Key Points:
•Dinosaurs of all clades laid hard-shelled eggs in nests.
•Some dinosaur nests seemed to have been covered with vegetation; others were open to the air..
•In the penneraptorans one or more parent brooded directly on top of the eggs to keep them warm.
•Baby dinosaurs were mostly very small relative to the adult body size.
•Compared to placental mammals, dinosaurs reproduced huge numbers of young. This indicates that chance of survival of any individual baby is very small.
•We can estimate the age at the time of death in dinosaurs by skeletochronology. It has been found that dinosaurs had rapid growth rates, reaching full body size in less than a decade for most groups, and less than two decades for even the largest.
•While not all dinosaurs were giants, all major branches of Dinosauria (Thyreophora, Ornithopoda, Marginocephalia, Sauropodomorpha, Theropoda) produced taxa equal to or exceeding in size the largest land animals of the modern word. In the case of Sauropoda, these included the largest animals to ever live on land.
•Some factors may have been necessary but were not sufficient to explain this giant size: parasaggital limb posture; efficient hearts & lungs; etc.
•Like placental large mammal communities, the food chains were very short for dinosaurian communities: hence more energy was available for the large predators and prey.
•One very important relevant difference is that dinosaurs--being egg-layers--did not have the limitations on gestation length and litter side that would restrict placentals from reaching their size.
•Another factor has to do with the joints of archosaurs vs. mammals; in archosaurs the amount of articular cartilage does not decrease in proportion as body size grows, meaning that it can continue to cushion the joints better than the decreasing-sized cartilage of mammals.
Living dinosaurs (birds) and their closest living relatives (crocodilians) share many derived features of reproduction; these are probably synapomorphies of Archosauria and so would be expected to be found in all extinct dinosaurs:
Fossil dinosaur eggs and nests found in the rock record, as have embryos of most major dinosaur clades.
ALL non-avian dinosaur eggs are basketball-sized or smaller: NO dinosaur hatched from eggs the size of people!!
Some dinosaur nests associated with covered mats of vegetation: probably helped to keep warm (as in croc nests).
Some dinosaurs (maniraptorans) found in "brooding position" over nests; unlikely to be found in dinosaurs which are too large (i.e., tyrannosaurids, hadrosaurids, sauropods, etc.) or lacking feathers (non-coelurosaurs). An ongoing study of the density of pores on archosaurian eggshells shows that most dinosaurs have characteristics of eggs from covered nests, but that maniraptorans show open nests. This is strongly consistent with the record of brooding.
In primitive modern birds it is the male rather than the female which broods the nest: paternal care. These nests are laid by multiple females. In these paternal care cases, the male rather than the female typically watches over the young after they hatch. When plotted against body size, the volume of eggs of nests of dromaeosaurids, troodontids, and oviraptorosaurs more closely resembles the pattern seen in paternal caring birds than in maternal caring or biparental caring birds; this suggests that paternal care may be an ancestral (maniraptoran?) trait that was passed on to primitive avians but modified in more derived birds. However, we do not have evidence yet to see if more basal dinosaurs had paternal care, maternal care (as in crocodilians), or biparental care. And indeed the statistics supporting the paternal care hypothesis in non-avian maniraptorans has been called into question.
Most primitive modern birds are ground nesters; suggests that tree nesting did not evolve until well into the modern bird (Aves) radiation.
Dinosaurs tend to have nests of about a dozen or so eggs each: more than found in modern birds, less than in (for example) turtles. This is regardless of size: troodontids to titanosaurids!
Implies that unlike placental mammals, dinosaurs could produce a dozen or so offspring a year regardless of size; among placental mammals, larger body size means LONGER gestation periods.
Two main potential life habits upon hatching:
Some evidence of these habits in hatchling dinosaurs:
Parental care of babies for at least several weeks is present in both modern crocodilians and modern birds, implying that this trait was present in archosaurs ancestrally. Birds have long been known to provision (bring food to) their babies; there are cases of crocodilians in captivity doing the same (see video below), but it is not yet known if they do this in the wild. But certainly crocodilian mothers bring their babies to safer places in ponds and lakes where they little ones could hunt.
After baby dinosaurs left the nest, at least seem to have travelled together. Packs/herds of of baby ankylosaurids, ceratopsians, and ornithomimosaurs are known; in at least the case of the ceratopsian Psittacosaurus and the ornithomimosaur Sinornithomimus these herds can include individuals of different ages, including adults. (The parents may have been with the other baby herds, but were too big to be buried.)
