On June 28th, 2018 we learned about

Fossil record suggests that solitary primates regrew claws to help with grooming

No matter how badly you may want them, no matter how long you let you nails grow out, you will never grow claws. Humans and most other primates seem to have traded in proper claws for our flimsy nails around 56 million years ago. It might seem like we gave up some crucial anatomy in this trade, as claws can be used as everything from weaponry to climbing gear, although the fact that we now have touch screen-friendly fingers certainly helps. Nonetheless, paleontologists are now realizing that not every primate has been truly served by the loss of claws, which is why some genera have apparently regrown one on each hand, although not for the more adventurous uses listed above. The task they couldn’t give up was grooming, which raises questions about why our fashion-conscious species is able to get by with nails alone.

The long-standing assumption about primate claws is that they were lost to enable our ancestors’ mobility. As arboreal climbing, leaping and grasping became increasingly important to these animals, thinner, flatter nails likely provided an advantage over more pronounced hooks and knobs. Nails would be less likely to snag on small branches, allowing a hand to rotate around a branch as an animal swung through the trees in a way embedded claws just wouldn’t allow for. As far as anyone knew, once a common ancestor switched to nails, there was no going back.

Caring for hair with claws

Except, of course, for all the living primates that still sport a tiny grooming claw on each hand. Lemurs, lorises, galagoes and tarsiers all have a tiny claw on their second digits, enabling them to pick debris and parasites like like lice and ticks out of their thick fur. These claws aren’t exactly fierce, and they had long been thought to be a hold-over from an alternate branch of the primate family tree. A new survey of fossil fingers has started to unravel some of this model though, suggesting that some modern grooming claws have actually developed again, restoring anatomy previous lost to fingernails.

As researchers pieced together all the distal phalanges, or finger-tip bones, held in various collections, they realized a pattern was emerging that might help explain why some species needed a grooming claw while others didn’t. More solitary species, such as the modern titi and owl monkeys, need these claws to groom themselves. Primates that live in social groups don’t need that extra tool, because they can rely on each other to clean up their fur. This study can’t prove conclusively that this is why some primates have regrown their claws while others haven’t, but if this trend holds true, it opens up a new avenue for understanding extinct animals from their fossils alone. Because the grooming claw may be tied to the social structure of a species, finding the right bony digit could presumably reveal a lot more about how an animal lived than just the size of its fingers.

Source: Fossils show ancient primates had grooming claws as well as nails by Natalie Van Hoose, Phys.org

On June 24th, 2018 we learned about

Many dinosaurs’ hyoid bone left their tongues immobilized in their mouths

You’ve probably never worried about this, but paleontologists have finally confirmed that dinosaurs like Tyrannosaurus rex couldn’t french kiss. They couldn’t lick lollipops, nor could they do that trick where you tie a knot in a cherry stem in your mouth. However amazing their teeth or jaws may have been, analysis of a bone called the hyoid has found that T. rex, as well as most other dinosaurs, couldn’t do much with their tongues besides swallow. Lacking the articulation of many modern birds, snakes and lizards, this sheds light how tongues evolve to support different animals feeding habits.

Degrees of tongue articulation

The hyoid is a small bone that, millions of years ago, was a gill arch in our fish ancestors. As animals have evolved along different ecological paths, the hyoid has been adapted to support a variety of anatomical needs. In humans, it sits above the larnyx, connecting soft tissues to help us manage the passage of air and food in our throats. In many birds, the hyoid juts further into the mouth, often acting as a mobile attachment point for the tongue. This enables a variety of tongue movements, including extreme cases like the curling, tube-like tongue of a hummingbird. Beyond tongues, hyoids also enable modern reptiles like the Mata Mata turtle to stretch and open its throat to suck in prey underwater.

However, in most theropod and sauropod dinosaurs, hyoids weren’t quite so active. They were found to usually be a relatively simple pair of rods under the tongue, suggesting minimal possibility for movement. This arrangement most closely resembles the hyoids of modern crocodiles and alligators, which isn’t entirely surprising. These crocodilians can position their heads well enough to lop off hunks of food, only needing their tongue to help push that food back to be swallowed. Without other demands, their tongues can essentially be fixed along the bottom of the animal’s mouth by muscle and other soft tissues. Dinosaurs like T. rex could probably use this sort of feeding pattern as well, relying on their heads and teeth to get food into their mouths to be swallowed.

