Like all animals, dinosaurs had to have babies to ensure the survival of their genes, and all known baby dinosaurs came from eggs. This might seem obvious, but when you stop and think about the trials and challenges of producing and tending to eggs with viable offspring inside, the more interesting and alive extinct dinosaurs become. A lot of the study of dinosaur reproduction is controversial; conjecture based on a limited fossil record and knowledge of related modern animals. But I’ll do my best here to talk as credibly as possible about where baby dinosaurs come from, the world of dinosaur eggs, and the life of a hatchling.
To make a baby dinosaur, obviously you first need parents. Parthenogenesis, the act of reproducing asexually (without the fertilizing of an egg by another individual) is reported in rare cases involving domestic birds (Schut et al., 2008), but seems to be much more common in lizards, and is unknown in dinosaurs. This means that you’re going to need a biologically male and female dinosaur if you want to make some eggs. And in many dinosaurs, the ways in which males and females found, completed for, and courted one another are both varied and elaborate.
Much has been written about the visual display features that dinosaurs possessed- crests, spikes, plates, horns, frills… All are features of the skeleton that scientists believe were used for showing off. As very visually-oriented animals, dinosaurs seem to have placed a lot of value in visual displays during breeding times, and it’s likely that various kinds of noise-making, plumage-flashing, and birdlike courtship ‘dances’ played a major part as well (Persons et al., 2014) (Saitta, 2015) (Lockley et al., 2016) (Brown, 2017). All sorts of modern animals do comparable things today, from frogs to lizards to ungulates to peacocks (just to name a few). There’s even evidence that some dinosaurs partook in fights with members of their own species, which might represent competitions for mates that went beyond display and into physical confrontation. Triceratops in particular seems to have been well-suited to battling each other using their horns and frills (Farke et al., 2009), and face-biting in some theropods might have been done for similar reasons (Tanke & Currie, 1998). Many modern animals have strange and creative ways of duking it out for mates (just look at giraffes and their sledge-hammer heads), so there may well have been other forms of combat between dinosaurian rivals that we can find no trace of yet.
After all that displaying and competing has resulted in breeding partners being chosen, the next step is to actually get down to making a clutch of baby dinosaurs. In the interest of keeping this blog PG-rated, we won’t go into the finer points about how this was achieved. Suffice it to say, dinosaurs did it in basically the same way as all other tetrapods. Dinosaurs had the same general reproductive organs that modern reptiles (including birds) have, requiring the transfer of gene-containing sperm from one individual to the egg cell of another. But the details of this are fuzzy. It’d make sense if many dinosaurs did the job using some sort of organ in the males to facilitate sperm transfer, same as crocodiles and some birds, but with no trace of this in the fossil record it’s hard to prove. Aside from that, the sheer mechanics of the whole thing in some groups are hard to imagine. How did sauropods weighing dozens of tons each or plated and spiked stegosaurs manage to do the deed without causing harm to themselves or each other? We’ll leave that to the imagination and move on.
After all that fun stuff is said and done, then comes the hard work of building a nest and laying and tending to the eggs. Mother dinosaurs preparing for egg-laying stored extra calcium in their bones in the form of a tissue called medullary bone. This type of bone has been found so far in Tyrannosaurus, Allosaurus, and Tenontosaurus at least (Lee & Werning, 2008), and is one of the more reliable ways to determine a dinosaur’s sex. Medullary bone is only present in females that were nesting or about to, though, so its absence in a dinosaur isn’t necessarily proof that you’ve got a male on your hands. We know that dinosaurs laid hard-shelled eggs like birds do, not leathery-shelled squamate-like eggs. For such delicate things, fossil dinosaur eggs have actually been found in a variety of locations. While early finds from France have turned out to be dinosaur eggs, the fist examples that were recognized as eggs at the time were from one of the famous expeditions to Mongolia put on by the American Museum of Natural History led by Roy Chapman Andrews (Andrews, 1932). This team had, infamously, been searching for evidence of human origins in Mongolia to support Henry Fairfield Osborn’s (racist) theories on human evolution. Instead, Andrews struck dinosaur eggs along with other fossils, and the rest is history.
