What evidence do we have that dinosaurs were mesotherms?

They went extinct millions of years ago, so what is the evidence supporting the claim that they were mesotherms? And are there any examples of modern-day mesotherms?

Mesotherms have two basic characteristics: [1]

  1. Elevation of body temperature via metabolic production of heat.
  2. Weak or absent metabolic control of a particular body temperature.

The first trait distinguishes mesotherms from ectotherms, the second from endotherms. For instance, endotherms, when cold, will generally resort to shivering or metabolizing brown fat to maintain a constant body temperature, leading to higher metabolic rates. A mesotherm, however, will experience lower body temperatures and lower metabolic rates as ambient temperature drop. [2] In addition, mesotherm body temperatures tend to rise as body size increases (a phenomenon known as gigantothermy [3] ), unlike endotherms. This reflects the lower surface area to volume ratio in large animals, which reduces rates of heat loss.

While extant mesotherms are relatively rare, good examples include tuna, lamnid sharks (e.g., the great white shark), the leatherback sea turtle, some species of bee, [4] naked mole rats, the hyrax, and the various monotremes.

Historically, the same term was used by Alphonse Pyramus de Candolle to describe plants that require a moderate degree of heat for successful growth. [5] In his scheme, a mesotherm plant grew in regions where the warmest month had a mean temperature greater than 22 °C (72 °F) and the coldest month had a mean temperature of at least 6 °C (43 °F).

The thermoregulatory status of dinosaurs has long been debated, and is still an active area of research. The term 'mesothermy' was originally coined [6] to advocate for an intermediate status of non-avian dinosaur thermoregulation, between endotherms and ectotherms. A more technical definition was provided by Grady et al, [7] who argued for dinosaur mesothermy on the basis of their intermediate growth rates, and the empirical relationship between growth, metabolism and thermoregulation in extant vertebrates.

This viewpoint was challenged by D'Emic, [8] who argued that because growth rates are sensitive to seasonal variation in resources, dinosaur maximum growth rates were underestimated by Grady et al. Adjusting dinosaur rates upwards by a factor of two, D'Emic found dinosaurs to grow similarly to mammals, and thus were likely endothermic. However, sensitivity to seasonal variation in resources should be true for all vertebrates. If all vertebrate taxa were similarly adjusted, the relative differences in rates does not change. [9] Dinosaurs remain intermediate growers and good candidates for mesothermy.

Nonetheless, the dinosaur mesothermy hypothesis requires further support to be confirmed. Fossil oxygen isotopes, which can reveal an organism's body temperature, should be particularly informative. Recently, a study of theropod and sauropod isotopes [10] offered some support for dinosaur mesothermy. Feathered theropods are probably the best candidates for dinosaur endothermy, yet the examined theropods had relatively low body temperatures 32.0 °C (89.6 °F). Large sauropods had higher body temperatures 37.0 °C (98.6 °F), which may be reflective of mesothermic gigantothermy. Future isotopic analysis of small, juvenile dinosaurs will better resolve this question.

Arctic Dinosaur Nursery Reveals Best Evidence Yet They Were Warm-Blooded

A pair of tyrannosaurs enjoying the brief summer in the high Arctic during the Late Cretaceous, when at least seven species have been found to nest within 10 degrees of the pole. Image Credit: James Havens.

By Stephen Luntz

The discovery in the 1950s of dinosaur bones in the Arctic set off a debate as to whether there was a permanent dino-presence at high latitudes, or if great beasts migrated there for the summer. Subsequent evidence supported the idea some dinosaur species survived the long nights in both the Arctic and Antarctic, but a new paper in Current Biology reveals even more: many species nested there.

Paleontologists have previously found evidence of nesting at sites close to the Arctic Circle. However, the species involved have not been identified. Dr Patrick Druckenmiller of the University of Alaska has easily eclipsed such finds. "We didn't just demonstrate the presence of perinatal remains – in the egg or just hatched – of one or two species, rather we documented at least seven species of dinosaurs reproducing in the Arctic," he said in a statement.

“This is the first time that anyone has ever demonstrated that dinosaurs could reproduce at these high latitudes,” Druckenmiller said.