Dinosaur Growth Rate and Skeletochronology
Like many animals, dinosaurs deposited Lines of Arrested Growth
(LAGs for short). These are basically growth rings like tree trunks, one per year. Various paleontologists use that information to
"age" individual dinosaur fossils. By comparing the age of the dinosaur to its size, and plotting all individuals of the species on a graph,
you can figure out the growth pattern and growth rate of that dinosaur species: that is, we can develop a skeletochronology for some species.
Actual rate was VERY HIGH compared to typical reptiles, particular in big dinosaurs. Estimates based on bone "growth rings" (and other features) indicate small dinosaurs were full grown at only a couple of years, hadrosaurids (e.g. Maiasaura) at only 7 years for, and only 15-20 years for big sauropods (e.g., Apatosaurus) and theropods (e.g., Tyrannosaurus) to reach adult size.
(In contrast, big crocodilians from the Late Cretaceous seem to have taken 50 years or so to reach the same size as big hadrosaurs).
Since most animal populations stay generally stable over time, more baby dinosaurs died before reaching adult size than in typical populations of modern birds or mammals (imagine herd of antelope where every female produced a litter of 12 every year!).
Unlike typical non-avian reptiles, dinosaurs seem to have determinate growth: reaching a fully adult size, than stopping growth (or at least slowing it WAY down: all the LAGs bunch up together).
Lifespans for dinosaurs seem to be shorter than those of similar sized mammals. For instance, the oldest known Tyrannosaurus individual is only 28, the oldest known sauropod studied so far is only 38, but elephants can live to 70 or more.
Life-History Strategies
Different types of organisms have different life history strategies: different ways of dealing with how many young are born at a time;
how many survive to adulthood; how long they live; etc. There are two general types of strategies that we see commonly in nature:
Non-avian dinosaurs are definitely more r-selected than typical larger placental mammals, but more K-selected than sea turtles.
So How Did (Some) Dinosaurs Reach Tremendous Size?
How Big Were Dinosaurs?
Calculating dinosaur size is a bit tougher that it might sound. The first issue is "what do you mean by size?" Do you mean "length" or "height" (i.e., linear dimensions)? Few dinosaur skeletons are complete enough and in the original position (giving the spacing of the intervertebral discs, so that the actual length is preserved) to give 100% fidelity of these linear dimensions. But in general it isn't TOO bad, particularly for taxa with complete necks and tails. And for those that are not complete, we can extrapolate total body length of an individual based on the parts that are known (with the caveats that the less-complete the fossil is, and/or the farther it is related from good specimens, the greater the error bars of the extrapolation.)
Dinosaurs include the longest vertebrate animals of all time, longer even than the biggest whales. (They are NOT the longest animals, though: that is the astonishingly-long ribbon worm Lineus longissimus, at 55 m (180 ft)!!!)
But consider that most of the length in dinosaurs is tail and neck, regions of the body that are (relatively speaking) slender. For instance, at 3 m (10') long Coelophysis sounds like it is bigger than humans, but half of the length of the dinosaur is tail, and about 2/5 of the rest is neck.
Thus, mass is the more important metric of size for an animal is most cases. It is a good estimate of how much animal is actually there: how much muscle, how much lung, ultimately how many cells. So how can we estimate dinosaur mass? We can do it the way you might do with a modern animal: i.e., get it onto a scale. (Alternatively, weigh yourself; weigh yourself holding your pet; and subtracting the former from the latter.) How can we estimate dinosaur body mass?
There are two major methods: volumetric models and linear regressions. Volumetric models involve creating a 3D model of the dinosaur (old style, a real physical model; today, a computer graphic). This should be based on as complete a fossil as possible. In the most complex models, the fraction of the body made up of bone, muscle, guts, fat, skin, and open space (inside the guts, lungs, and air sacs) is calculated, and each is multiplied by the observed density of these tissues in modern animals. (In simpler versions, some "average" density of archosaur is multiplied by the volume.) Of course, the actual amount of the soft tissues isn't known for these fossils, and indeed might vary widely between the fat and lean seasons. Additionally, the vast majority of dinosaur fossils are too incomplete to serve as the sources for accurate 3D models. Finally, this is a rather time-intensive approach, as such models could potentially be fairly complex.
