Thanks everyone for your questions and comments! Some of you have asked whether mammals and lizards were already occupying that niche, or whether mammals out-competed dinosaurs at smaller sizes-it’s certainly a possibility! Maybe dinosaurs were really bad at being small. The next question then becomes, why was that the case? That would tell us a lot more about dinosaur biology that we can’t just tell from their bones.

I thought that a relatively common hypothesis was that young dinosaurs occupied the niches for smaller animals, instead of competing with full-sized adults of the same species. We see a great deal of variation in anatomy between young and adult forms of several dinosaur species in the fossil record (including theoropods and ceratopsids).

It is amazing to think that the smallest dinosaur that we know of actually still exists with us

I'm thinking that at such small sizes they just couldn't compete with mammals and lepidosaurs. Birds prove that there were no physiological issues with being that small, and like he said, almost as soon as birds evolved some evolved to smaller sizes. But it's not hard to imagine that a tiny bipedal dinosaur would have a tough time burrowwing, scurrying through brush, etc, or otherwise moving around under circumstsnces that are much easier for quadrupedal mammsls and lepidosaurs. Then there is the fact that to tiny mammals the mammalian superpowers of chewing and mother's milk are as useful as ever, whereas a dinosaur's superpowers of hollow bones and highly efficient respiratory system don't help that much for a small creature that isn't flying. So tiny avian dinosaurs took advantage of small-size niches availabld to them, and other clades of dinosaurs faced too much competition.

It could just be that that ecological niche was already filled by other groups. Much like how large mammals didn't evolve while the dinosaurs were around. And the only reason why birds evolved to be small was because their ability to fly (or perhaps glide if we're talking early species) meant that they weren't in direct competition with a lot of the existing smaller species from other groups (or had an edge over them).

What if they could not compete with tiny mammals in the small size niche? I'm much more intrigued by the question: "where are all the mammals?" and it seems to me that the answer is "in micro-space" and "in jungle-space" maybe.

I am also of the camp of juvenile dinosaurs occupying niches that a "small adult" dino would occupy. When a catastrophic event like the KT event happened, ecosystems were ruined b/c the smaller dinosaurs that were juveniles were killed along with their large parents. Large parents were killed off, and could no longer produce the juveniles to fill those "small dino" niches. And the niches that the juveniles occupied had few to no competitor dino species to take over those empty collateral niches. It's not impossible to imagine that, in the centuries immediately after the KT event, some dino species might have downsized due to lack of resources due to the widespread destruction while plant species recolonized and reshuffled their ecological assemblages.Similar to how some traditionally large species become smaller when isolated on island ecosystems. Birds and mammals, however, and many forms of reptiles eventually would evolve larger sizes to fill those emptying niches during the Eocene and after.

I just love Tinysaurus Rex, Tinyplodacus, and Tinyceratops !

There's a mouse sized dinosaur in my yard. It's a sparrow...

Maybe that ecological niche was already filled by young dinosaurs. Unlike young mammals and birds which are raised by adults until they can take part in the same niche as adults, dinosaurs had to fend for themselves at a far smaller size compared to their adult size so there are plenty of tiny juvenile dinosaurs running around filling the small dinosaur niche.

I love that instead of just giving concrete answers, this asks questions and gives possibilities. That starts us all wondering, and that wonder drives curiosity.

Such a pleasant, low-key approach to a truly interesting question. And without the patronizing tone of so many nature channels. Great job!

Fascinating video. I learned some things from it. I like when it scientists in a particular field freely admit that they don't have an answer to an important question, at least not yet. My off the top of my head suggestion: Could it be that other groups of vertebrates were already dominating the small land animal niches and so tended to keep dinosaurs from successfully evolving in that direction?

One should also take into account that most likely there where many baby dinosaurs that were fully independent at birth so it wasn't something anatomical of their biology that prevented them from being small adults like animals that are born small but undeveloped like marsupials or something. My best guess is that small dinos just weren't efficient at occupying small body niches like reptiles,mammals and insects are, also maybe just being big was that much more evolutionarily advantageous

Excellent, well-written and illustrated.

I had the opportunity last fall to spend a couple days at the Natural History Museum and it was an amazing experience. A national treasure. Is it possible that the tiny mammals that were around at the end of the dinosaur era simply out-competed any tiny dinosaurs trying to occupy the niche?

