Baby Dinosaurs Hatched Into a World of Danger

Baby Maiasaura - like these in a nest mockup at London's Natural History Museum - hatched into a world of dangers. Photo by Brian Switek.
Baby Maiasaura – like these in a nest mockup at London’s Natural History Museum – hatched into a world of dangers. Photo by Brian Switek.

In 1985, three years before Littlefoot and gang would set off for The Great Valley, paleontologist Jack Horner, writer James Gorman, and artist Doug Henderson told the story of a different baby dinosaur on a perilous journey. Her name was Maia – short for Maiasaura – and while her adventure was fiction, it was informed by the science of what Horner and his colleagues were digging up at “Egg Mountain” in Montana.

Maia was not a miniature version of her parents. She hatched as a big-eyed, wobbly-legged infant, and spent her early days in the warmth of the nest as she grew and her limbs became stronger. But her salad days quickly ended when hungry Troodon raided the nesting ground, and as Maia matured she encountered even bigger, toothier dangers in the surrounding forests. Ultimately, though, Maia survived long enough to become a mother herself, and this, according to a new study by paleohistologist Holly Woodward and colleagues, would have made Maia a very exceptional member of her species.

The site that inspired the story of Maia isn’t just a horizon of tiny bones and eggshells. Over the past three decades paleontologists from Montana’s Museum of the Rockies have dug up hundreds of Maiasaura bones from a second horizon just above the nesting grounds. So many bones have been found here, in fact, that Woodward and her collaborators have been able to assemble the first good picture of what Maiasaura population biology was like.

Of all the dinosaurs that ever lived, only a fraction of a fraction of a fraction are preserved in the fossil record. And of the dinosaurs that have come down to us as petrified skeletons, many are only known from fragments or partial skeletons. This makes studying dinosaur biology incredibly difficult. While a biologist working on living animals might have hundreds of specimens to study for demographic information – and can often go observe their chosen species in life – paleontologists are lucky if they have a small smattering of beat-up bones to work with. And even well-known dinosaurs can be hard to study. While 50 or so partial Tyrannosaurus rex skeletons are known, for example, they don’t represent a single population but are from different places and slices of time from a window of about two million years. So if you’re a paleobiologist wanting to study variation between individuals, population structure, and other details, more often than not there just aren’t enough dinosaurs to get at those puzzles.

But the Egg Mountain bonebed is another story. Even though the bones were strewn into a jumble when buried, paleontologists have been able to recover elements from animals of different sizes and ages that died very close to each other in time. Within that sample the researchers have extracted at least 50 tibiae – the larger of the two lower hindlimb bones – and, as Woodward and colleagues demonstrate, this is a very useful bone to have.

Histology, or the study of bone structure, is one of the most important subfields within paleontology right now. By cutting into bones and looking at them under the microscope paleontologists can see how fast a dinosaur was growing, estimate its age, and get other biological information otherwise invisible to us. The most useful for studying dinosaurs have been leg bones – the femur and the tibia – as they often record a great deal of detail about an individual animal’s life in a way that’s relatively easy to see. To that end, Woodward and coauthors cut into and scrutinized the 50 leg bones – representing at least 32 distinct individual Maiasaura ranging from about 10 to over 23 feet in length – to investiage what the osteological details would reveal about this dinosaur population.

Maiasaura survivorship over time. From Woodward et al., 2015.
Maiasaura survivorship over time. From Woodward et al., 2015.

As it turns out, the fictional Maia was even luckier than even Horner initially understood. Within the sample, 31 of the bones lacked any lines of arrested growth (or LAGs) that are usually taken as markers of each passing year. This means that the majority of the dead dinosaurs were yearlings that were still growing extremely quickly. From this Woodward and her colleagues calculated that baby Maiasaura faced an 89.9% mortality rate in their very first year. If baby dinosaurs even survived to the moment they pushed out of their eggs, the odds were set against most of them surviving to adulthood.