Since then, eggs and nests from a few different types of dinosaurs have come to light, and it looks like each group did things a little differently. However, there are a few universally common things about nonavain dinosaur egg-laying. It looks like most dinosaur groups built nests of some kind to keep their eggs in, aside from sauropods who seem to have deposited and buried their eggs in a shallow excavation. Crocodiles lay multiple eggs at a time in a mound-shapes nest of vegetation, while modern birds can only lay one egg at a time due to them having only one functional oviducts as opposed to two in most other animals. Thanks to the pattern of eggs deposited in fossilized dinosaur nests and a remarkable oviraptorosaur pelvis with eggs intact, we know that these animals were intermediate in their egg laying between crocs and birds, laying two eggs at a time in clusters, circles, or spirals (Sato et al., 2005).
While we haven’t found eggs from every kind of dinosaur out there, the ones we can identify to the family level are differently shaped in different groups. Paleontologists have been able to narrow down some eggs and nests to specific genera of dinosaurs thanks to associated remains of hatchlings and parents, while others are more mysterious. It was once assumed that dinosaurs, like many modern reptiles, took little to no part in the hatching and care of their young. This assumption was proven incorrect, and also reflects something of a bias against modern reptiles. Some lizards stay with their nests, pythons keep their eggs warm, and crocodile mothers viciously guard their nests and hatchlings from predators. Turtles, on the other hand, are well known for burying and abandoning their young.
Animals tend to fall into two very general different categories as far as reproductive strategies go. One is called r-selection, which is characterized by having lots of young and providing them with limited, or no, parental care. The point is to flood the ecosystem with so many offspring that, even if many of them are taken by predators, enough will survive to make the whole ordeal worth it. r-selected strategists also tend to have relatively short lifespans, reach sexual maturity early, and often have young that are able to fend for themselves shortly after birth (an offspring trait known as precocial). Animals like sea turtles and rodents would be examples of r-strategists. The reverse of this are K-strategists, who tend to live long lives, have late sexual maturity, and produce fewer, more highly dependent (altricial) young. Elephants, whales, and humans are all K-selected. Bear in mind the r/K selection theory is regarded as more of a ‘rule of thumb’ than actual law nowadays, but in the broad strokes we often still think of animals in these terms.
From what we can tell, dinosaurs generally fell in the r-selection camp, producing large clutches of eggs and not lavishing as much attention on their young as, say, many songbirds today would. It was famously the study of the Montanan hadrosaur Maiasaura in the late 70’s that demonstrated that at least some dinosaurs tended to their young not only during incubation, but for some time after hatching (Horner & Makela, 1979). Since then, dinosaur nests have popped up in various other places, with evidence of varying parental care strategies and offspring behavior.
Hadrosaurs are often the poster-children for dinosaur nesting, thanks in no small part to Maiasaura. Nests of other species are known as well though, notably of the crested hadrosaur Hypacrosaurus from Montana and southern Alberta (Horner & Currie, 1994). Thanks to discoveries like these we know that both Maiasaura and Hypacrosaurus laid clusters of large, spherical eggs which might have been insulated with rotting vegetation added by the parents, themselves too large to brood their eggs like birds do. Strangely enough, even though Maiasaura and Hypacrosaurus weren’t that different in size when fully grown, the eggs of Hypacrosaurus were quite big, apparently about 20 cm long by 18.5 cm wide. This is larger than Masiasaura eggs, which were about the size of modern ostrich eggs at about 15 cm long. Perhaps Hypacrosaurus specialized in producing larger, more precocial young while Masiasaura invested in more parental care for its smaller hatchlings. Who knows, though…? Baby hadrosaurs of these species have noticeably different proportions from the adults, with comparatively large eyes and stubby snouts. It’s been suggested that this made them look cute to their parents as a way to garner more care and attention from them. It’s an idea that pretty tough to prove, but nice to think about at least. Who doesn’t love a baby hadrosaur?
As far as other ornithischians go, there’s some rare yet interesting finds. Juveniles of the Asian armored dinosaur Pinacosaurus have been found preserved in a group, perhaps caught while sheltering together from some kind of desert storm (Currie et al., 2010). Also known from central Asia are hatchlings of the basal ceratopsid Protoceratops (more on that later), and amazing mixed-age social groupings of young Psittacosaurus (Zhao et al., 2007) (Zhao, 2013).