Fascinating, would've never thought things were like this. This last summer I and my wife visited the AMNH and really enjoyed our visit, so if we ever return to NY in the future, we'll definitely repeat the visit

The best reason I can think of for why there might not be any tiny dinosaurs is that the ecological niche that such creatures would have filled, was filled by something else. Quite possibly something(s) that out competed them in the small world, just as they out competed everything in the large world.

The avian dinosaurs and their relatives have been around since the Jurassic, and are generally on the smaller side (eg. Microraptor, anchiornis, etc) - with anchiornis being only 34cm (roughly the size of a crow) and weighing a tiny 110g (which admittedly, is still considerably bigger than the 1-2g of other tiny vertebrates).

I always wondered about this because I wondered why no small dinosaurs survived the mass extinction

I would have thought that there was no pressing need to get smaller, considering mammals and other reptiles already dominated those smaller eco-systems. Birds did have pressure to get smaller because flying became increasingly problematic with increasing size and those niches for larger flying reptiles was already occupied by very successful groups of other flying animals. But they could compete with smaller mammals and reptiles because flight gave them a competitive edge and allowed escape from the more aggressive smaller predators. Meanwhile smaller sizes allowed more agility in the air and allowed the conquest of the arboreal environment. Dinosaurs had no such need, and in fact larger size was a successful survival strategy right up to the moment it was not. The asteroid impact and associated volcanism and climate change shifted the survival advantage to being small, and having a smaller stomach to fill. Sadly for them, dinosaurs could not evolve to smaller sizes to avoid extinction in the few years that they had left.

Palaeontologists used to get really really excited about very big bones. Perhaps they considered tiny bones irrelevant so didn’t particularly look for them let alone collect them - which would add another factor to explain the apparent lack of tiny dinosaurs.

Fascinating!

"Tiny dinosaur eggs." 'Who cares.' -proceeds to smash said eggs with butt of hand gun.

Fascinating

Great presentation…. And presenter.

Thank you! So cool!

Perhaps we are looking at things from a mammalian perspective, where the adult occupies a particular niche and the juveniles do not occupy a different niche because they are relatively large compared to the adult, and are strongly supported by the adults until independent in the adult niche. Newly hatched turtles cannot possibly occupy the same niche as the adults. Tadpoles do not occupy the niche of a frog. Lobster larvae do not occupy the niche of a lobster. If a juvenile dinosaur occupies a niche it must be outcompeting any species that would otherwise occupy it. Given the numbers of eggs laid, the hatchlings would strip the niche of all resources pretty quickly, so perhaps it is inevitable that as they grew they would successively exploit other niches until they reached adulthood.

Crunched up inside the big dinosaurs...?

It's winter now, but in the early spring they will be back. Besides, you should consider lowering the music volume in your videos, as it is now, it is hard to concentrate about what Roger Benson says. In particular don't play music in the same frequency range as the speaker, and prefer soft sounds instead of ringing and plinging sounds.

Wonderful talk. Thanks.

If you're interested in more of Roger Benson's research, check out our video on the dinosaurs of South Africa: ukposts.info/have/v-deo/kZGGqa2LrG9ysZ8.html

This sounds more like an environmental puzzle more than a biological puzzle. Change in the environment would have directly contributed to biological change: oxygen levels in the atmosphere, sunlight amount, change in plants as food, change in animals and insects as food, moisture in the atmosphere, flooding events, droughts, etc.

I watched an excellent video asking this same question a couple of years ago. Their theory was that the young of dinosaurs filled the ecological gap that a tiny adult dinosaur may have filled. As very briefly noted in this video, when they think they have found a very small dinosaur they discover it is a young example of an existing larger one. These could out compete an adult dinosaur of the same size.

I suppose one way to formulate the question is "why was it that the only way for a dinosaur to have a very small adult size was to be a bird?"

I think it was Steve Brusatte in his excellent Rise and Reign of the Mammals, who described the staggering diversity and success of Mesozoic mammals (if you want an easily readable but scientifically rigorous breakdown of Mesozoic mammals, buy this book). We think of them as just after thoughts living in the shadow of the dinosaurs, but while the success of dinosaurs prevented mammals from getting big you could say it's equally true that the success of mammals prevented dinosaurs from getting truly small. That ecological niche was already covered.

Interesting, thanks.