The Prince Creek Formation site sits above Alaska's Colville River and dates to the Late Cretaceous when it was 5-10 degrees from the pole.

The world was a warmer place then, but the average temperature year-round at the site is still thought to have been about 6ºC (42ºF). There would have been almost no Sun and precious little warmth for four months every winter.

Despite this, the location supported an abundance of life. Druckenmiller and colleagues have found hundreds of bones and teeth from baby dinosaurs. Some never left the egg, while others suffered unfortunate fates shortly after hatching. Rather than reflecting a variety of closely related species, their finds include hadrosaurids, ceratopsians (a family that includes triceratops), thescelosaurs, and even tyrannosaurs and other carnivores. Although the families are familiar, most of the individual species are new to science.

The baby dinosaur bones and teeth that have been found are so small they need to be filtered out like panning for gold. Image Credit: Patrick Druckenmiller

"It wasn't that long ago that the idea of finding any dinosaurs in such extreme latitudes and environments was a surprise," Druckenmiller said. "To then find out that most if not all of those species also reproduced in the Arctic is really remarkable. We have long been asked, 'Have you found any eggs?' [We] still answer 'no.' But, we have something much better: the actual baby dinosaurs themselves."

Lacking flight these journeys would have been beyond adults of the smaller species, let alone hatchlings. Co-author Professor Gregory Erickson of Florida State University has previously shown these sorts of dinosaur eggs took 2.5-6 months to hatch. Even if laid in early spring, the young would have been in no state to almost immediately migrate south after breaking free from their shells.

The finding is also possibly the best evidence yet on the question of whether dinosaurs were warm-blooded.

"Cold-blooded terrestrial vertebrates like amphibians, lizards, and crocodilians have yet to be found [so far north], only warm-blooded birds and mammals – and dinosaurs,” said Erickson, although he admits we still don't know how they survived the cold and limited food of the winters. Most likely smaller species hibernated and larger ones made do with tree bark and other low-quality forage. However, "I think that this is some of the most compelling evidence that dinosaurs were in fact warm-blooded," he said.

The Ten Biggest Dinosaur Mysteries We Have Yet to Solve

We know dinosaurs better than ever before. Paleontologists continue to find new species, naming a new one every two weeks or so, and more accurately reconstruct familiar dinosaurs such as Tyrannosaurus and Triceratops. Despite all our recent advances in understanding the Age of Reptiles, dinosaurs still present us with a slew of unresolved questions. Here’s a list of ten dinosaur mysteries that continue to perplex paleontologists.

1. What was the first dinosaur?

For paleontologists, the earliest species of any major lineage is always a sought-after critter. The trouble is that the fossil record is made up of snippets of life’s history, not the entire reel, so actually finding frames from the dawn of dinosaurs relies on luck as much as science.

Tracks found in Poland and skeletons from Tanzania belong to animals that were close, but not quite dinosaurs. So far, these finds suggest that the “terrible lizards” evolved around 245 million years ago, with the best candidate for the earliest dinosaur being a sleek, lanky, dog-size animal called Nyasasaurus. But further discoveries could still supplant this animal as the oldest known root of the dinosaur family tree.

2. Were dinosaurs hot-blooded or cold-blooded?

During the height of the “Dinosaur Renaissance” in the 1970s, the most contentious question of all was whether these celebrated animals were supercharged, hot-blooded creatures or the equivalent of cold-blooded giant lizards. Almost 40 years later, dinosaur physiology is still largely a mystery. Multiple lines of evidence – including their bone microstructure and growth patterns – suggest that dinosaurs were highly active animals that ran hot. But how they achieved this feat is a lingering question.

Paleontologists have suggested an array of arrangements, from a physiology that maintained a high, constant body temperature to big herbivorous dinosaurs warmed by fermenting vegetation in their guts. The latest hypothesis is that dinosaurs were mesotherms – they relied on the activity of their muscles to warm their bodies but had body temperatures that could fluctuate. Dinosaur experts will undoubtedly continue to investigate and debate the point, especially given that dinosaurs took forms ranging from pigeon-size, feathery raptors to 110-foot, long-necked titans.

3. What was the biggest dinosaur?

Of all the superlatives, the title of “biggest dinosaur” is among the most prized. But picking out a clear winner is confounded by quirks of evolution and the fossil record.