An alternative is to use some form of linear measurement (lengths or circumferences of a bone or bones), used in a calculation to yield an estimated mass. Considerable work in measurements of modern animals has found that the smallest circumference of the femur (or femur and humerus, for quadrupeds) scales mathematically fairly closely to body size in modern tetrapods. An allometric equation of linear measurements vs. body size can be calculated, and then used for estimating body mass in fossil animals by plugging in the preserved bone dimensions. This has the advantage of not requiring the complete skeleton (although obviously requiring at least these bones to be discovered!)
Size is a property of individuals, not taxa: One important point to remember with both these approaches (and indeed any other) is that taxa do not have masses!! By this I mean that, for instance, there is no "mass" for Tyrannosaurus rex. Instead, any given individual of T. rex had its own mass, and this needs to be estimated on a case by case basis. Think of it this way: what is the mass of Homo sapiens? It can obviously vary a lot: an NFL linesman is going to mass much more than a ballerina. So bear this in mind when considering what we mean by the size of a given dinosaur species.
Regardless of the approaches used, there is going to be some uncertainty in our estimates; after all, they are scientific calculations. And in fact slight differences of approaches might yield wildly different results. For instance, different methods used for the specimen of Tyrannosaurus rex nicknamed "Sue" (the most complete, and one of the largest, individuals known) vary from a mere 4 tonnes to a maximum of 18 tonnes!!
Just for grins, here are some of the body sizes of various dinosaur individuals as estimated in recent years. Note that these will almost certainly change as the methods improve. Masses are given in kg (multiply by 2.2 if you want to know the weight in pounds.) Note that the mid-point of the estimate is shown; in reality there are +/- values that might be 25% of this value either way. Unless otherwise indicated, these use linear regressions and are based on the largest individuals measured in these studies:
Heterodontosauridae and other Basal Ornithischia:
Fruitadens haagarorum: 0.75
Tianyulong confuciusi: 0.89
Eocursor parvus: 4.2
Heterodontosaurus tucki: 5.2
Basal Thyreophora:
Scutellosaurus lawleri: 26
Scelidosaurus harrisonii: 645
Stegosauria:
Huayangosaurus taibai: 301
Tuojiangosaurus multispinus: 1134
Kentrosaurus aethiopicus: 1597
Gigantospinosaurus sichuanensis: 2221
Loricatosaurus priscus: 2840
Stegosaurus stenops: 4869
Hesperosaurus mjosi: 4950
Stegosaurus armatus: 6953
Dacentrurus armatus: 7515
Ankylosauria:
Ahshislepelta minor: 820
Gargoyleosaurus parkpinorum: 891
Hoplitosaurus marshi: 1076
Hylaeosaurus armatus: 1354
Panoplosaurus mirus: 1372
Gastonia burgei: 1411
Edmontonia rugosidens: 1520
Sauropelta edwardsorum: 1643
Dyoplosaurus acutosquameus: 2195
Edmontonia longiceps: 2256
Euoplocephalus tutus: 2330
Nodosaurus textilis: 2538
Ankylosaurus magniventris: 7950
Basal Neornithischia:
Yueosaurus tiantaiensis: 4.6
Gongbusaurus wucaiwanensis: 4.9
Xiaosaurus dashanpensis: 5.3
Lesothosaurus diagnosticus: 5.9
Jeholosaurus shangyuanesis: 10
Orodromeus makelai: 19
Hypsilophodon foxii: 25.4
Oryctodromeus cubicularis: 60
Thescelosaurus neglectus: 356
Ornithopoda:
Gasparinisaura cincosaltensis: 12.9
Dryosaurus altus: 169
Zalmoxes robustus: 185
Rhabdodon priscum:947
Tenontosaurus tilletorum 1019
Camptosaurus dispar: 1324
Mantellisaurus atherfieldensis: 1429