Excellent presentation and explanation. The mystery remains as to why there are no tiny dinosaur fossils found even though tiny mammal and reptile have been found alongside dinosaurs,

Would love to know how your research is coming along on this time period

Very interesting ! If we should look at earth holistically during the jurassic period, and know how the continents were placed, and the weather systems, then it might shine some light on where to look ! Hopefully you'll get some species, and answers soon. Goodluck.

I would really appreciate some details in the mathematics - square-cube law, weight and volume ranges or present species and those of dinosaur ranges, and any patterns in those numbers. I guess one has to google the papers for now.

"Where are the tiny dinosaurs?" Inside the big dinosaurs.

The baby dinosaurs took up separate niches to their parents, which prevented small dinosaurs from evolving into those niche. Birds could reduce in size (head remains the same) - because they were evolving into new niche's, where better flying and more brain power, to catch insects or take advantage of the new flowering plants, accelerated the evolution to a smaller size.

One thing I wonder is if it’s possible the lack of small dinosaurs is because of baby dinosaurs. For example, sauropod hatchlings likely occupied completely different niches from the adults, they are practically two different worlds. Now sauropod hatchlings were probably larger than the smallest vertebrates today, but there are still many other species that would have had no parental influences on young, and many of these like sauropods may have been occupying entirely different niches. It’s one of the reasons beetles for example are one of the most successful groups on the planet, because the grubs don’t directly compete with their adult forms. I don’t think that completely explains it and I’m 100% certain that there were at least some micro dinosaur species (the chances of their not being are staggering) but it’s an idea.

That is very interesting.

So several things going on. 1) Easier to see Large Dinos 2) Survivability / Fossilization of smaller creatures 3) Relationship of the frequency of smaller dinos in the environments likely to fossilize them. 4) Sampling. I gave up Geology for Prob's and Stats 35 years ago but where do I find the studies to talk about fossil statistics?

very interesting

Bring back the Roosevelt statue

As Steve Brusatte says, dinosaurs may have stopped mammals from getting big, but mammals stopped dinos from getting small...

My thought this evening is that maybe the niches that could have been filled by smaller non-avian dinosaurs either didn't exist or were already filled by other animals. So it's no surprise that after the K/T really tiny birds like hummingbirds evolved to fill new and vacant niches afforded by the flowering plants. Also time is needed for things to get smaller, or larger, as they evolve into the available morphospace.

Beneath a certain size, the things that made dinosaurs dominant may no longer have been competitively advantagous.Upright posture isn't an advantage in microspecies-- probably why there aren't truly tiny ungulates, for example. Also micro-species generally require a refuge to flee to-- a burrow, a tree, or the water. The dinosaur body plan, especially obligate bipeds (until birds) is not necessarily competitive with mammals and other reptiles for burrowing, climbing or swimming. Also, yearly they would be in competition with swarms of the young of small dinosaurs. Lastly, the hatchlings of micro-dinosaurs would be very tiny indeed, magnifying the challenges listed above. I would also mention a recent study concluding that perhaps because of trampling and consumption by dinosaurs, thick, impenetrable forests may have been rare or largely absent in the Jurassic and much of the Cretaceous, at least until quick growing angiosperms (which tend to dominate thick undergrowth where it occurs) evolved. If true, that would massively cut down the number of niches available to micro-species in general. We'll probably never know, but the faunas of high elevation, mountainous areas may have had very different ecosystems, with more room for "experimentation" for tiny creatures, more free from theropod predation. It may even be why China, which was very mountainous in the Mesozoic, seems to have been a center for small theropod (maniraptor) evolution, as "wings", even non-volant ones, would be of great value--maybe even be a necessity-- for small, bipedal predators in a steep, rock and boulder strewn environment.

Another possibility might be thermal losses. At smaller sizes there's less mass per surface area, so it's harder to maintain body temperature. Then you would have to find what birds might be doing different than earlier dinosaurs to allow them to shrink without freezing.

Why is there not more inquiry into why sauropods were so big. Some engineers calculate that sauropods were too heavy for their joints and bone to support in today's gravitational force. This size enigma is also found in pterosaurs and other very heavy flying reptiles, that were clearly too heavy to fly today.

Where are all the tiny dinosaurs? I had one for lunch.

Maybe small dinosaurs simply couldn’t compete against small mammals, small amphibians, small lizards, and so on

Maybe the dingo ate your tiny dinosaur.

Here's something to consider: Maybe all the "small niches" were occupied by insects, other arthropods, and mammals, and that dinosaurs had no physiological advantage that would let them win against established competition. Also, based the size of eggs and fossils of juveniles, how did the young offspring of large dinosaurs compare to the small animals of the day?