Instead of just getting bigger on a straight trajectory through the entire Age of Dinosaurs, titanic sauropods evolved multiple times. This has given paleontologists a slew of contenders from different sauropod groups that lived in different places and in different time periods. Length estimates for the largest of these – such as Supersaurus, Diplodocus, Argentinosaurus, Futalognkosaurus and more – all come out around 100 to 110 feet or so, with variations in weight depending on the reconstructions.

There’s so much leeway in those numbers because the biggest dinosaurs are only known from partial skeletons, typically less than half the skeleton down to maybe one part of a single bone. That means paleontologists have to rely on smaller, more complete cousins of the giants to come up with size estimates, and these figures are often revised as researchers unearth new fossils.

With so many huge dinosaurs topping out at around the same size, we need more complete fossils for a definitive size check. And given how many times hefty sauropods evolved, along with the amount of fossil outcrops that are yet unexplored, the Big One could still be awaiting discovery.

4. How did dinosaurs mate?

Every dinosaur started life by hatching from an egg. That much we know for sure. But how parent dinosaurs came together to start the next generation isn’t as clear. Dinosaur mating displays didn’t fossilize, and paleontologists have yet to find telltale trackways showing, say, two amorous Allosaurus coming together, which could leave a trace of a dinosaurian embrace.

Even the basic sexual anatomy of dinosaurs is a bit of a mystery. They must have had a cloaca, a single orifice for the urinary, excretory, and reproductive tracts shared by birds and crocodiles. It’s also likely that male dinosaurs had an “intromittent organ” similar to that of ducks and ostriches. But since no one has found an impression or other trace of such an organ, we don’t know whether male Apatosaurus were modestly equipped or hung like an Argentine lake duck.

5. What’s with the funky headgear?

Many of our favorite dinosaurs – Triceratops, Stegosaurus and more – were decked out in all manner of horns, spikes, plates, crests and other adornments paleontologists group together as “bizarre structures.” Why these dinosaurs evolved to be so striking is a long-debated point among experts.

Despite early ideas that bizarre structures primarily evolved for functions like defense or temperature regulation, paleontologists have largely tossed out these notions and focused instead on the social implications of looking so garish. The horns and frill spikes of dinosaurs like Styracosaurus, some paleontologists suggest, evolved as species-specific signals that allowed dinosaurs to easily identify members of their own kind. Other experts disagree, suggesting that the various bits of armor, crest and horn were sexy structures that evolved as billboards meant to impress mates. Both scenarios might have had roles to play, but for now paleontologists are actively debating why so many dinosaurs looked so strange.

6. Did dinosaurs hunt in packs?

Much of the tension in the movie Jurassic Park relied on the idea that raptors were clever girls able to hunt in packs. The truth is that we don’t know whether carnivorous dinosaurs coordinated to bring down prey.

While rare trackways have shown that some predatory dinosaurs like raptors and tyrannosaurs may have walked together, these strolls fleetingly preserved in the rock don’t tell us why the dinosaurs walked side-by-side. Paleontologists would need to find something as unlikely as a set of predatory dinosaur tracks intercepting a victim’s trackway, preferably with signs of a scuffle or even a skeleton at the end. Bonebeds with multiple dinosaur carnivores are even more problematic. These assemblages tell us about the deaths and burials of the dinosaurs, but are frustratingly unclear on whether those animals formed a social group or an unrelated gaggle that was fighting over a food source.

7. Which dinosaurs roamed the night?

One of the most common tropes in descriptions of the Mesozoic world is that small, snuffling mammals eked out a living in the Age of Reptiles because the little beasts were active at night, when dinosaurs slumbered. The trouble is that it’s very difficult to tell when dinosaurs were awake.

Since we can’t watch extinct dinosaurs directly, we have to rely on the evidence they left behind. In terms of their daily schedule, one study suggested that a set of delicate bones in their eyes – called sclera rings – held telling evidence for both the anatomy of the eye and pupil that would have let in light. Based on these clues, the study suggested small predatory dinosaurs such as Juravenator and Velociraptor were most likely active at night. But a follow-up comment argued that sclera are not actually very informative for determining when dinosaurs were active.