Gongpoquansaurus mazongshanensis: 1488
Ouranosaurus nigeriensis: 1601
Maiasaura peeblesorum: 3656
Hypacrosaurus altispinus: 3689
Gryposaurus notabilis: 4311
Corythosaurus casuarius: 4361
Brachylophosaurus canadensis: 4458
Parasaurolophus cyrtocristations: 4937
Parasaurolophus walkeri: 5117
Muttaburrasaurus langdoni: 5196
Lambeosaurus lambei: 5585
Lurdusaurus arenatus: 5924
Edmontosaurus annectens (although larger specimens are known): 6610
Saurlophus osborni: 6638
Edmontosaurus regalis: 7587
Magnapaulia laticaudus: 8590
Iguanodon bernissartensis: 8680
Iguanodon seelyi (possibly a very large specimen of I. bernissaertensis): 15336
Shantungosaurus giganteus: 17370
Pachycephalosauria:
Stegoceras validum: 16.9
Homalocephale calathocercos: 40.1
Prenocephale prenes: 63
Pachycephalosaurus wyomingensis: 370
Ceratopsia:
Psittacosaurus sinensis: 4.6
Psittacosaurus mongoliensis: 14
Protoceratops andrewsi: 82
Leptoceratops gracilis: 416
Achelousaurus horneri: 1571
Centrosaurus apertus: 2492
Agujaceratops mariscalensis 2622
Anchiceratops ornatus: 2958
Pachyrhinosaurus lakustai: 2960
Chasmosaurus belli: 3097
Chasmosaurus russelli: 3141
Styracosaurus albertensis: 4370
Pachyrhinosaurus candensis: 4440
Torosaurus latus: 9658
Triceratops horridus: 7400
Pentaceratops sternbergii: 10802
Triceratops prorsus: 11393
Basal Saurischia:
Staurikosaurus pricei: 23
Guaibasaurus candelariensis: 33
Herrerasaurus ischigualastensis: 292
Basal Sauropodomorpha (i.e., "prosauropods"):
Pantydraco caducus: 1.1
Pampadromaeus barberenai: 8.5
Saturnalia tupiniquim: 11
Panphagia protos: 12.5
Cromogisaurus novasi: 13
Eoraptor lunensi: 17
Efraasia minor: 37
Anchisaurus polyzelus: 236
Massospondylus carinatus: 510
Plateosaurus engelhardti: 855
Plateosauravus cullingworthi: 1352
Eucnemesaurus fortis: 1429
Riojasaurus incertus: 2233
Sauropoda:
Magyarsaurus dacus: 750
Europasaurus holgeri: 1050
Lirainosaurus astibae: 1800
Isanosaurus attavipachi: 3505
Gondwanatitan faustoi: 3600
Antetonitrus ingenipes: 5639
Saltasaurus loricatus: 5769
Euhelopus zdanskyi: 5924
Vulcanodon karibaensis: 8662
Amargasaurus cazaui: 10194
Diplodocus hallorum (subadult at the Smithsonian): 10940
Haplocanthosaurus priscus: 11897
Camarasaurus lewisi: 13810
Diplodocus carnegiei: 17008
Mamenchisaurus constructus: 17591
Camarasaurus grandis: 18174
Opisthocoelicaudia skarzynskii: 25418
Cetiosaurus oxoniensis: 27254
Apatosaurus ajax (but some specimens may be much larger, rivaling the biggest titaosaurs): 29235
Diplodocus hallorum: 30378
Brontosaurus excelsus: 30541
Lourinhasaurus alenquerensis: 30605
Jobaria tiguidensis: 31754
Giraffatitan brancai: 35780
Futalognkosaurus dukei: 38138
Apatosaurus louisae: 41268
Argyrosaurus superbus: 41937
Losillasaurus giganteus: 47000
Paralititan stomeri: 50768
Turiasaurus riodevensis: 50953
Ruyangosaurus giganteus: 53996
Brachiosaurus altithorax: 57680
Dreadnoughtus schrani: 59367
Patagotitan mayorum: 69092
Argentinosaurus huinculensis: 94844
Basal Theropoda (incl. Ceratosauria):
Procompsognathus triassicus: 1.1
Tawa hallae: 3.2
Eodromaeus murphi: 7.1
Coelophysis bauri: 9.8
Megapnosaurus rhodesiensis: 13
Limusaurus inextricabilis: 20
Liliensternus liliensterni: 84
Gojirasaurus quayi: 190
Elaphrosaurus bambergi: 263
Dilophosaurus wetherilli: 349
Cryolophosaurus ellioti: 349
Sinosaurus triassicus: 561
Aucasaurus garridoi: 851
Ceratosaurus nasicornis: 966
Skorpiovenator bustingorryi: 1112
Eoabelisaurus mefi: 1143
Megalosaurus bucklandii: 1442
Majungasaurus crenatissimus: 1690
Carnotaurus sastrei: 1743
Basal Tetanurae (incl. basal Coelurosauria):