When to switch from simply swallowing

This doesn’t mean that hyoids only became complex in the last 65 million years though. Plant-eating ornithischian dinosaurs, like Stegosaurus or Triceratops, had more sophisticated hyoids than many of their compatriots. Pterosaurs were also found to have hyoids that would have facilitated more tongue waggling and lapping. Taken together, these ‘exceptions’ help prove what a mobilized tongue could be good for— if T. rex could simply aim its mouth at its food, Triceratops and pterosaurs likely had to use their tongues to grasp and manipulate their food before eating it. This could be necessary for grasping and chewing twiggy plants, or getting a hold of small prey with long, skinny beaks. In modern birds, this kind of tongue use is clearly demonstrated by parrots who use their tongues to pluck and hold nuts and seeds, almost like a finger built into their mouth.

To really make the point about what pressures push a species to develop a more complex hyoid, researcher noted the difference between tyrannosaurs and avian dinosaurs more directly related to modern birds. While both groups were theropods, the larger, bitier predators had the more crocodilian tongue arrangements. Flying theropods like microraptors had mouths that look much more like modern birds, showing that taking to the sky created a need for a more complex mouth.

Source: Tongue-tied: T rex couldn't stick out its tongue by Nicola Davis, The Guardian

On May 24th, 2018 we learned about

A look at modern quail ligaments suggests that pterosaur legs had a limited range of motion

It’s hard to study extinct animals with no surviving descendants, which is why researchers have had to use modern quail skeletons to prove that pterosaurs didn’t move like bats. It sounds silly, but is also somewhat appropriate for animals that are often confused with dinosaurs and were also strongly compared to flying mammals when their fossils were first discovered. Because pterosaurs also flew on wings made of skin that extended from the animals’ torsos to their specialized fingers, the assumption was that their hind legs would also splay out as if they were a bat. That idea may be falling out of favor though, and a recent study with modern quail corpses may narrow the range of possible poses even further.

Which poses were possible?

With very rare exceptions, fossils usually only preserve skeletal anatomy in an animal. We can learn a lot from bones alone, as they will generally be marked by specific textures where muscle once attached, as well as stress damage from the pull of strong tendons. Of course, not all wear and tear adds useful information to a fossil, and flattened or broken bones make it harder to know how a skeleton once interacted with softer tissues like muscles, tendons and ligaments. Just because a thigh bone could appear to fit in a hip socket in specific orientation doesn’t necessarily mean that we can be sure the animal actually held itself in that posture when it was alive.

To see just how much of a difference soft tissue can make, Armita Manafzadeh from the Brown University dissected and analyzed the femurs and hips of modern quails. She found that a few ligaments in particular greatly reduced the range of movement in the bird’s legs, ruling out close to 95% of the leg positions that the bones alone could adopt. Among those positions are the bat-like leg poses seen in early reconstructions of pterosaurs, suggesting that pterosaurs never flew this way. This doesn’t take a huge stretch of the imagination, since while bats and pterosaurs could have independently evolved this posture in a case of convergent evolution, they’re so distantly related from each other there’s no way one animal inherited the pose from the other.

How close are these connections?

On the other hand, pterosaurs aren’t closely related to just about any animal alive today, including the quail used in this study. Pterosaurs weren’t birds or even dinosaurs, and so they wouldn’t have a direct connection to modern bird anatomy, especially as far as flight anatomy is concerned. Instead, this research is relying on the idea of phylogenic bracketing, in which a trait is inferred in one species by looking for shared traits in nearby branches of their family tree. Since the key ligaments that limited leg movement are similar in both birds and modern reptiles like lizards, it’s somewhat fair to assume that pterosaurs would have inherited similar anatomy from these groups’ last shared ancestor. Of course, reptiles nor birds flew with elongated pinkies either or developed a pteroid, so there’s also a chance that pterosaurs had time to evolved their own, specialized version of a more flexible ligament.