Old dinosaurs didn’t fare so well, either. Woodward and colleagues counted 11 tibiae from Maiasaura that were seven years old or more, and five of these had a special growth marker – the external fundamental system (EFS) – which signaled that these were old dinosaurs whose skeletons had stopped growing. These fully mature dinosaurs had a mortality rate of 44.4%, Woodward and coauthors concluded, and probably represent the attrition that comes with old age.

There were a few dinosaurs in the middle of these two extremes, but, for the most part, young, vigorous Maiasaura were absent from the sample. And this seems to fit with the general picture of how the bonebed formed. The profile seems to suggest some kind of local catastrophe, like a drought or disease. Yearlings and old individuals were hit hardest, as is the case with many large animals alive today, while the young dinosaurs in their years of “peak performance” were better able to withstand the stress. Not that being a young, vigorous dinosaur was without its challenges, though. The few prime-aged Maiasaura in the sample might have been undergoing some unique growing pains that made them more susceptible, Woodward and colleagues write, which might be related to the onset of sexual maturity at around three years of age.

This sort of life history wasn’t unique to dinosaurs. Maiasaura grew at rates comparable to their living avian cousins, and the survivorship estimates Woodward and colleagues were able to calculate compare well with those biologists have recorded for red deer and Dall sheep. And the fact that such conclusions are possible at all is not only owed to the fortuitous discovery of so many dinosaurs in one place, but to the decades of work Museum of the Rockies researchers and volunteers have put into the site.

Best of all, all that hard word raises the possibility that paleontologists can apply the same lessons to other places where dinosaurs died en masse. The disarticulated remains of at least 48 individual Allosaurus have been found at the Cleveland Lloyd Dinosaur Quarry in Utah, for example, and New Mexico’s Ghost Ranch quarry is famous for producing hundreds of Coelophysis skeletons. These collections of bones, as well as others scattered around the world, offer additional opportunities to gain new insights into dinosaur life if we can grit our teeth and comes to terms with this simple fact – we won’t really be able to understand dinosaurs without slicing up a few bones.

Reference:

Woodward, H., Freedman Fowler, E., Farlow, J., Horner, J. 2015. Maiasaura, a model organism for extinct vertebrate population biology: a large sample statistical assessment of growth dynamics and survivorship. Paleobiology. doi: 10.1017/pab.2015.19

7 thoughts on “Baby Dinosaurs Hatched Into a World of Danger

  1. This is a thought-provoking article. If the mortality rate among infant Maiasaurs was so high, then the birth rate must have been or must necessarily have become proportionate. Could you please tell me, is it known how many eggs a female Maiasaur usually laid at a time?

    These herbivorous dinosaurs, Maiasaurs and their relatives, were certainly not well equiped for defense; how could they have protected their offspring from hungry predators? Or from sandstorms, for that matter? At what age might a juvenile Maiasaur been old enough to leave the nest? Surely the young ones would not have waited until sexual maturity to venture out on their own…I wonder if these adolescents might have all left their nest together, going out to face the dangerous environment in packs, for the sake of protection…

    1. As Brian explained, Maiasaura had a high mortality rate. In order to keep the species going, Maiasaura employed several survival techniques, including having a lot of young at a time. Fossil Maiasaur nests average about 30-40 eggs at a time, this way predators didn’t consume the entire next generation.
      (https://bioweb.uwlax.edu/bio203/s2014/fischbac_sama/interactions.htm)

      Maiasaura, like most other hadrosaurs, didn’t possess strong armor, spikes or tail clubs like some other herbivores. However, even though I suspect a hadrosaur could defend itself using its tail or kicking a predator with its “hands”, this was probably a last resort. Recent studies indicate that while hadrosaurs weren’t as fast as their larger predators, they could retain a set speed for a longer period of time.
      (http://news.discovery.com/animals/dinosaurs/dino-life-or-death-chases-recreated-1411051.htm)

      Sandstorms wouldn’t have been a problem in the Maiasaura’s habitat, as it lived not in a desert, but an area of North America that was filled with open plains and forests.
      (https://bioweb.uwlax.edu/bio203/s2014/fischbac_sama/habitat.htm)

      As for your last question, while baby Maiasaura probably didn’t stay with their mother their entire lives, they don’t seem to have ventured off “on their own”. Evidence from fossil deposits indicate many species of hadrosaur, including Maiasaura, indicates that they lived in large herds. Living in herds would have decreased the chances of any one individual being picked off by a predator.
      (https://bioweb.uwlax.edu/bio203/s2014/fischbac_sama/interactions.htm)

      I hope this helps.

      1. Dear Ryan,

        Thank you for your response to my post. It’s interesting to know that the female Maiasaura could lay so many eggs at a time…I wonder, though, if the fertilization process might have occured over the space of several days, to avoid such a bulky mass of eggs inside the female…Otherwise the eggs must have been very small.

        As for sandstorms, I mentioned these in connection with hadrosaurs in general, those which might have lived in more arid zones or near beaches. Maiasaura may have had to face tempestuous rainstorms, dust storms, hail etc.

        Your idea about hadrosaurs living in herds is quite likely, even though I suspect that young adults did leave home to found new communities, especially in times of scarcity or drought. Hadrosaurs may well have been social creatures; perhaps they mated for life, males and females sharing the tasks of “bringing up baby”.

        Their practical defense system was almost nil. Poor hapless creatures: they were among the most vulnerable of all dinosaurs. As we both have stated, their best bet for survival was always running away.

    2. Is it wise to calculate a “theorized” mortality rate among juveniles from just “this bone bed”? Obviously, the event that caused this catastrophe had a profound impact on this population of Maiasaura. Is it indicative, however; of Maiasaura population in an other “catastrophe-free” environment. How would Maiasaura juveniles fare under these circumstances? From the data substantiated and gathered for this article, a “picture is painted” of a desperate species seeking to survive a rather high mortality rate within one year of life. There is much more to this species that has yet to be determined by a paleoecologist….

      1. Hello Rob,

        What you say makes sense; but as I commented previously, it seems to me that adult Maiasaurs were singularly ill-equiped to defend their young or even themselves, in spite of their dubious reputation as “good mothers”. Hadrosaurs in general, lacking sharp teeth and claws, spines or tails long and strong enough to really lash out and deliver a disabling blow, must have been easy prey for medium to large carnivores. Their only effective solution would have been to run away as fast as their legs could manage…But the nestlings were too small and weak to outrun their predators; unless successfully defended on the spot by their parents, which is unlikely, they must have been devoured in great numbers. Which is why I believe that the natural impulse for survival would have provided these mostly helpless species with an elevated birth rate (as in the case of fish).

  2. “Best of all, all that hard word raises the possibility that paleontologists can apply the same lessons to other places where dinosaurs died en masse. The disarticulated remains of at least 48 individual Allosaurus have been found at the Cleveland Lloyd Dinosaur Quarry in Utah, for example,”

    That reminds me of the following quote.

    Quoting Bakker/Bir ( http://www.amazon.com/Feathered-Dragons-Studies-Transition-Dinosaurs/dp/0253343739 ): “Intense parental care should reduce mortality from adolescent to full-grown adult. This prediction is met (fig. 14.13). It’s useful to divide the shed teeth into 3 classes: (a) smallest to one-third grown; (b) one-third to two-thirds grown; and (c) two-thirds grown to maximum size. Among Como crocodiles, only 4 percent of the (b) class survive to grow into the final (c) size class. The distribution of allosaur crowns shows a much less catastrophic decrease; 33 percent of the (b) class survive to grow up to he the (c) class (difference significant at 0.05 level, two-sample K-S test). In other words, if you are an adolescent allosaur, your chances of surviving into adulthood are much greater than they would he if you were a Jurassic crocodile.”

Leave a Reply

Your email address will not be published. Required fields are marked *