Eggs with embryos are known from the basal sauropodomorph Massospondylus of South Africa (Reisz et al., 2005). These are to date the geologically oldest dinosaur embryos known, dating back to the early Jurassic. Massospondylus seems to have been born fairly small and helpless for a dinosaur that would grow into a mid-sized bipedal herbivore, and seems to have required parental care for some time after hatching. Another basal sauropodomorph, Mussaurus, has known hatchlings similar in size and shape to those of Massospondylus, suggesting that young early sauropodomorphs were born small and in need of parental care. Big clusters of large, round eggs are also known from the most derived of sauropods, the titanosaurs (Powell, 1992).
The stars of dinosaur reproductive science are the theropods, and the best examples come from birdlike coelurosaurs in Asia. Many species of this group not only tended to their eggs prior to hatching, but were able to sit over and insulate their eggs with their warming coat of feathers just as modern birds do, a behavior known as brooding. Some spectacular fossil nests complete with preserved parents atop their eggs prove this.
Let’s start out small, though. A specimen of the small proto-feathered Sinosauropteryx was found to contain two small, oval-shaped eggs in its pelvic region (Chen et al., 1998). While it could be argued that the dinosaur had simply swallowed some other animals eggs just prior to dying, their position in the abdomen and the intact shells suggest otherwise. Eggs and embryos are known from the bizarre, scythe-clawed therizinosaurs from China (Kundrát et al., 2008). The describers of this clutch noted the advanced development of the embryos prior to hatching, which suggests they were very precocial at birth.
In Montana, paleontologists have also found troodontid nests, which include eggs, embryos, and the partial skeleton of a brooding adult (Varricchio et al., 1997). The teardrop-shaped eggs were laid in circular nests of raised dirt and sand, with the more pointed end of the eggs partially embedded in the sediment of the nest. The eggs appear to have hatched around the same time, even though they weren’t laid all at once, which means the parent didn’t begin brooding until the last egg was laid (Varricchio et al., 2002). The young troodontids seem to have been precocial too, as they were born fairly well developed and the nests lack crushed eggshells that would indicate they spent a lot of time in the nest. Seeing as how both crocodilians and many basal modern birds also have precocial young, this isn’t surprising. It’s been suggested that the brooding troodontid parent might have actually been a male, the father of the offspring (or at least some of them), a trait also seen in some birds (Varricchio et al., 2008).
The real stars of theropod reproduction in the fossil record have to be the oviraptorosaurs. These beaked, crested, feathered theropods are known from Asia and North America, and some very remarkable specimens have been found in the former continent. The first of these dinosaurs described by science was Oviraptor of Mongolia, found by Andrews during one of those 1920’s expeditions (Osborn, 1924). The story of this dinosaur’s name is famous, but bears repeating here. The remains of this animal, which included a partial skull and some postcranial material, were found closely associated with a nest of dinosaur eggs. Given the abundance of Protoceratops in the area, it was assumed that the dinosaur had been raiding a Protoceratops nest for the eggs inside when it died. After all, it was clearly a theropod, what else could it have possibly been doing there? Never mind that it had a toothless, birdlike beak- this must have been an adaptation for crushing eggshells. Its describer, Henry Fairfield Osborn, named it Oviraptor philoceratops, which roughly translates to ‘ceratopsian-loving egg thief’. To his credit, in the 1924 description Osborn left room for doubt in his assessment of the dinosaur’s behavior, noting that he might have burdened it forever with a misleading name. Despite this, for decades oviraptorosaurs were almost universally depicted in popular media as sneaky, conniving egg-thieves living off the unborn offspring of poor old Protoceratops.
Then Citipati was discovered. Another central Asian oviraptorid, the first described specimen of this genus had been fossilized while closely brooding a nest of elongate, oval-shaped eggs (Norell et al., 1995) (Clark et al., 1999). The brooding adult was found lying down with its arms spread out over its eggs, presumably using its pennaceous ‘wing’ feathers to protect them. This was some of the earliest, and best, evidence of dinosaurs brooding their eggs in a similar manner to modern birds. Further discoveries of Citipati on the nest have been made as well, and this soon inspired paleontologists to question if Osborn had been correct about the assumption he made about Oviraptor. As it turns out, those incriminating eggs found by Andrews’ team were had been oviraptorosaur eggs all along, and what was thought to have been an egg-thieving predator was really a caring parent.