Only this: thank you so much! Thanks to those who commented/queried, also. A marvelous planet we have...if we can keep it, to paraphrase Ben Franklin.

If your thermal regulation processes are inefficient and/or imprecise, there would presumably be an evolutionary advantage to maintaining a larger mass:surface area ratio, to reduce the rate of internal temperature changes caused by thermal flux.

Perhaps a few tiny dinosaur did exist but did not proliferate due to competition from mammals and larger dinosaurs.

The only tiny dinosaur species I can think of are Microraptor, Compsognathus, Archaeopteryx and Mei Long.

Why they did not get small (ignoring the lack of fossils ) -> Those niches were already filled by other groups (other reptiles, mammals etc) Why avians (birds) got small -> The end Cretaceous mass extinction put a strain on the ecosystem and resulted in a lot of empty niches which were quickly filled by mammals Thus leaving plenty and suitable niches for small bodied animals ,birds And birds did find great success in these forms and diversified successfully

Right here ! Two dinosaurs in a cage. two budgies

Seems like dinosaurs got very big, very quick. The small animal niche had plenty of competition from mammals, amphibians, and lizards …so dinos won the race to get large. The window of time for the transition being small, makes it a needle in a haystack to find fossils. Birds got small quick and created a new niche for small animals with flight.

8 seconds in and the answer was given. Well that's refreshing 😊. Wish all videos i only had to see 8 seconds and then move on.

Would it be fair to say that mammals dominated the small size niches of the era? Dinos were niche partitioned into being larger?

Species? - Perhaps outcompeted by mammals? Individuals (meaning hatchlings) ? - idk

There are two competing processes: decay and fossilization. It takes longer for larger bones to decay, providing enough time for fossilization to begin.

Is the ‘big bone room’ a big room with bones in it or a room with big bones in it? Could birds get small because they had more insulation and would not lose heat as quickly as a non avian dinosaur of the same size?

Maybe you would know. I always wondered whether oil and gas deposits are checked for fossils that could be in the raw mix ? Or do they discard whatever is in the filter ?

I was raise on a gravel road make of crushed limestone and trilobites and coral fossils are common. I'll go with their bodies were easy to chew into smaller pieces, their bones were crushed/dissolved during digestion, those in "scat" would decay before they could fossilize. Evidence of these smaller dinosaurs may only exist in marshes, peat bogs, coal, limestone,etc where "environmental" factors protected small bones from deteriorating.

In the ground.

They're everywhere eating seeds in front of my face

Maybe the dinosaurs (and other archosaurs) had to either become big, learn to fly or take to the ocean, because being small was the niche dominated by the badass mammals.

Aren't all the tiny dinosaurs...still alive?

I wonder if it is because dinosaurs are inherently fragile. Their air sacs that take in oxygen are not only to be found in their lungs but in other parts of their body. This allows more efficient taking in of oxygen since air can circulate in a uni-directional manner. A more efficient way of breathing than mammals method. But it leaves the more easily damaged air sacs vulnerable to injury, whereas our air sacs are all located in the lungs that are protected by the rib cage. Which means that dinosaurs cannot survive, unless they are large enough. At least half a kilogram. Or, are so mobile (ie: can fly) that they can largely avoid confrontations with other animals that would too easily damage an animal with such a circulatory system if they weighted under half a kilogram. For large dinosaurs, they are so large, this 'Achilles Heel' of their design can be lived with. I wonder if by the end of the Cretaceous if all non bird dinosaurs had an adult weight well over half a kilogram, making all of them vulnerable to the asteroid impact. I wonder what the minimum adult non bird dinosaur weight was at the end of the Cretaceous.

Small four legged mammals are often good at at digging and hiding in tight spaces, so they had an advantage over tiny dinosaurs.