8. How did dinosaurs learn to fly?

Dinosaurs undoubtedly learned to fly. We can see them do so today as swallows, hawks and other birds take to the air. But how did dinosaurs along the bird branch gain this exceptional ability?

Paleontologists have traditionally thought of dinosaurs gaining flight in several ways. The “trees down” hypotheses, now out of favor, envisioned arboreal dinosaurs that could glide before they started flapping. The more popular “ground up” scenarios expect that dinosaurs started flapping on the ground – perhaps to better run up inclined surfaces or pin down prey – as a run-up to becoming airborne. Ongoing aerodynamic research on feathery dinosaurs is starting to provide a new look at when and how dinosaurs learned to fly, but for the moment, the details are waiting to be teased out of the fossil record.

9. Which kinds of dinosaur were fluffy?

Dinosaurs were fuzzier than anyone ever expected. In addition to species closely related to early birds, like Anchiornis and Microraptor, a variety of dinosaurs have been found to sport feather-like coverings, from fuzzy, 30-foot tyrannosaurs to early horned dinosaurs with shocks of bristles on their tails.

The wide spread of these weird body coverings suggests that many other dinosaur lineages – perhaps all of them – had fuzzy members in their ranks. But which ones artists should start drawing as fluffy isn’t so clear. We don’t yet know whether dinofuzz was an ancient trait present in the last common ancestor of all dinosaurs or something that evolved later multiple times. Paleontologists will undoubtedly uncover protofeathers and bristles in further unexpected dinosaur lineages, but which ones remain a mystery.

10. Why are so many dinosaurs extinct?

We’ve still got avian dinosaurs – birds – but all of their awesome relatives died out in a geological instant 66 million years ago. Paleontologists still don’t know why. Yes, a massive asteroid struck the planet at that time, following a protracted period of global ecological change and intense volcanic activity in a spot called the Deccan Traps. But paleontologists haven’t fully pieced together how all these triggers translated into a mass extinction that killed off all the non-avian dinosaurs. Not to mention that most of what we know about the catastrophe comes from North America, even though dinosaurs lived around the globe. Paleontologists know the victims and the murder weapons, but they have yet to fully reconstruct how the ecological crime played out.

About Riley Black

Riley Black is a freelance science writer specializing in evolution, paleontology and natural history who blogs regularly for Scientific American.

'Problematic' or 'unsurprising'?

The idea that dinosaurs fit poorly into existing categories is not entirely new. In a study published in February, researchers from the University of Mainz placed dinosaurs in the middle of their own growth-rate survey.

"Personally I don't find the result very surprising," commented Dr Paul Barrett from the Natural History Museum in London. But he was impressed that the new paper had convincingly added metabolic rates to the picture.

"It's nice that they've taken real, empirical data from living animals and come up with a model," he told BBC News. "There's always been some discussion over whether you can use growth rate as a proxy for metabolic rate. What this shows is that maybe we can have more confidence."

"They've brought a lot of data together and make a nice case," Dr Barrett said.

Creating a whole new mesothermic category, something also discussed by Canadian palaeontologist Dr Scott Sampson in a 2009 book chapter called "The Goldilocks Hypothesis", seems to split opinion.

"Mesothermy I see as a little bit problematic," offered Dr Jan Werner, lead author of the Mainz study. He is uncertain about the mixing of classical endotherms and ectotherms within an additional group.

"I'm not sure if this is a useful or necessary category," Dr Werner said. But he agrees that the existing system is too simplistic, saying a dichotomy of endotherms and ectotherms is "too easy".

"It's basically a spectrum," observed Dr Barrett. "I don't think it's bad to have a new word that describes animals that are in the middle of that spectrum."

This also appeals to Prof Mike Benton, an eminent palaeontologist from the University of Bristol. "It's a neat term for a few, perhaps odd, living forms, and for dinosaurs as a whole," he told the BBC.

"It reminds us that living organisms don't always show the only or best adaptations dinosaurs did something else and what they did had many advantages - they could achieve the benefits of stable, warm body temperatures without having to eat so much."

Other experts, however, will take more convincing that dinosaurs sat on the fence, metabolism-wise.