At the very least, these more bird-like postures don’t necessarily conflict with some of the best evidence we have about pterosaur legs— their trackways. Many trace fossils of pterosaur footprints have been found, which has helped build the hypothesis that larger pterosaurs used their legs and arms to hop, jump and catapult themselves into the air. What their legs did once airborne may need to be debated a bit further though.

Source: Study casts doubt on traditional view of pterosaur flight by Kevin Stacey, News From Brown

On May 23rd, 2018 we learned about

Two types of taste-buds allow bees to enjoy their nectar more than other insects

My daughter might be part honeybee. While her younger brother seems to think the best way to eat ice cream or candy is to gobble it up quickly, she figures the experience should be extended for as long as possible, even if that means licking a single Skittle for minutes at a time instead of just chewing it up in her mouth. Now, bees don’t necessarily ration our their enjoyment of nectar from flowers in quite this manner, but they do spend a lot more time than other insects slurping up the sugary liquid, eventually collecting more nectar and pollen as a result. While some of that nectar will need to be shared with their hive, the bees aren’t strictly being dutiful about their nectar collection— their mouths are actually adapted to help maximize their enjoyment of every sugary sip in a way my daughter would be envious of.

Like many foods that non-carnivores enjoy, the sugars in nectar activate specialized sensory cells on a bee’s proboscis, or mouth parts, which send a signal back to the brain. This usually coincides with a pleasurable release of neurotransmitters, providing positive feedback for acquiring some calorie-rich food. Over time, this response is diminished, presumably to make sure an animal doesn’t just burn out its taste buds by constantly soaking them in the first source of sugar it can find, which is why a tenth piece of candy might not feel as exciting to eat as the first.

Repeating the first sugary sip

Since bees are gathering nectar and pollen for more than their own consumption, evolution has set them up with an incentive to linger in flowers a bit longer than other insects. In addition to the sweet-sensitive taste receptors, they have secondary nerve cells that intermittently interrupt the process, causing the timer on their enjoyment to repeatedly reset. So by hitting restart on this process over and over, the bee is incentivized to suck up nectar for a longer period of time, maximizing each visit to a flower which can last as long as 10 seconds.

Aside from explaining what keeps bees on a particular flower for extended nectar-slurping sessions, this is also the first time scientists have ever seen taste-oriented nerve cells interact with each other, rather than simply pipe information straight to an animal’s brain.

Source: What gives bees their sweet tooth? by Newcastle University, Phys.org

On May 21st, 2018 we learned about

Comparisons with animals reveal how human anatomy limits our ability to be loud

Despite their efforts to prove otherwise, my kids just aren’t that loud, relatively speaking. Sure, their shrieks seem like they could peal the paint of the walls sometimes, but as humans they just can’t generate coherent sound as well as most other birds and mammals. After studying the physics of many species’ bodies, researchers have figured out the three main factors that keep humans from being able to shout as loudly as you might expect for an animal our size.

Air-pushing power

The first limiting factor in human vocalizations is just how well we can push air with our mouths. Generating a specific sound requires pushing and shaping air from the lungs, through the throat and out our mouths. Each piece of anatomy that that air passes by may help vibrate it at specific frequencies to produce a desired note, but they also absorb energy.  This means that a significant amount of the initial power your lungs may try to provide is reabsorbed by your body before it has a chance to be broadcast to listeners’ ears.

Open wider

Many animals make up for this side effect of being squishy by maximizing how much air they push out of their mouths. Opening a beak or maw just a bit wider increases the amount of air, and thus the volume of a vocalization, substantially. So while human tongues, lips and cheeks may help us speak, the average human mouth can only open to a two-inch diameter. Essentially, the anatomy that helps us shape our speech also gets in the way of outgoing air, forcing us to be a bit quieter.

Speak or squeak

As it turns out, we may not be picking the best sounds to say, either. If humans really wanted to speak more loudly, we’d do so at a more efficient pitch. Based on the shape of our vocal tract, we’d be louder if we shouted at around 6,860 Hz, although our ears are probably grateful that we don’t. Instead, an adult human usually speaks at around 300 Hz, giving up volume for something a bit more pleasant to listen to.

