A Study of Dinosaur Eggs and Nesting

The typical image of a dinosaur  to most people is a single gigantic sized egg, maybe green or yellow in color with some dark shading, cracking open to reveal a pint-sized dinosaur ready for the hunt. This image has been reinforced by media portrayals in television and the movies but scientific research and discovery has proved it to be nothing more than a myth. The largest dinosaur eggs ever found were hardly as big as a football, and adult dinosaurs may have been a more important part of their offsprings lives than normally thought.

The interactions of the shape and size of an egg, the eggshell structure, nest structure and egg distribution, and the nesting behavior of adult dinosaurs, and the environmental conditions were vital to the healthy development of the dinosaur embryos. There seems to be a classification system for almost everything in the scientific world, and dinosaur eggs are no exception. There have been many changes, variations, and designers of the system over time, some delineating by shape while others rely on the different crystalline structures of the shells.

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However, there is no universal parataxonomic system for classifying dinosaur eggs, and this leads to confusion when each paleontologist works within the boundaries of whatever system he has chosen to use (Website).

When examining the dinosaur egg itself, there are three aspects that need to be taken into consideration; the structure of the shell, the size, and the shape. The eggshell may be the most complex aspect of the physical egg itself.

The thin shell is covered in pores, and their size and number determine the amount of gases that can be exchanged into the egg and out, mainly O2, CO2, and water vapor. This  is 8 to16x that of present day bird eggshells and suggests a humid environment of low oxygen and high carbon dioxide (Paleobiology, p 42). The texture of dinosaur eggs also varies from smooth to nodular, ridged, and striated.

Some scientists believe that the texture of the shells helped to strengthen them without increasing their thickness. Another idea is that they helped to facilitate the movement of gases around the egg when it was surrounded by dirt or vegetation so that the embryo would remain incubated and still receive the proper exchange of gases (Dinosaur Eggs and Babies, p. 44). Egg size is directly related to the eggshell structure in that volume is limited by the strength of the shell to contain it (Website). If eggs become too big, the eggshell will become too thick for gases to pass in and out to the embryo, thus suffocating it. Also, thickness of the shell could make it impossible for the baby dinosaur to break out when it comes time to hatch. Therefore, dinosaur egg size was limited by their pores, and never were larger than an ostrich egg (Dinosaur Eggs and Babies, p.44). There are two recognized shaped for dinosaur eggs. The first is an elongated oval shape with a blunt end, where the length is approximately twice that of the maximum diameter, while the other (though less common) is more spherical. Most of the eggs that have been discovered fit into the first category (those laid by Protoceratops and Maiasaurs), but the second type of egg has been found in sites in France (Paleobiology, p.41).

Adding another behavioral link to present day birds is the knowledge that dinosaurs actually created nests for their eggs, as opposed to just laying them on the ground exposed to the air. Nests were crater like excavations, dug out mostly likely using their hind legs, where eggs 12-24 eggs were laid and then covered with dirt and/or vegetation (Paleobiology, p.42). The eggs were not just randomly arranged but oriented and spaced out to the point that some scientists are now considering that dinosaurs had the ability to recognize geometric patterns; they were close together but not touching, and used the minimum amount of space necessary. (Dinosaur Eggs and Babies, p.44). There are two types of nests; clutches and linear. Clutches can be broken down further by the arrangement of eggs in concentric circles, spirals, or inverted cones. Linear nest eggs were usually arranged in parallel rows or arcs. Another thing to consider is the ornamentation of the eggs themselves. Bidirectional ornamentation occurs with longitudal eggs with ridges that are arranged vertically in circular nests (like the Ornithischians, below), while multidirectional ornamentation occurs with nodular eggs arranged randomly or in both clutches and linear ways (Website).

Fossils and nests found all over the world have allowed for a classification of nest arrangement and egg shape by dinosaur. Ornithischians eggs are elongated, with a blunt end and ridged surface. They utilized the concentric circle or spiral nest arrangement, putting eggs vertical with the blunt end up. Saurischians are a bit more varied in their egg distribution styles; Prosauropods are assumed to have made nests but there exists no evidence of how they arranged them and the shell fragments found do not specify their texture. Sauropods had the more spherical shaped eggs with a nodular surface, and they used the circular clutches and parallel rows and arcs to arrange their nests. The final type of Saurischian dinosaur, the Theropods, had elongated eggs that were found in linear arrangements (Dinosaur Eggs and Babies, p. 38-42). Scientists can really learn the most about the nesting and nurturing behaviors of the dinosaurs from the actual nest sites that have been discovered around the world. Even though it is thought that nesting developed in the Triassic period, most nest sites are from the Upper Cretaceous strata, therefore making the discussion of dinosaur nesting behavior a little bit slanted (Dinosaur Eggs and Babies, p.37). It is not possible to discuss how

nesting rituals developed throughout the Mesozoic, and even in the Cretaceous, of the roughly 285 genera and 336 species of dinosaurs that existed, the behaviors of only about five of them are thought to be well studied and recreated; especially those of the Protoceratops and Maiasosaurs (Website). Jack Horner, fondly called the discovered the first dinosaur nest in North America, and kept on digging from there, eventually unearthing the richest supply of duck-billed dinosaur fossils in the world. Egg Mountain discovered in 1979 and 1983, are located in the hills of western Montana (Digging Dinosaurs, p.139-162). Using his knowledge of the existence of a low land, alkaline, shallow sea, Horner predicted where to find the eggs (in the uplands where the soil is free of acidity). Here he unearthed several nests located the same distance away from one another.

This suggested that these dinosaurs were social, herding animals that bred communally, returning to the same spot year after year to lay their eggs (Digging Dinosaurs). Horner also found not only an abundance of eggshells and eggs with embryos inside, but also the fossils of adult and juvenile dinosaurs, dung, mounds of vegetation, and very chewed food that he thinks was regurgitated by the adult to their offspring. These suggest that the mother dinosaur was on hand to feed her young when they hatched, and this nurturing, caring nature led him to created the term to describe the Maiasaura at the nest (Kings of Creation, p.264). The site is extremely rare not just in the number of intact eggs and bones but it is very uncommon for a site to yield enough evidence to be able to recreate the habitat and behaviors of the dinosaurs that roamed there (Kings of Creation, p. 263). Along with the Montana site, sites in Alberta yield even more insight into the nesting behavior of these ancient reptiles. Evidence of fossilized vegetation has been found inside nests, showing how dinosaurs buried their eggs instead of leaving them out in the open.

Decaying plant matter creates heat, and this technique was used so dinosaurs did not have to use their bodies to sit on and cover their fragile eggs (Website). Also, an egg incubated in the open air could have detrimental water loss problems due to the environmental conditions of the time, so an underground environment in a nesting mound was the best place for an egg to be (Dinosaur Eggs and Babies, p. 43). In contrast to this discovery, a nesting site in the Gobi Desert of Mongolia seems to show the exact opposite. An Oviraptor skeleton was found sitting on top of a clutch of eggs, like the way a bird sits on its eggs to incubate them.

This may be proof that it was possible for some dinosaurs to brood their own eggs, although this is the only nest found in this condition so far (Website). The location of adult dinosaurs next to nests was probably for at least one of three reasons: to ward off predators, to provide shade or warmth for the eggs, and to be there and ready to care for the babies once they hatched (Paleobiology, p. 44). Most sites show dug out nests with enough space in between for an adult dinosaur to have had laid and protected or watched over her eggs. Some of the Maiasaura nests discovered by Horner contained the bones of lizards, small predatory dinosaurs, and primitive mammals that most likely tried to steal the eggs as food, therefore, this attendance of the adult dinosaur was as necessary for the survival of their offspring as the correct nest construction and egg structure.

Reproduction is a necessary process for the continued existence of a species, and drastic environmental changes can have a huge impact on this. Seemingly straight out of a sci-fi movie, consider for a moment the fact that the temperature during the incubation of a dinosaur egg could actually determine the sex of a dinosaur embryo (much like turtles, crocodiles, and alligators today). The nesting sites in the uplands of Montana were subject to the changing temperatures of the Late Cretaceous slowly deteriorating environment, and too many of one sex of dinosaur born could have increased a species chance of extinction. The habitat alterations, changes in the food supply, and changing global climate at the end of the Cretaceous would have affected the dinosaur eggs sooner, and more intensely, than it would the adult population.

Just a shift of even two degrees Celsius would have had important demographic effects in a few generations. The effects of depressing the numbers of large herbivorous dinosaurs such as hadrosaurs and ceratopsians by disrupting their population breeding structure would ripple through the food chain and affect carnivorous dinosaurs as well (Paleobiology, p. 66). Dinosaurs may have had to rearrange the time in which they laid and nested their eggs in order to balance of the sexes and to give offspring sufficient time to develop after they hatched, before the migration period. If the young had not reached an adequate size then they might night survive or be left behind, almost destroying their chance or survival anyways. Such a scenario and its required mechanisms are consistent with known geologic, climatic, physiologic, evolutionary and ecologic processes.

This hypothesis, together with other biological factors such as changing food supply and available niches, is sufficient to account for the extinction of the dinosaurs, and it is not necessary to invoke a celestial dues ex Machina to account for this phenomenon (Paladino, et al., Paleobiology, p. 68). Taking this extinction theory into consideration, along with all the other previously stated facts about the necessary conditions for an embryo's correct development, it is obvious that dinosaurs were not careless with their reproduction and nesting. From the frail eggshells to the strong guardianship of the mother dinosaur of her clutch, the process to ensure the life of a young dinosaur of future generations was a complex and amazing process.