"They could find only eight species living today that are mesotherms," said Prof Roger Seymour, a reptile physiologist at the University of Adelaide in Australia. "Doesn't that suggest that there's an adaptive value in being either a good ectotherm or a good endotherm, but not in the middle?"

Prof Seymour has argued since the 1970s that dinosaurs were warm-blooded. He points to the large blood vessel tracks found in dinosaur bones, and other evidence suggestive of high blood pressure and a big, warm blood supply.

"It would be disadvantageous to be in the middle," Prof Seymour told BBC News. He remains convinced that only powerful, genuinely endothermic dinosaurs would have wielded sufficient energy to remain dominant for 150 million years.

"You have to realise that this group of dinosaur physiologists, including me, are very hesitant to change our minds. Each one of us has the smoking gun, we think, that proves one thing or another."

And so the debate goes on. Mr Grady seems pleased to have made a splash as a graduate student.

He chuckled: "It's a good start. Maybe I'll get a job!" And he added: "It's been an exciting week."

12 How Did They Mate?

Unfortunately, behaviors don’t typically fossilize. Sometimes, bones can give us clues to an animal’s lifestyle, though. For example, the type of teeth an animal has may indicate their diet. But, their hunting or foraging behavior is usually just a guess. The same thing goes for mating behaviors of dinosaurs. We know that dinosaurs lay eggs. But, we don’t know what leads up to that point. Did the males fight each other off so that the winner gets the female? Did the males do mating calls or a special displays to impress the females? These questions all still remain unanswered and it’s likely that they will always remain that way.

We don’t even know much about the sexual anatomy of dinosaurs. But, it’s believed that they were similar to birds in the sense that they may have had a cloaca (which is used for excretion and reproduction).

Dinosaurs were neither cold-blooded nor warm-blooded, study finds

Dinosaurs have long been thought of as slow, lumbering, cold-blooded animals, akin to reptiles like the crocodile and the lizard – but there’s been increasing signs in recent years that they may have been warm-blooded, as mammals and birds are. New research out of the University of New Mexico in Albuquerque, however, shows that these long-dead creatures may have been a little bit of both.

The new findings in the journal Science hint that dinosaurs’ metabolic systems might be far more complicated than previously thought.

Researchers have long thought the ancient lizard-like beasts were ectothermic (colloquially referred to as cold-blooded) – that is, they rely on the temperature outside of their bodies to help regulate their internal temperature. That’s why many reptiles tend to turn sluggish in cold weather. Mammals, on the other hand, are endothermic (or warm-blooded), in that we’re able to regulate our own temperature from within. Keeping the body’s internal temperature high requires a lot of energy, but it also means mammals can be more active and grow faster.

But researchers have examined dinosaur bones and found signs of surprisingly fast growth – which shouldn’t be achievable with an ectothermic animal because it requires so much energy. Was it possible the creatures were actually endothermic?

For this paper, the scientists decided to perform a metabolic census of sorts. They compared the growth rate, adult size and metabolism of 381 vertebrate species, living and extinct, including 21 dinosaur species. The idea was that the higher the growth rate, the higher the animal’s rate of metabolism would probably be. And higher metabolisms would probably tend to be endothermic, warm-blooded systems.

Sure enough, they found that high growth rates seemed to match up with the higher metabolisms of warm-blooded animals, whereas lower growth rates were linked to the lower metabolic rates of cold-blooded animals. The dinosaurs seemed to sit right in the middle, overlapping with some endothermic and some ectothermic animals. The scientists aptly called them “mesotherms” (“meso” basically means “middle” in Greek.)

There are some intermediate, mesothermic species that exist today, the authors point out: Animals such as tuna and the leatherback turtle are able to somewhat regulate their internal temperatures, though not to the extent that warm-blooded animals do.

“Dinosaurs dominated the flux of matter and energy in terrestrial ecosystems for more than 135 million years,” the study authors wrote. “Consequently, our results have important implications for understanding ancient Mesozoic ecosystems.”

Need more dinosaurs in your life? Follow me @aminawrite for more news on extinct charismatic megafauna.

Dinosaurs were Mesotherms, neither warm nor cold blooded

So, what does that actually mean? How did they regulate their body temperature if not through external or internal means? A bit of an annoying article to be honest, to not explain what mesotherm means, despite talking to an expert!

Meso means intermediate, or middle, so mesotherm is referring to following an intermediate path for temperature regulation. Endotherms, like mammals, and birds, primarily regulate their temperature internally (through metabolic control and other basic physiological features, like sweating, or adjusting circulation, etc.), while ectotherm would refer to a creature whose temperature is entirely dependent on its environment. True ectotherms are pretty much absent among organisms complex enough to have a circulatory system, just like there aren't many true endotherms (you're outside on a hot, sunny day, roasting terribly in the sunlight, and feeling faint because you're starting to develop heat stroke.. you notice shade nearby.. do you step into the shade to cool off? If you do, congratulations, you're not a pure endotherm, because you just changed your environment to regulate your temperature), complex animals (and many insects) are a combination of the two across a broad spectrum. What's more useful are the terms poikilotherm (referring to an organism that normally experiences significant change in internal temperature), and homeotherm (maintains a very narrow, strict range of internal temperatures). Again, broad spectrum, but more useful in defining the nature of an organism's metabolism.
tldr mesotherm indicates the dinosaurs were using some behavior, like basking, and a much greater degree of internal heat than a reptile, to regulate their body temperature, pretty much in the middle of the spectrum.

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Were dinosaurs warm blooded or cold blooded? (r/askscience question answered)

This is a really common question that gets asked and I figured I would highlight what the current understanding of dinosaur metabolism is. The r/askscience thread is pretty barren so imma just plop this here.

So, were dinosaurs warm blooded or cold blooded?

In layman’s terms: somewhere in between.

Warm vs. cold blood is an antiquated terminology for endotherms and ectotherms. Endotherms are able to create their own body heat and are not reliant on the outside system to regulate their body temperature. Mammals and birds belong to this group. Ectotherms on the other hand are reliant on the outside temperature to warm their bodies. Reptiles are ectotherms.

What this boils down to is metabolism. To be endothermic means you have to generate your own heat, requiring a lot of food. Think of having a heater in your house, but you keep your windows open. Yeah your house is warm, but you have to be constantly running that heater and need a ton of fuel to keep it running. Ectotherms are a house with no heater, but the windows are closed. You can’t warm yourself up, but just need the sun to do the work for you.

Dinosaurs were originally thought to be cold blooded reptiles because by definition, reptiles are cold blooded. But the fossil evidence (bone histology, ecology, biology) tells us another story. We can look at the holes in the bones of juvenile dinosaurs and we see they are heavily proliferated with holes for blood vessels. Large blood vessels means a lot of blood for quick growth. Growth that is this quick requires a lot of food and a high metabolism. While the number of holes is much higher than that of a low metabolism reptile, they pale in comparison to birds. If we look at the bones of adult animals, the holes become larger and less numerous, indicating a slowness of growth. This tells us that the animals probably started with very high metabolism but gradually slowed down with age.

Another bit of evidence is the presence of feathers. It is now commonly believed that feathers are the ancestral character for all dinosaurs (meaning the ancestor of all dinosaurs possessed them and any dinosaurs that lacked them lost them independently). Feathers are used today for both insulation, display and of course flight. In their earliest form, feathers were very simple and would have resembled fuzz. They were used primarily for keeping warm. This is a way of manual body temperature regulation and would only be needed if the animals were endothermic.

Migratory behavior is another one. Dinosaurs are found on every continent (including Antarctica) and as far north as Alaska. Even though the climate was quite different in the past, many of these locations freeze in the winters, because we find no crocodiles or small reptiles. We find large herds of dinosaurs there. Either these animals were living there year round (meaning they had to have a reliable way of keeping themselves warm) or they migrated. Migration is an extremely high energy activity that is very rarely practiced by reptiles (sea turtles are a special case). You need to have a high metabolism to do it.

There are other bits of evidence you can look at (ecology-carnivore:herbivore ratios are different when comparing endotherms and exotherms). Today, the consensus seems to be that dinosaurs were somewhere in between (mesotherms). A 1 kg mammal needs

Watch the video: Most AMAZING Fossil Discoveries Ever! (December 2021).