Stop standing

Finally, human posture also dampens our auditory output. Ideally, we would be able to use our anatomy as a sort of sound-reflective wall, or baffle, directing our voice in one direction as much as possible. Birds, for instance, can tuck their head back a bit making their body shape a big tube, pushing air from their lungs straight out of their mouths. For people, our upright posture and relatively inflexible neck doesn’t let us adopt a pose that would really optimize our lung capacity in this way.

Shouting vs. cell phones

Of course, in an age of ubiquitous cell phones, why should we worry? Most of us don’t plan on singing opera, and can just use technology to broadcast our voice around the world. However, there is an interest in knowing how far a human voice an intelligibly travel, particularly in search and rescue scenarios. If we know exactly how far a human can push coherent sound, it may help first responders know how close they need to be before they should expect a call for help. Either that, or we all need to learn to whistle a lot better.


My third-grader asked: What about whales? I thought blue whales had really deep voices.

Blue whales can make sounds that are so low we can’t hear them with our ears. Those low-frequency vibrations can then travel huge distances through the water, which is actually a bit better at transmitting sound than the air is. However, the whales aren’t expelling air to make any of these sounds, so the above “rules” about mouth sizes and optimal pitches don’t necessarily apply.

What about elephants? And… wait, do other animals use their noses to make sound?

Elephants may be the only animal that really need push air out their nose for vocalizations, although they also make sound with their mouths. Studies have found that both kinds of exhalations are pretty loud, although a lot elephant communication happens with low grumbles and grunts, many of which are at least partially transmitted through the ground instead of the air, making this a less direct comparison. In fact, elephants have specialized in that range of sounds so much that they don’t hear some of the higher-frequency vibrations that our ears can pick up. One of the big effects of their trunks isn’t to be louder, but to help make their overall vocal tract larger, making it easier to produce lower sounds.

Speaking of long anatomy, giraffes’ long necks apparently make it hard to push large amounts of air out of their mouths, leaving them a bit mute. On the other hand, a study of 1000 hours of giraffe recordings did discover that they can use their body, and presumably their neck, as a resonating chamber. While they’re not easy to hear, giraffes can make low-pitched set of humming noises, in addition to smaller, more obvious snorts from their noses.

Source: How animals holler by University of Utah, Phys.org

On May 3rd, 2018 we learned about

Scanned Ichthyornis skulls show a mash-up of modern bird beaks on ancient dinosaur jaws

When the first Ichthyornis dispar fossils were discovered in the 1870s, paleontologists thought it was actually two animals buried in the same spot. The body was decidedly bird-like, but the jaw looked like it came from a marine reptile. Further excavation convinced people that the strong, tooth-filled mouth was indeed part of the accompanying seagull-shaped body, and instead of a chimera, no less than Charles Darwin himself suggested that it was a great example of a transitional species. More recent studies, including reexaminations of those original specimens, have basically confirmed this view, especially as new details reveal even more features found in modern birds in this 100 million-year-old dinosaur.

Details in three dimensions

The latest study of Ichthyornis gathered data from museum collections, plus a new specimen that was never fully excavated from the rock it was buried in. Unlike earlier finds that were partially damaged over time, this new specimen retained its original shape in all three dimensions. CT scans allowed researches to build a digital 3D model, then compare that to earlier Ichthyornis skulls. This not only confirmed more obvious anatomy, like the toothed jaw, but also revealed more subtle features, including an independently-mobile upper jaw. This is something previously seen only in modern birds, making this the earliest known example such anatomy.

Blending toothed jaws with a touch of bird beak

The tip of the mouth was also a first. While the smooth, curved beak may look like your everyday seagull snout, the shape, keratin covering and aforementioned mobility make it the first known snout to function like a modern bird’s beak. This beak could have been used like pinching fingers, allowing Ichthyornis to peck, preen its feathers or pick up small objects. Paleontologist Bhart-Anjan Bhullar, who lead the investigation, even commented that the precise grip offered by Ichthyornis‘ beak may have helped it compensate for its lack of hands, which were essentially flight-ready wings instead of the clawed digits seen on theropods like a Velociraptor.

