dinosaur eggs

Dinosaur eggs are the organic vessels in which a dinosaur embryo develops. When the first scientifically documented remains of non-avian dinosaurs were being described in England during the 1820s, it was presumed that dinosaurs had laid eggs because they were reptiles. 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 (birds were not yet recognized as dinosaurs at the time).

The first scientifically recognized non-avian dinosaur egg fossils were discovered in 1923 by an American Museum of Natural History crew in Mongolia. 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.Skinner, Justi
"ROM Puts Oldest Dinosaur Eggs Ever Discovered on Display"
insidetoronto.com. May 6, 2010.Moskvitch, Katia
"Eggs with the Oldest Known Embryos of a Dinosaur Found"
BBC News. November 12, 2010.

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 Pouech; he thought, however, that they were laid by giant birds."First Discoveries," Carpenter (1999); page 5. 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."First Discoveries," Carpenter (1999); page 4. 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.

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. However, there are many kinds of naturally occurring objects which can resemble fossil eggs. These can fool even professional paleontologists.

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. Sometimes tiny dimples cover the surface of a stomach stone, which can fool observers into thinking they are the pores of an egg. Fossil egg expert Ken Carpenter has described stomach stones as the most egg-like natural objects, noting that they are "the trickiest gg-likeobjects to correctly identify". 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. 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. 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. Stomach stones also lack distinct shells with their attending structural components like continuous or prismatic layers, mammillae, and pores.

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 is egg-shaped. Most egg-shaped concretions have uniform interiors, however some form through the accumulation of mineral in layers. 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.

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.

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.

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

Structure

Paleontologists' knowledge of the structure of dinosaur eggs is limited to the hard shell. However, it can be inferred that dinosaur eggs had an amnion, chorion, and an allantois, the three major membranes in modern bird and reptile eggs. Dinosaur eggs vary greatly in size and shape, but even the largest dinosaur eggs ('' Megaloolithus'') are smaller than the largest known bird eggs, which were laid by the extinct elephant bird. Dinosaur eggs range in shape from spherical to highly elongated (some specimens three times longer than they are wide). Some elongated eggs are symmetrical, whereas others have one rounded end and one pointed end (similar to bird eggs). Most elongated eggs were laid by theropods and have an avian-like eggshell, whereas the spherical eggs typically represent non-theropod dinosaurs.

Fossil dinosaur eggshells, like modern bird and reptile eggshells, are made up of calcium carbonate crystal units. The basic arrangement and structure of these eggshell units (called the ultrastructure) is used to divide fossil eggs into several basic types, including the spherulitic, prismatic, and ornithoid basic types, which contain dinosaurs. Dinosaur eggs further classified by the microstructural aspects of the crystalline structure of the eggshell units and by the type of their pores and their shell ornamentation.

Layers

Dinosaur eggshells are divided into one, two, or three layers of distinct ultrastructure.

The innermost layer, known as the mammillary layer or the cone layer, is only found in theropod eggs (the prismatic and ornithoid basic types). It is composed of cone-shaped structures called mammillae at the base of each shell unit. Mammillae are the first part of the eggshell to form. Each mammilla forms from crystals radiating outward from an organic core until they touch neighboring mammillae and grow upwards into the next layer.Laura E. Wilson, Karen Chin, Frankie D. Jackson, and Emily S. Bray
II. Eggshell morphology and structure
''UCMP Online Exhibits: Fossil Eggshell'' In spherulitic eggs, the eggs of non-theropod dinosaurs, the eggshell units grow upward from their organic cores; the base of each eggshell unit is rounded, but is not a true mammilla because it does not have a distinct ultrastructure from the top of the unit.

The second layer is alternately called the prismatic layer, the columnar layer, the continuous layer, the crystalline layer, the cryptoprismatic layer,Simon, D. J. (2014).
Giant Dinosaur (theropod) Eggs of the Oogenus Macroelongatoolithus (Elongatoolithidae) from Southeastern Idaho: Taxonomic, Paleobiogeographic, and Reproductive Implications.
(Doctoral dissertation, Montana State University, Bozeman). the palisade layer, the spongy layer, or the single layer. In this layer, the shell units can be distinct, partially fused together, or entirely continuous. In some dinosaur eggs, the prismatic layer exhibits squamatic ultrastructure, where the prismatic structure is obscured by a rough texture resembling lizard skin.