I was first struck by the fact that embryos and newborn dinosaurs fossils have been found; I think in the Gobi Desert by Andrews. But these animals probably reached non-predatory size fairly rapidly. I've seen videos of fifty foot high pteranodons chasing down and eating small dinosaurs, so I guess these animals conducted their terrifying ground raids during hatchling season to get the smaller animals before they grew larger. Is there any physical evidence that these animals ate small dinosaurs? So maybe the evolution of dinosaurs was part of the general arms race involving all higher animals for different strategies as a protection against predators, here emphasizing size. So why this emphasis on developing huge size at this time? Is it because the Jurassic and the Cretaceous represented the periods of the proliferation of the massive dinosaurs? But also must have proliferated were numerous top predators among the group who were able to finally tackle the largest sauropods down to the smallest prey, as well as pterosaurs in the skies, proto-birds with teeth, mammals, arthropods, ants, etc. In this ground setting any smaller dinosaurs or mammals or lizards or birds would have been at a disadvantage on the forest floor because they would have to escape the larger beasts, fight for food with other types of smaller predators and compete with each other. The only strategy for dinosaurs for competing in this scaled down environment involved apparently an evolutionary decision not to compete for larger prey but to start confining their diet to fruits, seeds, worms, insects - and flowering plants -out of the mainstream of the predatory food chain, lose their teeth, scale down their size and - very importantly for the survival of dinosaurs at the end of the Cretaceous! - develop wings not only to escape forest fires but to also escape predators. Even mammals, soon to inherit the earth, were not common creatures at this time because giant Tyrannosaurs kept eating the bigger ones. Maybe the fight on the forest floor is a key to why birds became the only successful surviving group within the clade Dinosauria. So why all this happening in the Mesozoic so dramatically in which an entire group of animals which we would classify as dinosaurs abandoned the understory? It may be because of the rapid rise of the dinosaurs as a group themselves, a process which overwhelmed the food chain in effect leaving any smaller related animals requiring a strategy to keep from being stomped underfoot by their larger cousins' ironic success or perhaps even competing with the young of larger dinosaurs. A huge evolutionary shift indeed. SZ BA MA

There are plenty of tiny dinosaurs. We see them every day in the trees, in the sky, in the grass and standing on cables.

Wow! Fascinating! The Bible talks of the second act of Creation (the first being the Earth and the Universe) as "The *great* reptiles and the fowls of the air (birds)". Somehow, that blows my mind.

good chance many of them have been here the whole time: bluebellies, gator lizards, king snakes, racers, ensatinas, toads, turtles, etc. desert iguanas could tell us stories about the dinosaures if we could only understand

Maybe focusing on possible ecological dynamics and the limits of available fossils... ... caused by gaps in localized conditions that allow fossils to survive

they are actually otters running backwards

I think it has to do with the biological niche of smallness which already had been taken up by mammals.

It could be possible that there were a ton of tiny dinosaurs but they lived in areas like jungles where fossilization just doesn't happen.

I dont see why it cant be thay dinosaurs are a branch if species that went big while others filled the niche on the lower end. Big makes you more competitive but it a world of giants its easier to start small and stay small. Take the ocean for examples. As a whole fish are generally quite large whereas crabs or snails that live in the same medium are tiny comparable.

What I'm curious about is how these really huge creatures (~70 tons) could exist on land. Physically, it seems that there's a fundamental problem of supporting that weight.

Being larger means less surface area to mass. Is it to do with temperature regulation?

Could it be that the physiology of dinosaur eggs have some fundamental difference from the other egg laying critters of the time?

1 lb. is not a big rabbit, its a baby rabbit. When I raised meat rabbits, I expected them to be 4lbs. by 8 weeks old. The adults were 8-10lbs.

Its amazing to think that in the current day we have the largest ever known animal (the blue whale) and the smallest known dinosaur (bee hummingbird)

I always think about how bigger is better basically fell off to be an advantage to being small, specialized and so various

They are outside my window, sleeping in the feeder.

When you force an answer you get mistakes but it looks like the the small dinosaurs evolved into birds so they could fly away from their predators . The dinosaurs , defined by the supporting legs pointing down rather than out to the side, were selected by Nature for size which was facilitated by weight support efficiency, for the purpose of self defence ( herbivores) or their ability to overcome those defences.

Look, a tiny dinosaur: 🐦‍⬛

Wait for a fertile chicken egg to hatch. Voila'! A tiny dinosaur.

There are no humminbgird-sized penguins. There aren't any tiny ocean mammals presently. The animal sizes are those that best fit the ecosystem. We just don't know enough about the entire ecosystem of the dinosaurs.

You don't talk about baby dinosaurs. I thought there was a physical limit to how big a dinosaur egg could be?

Seems obvious to me that crown dinosaurs early on mutated a gene for gigantism, and birds happened to turn it off.

I think they could have evolved to be very small but mammals and lizards already occupied that niche and were better adapted to it.