Of course, as a transitional species, Ichthyornis wasn’t exactly an ancient seagull. Keratin only covered the tip of its snout, leaving plenty of room for a more reptilian mouth, complete with teeth. The lower jaw also had considerable muscle-attachment points, suggesting that unlike many modern birds, Ichthyornis could bite with considerable force. All these features may have then added up to a dinosaur that could nimbly pluck prey like fish or crustaceans out of the water before chomping them into bits with its powerful teeth.

First features of our feathered friends

Scientists can’t say for sure that Ichthyornis is a missing link in the evolution of modern birds. Just because it looked like a collection of both ancient and modern theropod features doesn’t mean that it was directly related to any bird species alive today. However, it does help establish when in history these features evolved, making many modern bird features a product of the Mesozoic era.

Source: This ancient fowl bit like a dinosaur and pecked like a bird by Carolyn Gramling, Science News

On April 5th, 2018 we learned about

Four-eyed lizard fossil helps explain the evolution of “extra” eyes in vertebrates

At first glance, the skull of a Saniwa ensidens doesn’t look terribly different than other monitor lizards. Even though this species lived 48 million years ago, it would have closely resembled monitor lizards alive today. A closer look would reveal two important features though— S. ensidens had two tiny holes in its head that were there for its third and fourth eyes. While that sets a record for ocular organs in a jawed vertebrate, the most important part of these extra peepers is how they differed from each other, a fact that may help explain skull-top eyes in animals alive today.

Two evolutionary tracks for third eyes

As unfamiliar as it may sound, plenty of vertebrate animals sport a third eye on top of their head (with lampreys also having four.) They’re not eyeballs in the way we’re used to, but are light-sensitive anatomy known as parietal eyes, growing from either the parapineal or pineal glands through the top of an animal’s head. Fishes and frogs generally have pineal organs, while lizards (and lampreys) have parapineal organs. Mammals and birds lack these third eyes, raising questions about how this anatomy ended up in some terrestrial species but not others.

One possible explanation was that lizards had basically kept and modified the extra eye they inherited from fish and amphibians. It’s not clear why mammals and birds would have given this up (hair and feathers in the way?) but this explanation also doesn’t explain why lizards’ parapineal organs closely line up with distant relatives like lampreys. Fortunately, CT scans of S. ensidens were able to capture enough detail in skull anatomy to reveal that this extinct lizard had both the fish and lizard versions of these eyes, clearly demonstrating that the two variations developed in parallel to each other, rather than in succession.

The view from the top of one’s head

There are still some questions about the utility of parietal eyes. The organs are formed when an embryo’s developing brain comes in contact with a patch of skin. That contact triggers a cascade of activity in the affected cells, creating any one of an animal’s three eyes. So far, pineal and parapineal eyes have been linked to endocrine functions (such as circadian rhythm management) and navigation. Some vertebrates seem to be able to detect the polarization of light with their third eye, then use that information to better orient themselves in their local environment.

Source: A four-eyed lizard offers a new view of eyesight’s evolution in vertebrates by Jim Shelton, Yale News

On March 15th, 2018 we learned about

Scans show that Archaeopteryx’s arm bones were able to flap like a pheasant

For over 100 years, the biggest point of fascination on Archaeopteryx fossils wasn’t the animal’s bones, but its feathers. When first discovered in the 1860s, people were understandably fixated on the impressions of the specimens long feathers left in the rock around the skeleton. They appeared to be long and rigid like a modern bird’s feathers, right down to tiny, interlocking barbules that would give each feather more strength. On the other hand, Archaeopteryx’s skeleton seemed to contradict this bird-like anatomy, as its long tail and toothed mouth aren’t found in any modern avians, and its breast bone lacked the large keel that modern birds use to attach powerful chest muscles needed for flapping. To dig in a little deeper, the latest study of Archaeopteryx looked inside the animal’s bones, and found that they probably could fly like a bird, but only those birds that stay close to the ground.

