rearrangement” of claws for aesthetics.

The top second-to-the-left crab in the second picture is noteworthy in that it hosts several 33-35 MYO tube worms. Two other tube-worm infested crabs were sent to Kent State where they were studied, published and donated to the Rice NW Museum of Rocks & Minerals here in Oregon.

While hunting for fossil crabs, I stumbled upon three concretions containing bones of a large penguin-like flightless bird, a Plotopterid, that turned out to be a new genus and species, published and named Olympidytes thieli, given to the Senckenberg Museum, in Frankfort, Germany.
See more crabs and info about the prep process at:

Sue’s new digs

December 31, 2018 • 12:30 pm

by Greg Mayer

Sue, the remarkably complete Tyrannosaurus rex discovered by (and named for) Sue Hendrickson, and excavated by Pete Larson and the Black Hills Institute of Geological Research, has long graced Stanley Field Hall at Chicago’s Field Museum of Natural History. Last year, the Museum announced that Sue would be moved upstairs, into the “regular” dinosaur hall, and that her place would be taken by a model of Patagotitan, a very large sauropod dinosaur. We’ve been following Sue’s progress here at WEIT, and earlier this month the new arrangements were completed and opened to the public.

Let’s start with what’s taken Sue’s place in Stanley Field Hall.

Stanley Field Hall, December 28, 2018.

Standing more or less where Sue once stood is Patagotitan. Unlike Sue, who is 90% actual fossil by volume, Patagotitan is a cast. (Note the elephants and people for scale.)  Swooping over the elephants is a life reconstruction of Quetzalcoatlus, the largest known pterosaur, who shares etymological roots with Jerry’s favorite beuatiful bird. Several smaller, long-tailed pterosaurs– Rhamphorhynchus, I think– can be seen over the Patagotitan. Hanging from the ceiling are several large planters, which resemble the “floating islands” from Avatar.

Hanging planters in Stanley Field Hall.

There were many small lights attached to each planter. I could not discern how the plants were watered.

From the second floor we could look down upon Patagotitan,

Stanley Field Hall.

and look Quetzalcoatlus in the eye.

Quetzalcoatlus in Stanley Field Hall.

Sue is now housed in a special section of the longstanding exhibit, Evolving Planet, which is organized as a walk through time, from the pre-Cambrian to the Cenozoic. Another life reconstruction of Quetzalcoatlus, this one in standing posture, has been placed at the entrance. They were big!

Quetzalcoatlus at entrance to Evolving Planet.

The new Sue hall is located in Evolving Planet in the appropriate spatial and chronological location– the end of the Cretaceous.

Note that the signage is bilingual, in English and, in a smaller font, Spanish. The Field has adopted this convention for all it’s new exhibits.

So here’s the old girl herself!

Sue, the Tyrannosaurus rex.

(Sue’s sex is actually unknown, so, properly, it’s “itself”.) Here’s a video overview of all of her.

In the video, you may have noticed that, compared to her previous mounting, Sue now has a second set of “ribs”, the gastralia, or “abdominal ribs”. These were part of the original excavation, but not included before. The true ribs have also had their distal ends extended a bit laterally, giving Sue a more barrel-chested appearance.

The new, barrel-chested Sue.

This is also a life reconstruction mural in the exhibit. I believe it is a new reconstruction; it is not the one by John Gurche that was found in the old second floor exhibit.

The new, barrel-chested Sue, as she might have appeared in life.

The revisions in the mounting are explained in this ‘science makeover‘ explainer on the Field’s website. Sue’s skull is still housed in a case separate from the mount, and the other Sue materials (most notably bronze models of various bones) from the old second floor overlook have been moved in to the new exhibit.

An engineer friend who I showed some of these pictures to thought that Sue was depicted as too front-end heavy, and that she would topple over forward. The current view is that the tail was massive and muscular, and provided a counterweight, but I, too, thought that, especially with the new barrel chest, she did look a bit over-extended, the heavy front end held too horizontal to readily balance over the hind legs.

A number of associated fossils from the late Cretaceous are also in Sue’s part of the hall, most notably this Triceratops skull.