The eggs of Citipati are fairly big for a dinosaur that didn’t grow terribly large, averaging over half a foot long, with Oviraptor eggs coming out a bit smaller at 14 cm long (Clark et al., 1999). This makes them comparable to ostrich eggs in length, but much narrower down their length. These are far from the biggest oviraptorosaur eggs that have been found, however. Giant eggs measuring around half a meter in length laid in enormous rings have been found in Asia that seem to have come from enormous oviraptorosaurs like Beibeilong and Gigantoraptor (Paul, 2010) (Pu et al., 2017). The arrangement of these eggs in circles with an empty spot in the middle, just like that of much smaller species, suggests that they were brooded by their enormous parents. What a sight that would have been to see.
Since the days of the Mesozoic, the largest eggs produced by a dinosaur have been those of the elephant birds, or aepyornithids. These could apparently grow to over a foot long (Mlíkovsky, 2003). This is over twice as large as a modern ostrich egg, and it’s interesting to note that ostriches lay comparatively small eggs for their body size. At the other end of this spectrum is the chicken-sized kiwi of New Zealand that famously lays an egg that takes up much of its abdominal room before being laid.
In short, dinosaur nests and eggs came in all shapes and sizes. Some had small young for their size, while some had pretty big babies. Most seem to have cared for their eggs at least to some degree, but in most parental care was limited after the offspring hatched. Young dinosaurs had to make their own way pretty soon in a hard world. There’s still lots known about dinosaur eggs and babies that we couldn’t cover here, and much more still to learn about the reproductive biology of many other dinosaur groups. As always, there might be some real surprises awaiting us.
By Nicholas Carter
References
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As far as we know, all dinosaurs reproduced by laying eggs, as do most other sauropsids (reptiles). It is very difficult to determine what species of dinosaur laid the eggs that have been discovered, because only a few dinosaur embryos have been found inside the fossil eggs.
Did dinosaurs lay big eggs? Some probably did, but there is a limit to how big an egg can be. Bigger eggs need thicker shells for support, but
Mark Norell, Macaulay Curator and chair in the Division of Paleontology, discusses dinosaur reproduction in the video above. Find answers to other questions in the Museum’s dinosaur videos.
All dinosaurs hatch from eggs, including extinct dinosaurs and modern birds; as do crocodiles, the living group most closely related to dinosaurs. Until the 1980s, discoveries of fossilized eggs and bones of young dinosaurs were extremely rare, but dinosaur eggs have now been discovered on several continents, and fossils of hatchlings, juveniles, and adults have been found for most major groups.
One remarkable find was in Montana, where fossils of duckbill dinosaurs, including eggs, nests, hatchlings, juveniles, and adults were found together in one death assemblage, or mass grave. The eggshells in the nests were badly broken, arousing speculation that the hatchlings might have crushed the eggs while moving around the nests. Some paleontologists think this site was a nesting colony, where adult dinosaurs cared for their young during the first several months after hatching.
One of the greatest highlights of the American Museum of Natural History’s expedition to Central Asia occurred in 1923 at the Flaming Cliffs of Mongolia. It involved the discovery of eggs that, after first analysis, were thought to belong to the dinosaur Protoceratops. Roy Chapman Andrews, the leader of the expedition and future director of the Museum, described the scene in his field notes:
“On July 13, George Olsen reported at tiffin [lunch] that he had found some fossil eggs...We felt quite certain that his so-called eggs would prove to be...geological phenomena. Nevertheless, we were all curious enough to go with him to inspect his find. We saw a small sandstone ledge, beside which were lying three eggs partly broken…The brown striated shell was so egg-like that there could be no mistake. Granger finally said, ‘No dinosaur eggs have ever been found, but the reptile probably did lay eggs. These must be dinosaur eggs. They can’t be anything else.’ The prospect was thrilling, but we would not let ourselves think of it too seriously, and continued to criticize the supposition from every possible standpoint. But finally we had to admit that ‘eggs are eggs,’ and that we could make them out to be nothing else. It was evident that dinosaurs did lay eggs and that we had discovered the first specimens known to science." (The New Conquest of Central Asia, 1932.)
Paleontologists presumed that the fossil eggs at Flaming Cliffs were laid by Protoceratops because it was the most common dinosaur at the locality where the eggs were found. However in the 1990s Museum expeditions discovered identical eggs, one of which contained the embryo of an Oviraptor, like dinosaur—which changed scientists' view of which dinosaur laid these eggs. Turns out Oviraptor was a parent, not an egg thief.