Sinosauropteryx prima (subadult): 1.6
Compsognathus longipes: 3.8
Ornitholestes hermanni: 14.3
Dilong paradoxus: 16
Guanlong wucaii: 71.6
Ornithomimus edmontonicus: 87.5
Anzu wylei: 239
Eustreptospondylus oxoniensis (subadult): 365
Struthiomimus sedens (volumetric): 443
Gallimimus bullatus: 558
Piatnitzkysaurus floresi: 755
Appalachiosaurus montgomeriensis: 889
Afrovenator abakensis: 1037
Metriacanthosaurus parkeri: 1270
Albertosaurus sarcophagus: 2012
Sinraptor hepingensis: 2045
Gigantoraptor erlianensis: 2092
Torvosaurus tanneri: 2433
Gorgosaurus libratus: 2487
Allosaurus fragilis: 2681
Daspletosaurus torosus: 2840
Tarbosaurus bataar (larger specimens are known): 3017
Suchomimus tenerensis: 3260
Carcharodontosaurus saharicus: 3300
Acrocanthosaurus atokensis: 3613
Chilantaisaurus tashuikouensis: 3710
Saurophaganax maximus (?= Allosaurus fragilis): 3830
Mapusaurus roseae: 4342
Tyrannotitan chubutensis: 6260
Giganotosaurus carolinii (most complete specimen): 6349
Deinocheirus mirificus: 7328
Tyrannosaurus rex: 8870
Maniraptora (exclusive of Avialae):
Parvicursor remotus: 0.15
Microraptor zhaoianus:0.43
Mahakala omnogovae: 0.79
Mei long: 0.84
Shuvuuia deserti: 2.8
Bambiraptor feinbergorum: 3.6
Caudipteryx zhoui: 12.8
Saurornithoides mongoliensis: 18.8
Velociraptor mongoliensis: 19.7
Troodon formosus: 47.4
Stenonychosaurus inequalis:57.4
Deinonychus antirrhopus: 99.8
Achillobator giganteus: 521
Nothronychus mckinleyi: 570
Beishanlong grandis: 580
Austroraptor cabazai: 600
Nothronychus graffami: 1188
Gigantoraptor erlianensis: 1997
Suzhousaurus megatherioides: 3100
Segnosaurus galbinensis: 4600
For comparison, here are some masses of typical land modern animals to judge the above:
Columba livia (rock dove, aka pigeon): 0.38
Didelphis virginiana (Virginia opossum): 6.4
Odocoileus virginius (white-tailed deer, typical large male): 136
Panthera leo (lion, large adult male): 200
Alces alces (moose, large adult male from Siberian subspecies): 725
Hippopotamus amphibius (hippo, male): 1500
Ceratotherium simum (white rhino, large male): 3600
Elephas maximus (Indian elephant, large male): 5000
Loxodonta africana (African bush elephant, largest modern land animal, large male): 6000
The extinct giant hornless rhino Paraceratherium transouralicum may have reached masses around 17000 kg, and the extinct elephants Mammut borsoni and Palaeoloxodon namadicus were even larger: the latter maybe up to 22000!
However, whales dwarf typical dinosaurs, and the largest whales are bigger than even the largest dinosaurs:
Orcinus orca (orca, large male): 6000
Physeter macrocephalus (Sperm whale, large male): 57000
Balaenoptera physalus (fin whale, second largest living animal, largest confirmed individual): 74000 (but maximum estimated is 114 tons)
Balaenoptera musculus (blue whale, largest living animal, largest confirmed individual): 173000
It isn't just that some dinosaurs were large. It is that giant (>1 t) size is fairly common in dinosaur groups, and that >5 t size occurs in both carnivorous and ornithischian dinosaurs (with many independent cases), and that >10 t is actually a STANDARD body size for the sauropod dinosaurs! Indeed, the very largest dinosaurs reached masses of 80-100 t (or, in other words, the mass of a herds of elephants!). In contrast, where there have been a few 10 t land mammals, most mammals were much smaller. (Of course, even dinosaurs were dwarfed by the modern blue whale). So how did dinosaurs get so big?
There are lots of unsupported, less-than-reasonable, pseudoscientific explanations:
So, let's take a look at other possibilities.