As some readers may know, Sue came to the Field Museum by a roundabout and unsavory process, involving civil and criminal legal battles, and major financial intervention by McDonald’s (the burger chain) and Disney. The Field Museum’s part in this was largely, if not wholly, salutary, but nonetheless, as in the last exhibit, there is little or no mention of these circumstances in the new exhibit, other than a prominent nod to Sue Hendrickson. Some of the Black Hills Institute’s photos and field notes figure in the exhibit, and are subtly, but properly, acknowledged.

What were the first animals?

October 31, 2018 • 11:45 am

by Matthew Cobb

I’ve just finished making a BBC World Service radio programme about the first animals. Anyone, anywhere in the world, can listen to it (it’s only 28 minutes long!) – you just have to register with the BBC (free, rapid and cost- and spam-free). Click on the pic to go to the BBC website:

The programme deals with two different ways that researchers are studying this question – by looking at fossils, and at DNA. In both cases I interview researchers and – in the case of the Ediacara – get to handle some fossils. I also ate some 600 million year embryos at Bristol University (to see what they tasted like, obviously), but we didn’t include that in the programme. . .

The fossil data relate to what are called the Ediacaran biota – strange fossils from before the Cambrian, around 570 million years ago. The fossils are very hard to interpret – they don’t look like much alive today – but an amazing technique for analysing cholesterol molecules in the rock, so organic molecules preserved for all that time, has confirmed that Dickinsonia, the thing in the picture above, was an animal. Other techniques involve looking at large numbers of Ediacaran fossils and seeing how their distribution relates to those of modern animals. All the data suggest that some of the Ediacaran weirdos were indeed animals, although we cannot know if they are the ancestors of any animal alive today.

The DNA data focuses on a different question, which DNA can answer – which of the groups of animals alive today was the first to branch off the tree of life? Traditionally there has been a straightforward answer to this: sponges, which are nerveless and tissueless. But 10 years ago comparative genomic studies dropped a bombshell – they suggested that the first group to branch off were the ctenophores or comb jellies. This has caused a huge row because it would mean either that nerves evolved twice – once in the ctenophores, and once in our ancestors, after the nerveless sponges branched off – or that the huge sponge group somehow lost the genes for producing nerves.

Many biologists (myself included) don’t like either of these options, and prefer the sponges as the first model, but the data are persistent. Or are they? I spoke to experts on both sides of this argument, which has caused quite a hoo-haa in the zoological community for the past decade.

Anyway, go ahead and have a listen – download it and listen to it on public transport or while you are exercising. NB: I made the programme with ace producer Andrew Luck-Baker.

If you are a teacher, especially if you teach animal evolution, please get your students to listen to it.

Sue update

March 2, 2018 • 10:15 am

by Greg Mayer

She’s gone. I was at the Field Museum on Wednesday for the first time since the previous month, and the removal of Sue the Tyrannosaurus rex has been completed.

Stanley Field Hall, where Sue used to be.

Viewed from the balcony above, visitors walk through Stanley Field Hall, seemingly unaware of the ghostly white outline of Sue’s now departed plinth.

Where Sue used to be, from above.

A sign explained where Sue will eventually show up.

Sue’s actually not gone away entirely, for the second floor balcony display, featuring Sue’s real skull, remains in place. [JAC: the skull was always up there as it was too heavy to mount on the skeleton downstairs.]

The second floor display also includes touchable, life-size, bronze models of various parts of Sue, including the (relatively) tiny forearm. Devotees of the concept of unity of type, and Neil Shubin‘s Your Inner Fish in particular, will recognize the “one bone, two bones, many bones” pattern found throughout the tetrapod vertebrates and their piscine forebears.

A bronze model of Sue’s forearm.

A closeup of the digits; the two distalmost phalanges of the outer (lower, in this photo) digit were among the few bones missing from Sue’s skeleton, and the ones in the model are based on Albertosaurus, a related theropod dinosaur.

Sue’s fingers.

From up on the balcony, I could also get a better look at the model of Pteranodon longiceps hanging from the ceiling.

Pteranodon longiceps in the Field Museum.

And zooming in a bit.

Does the position of this model mean that Pteranodon is Ceiling Reptile?

So long, Sue…. see you upstairs!