X-ray scanning for signs of strength

With the exterior of Archaeopteryx’s fossil having been extensively documented, researchers opted to look at the inner structure of each bone in the European Synchrotron Radiation Facility. The powerful x-rays would let them look at delicate structures inside these 150 million-year-old fossils with amazing resolution without needing to damage them in the process. The goal was to measure the arm bones’ torsional resistance, which is how well they would stand up to being twisted when used in flight. Since modern birds that do a lot of continuous flying have higher torsional resistance than birds that don’t, this measurement could be used as another way to assess how flight-ready Archaeopteryx was, regardless of feathers or breast bones. To make sure they weren’t missing a larger pattern, the bones were also compared to crocodilians and pterosaurs as well.

To nobody’s surprise, Archaeopteryx didn’t soar like an eagle, or even a Quetzalcoatlus. However, its arms did appear to handle more than just crawling around, most closely resembling birds like quails and pheasants that are known for short bursts of flight, usually to avoid danger. The x-ray scanning also revealed a large number of blood vessels in Archaeopteryx’s skeleton, a trait associated with high growth rates and metabolism. This would indicate that while the dinosaur wasn’t a bird itself, it probably grew and moved like one.

Not a fully-fledged flyer— yet

This still doesn’t make Archaeopteryx the world’s first bird, or even a bird ancestor. Other species have been found with feathers, even predating Archaeopteryx. We also don’t believe that Archaeopteryx was part of the raptor lineage that eventually developed into modern birds, and instead was a case of convergent evolution. In this case, that convergence would be the capacity for short, evasive flight, which makes sense as avoiding predators has been found to be the most likely reason any species develops wings in the first place. The one catch is how Archaeopteryx would ever get off the ground in the first place. Until evidence of something like a breast keel made from cartilage can confirm its flapping strength, we’re still not sure how well the animal could defy gravity to get itself airborne.

Source: This Famous Dinosaur Could Fly— But Unlike Anything Alive Today by Michael Greshko, National Geographic

On February 12th, 2018 we learned about

Cheetahs’ high speeds are viable thanks to their uniquely-sized inner ears

Cheetahs don’t run fast to set records. They run to catch food. From that perspective, it makes sense that some of their most important adaptations aren’t only found in their long legs or flexible spines, but in their heads to help guide them towards their prey. A study of the big cats’ inner ears has found that they’re possibly the most advanced, and most recently developed, feature that helps cheetahs not just run after, but also catch their elusive prey.

Inner ears aren’t used for hearing. They’re a specialized structure in the skull that tells vertebrates how their heads are oriented, like the accelerometers in your phone or video game controller. Each inner ear consists of three semicircular canals that contain fluid and sensitive hair cells. The fluid levels off thanks to gravity, tickling some hair cells but not others. With each canal handling a different direction of motion, such as side-to-side versus tilt, the combined data can be used by the brain to figure out how a head is not only oriented, but how it’s moving through space. This kind of information is crucial for movement and balance, especially when that movement is coming from a 65 mile-per-hour sprint after an unpredictable gazelle.

Steering at top speed

For a cheetah (Acinonyx jubatus) to stay on target, it needs to maintain visual contact with its prey. Even when their body is careening in a new direction, cheetahs do a great job at keeping their head steady so they can keep track of how their prey is moving. Researchers suspected that their inner ears were somehow enhanced to make this possible, and started scanning the skulls of cheetahs and other big cats with X-ray computed tomography, giving them a detailed, 3D view of how each animal’s ears worked. As expected, cheetahs were found to have larger and longer inner ear canals than other cats, which would give the fluid and hair cells more granularity in the signals they could provide. To put it another way, these super-sized sensors could pick up smaller variations in tilt and movement, plus have a slightly bigger range before those signals were “maxed out” at one extreme or the other.

