Dinosaur Eggs

Dinosaur Eggs and Nesting

The typical image of a dinosaur "birth" 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 offspring's 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. 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 "exchange capability" 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 "Egg Man," 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" and "Egg Island," 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