February 20, 2018 • 7:45 am

by Greg Mayer

Sue, the iconic Tyrannosaurus rex that has inhabited the Field Museum of Natural History‘s Stanley Field Hall since 2000, is coming down. But, shortly after she comes down, she’ll be going up– upstairs that is.  The Museum announced plans last year to replace Sue in Stanley Field Hall with a model of Patagotitan mayorum, a much larger dinosaur than Tyrannosaurus rex. At the same time, they’ll be adding plants and pterosaurs to the Hall. Bill Simpson (who for some reason appears to be being assisted by Ricky Gervais) explains what’s going to happen to her in this video. (And continue watching the next video, also featuring Bill, that comes up after the first finishes.)

A similar model of Patagotitan has been on display at the American Museum of Natural History in New York for a couple of years now. It doesn’t even fit in the dinosaur hall there, and its head and neck poke out into the hall way to greet visitors arriving by elevator! Sue will be moving upstairs to the Field’s second floor, whose balconies overlook the Hall, where she’ll join the rest of the dinosaurs in the Evolving Planet exhibit. Sue is a theropod, and though in the same order of dinosaurs as Patagotitan, which was a sauropod, Sue and her kin ate creatures like Patagotitan and its kin.

I had gotten to see Sue up close during the study and preparation phases prior to her being placed on exhibit, and wanted to say farewell (for a little while), so I went down to see her before the deconstruction. These are pictures from a visit in late December.

Sue towers over her human prey admirers in Stanley Field Hall.
Getting closer to Sue’s business end.
The better to eat you with.
The somewhat old-fashioned painted reconstruction on the second floor, overlooking Sue down below. Sue’s skull, which is too heavy to be supported on the body of the mounted skeleton in Stanley Field Hall, has always resided in a separate display case on the second floor balcony, just below this painting.
One of the pterosaurs is already in position.

I went down again last month, and took a few more pictures, mostly closer shots of interesting parts of her anatomy.

A closer view of her teeth.
Her reduced, two-fingered, forelimbs. The functional significance of this feature is much speculated on, but unknown.
Her strong, 4-toed (3 forward, 1 back) hind foot. These provided a powerful mode of locomotion.
Au revoir, Sue!

The tale of how Sue got from South Dakota to the Field Museum is a long and tortuous one, and not very edifying; but that’s a story for another post.

Here’s the organism (well, sort of. . . .)!

December 8, 2017 • 12:00 pm

Did you guess what organism made the pattern below, found on a recent dive around the hydrothermal vents off Tonga?

Here’s the answer in the second tweet:

How big is that thing? The laser beam images are 10 cm (about 4 inches apart): The paper from which this comes (below) adds, “Note the shield-shaped elevation, marginal elevated rim and mote, and color (pale pink) of the area of the pattern compared with the surrounding veneer of gray calcareous lutite (image courtesy The Stephen Low Company).” You can find thousands of these things on the wall of the mid-Atlantic Ridge.

The pattern is similar to that described in a 2009 paper in Deep Sea Research (click on screenshot to go there):

It’s called a “living fossil” because the patterns are nearly identical to those found in ocean sediment cores from about 50 million years ago. That doesn’t mean, of course, that the organism that made (or left) this pattern is the same as the ancient one, for it may be not a fossil but a burrow.

But what IS the organism involved? The paper above doesn’t say, because they haven’t recovered an organism from whatever makes this pattern. DNA sequencing of material recovered from the holes shows genetic material from foraminiferans, protists that probably settled in the holes rather than making them.

When the holes are injected with resin underwater, and then the cast recovered, it looks like this (caption from paper):

Fig. 8. Photo of plasticine reconstruction (3-D) of the modern P. nodosum pattern based on observation of the hexagonal pattern of holes at the sediment–water interface and vertical shafts connecting with an underlying horizontal hexagonal network of tunnels or tubes (model and photo by Hans Luginsland).

The raised nature of the pattern as well as the rim can, according to the authors’ models, enhance water flow over the openings, suggesting that either this is a burrow of some sort or the 3-D remains of an organism that filtered microbes out of the water.  The authors suggest this could be a remnant of one of two types of organisms:

1.) Xenophyophores: Giant single-celled foraminifera that have multiple nuclei and form a “test”, a hard skeleton made from minerals extracted from seawater.

2.) The remains of a sponge. As the authors say:

Alternatively, the modern form is the compressed body of a hexactinellid sponge adapted to an unconsolidated sedimentary substrate (Rona and Merrill, 1978). If this interpretation is correct, then the fossil form is a body rather than trace fossil.

These sponges have hard parts as they contain spicules (small bits of the body) made of silicon.

Alternatively, it could be something else. The authors don’t consider that it might be burrows of a worm, but this site suggests that:

The short answer is, “We have no fricking idea.” There are many mysteries on the ocean floor.

Identify the organism that made this pattern

December 8, 2017 • 10:00 am

Here’s a new tweet that Matthew sent, showing a pattern found underwater by ROV SuBastian dive #96 (dive #97 starts at 11 a.m. Chicago time, and you can watch it here).  These dives are sponsored by the Schmidt Ocean Institute, and are currently investigating hydrothermal vents around Tonga in the Pacific.

Your job: Guess what kind of organism made that pattern. It’s not a human creation, but a genuine trace left by living organisms. What is it? I’ll put up the answer at noon Chicago time. Paleodictyon trace fossils are of unknown origin, but we’re pretty sure what made this one. You’ll find out in two hours.

Today’s dive looks cool, and here’s the info:

This is the twelfth ROV dive of the Underwater Fire expedition. This dive will visit the known hydrothermal vent field at Mata Fitu volcano, one of the North Mata group of volcanoes. This is the second dive of this expedition at Mata Fitu, but first visit to the hydrothermal vent field.The dive will start downslope of the area of known venting and will traverse back-and-forth upslope to establish the aerial extent of venting. The dive will be a mix of geo-transects to visually explore the area, sample lavas and sediments, and will also do chemical and biological sampling at the hydrothermal vents.

Watch it here in about an hour:

Readers’ wildlife photos

November 11, 2017 • 7:45 am

Reader Mark Sturtevant sent some diverse photos from his Ikea Cabinet of Mystery.

About a year ago I had posted pictures showing a few specimens from my “Cabinet of Mystery” (from Ikea!). It is once again time open its glass paneled doors and peer inside for more oddities and curiosities.

We begin with a fossil that I picked up while on a class field trip to the famous Mazon Creek formation in Illinois. This site contains ironstone nodules with numerous fossils from the Carboniferous period. I found it pretty much impossible to split these nodules successfully, but there were many that were already split open by the regional freeze-thaw cycle, revealing whatever there was to reveal. Anyway, this large fossil leaf is from a seed fern known as Neuropteris. I found several of these, plus other kinds of ferns and horsetail fossils. All pretty common. Animal fossils are also known from this site, including some very famous ones. One observant member of our class found a horseshoe crab fossil!

Next we have an ammonite. I had long ago misplaced information about this purchased fossil, but a quick search online identifies it as a Cretaceous species named Discoscaphites. These are famous for their pearlescent surface, which one can see here. I suspect that what is going on is that the outer layers have been removed, revealing the pretty inner layer of the shell.

The next picture is a belemnite shell. Like the ammonites, these were cephalopods with a shell, but this piece is the rear-most tip of a larger shell which was elsewhere quite thin and so not often preserved. When a teenager, I convinced my parents to let me join an extended school-sponsored summer field trip. We hiked and camped through most of the major parks in Wyoming and South Dakota, and at an enormous and isolated hogback ridge formation in Wyoming we came across a huge deposit of belemnite fossils. I would be surprised to learn that kids get to go on field trips like that any more. It was amazing (and at times a little dangerous), but I thought it was an important part of growing up.

Back to some more purchased fossils. Next is a set of Eocene fossil bird footprints that are from the Green River formation in Utah.

Following that, the next fossil appears to be a crane fly, also from the Green River formation.

I like to collect fossils, and the next picture is a mystery fossil that my oldest son had collected when we lived in Flagstaff Arizona. The geology of Flagstaff is a complex. Right in the town you can drive by road cuts containing sandstone layers that date to the Permian and Triassic periods, and at various places there are much older deposits. Anyway, my son picked up this specimen while hiking in the forest in the back of our house. Note that one side is convex and the other is concave, and there are different textures on the two sides. The fossil could be something rather mundane like a shelled animal (Brachiopod?), but I don’t know of any species like this. I am wondering if it is a scute (a kind of bony dermal plate) from a reptile. Phytosaurs were Permian reptiles that were convergent on crocodiles, as shown here, and they had bony scutes. I know there are large bone beds of Phytosaurs just outside of town.

Not all treasures in my Cabinet of Mystery are fossils. The human skull in the next picture is one of the main prizes. I have always called it ‘Uncle Herbert’ for some reason, and I do suspect it to be from a male because of certain craniofacial features like the brow ridges and squarish eye orbits. In any case it was an amazing gift from my very generous parents. They were teachers, and so growing up we always had catalogs from Carolina Biological Supply around the house, and I was always poring through them and bringing up this or that cool thing. There were many Christmas mornings when I would awake to find a special box from CBS sitting under our Xmas tree. These might include things like embalmed animals for dissection, or insects for mounting. But one special morning there was a box that contained… dear Uncle Herbert.

Another item from CBS is shown in the next picture. This is a lump of tar from the famous La Brea tar pits, containing a beetle which I suspect is a species of water scavenger beetle. The La Brea tar pits are a system of tar seeps in the middle of Los Angeles, California. These seeps have been trapping animals and plants of all sorts for tens of thousands of years. Although their more famous victims include numerous saber-toothed cats and mammoths, they will ensnare anything. I don’t know how old this specimen is, but it is supposed to be ancient and the chunk of tar is as light as a feather, not sticky at all, and it smells like a plain rock.

Finally, we finish with a fossil tooth from an Albertosaurus, which is a smaller relative of T. rex. Dinosaurs would regularly shed their teeth and replace them with new ones, and Albertosaur teeth are presumably fairly abundant in some sites as I have seen a lot of these in rock shops.

A ten pound frog frog lived in ancient Madagascar

October 9, 2017 • 1:15 pm

A frog that could swallow a small theropod dinosaur? Well, maybe: it was large enough, and weighed in at a hefty ten pounds (4.5 kilograms). This animal, with the clever name of Beelzebufo, was first described in 2008, but a new paper in PNAS by Susan Evans et al. (reference and free link below), describes a full species, Beelzebufo ampinga, based on a larger sample of fossils from Madagascar.  The name is scientifically given and described as follows:

Etymology. The generic name is based on Beel’zebul (Greek), Devil, and Bufo (Latin), toad, in reference to the size and probable life appearance of this anuran; the specific epithet, ampinga (Malagasy), means shield, in reference to cranial hyperossification.

Here’s an artist’s reconstruction of B. ampinga next to a normal frog and a pencil. Its skull was up to eight inches wide, its length was about sixteen inches (40 cm) and it’s been described as having the size and appearance of  a “squashed beach ball.” It lived about 65-70 million years ago, and may be the largest frog that ever lived. We’ll never know for sure given the incompleteness of the fossil record and the fragility of frog bones, but it’s a big ‘un:

The animal was reconstructed from bits of its skull and vertebrae, and part of its pelvis. Here are some fragments; if you want to see what they are, go to the paper.

And the reconstruction, with the discovered fossilized parts in white and the presumed remainder stippled. The caption gives two living frogs for a size comparison:

(Fig 2 in paper): (A) Skull reconstruction showing parts preserved (white areas, Left) and distribution of pit-and-ridge ornament (stippling, Right). (B) Skeletal reconstruction and inferred body outline of average-sized (skull width, 200 mm; SVL, 425 mm) adult female B. ampinga based mainly on Lepidobatrachus asper (32). White areas indicate parts represented by fossil specimens. For size comparison, dorsal view silhouettes of Ceratophrys aurita (the largest extant ceratophryine) (C), and Mantidactylus guttulatus (the largest extant Malagasy frog) (D), are shown. cp, crista parotica; fm, foramen magnum; frp, frontoparietal; mx, maxilla; n, nasal; pmx, premaxilla; qj, quadratojugal; qu, quadrate; sq, squamosal. (Scale bars: 50 mm.)

A new article in National Geographic gives a layperson’s take (the paper itself is full of arcane vertebrate paleontology):

These largely terrestrial frogs may have been as ill-tempered and aggressive as their living relatives, the ceratophyrines of South America, scientists say. Ceratophyrines are nasty sit-and-wait predators that are eager to snap at just about anything that happens by, experts note. The ancient devil frogs may have snatched lizards, small vertebrates, and possibly even hatchling dinosaurs with their huge mouths and powerful jaws.

Scientists announced Beelzebufo in February 2008 more than a decade after the first bits of fossilized remains from the species were found. Its name is derived from Beelzebub, Greek for “devil,” and bufo, Latin for toad. Ampinga means “armored,” describing the prominent cranial shield the species had on its head.

One sidelight of interest: its closest relatives are not on Madagascar, but on the South American mainland, and it’s very different from living and fossil frogs on Madagascar. The conventional wisdom is that the land that now comprises the conglomorate of Seychelles + Madagascar + the Indian subcontinent drifted away from Antarctica/Australia/South America about 120 million years ago. But there is some evidence from other groups that there were physical links between Madagascar + India and South America up to about 80 million years ago in the late Cretaceous—when this frog lived.  The fact that this frog lived at that more recent period, and has its closest affinities with frogs from South America, supports the latter hypothesis, though not strongly. Another alternative is that the ancestors of this frog were simply one-off survivors that floated away from South America over 100 million years ago and weren’t part of the radiation of other frogs on Madagascar. A third alternative, which is the least likely, is that its ancestors somehow made it over the ocean from South America about 70 million years ago, perhaps on a floating raft of vegetation. That seems unlikely, however, as salt water is deadly to amphibians.

It would be lovely to see this thing alive, but alas, the inexorable course of evolution prevents that. Maybe George Church can bring it back!


Evans, S. E., M. E. H. Jonesand D. W. Krause A giant frog with South American affinities from the Late Cretaceous of MadagascarPNAS 2008 105 (8) 2951-2956; published ahead of print February 19, 2008, doi:10.1073/pnas.0707599105


Giant “paleoburrows” dug by extinct mammals

June 24, 2017 • 3:15 pm

In the last few years, geologists have been finding—mostly in Brazil—large “paleoburrows” that were almost certainly dug by large, extinct mammals such as giant ground sloths. These burrows can be up to 3000 feet long (!) and 5 feet wide, though the very long ones were surely dug by many individuals over many generations. The shape of the burrows, and the fact that they bear distinct claw marks, are sure signs they were dug rather than being natural caves (the beasts could also have “improved” existing caves, but geologists think that’s unlikely).

Here’s what the excavations look like, from a summary of paleoburrows in Discover Magazine (captions from the site):

Looking into a large paleoburrow in Brazil. (Courtesy: Heinrich Frank)
Inside the first paleoburrow discovered in the Amazon. It’s nearly twice as large as the second-largest known burrow, located elsewhere in Brazil. (Credit: Amilcar Adamy/CPRM)

Here’s evidence that these are the products of animal activity: claw marks:

Claw marks are clear signs from the engineers who dug the tunnel. (Courtesy: Heinrich Frank)

A close-up of the scratches (sadly, there’s no scale, but you can see from the above that the animals must have been BIG.

A closer look at those claw marks. (Courtesy: Heinrich Frank)

The caves are at least 8,000 to 10,000 years old, which is when the suspected excavators went extinct. And those suspects are both giant ground sloths (Megatherium) and giant armadillos (glyptodonts). The sloths, one of the largest land animals that ever lived, were this big (below) and, at least according to Wikipedia, could weigh up to 4000 kilograms (4 “tonnes”) and stretch 20 feet from head to tail. Some were as big as modern elephants, and, like the pachyderms, they were herbivores:

Another candidate, the giant armadillos, or glyptodonts, were also huge, weighing about 2 “tonnes” (2000 kg) and extending about 11 feet: they were the size and weight of a VW Beetle. Here’s one:

But the caves with scratches on the roof were probably made by sloths, which could rear up on their hind legs. Based on the shape of the excavations, Heinrich Frank, a Brazilian geologist, thinks they were made this way:

But why did they need these caves, particularly such long ones? It’s still a mystery:

. . .  the sheer size of the burrows is something that Frank and his colleagues are still trying to explain. Whether prehistoric sloths or armadillos were responsible, the burrows are far larger than would be necessary to shelter the animals that dug them from predators or the elements.

The giant armadillo, the largest living member of the family, weighs between 65 and 90 pounds and is found throughout much of South America. Its burrows are only about 16 inches in diameter and up to about 20 feet long.

“So if a 90-pound animal living today digs a 16-inch by 20-foot borrow, what would dig one five feet wide and 250 feet long?” asks Frank. “There’s no explanation – not predators, not climate, not humidity. I really don’t know.”

You tell me! It’s cool enough that we have fossil habitats like this.

h/t: Michael