By including the skulls of extinct cat species in this study, researchers were also able to estimate when cheetahs acquired this degree of sophistication in their inner ears. An extinct relative, Acinonyx pardinensis, was also specialized for running, but its ear canals wouldn’t have provided the same feedback found in a modern cheetah. That bulkier relative lived only a few hundred-thousand years ago, showing just how recently this line of cats evolved this degree of sensitivity. That timing may make sense, since A. pardinensis probably wasn’t as fast as a modern cheetah either, and thus didn’t need quite as much control and maneuverability as today’s record-holding runners. As top speeds increased over time, it seems that the sensors and feedback systems to help the animal steer grew as well.


My third-grader asked: If balance is connected to your ear, do deaf people have problems with balance?

They can. One estimate says that as many as 30 percent of deaf people may experience some kind of persistent balance problem. Apparently the cause of the hearing loss, such as meningitis versus  something like Usher’s Syndrome, can affect how severe the balance issues may be.

Source: Cheetahs' inner ear is one-of-a-kind, vital to high-speed hunting by American Museum of Natural History, EurekAlert!

On January 18th, 2018 we learned about

Survey of species parses the prerequisites for a wielding a weaponized tail

It’s hard to fight with your rear end. Aside from a creature like a horse chasing away flies with its tail, few creatures can be said to be more intimidating from behind. There are exceptions to this rule, and they’re unusual enough that researchers have studied every tail-swinging tough-guy to see why those species evolved to fight with their rears instead of their claws or faces. Even though using one’s head or limbs would seem perilous in its own way, the number of anatomical requirements needed to make a tail weapon functional may have curtailed (ahem) their popularity in both living and extinct animals.

There’s obviously some variety to how a weaponized tail can operate. Dinosaurs like Stegosaurus carried spiked thagomizers, while some ankylosaurs ended up with bludgeoning clubs. Living species are generally a tad less ornamented, but the quills on a porcupine or the speedy snap of a monitor lizard’s tail can still make a foe think twice about attacking. Once a list of 286 species was assembled, researchers categorized them by an array of traits, from diet to size to the presence of bone or spikes on the tail’s tip. At that point, the data could be sorted and sifted to find what common threads united the reptiles, mammals and dinosaurs that have ever purposely backed into a fight.

Two traits proved to be universal among all the tail-swingers. Every animal that did battle with its rump was an herbivore, and as such these weapons weren’t being used for hunting. Most living species with dangerous derrieres only use them defensively, although exceptions, like rainbow agama lizard, pick fights to compete for mates. Still, it seems that teeth and claws have an edge (sorry) for predatory activities, beating out clubbed behinds as the anatomy of choice for predators around the world.

Other traits that were tied to tail weapons weren’t quite as obvious. Creatures with enlarged tail tips, like Ankylosaurus, Glyptodon or Shunosaurus, were all huge, with specially formed vertebrae in their tails. Stiff tails seem to require a smaller minimum body size, but shared a likelihood for osteoderms, or bony skin armor. Tail spikes, such as those found on a Stegosaurus or meiolaniid turtles, were tied to more head ornamentation, wide hips and some kind of bony but discontinuous armor on the torso. All the extinct species were also found to share osteoderms enveloping their tail tips, as well as an unusually stiff trunk, possibly to provide better leverage for lateral tail-swings. Living backside-battlers still follow many of these rules, although the minimum size requirements have shrunk down to around three feet in length since the age of dinosaurs.

Putting these parts in perspective

This may seem like a very roundabout way to describe animals we already know of, but organizing these traits helps bring a few things into focus. Requirements like stiff bodies and protective skin covering had to evolve before tails were weaponized, since a tail club or spike is the less common feature, and there’s evidence that anatomy like osteoderms actually fused together to create those weapons in the first place. However, most large herbivores found other ways to protect themselves before actually developing a rear-mounted defense mechanism. This isn’t to say that tail weapons were ineffective at their jobs, but that they require so many other anatomical conditions first, they’re very unlikely to evolve in the first place.

It also seems that, as cool as a tail bludgeon or thagomizer may have been, the complete package of bulk, armor and bone didn’t necessarily out-compete other forms of defense once they were assembled. Instead, brawling with one’s butt remains a bit of a niche adaptation that evolution keeps playing with, but rarely fully commits to.

Source: Where did all the tail clubs go? by Victoria Arbour, Pseudoplocephalus