Was It Limb Posture (i.e., the Parasaggital Stance)? It is true that all the supergiant land animals (dinosaurs, elephants, deinotheres, indricothere rhinos, etc.) all have an upright stance. While there are giant 10-15 t crocodilians contemporaneous with and after the Age of Dinosaurs, these semi-upright animals were primarily aquatic and in a sense don't count.
But other groups with parasaggital stance--such as most pseudosuchians, marsupial mammals, modern birds, etc.--never reach these sizes. So it might be that a parasaggital stance is necessary but not sufficient for the >10 t size.
Was It From a REEAALLLLYYYYY LOOOOONNNNNGGGGG Period of Growth? Giant tortoises and big modern crocodilians live a very long time (centuries in the case of tortoises), growing throughout their life. Perhaps dinosaurs just grew for a tremendously long time? But as we saw previously, dinosaurs had a rapid growth rate and relatively short life spans.
So we can REJECT the idea that a long growth phase led to giant dinosaur size!
Very Efficient Dinosaur Hearts and Lungs? Yes, they had both, as we will see. These were probably a necessity for giantism, but not sufficient.
Was it Exceedingly Short Food Chains? When we watch nature specials on TV that focus on large charismatic modern land mammals, we get the impression that the simple food chain of plant → large herbivore → large carnivore (e.g., grass → zebra → lion) is typical. In fact, while it does apply to the world of Cenozoic hoofed mammals and their predators, it actually is not the case for most terrestrial life. As in the marine realm, most terrestrial food chains have many steps from primary productivity to apex carnivore, often through insects and other herbivorous invertebrates, to carnivorous arthropods, to small-bodied insectivorous animals, to predators of those, and so on. (For example, plants → aphids → ants → spiders or mantids → small insectivorous birds → snakes → hawks). And of course there is loss of energy every step up the food pyramid.
Ecosystems of protomammal synapsids of the Carboniferous, Permian, and Triassic, and perhaps even protocroc pseudosuchians of the Triassic, seem to have had relatively few large bodied herbivores, and may have had these longer, energy-leaking food chains. And thus, there is less energy available towards the tops. In contrast, dinosaurs produced a broad variety of herbivores able to capture the energy at the base of the pyramid, with a set of carnivores feeding on these. These short food chains may have allowed much more of the solar energy to make its way into meat.
Of course, there is still the observation that even mega-mammals with similarly short food chains fail to achieve dinosaurian size, so this cannot be the entire story. However, it is worth noting that said mega-mammals are the ONLY other case of terrestrial vertebrates which get anywhere near dinosaurian size.
Was it Changing Atmospheres? Even as dinosaurs were evolving, the atmosphere they were breathing was evolving, too. Geochemists have seen that the ratios of various gases, including oxygen, have varied over geologic time. At least some models suggest that the Middle Jurassic though the end of the Cretaceous had oxygen levels exceeding the present 20%. This would mean that every breath a dinosaur took would have more oxygen, making it easier to power a high metabolism.
Furthermore, experiments of growing some plants of Mesozoic varieties under Mesozoic-style atmospheres suggests that their productivity (essentially, the amount of nutrients they produce per area per unit time) could go up 2 to 3 times present day conditions. If so, then there would have been more food available per unit area for the herbivores (and from this up the energy pyramid), again making it easier to be an endotherm in these conditions. (On the other hand, some plants do more poorly under the higher carbon dioxide situations.)
Environmental changes in Mesozoic discussed above may have supported giantism:
The second aspect is probably easier to deal with. There are several possible advantages:
However, we may be asking the wrong question. The problem might not be "how did dinosaurs get so big?"; it might be "why DON'T placental mammals get so big?" This may in part be due to the lower oxygen level and productivity during the Cenozoic. However, it may have to do with mammalian vs. dinosaurian reproduction:
In contrast, even giant dinosaurs could could lay clutches of a couple dozen eggs a year or more. Thus, they could survive environmental change much more easily than big mammals. So mammals may not be able to reach giant size because they become extremely vulnerable to extinction.
Additionally, the thicker cartilage of dinosaurs was present at all body sizes, whereas the size of the articular cartilage in mammals decreases with increasing body size. As a result, larger mammals have less cushioned joints, which may have been a hinderance to activity. So it may simply be that larger land mammals could not actually move effectively, but that dinosaurs continued to be mobile.
And another:
