Dinosaur egg

Dinosaur eggs are eggs laid by dinosaurs. When the first scientifically documented remains of dinosaurs were being described in England during the 1820s, it was presumed that dinosaurs had laid eggs because they were reptiles.^[1] In 1859, the first scientifically documented dinosaur egg fossils were discovered in France by Jean-Jacques Poech, although they were mistaken for giant bird eggs. The first scientifically recognized dinosaur egg fossils were discovered in 1923 by an American Museum of Natural History crew in Mongolia. Since then many new nesting sites have been found all over the world and a system of classification based on the structure of eggshell was developed in China before gradually diffusing into the West. Dinosaur eggshell can be studied in thin section and viewed under a microscope. The interior of a dinosaur egg can be studied using CAT scans or by gradually dissolving away the shell with acid. Sometimes the egg preserves the remains of the developing embryo inside. The oldest known dinosaur eggs and embryos are from Massospondylus, which lived during the Early Jurassic, about 190 million years ago.^[2][3]

History

In 1859, the first scientifically documented dinosaur egg fossils were discovered in southern France by a Catholic priest and amateur naturalist named Father Jean-Jacques Poech; he thought, however, that they were laid by giant birds.^[4] The first scientifically recognized dinosaur egg fossils were discovered serendipitously in 1923 by an American Museum of Natural History crew while looking for evidence of early humans in Mongolia.^[5] These eggs were mistakenly attributed to the locally abundant herbivore Protoceratops, but are now known to be Oviraptor eggs. Egg discoveries continued to mount all over the world, leading to the development of multiple competing classification schemes. In 1975 Chinese paleontologist Zhao Zi-Kui started a revolution in fossil egg classification by developing a system of "parataxonomy" based on the traditional Linnaean system which classified eggs based on their physical qualities rather than their hypothesized mothers.^[6] Zhao's new method of egg classification was hindered from adoption by Western scientists due to language barriers. However, in the early 1990s Russian paleontologist Konstantin Mikhailov brought attention to Zhao's work in the English language scientific literature.^[7]

Identification

Fossil dinosaur eggshell fragments can be recognized based on three important traits. Their thickness should be roughly uniform, they are usually slightly curved, and their surface is covered in tiny pores. Less frequently, the concave underside of the eggshell fragment will preserve bumps known as mammillae. Sometimes the embryo had absorbed so much of the calcium that the mammilae need a magnifying glass or microscope to be seen.^[8] However, there are many kinds of naturally occurring objects which can resemble fossil eggs. These can fool even professional paleontologists.^[9]

False eggs

Calculus: Calculi are egg-like objects formed in the stomachs of ruminants such as cattle, deer, elk, and goats. Calculus formation is a defense mechanism protecting the ruminant's stomach from damage if it swallows a foreign object while grazing. After ingestion, the object is covered by the same material composing bone, calcium phosphate, and eventually vomited out of the animal's system. These "stomach stones" tend to range in size from 1 to 6 centimeters. Larger sizes are known but very rare.^[10] Sometimes tiny dimples cover the surface of a stomach stone, which can fool observers into thinking they are the pores of an egg.^[11] Fossil egg expert Ken Carpenter has described stomach stones as the most egg-like natural objects, noting that they are "the trickiest [egg-like] objects to correctly identify".^[12] Calculi are so egg-like that on one occasion a detailed description of a stomach stone misidentified as a fossil egg was published in the scientific literature.^[11] Calculi can be distinguished from real egg fossils because when they are broken open, they show the layers of calcium phosphate and the foreign object at the core.^[11] Multiple layers of eggshell are known in pathological eggs, but these layers don't go all the way down to its core the way a stomach stone's do. Calculi are often suspiciously intact, unlike fossil eggs, which are usually damaged.^[10] Stomach stones also lack distinct shells with their attending structural components like continuous or prismatic layers, mammillae, and pores.^[11]

Concretions: Concretions are formed when decaying organisms change the chemistry of their immediate surroundings in a manner that is conducive to minerals precipitating out of solution. These minerals accumulate in a mass roughly shaped like the region of altered chemistry. Sometimes the mass produced is egg-shaped.^[13] Most egg-shaped concretions have uniform interiors, however some form through the accumulation of mineral in layers.^[14] These layered concretions can be even harder to recognize than those with uniform interiors because the layers can resemble egg white and yolk. The yellow of the false yolk comes from minerals like limonite, siderite, and sulfur.^[11]

Concretions also generally lack distinct shells, although sometimes they can appear to have them if their outside surfaces have been case-hardened. Since their interiors are softer, erosion can separate the two, creating eggshell pseudofossils. Real egg fossils should preserve eggshell structures like pores, mammillae, and prismatic or continuous layers, which are not present in concretions. Any given concretion is unlikely to be exactly the same size as any other, so associations of egg-like objects of different sizes are probably not real eggs at all. Concretions can also be far larger than any real egg so an apparently unnaturally large "egg" has probably been misidentified.^[11]

Insect trace fossils: Sometimes the living or breeding chambers of an insect burrow are so perfectly egg-shaped that even a paleontologist can mistake a natural cast of these chambers for a fossil egg. Insect burrow fossils can sometimes be distinguished from real egg fossils by the presence of "scratch marks" on their surface left by the insect during the burrow's original excavation. Fossil insect pupae can also resemble eggs. After death and burial, the decomposition of a deceased pupa would leave a gap in the sediment that could be filled with minerals carried by groundwater, forming an egg-like cast. These pseudo-eggs can be recognized by their small size (usually not much longer than a centimeter or two) and lack of an eggshell with its typical anatomy.^[11]

Stones: The erosive effects of water can sometimes round rocks into egg-like shapes.^[13]

Classification

The classification of dinosaur eggs is based on the structure of the egg shells viewed in thin section via microscope. There are three main categories of dinosaur eggs: (sauropods and hadrosaurs,)^[15] Prismatic,^[16] and ornithoid (theropods, including modern birds).^[17]

Oogenera

Oogenera are taxonomic names for types of eggshell. Nearly three dozen oogenera have been named for dinosaur eggs:

Ageroolithus Apheloolithus[18] Boletuoolithus[19] Cairanoolithus[20] Continuoolithus[18] Dendroolithus[18] Dictyoolithus[21] Dispersituberoolithus Dughioolithus[22] Ellipsoolithus[23]

Elongatoolithus[24] Faveoolithus[18] Heishanoolithus[25] Laevisoolithus Macroolithus[26] Macroelongatoolithus[27] Megaloolithus[28] Nanshiungoolithus[29] Oblongoolithus Ovaloolithus[30]

Paraspheroolithus[31] Phaceloolithus[18] Placoolithus[32] Porituberoolithus Preprismatoolithus[18] Prismatoolithus[33] Protoceratopsidovum[34] Pseudogeckoolithus Shixingoolithus[35] Sphaerovum[18]

Spheroolithus[36] Spheruprismatoolithus[34] Spongioolithus[37] Stromatoolithus[18] Subtiliolithus[18] Tacumarembovum[18] Trachoolithus[38] Tristraguloolithus Youngoolithus[18]

Therizinosaur nest and eggs in from Dinosaurland in Lyme Regis, England.

Embryos

Dinosaur embryos, the animal inside the eggs, are very rare but useful to understand ontogeny, heterochrony, and dinosaur systematics. Embryo fossils are known from:

• Citipati^[39]
• Massospondylus^[40]
• Lourinhanosaurus^[41][42]
• Troodon^[43]

Taphonomy

The formation of fossil eggs begins with the original egg itself. Not all eggs that end up fossilizing experience the death of their embryo beforehand. Fossil eggs with open tops are common and could result from the preservation of eggs that hatched successfully.^[44] Dinosaur eggs whose embryos died were likely victims of similar causes to those that kill embryos in modern reptile and bird eggs. Typical causes of death include congenital problems, diseases, suffocation from being buried too deep, inimical temperatures, or too much or too little water.^[45]

Whether or not hatching was successful, burial would begin with sediments gradually entering any large openings in the shell.^[44] Even intact eggs are likely to fill with sediment once they crack under the strain of deep buial. Sometimes, though, fossilization can begin fast enough to prevent the eggs from being cracked. If the water table is high enough dissolved minerals like calcite can percolate through the pores of the eggshell. When the egg is completely filled it can become sturdy enough to withstand the weight of the overlying sediments.^[45] Not all fossil egg specimens are of complete specimens, however. Individual pieces of eggshell are much more robust than the entire egg and can be transported intact long distances from where they were originally laid.^[46]

When the egg is buried deeply enough, the bacteria decomposing it no longer have access to oxygen and need to power their metabolisms with different substances. These physiological changes in the decomposers also alter the local environment in a way that allows certain minerals to be deposited, while others remain in solution.^[45] Generally, however, a fossilizing egg's shell keeps the same calcite it had in life, which allows scientists to study its original structure millions of years after the developing dinosaur hatched or died.^[47] However, eggs can also sometimes be altered after burial. This process is called diagenesis.^[47] One form of diagenesis is a microscopic cross-hatched pattern imposed on the eggshell by the pressure of being buried deeply.^[48] If the pressure gets severe enough, sometimes the eggshell's internal microscopic structure can be completely destroyed. Diagenesis can also happen chemically in addition to physically. The chemical conditions of a decomposing egg can make it easy for silica to be incorporated into eggshell and damage its structure. When iron-bearing substances alter eggshell it can be obvious because compounds like hematite, pyrite, and iron sulfide can turn the shell blackish or rusty colors.^[49]

Depositional environments

Dinosaur eggs are known from a variety of depositional environments.

Beach sands: Beach sands were a good place for dinosaurs to lay their eggs because the sand would be effective at absorbing and holding enough heat to incubate the eggs. One ancient beach deposit in northeastern Spain actually preserves about 300,000 fossil dinosaur eggs.^[50]

Floodplains: Dinosaurs often laid their eggs on ancient floodplains. The mudstones deposited at these sites are therefore excellent sources of dinosaur egg fossils.^[46]

Sand dunes: Many dinosaur eggs have been recovered from sandstone deposits that formed in the ancient dune fields of what are now northern China and Mongolia.^[51] The presence of Oviraptor preserved in their life brooding position suggests that the eggs, nests, and parents may have been rapidly buried by sandstorms.^[50]

Excavation and preparation

Usually the first evidence of fossil dinosaur eggs to be discovered are shell fragments that have eroded away from the original eggs and been transported downhill by the elements.^[8] If the source eggs can be found the area must be examined for more unexposed eggs. If the paleontologists are fortunate enough to have found a nest, the number and arrangement of the eggs must be estimated. Excavation must proceed to significant depth since many dinosaur nests include multiple layers of eggs. As the underside of the nest is excavated, it would be covered by material like newspaper, tin foil, or tissue. Afterwards, the entire block is covered in multiple layers of plaster-soaked strips of burlap. When the plaster is dried, the block is undercut the rest of the way and turned over.^[52]

The fine work of cleaning the egg fossils is performed in a laboratory. Preparation usually begins from the underside of the block, which tends to be the best preserved.^[52] Because of their fragility, cleaning fossil eggs requires patience and skill.^[53] Scientists use delicate instruments like dental picks, needles, small pneumatic engraving tools, and X-Acto knives.^[52] Scientists must determine at what point to stop cleaning based on their own criteria. If eggs are fully extracted they can be more fully studied individually at the cost of information regarding the spatial relationships between eggs or if the eggs had hatched. Commercial fossil dealers tend to expose only the bottom of the eggs since the topsides might be damaged by hatching and therefore less visually appealing to potential customers.^[54]

Research techniques

Acid dissolution

Acids can be used to learn more about fossil eggs. Diluted acetic acid or EDTA can be used to expose the microstructure of shell that has been damaged by weathering. Acids are also used to extract embryo skeletons from the egg encasing them.^[55] Even fossilized soft tissue like muscle and cartilage as well as fat globules from the original egg yolk can be uncovered using this method.^[56] Amateur paleontologist Terry Manning has been credited with groundbreaking work developing this technique. First, the paleontologist must submerge the egg in a very dilute phosphoric acid bath. Since the acid solution can penetrate the egg, every few days the specimen must be soaked in distilled water to prevent the acid from damaging the embryo before it is even exposed. If embryonic fossil bone is revealed after drying from the water bath, the exposed fossils must be delicately cleaned with fine instruments like needles and paint brushes. The exposed bone is then coated with plastic preservatives like Acryloid B67, Paraloid B72, or Vinac B15 to protect it from the acid when submerged for another round. The complete process can take months before the whole embryo is revealed.^[55] Even then only about 20% of the eggs subjected to the process reveal any embryo fossils at all.^[57]

CAT scans

CAT scans can be used to infer the 3D structure of a fossil egg's interior by compiling images taken of slices through the egg in small regular increments. Scientists have tried to use CAT scans to look for embryo fossils contained inside the egg without having to damage the egg itself by physically extracting them. However, as of Ken Carpenter's 1999 book on dinosaur eggs, Eggs, Nests, and Baby Dinosaurs, all alleged embryos discovered using this method were actually false alarms. Variations in the type of infilling mineral or cement binding the infilling sediment into rock sometimes resemble bones in CAT scan images. Sometimes eggshell fragments that fell back into the egg when it hatched have been mistaken for embryonic bones.^[55][58] The use of CAT scans to search for embryonic remains is actually conceptually flawed since embryonic bones have not yet mineralized. Since the infilling sediment is their only source of minerals they will be preserved at basically the same density and therefore have poor visibility in the scan. The validity of this issue has been confirmed by performing Cat scans on fossil eggs known to have embryos inside and noting their poor visibility in the scan images. The only truly reliable way to discover a dinosaur embryo is to cut the egg open or dissolve some of its eggshell away.^[55]

Cathodoluminescence

Cathodoluminescence is the most important tool paleontologists have for revealing whether or not the calcium in fossil eggshell has been altered.^[59] Calcite in eggshell is either pure or rich in calcium carbonate. However, the calcite composing the egg can be altered after burial to include significant calcium content. Cathodoluminescence causes calcite altered in this fashion to glow orange.^[60]

Gel electrophoresis

Gel electrophoresis has been used in attempts to identify the amino acids present in the organic components of dinosaur eggshell. Contact with human skin can contaminate eggs with foreign amino acids, so only untouched eggs can be investigated using this technique. EDTA can be used to dissolve the calcite of the eggshell while leaving the shell's organic content intact. The resultant organic residue would be blended and then implanted into gel. Electricity would then be run through the sample, causing the amino acids to migrate through the gel until they stop at levels determined by their physical properties. Protein silver stain is then used to dye the amino acids and make them visible.^[59] The bands of amino acids from the dinosaur eggs can then be compared with the banding of samples with known composition for identification.^[59]

Gel electrophoresis is not necessarily a perfect means of discovering the amino acid composition of dinosaur eggshell because sometimes the amount or type of amino acids present could be altered during or after preservation. One potential confounding factor would be the heating of deeply buried egg fossils, which can break down amino acids. Another potential source of error is groundwater, which can leach away amino acids. These issues cast doubt as to whether the results these sorts of studies give are reliable as the actual composition of the eggshell's organic material in life. However, studies applying these techniques have made suggestive findings, including amino acid profiles in dinosaur eggs similar to those in modern birds.^[59]

Geneva lens measure

The Geneva Lens Measure is a device used to measure curved surfaces. It is most commonly used by opticians to measure lenses but can also be used by paleontologists to estimate the life size of dinosaur eggs from shell fragments. The instrument can be used to help estimate the size of fossil eggshells by measuring their curved surfaces. Since most eggs aren't perfectly round measurements from multiple parts of the egg with varying shell curvatures may be needed to get a full idea of the egg's size. Ideally an eggshell fragment being used to estimate the full size of an egg should be more than 3 cm long. Smaller eggshell fragments are better suited to other methods of study, like the Obrig radius dial gauge. The Geneva Lens measure gives units in diopters which must be converted to the radius in millimeters. Use of the Geneva Lens Measure to estimate the size of a fossil egg was first done by Sauer on fossil ostrich eggs.^[60]

Light microscopy

Light microscopy can be used to magnify the structure of dinosaur eggshell for scientific research. To do so an eggshell fragment must be embedded in epoxy resin and sliced into a thin section with a thin-bladed rock saw. This basic method was invented by French paleontologist Paul Gervais and has remained almost unchanged ever since. Horizontally cut thin sections are called tangential thin sections while vertically cut thin sections are called radial sections. Regardless of direction, the sample must be abraded by fine-grit sand or emory paper until it is translucent. Then the structure of the shell's calcite crystals or pores can be examined under a petrographic microscope.^[61] The calcite crystal structure of dinosaur eggshell can be classified by their effect on polarized light. Calcite is capable of acting as a polarizing light filter.^[62] When a microscopic thin section sample is rotated relative to polarized light it can eventually block all the light and seem opaque. This phenomenon is called extinction. Different varieties of dinosaur eggs with their different calcite crystal structures have different light extinction properties that can be used to identify and distinguish even eggs that seem very similar on the surface.^[63] To reconstruct the three-dimensional structures of the shell's pore channels scientists require a series of multiple radial sections.^[61]

Scanning electron microscopy

Scanning electron microscopy is used to view dinosaur eggshell under even greater magnification than is possible with light microscopy. However, this does not mean that scanning electron microscopy is necessarily the superior research method. Since both techniques provide differing amounts and types of information they can be used together synergistically to provide a more complete understanding of the specimen under scrutiny. Eggshell specimens best suited for scanning electron microscopy are those recently broken because such a break will usually occur along the plane of the eggshell's calcite crystal lattice. First, a small specimen would be covered with a very thing layer of gold or platinum. The specimen would then be bombarded with electrons. The electrons bounce back off the metal and due to their small size, can be used to form a detailed image of the specimen.^[63]

Mass spectrometry

Mass spectrometry is a method for determining eggshell composition that uses a device called a mass spectrometer. First, the eggshell sample must be powdered and placed in the mass spectrometer's vacuum chamber.^[56] The powder is vaporized by the heat of an intense laser beam. A stream of electrons then bombard the gaseous eggshell molecules, which breaks down the molecules in the eggshell and imbues them with a positive charge. A magnetic field then sorts them by mass before they are detected by the spectrometer.^[64] One application of mass spectrometry has been to study the isotope ratios of dinosaur eggshell in order to ascertain their diets and living conditions. However this research is complicated by the fact that isotope ratios can be altered post mortem before or during fossilization. Bacterial decomposition can alter carbon isotope ratios in eggs and groundwater can alter the oxygen isotope ratios of eggshell.^[65]

X rays

X-ray equipment, like CAT scans, are used to study the interior of fossil eggs. Unlike CAT scans, x-ray imaging condenses the entire interior of the egg into a single two-dimensional image rather than a series of images documenting the interior in three dimensions. X-ray imaging in the context of dinosaur research has generally been used to look for evidence of embryonic fossils contained inside the egg. However, as of Kenneth Carpenter's 1999 book Eggs, Nests, and Baby Dinosaurs, all putative embryos discovered using x-rays have been misidentifications. This is because the use of x-rays to find embryos is conceptually flawed. Embryo bones are incompletely developed and will generally lack their own mineral content, as such the only source of minerals for these bones is the sediment that fills the egg after burial. The fossilized bones will therefore have the same density as the sediment filling the interior of the egg which served as the source for their mineral content and will be poorly visible in an x-ray image. So far the only reliable method for examining embryonic fossils preserved in dinosaur eggs is to physically extract them through means such as acid dissolution.^[55]

X-rays can be used to chemically analyze dinosaur eggshell. This technique requires pure shell samples, so the fossil must be completely free of its surrounding rock matrix. The shell must then be further cleaned by an ultrasonic bath. The sample can then be bombarded by electrons emitted by the same sort of probe used by scanning electron microscopes. Upon impact with the samples x-rays are emitted that can be used to identify the composition of the shell.^[56]

X-ray diffraction is a method for determining eggshell composition that uses X-rays to directly bombard powdered eggshell. Upon impact some of the x-rays will be diffracted at different angles and intensities depending on the specific elements present in the eggshell.^[56]

Footnotes

References

Wikimedia Commons has media related to Dinosauria eggs.

• Carpenter, Kenneth (1999). Eggs, Nests, and Baby Dinosaurs: A Look at Dinosaur Reproduction (Life of the Past), Indiana University Press; ISBN 0-253-33497-7.
• Deeming, D. C. and M. W. J. Ferguson (eds.) 1991. Egg incubation: its effect on embryonic development in birds and reptiles. Cambridge University Press, UK. 448pp.
• Glut, Donald F. (2003), "Appendix: Dinosaur Tracks and Eggs", Dinosaurs: The Encyclopedia. 3rd Supplement, Jefferson, North Carolina: McFarland & Company, Inc., pp. 613–652, ISBN 0-7864-1166-X 
• Horner, John R., Weishampel, David B. (1996) "A comparative embryological study of two ornithischian dinosaurs - a correction." "Nature" 383:256-257.
• Mateus, I, H Mateus, MT Antunes, O Mateus, P Taquet, V Ribeiro, G Manuppella 1998. Upper Jurassic theropod dinosaur embryos from Lourinhã (Portugal). Memórias da Academia das Ciências de Lisboa 37, 101–110.
• Moskvitch, Katia. "Eggs with the Oldest Known Embryos of a Dinosaur Found". BBC News. November 12, 2010.
• de Ricqlès, A., Mateus, O., Antunes, M. T., & Taquet, P. (2001). Histomorphogenesis of embryos of Upper Jurassic theropods from Lourinhã (Portugal). Comptes Rendus de l'Académie des Sciences-Series IIA-Earth and Planetary Science, 332(10), 647–656.
• Reisz, Robert R.; Diane Scott, Hans-Dieter Sues, David C. Evans, and Michael A. Raath (2005). "Embryos of an Early Jurassic prosauropod dinosaur and their evolutionary significance". Science 309 (5735): 761–764. Bibcode:2005Sci...309..761R.
• Skinner, Justin."ROM Puts Oldest Dinosaur Eggs Ever Discovered on Display". insidetoronto.com. May 6, 2010.
• "What are dinosaur eggs?", University of Bristol Earth Sciences, retrieved June 20, 2013