Though rare in non-avian dinosaurs, some theropod eggs and most bird eggs have a third layer (known as the external layer) made up of vertical calcite crystals.

Pore canals

In all eggs, the embryo must breathe. In egg-laying amniotes (including dinosaurs), pore canals cutting through the eggshell allow gas exchange between the embryo and the outside world. Dinosaur eggshells exhibit a lot of diversity in pore size, density, and shape. One early attempt at classification of dinosaurian eggs, proposed by the Soviet paleontologist A. Sochava, was based on grouping eggs by their pore systems. This system was abandoned when it was discovered that different eggs could have very similar pores, but pore systems do play an important role in modern eggshell parataxonomy. The density and width of the pores, combined with the eggshell's thickness can be used to predict the gas conductance of a dinosaur's egg. This can provide both information about nesting behavior and about the climate: eggs buried in sediment have higher rates of gas conductance than those laid in the open, and eggs laid in arid environments have lower gas conductance (to prevent water loss) than those laid in more humid conditions.Laura E. Wilson, Karen Chin, Frankie D. Jackson, and Emily S. Bray
V. Paleobiology and eggs
''UCMP Online Exhibits: Fossil Eggshell''

Paleontologist and fossil egg expert Kenneth Carpenter catalogued six types of pore systems:
#Angusticanaliculate - Long, narrow, straight pores with low pore density. These eggs would have a low gas exchange rate, and therefore they were typically laid in dry areas.
#Tubocanaliculate - Large diameter pores with funnel-shaped openings on both inner and outer surfaces of the shell. These eggs would have a high gas exchange rate, and therefore were probably buried in humid mounds.
#Multicanaliculate - Numerous large, branching, and closely spaced pore canals. They have a high gas exchange rate, so like tubocanaliculate eggs they were probably also buried humid mounds.
#Prolatocanaliculate - Pores vary in width throughout their length. Gas exchange water loss rates are variable, so these eggs could have been laid in many different environments. This type is subdivided into foveocanaliculate with larger pore openings, and lagenocanaliculate with narrower pore openings.
#Rimocanaliculate - Very narrow slitlike pore canals. This pore system is seen in modern ostriches, so these eggs were laid in open nests, similar to how ostriches do today.
#Obliquicanaliculate - These canals cut diagonally through multiple eggshell units instead of going between them like in other pore systems. Obliquicanaliculate pores are only found in a single oogenus: ''Preprismatoolithus''.

Ornamentation

Unlike most modern eggs, many dinosaur eggs had a rough texture formed by nodes and ridges ornamenting the surface of their shell. This is predominant in Cretaceous dinosaur eggs, but very rare in eggs from the Jurassic or Triassic. Because of the lack of modern analogues, the purpose of eggshell ornamentation is unknown, but many functions have been proposed. Possibly, they provided extra strength to the eggshell without having pore canals too long for adequate gas exchange. They could also have helped keep substrate away from the pore openings of eggs that were buried, but modern turtles and crocodylians which bury their eggs have smooth eggshells, so this adaptation is not necessary for animals which bury their eggs. Another hypothesis, proposed by R. M. Mellon in 1982 in his senior thesis at Princeton University, is that the ridges and nodes would have formed pathways for gas to flow across the surface of the eggshell, preventing accumulation of too much CO₂ and aiding the flow of oxygen and water vapor.

Since it varies from egg to egg, the texture of an eggshell's ornamentation is useful for classification. Six types of ornamentation were catalogued by Carpenter in 1999:
#Compactituberculate - The dome-shaped tops of the shell units form a dense covering of nodes on the surface of the eggshell. This type of ornamentation is most commonly seen in megaloolithids.
#Sagenotuberculate - The nodes and ridges form a netlike pattern interspersed with pits and grooves.
#Dispersituberculate - Scattered nodes. This ornamentation is seen on the poles of elongated eggs, which may have allowed accumulations CO₂ at the poles to escape between the nodes.
#Lineartuberculate - Ridges, and chains of ridges and nodes form lines parallel to the long axis of the egg.
#Ramotuberculate - Irregular chains of nodes, typically found as a transition between the lineartuberculate midsection and dispersituberculate ends of elongated eggs.
#Anastomotuberculate - Ridges similar to lineartuberculate, but instead form wavy, branching, or anastomosing patterns resembling the water ripple marks in sand.

Classification

The classification of dinosaur eggs is based on the structure of the egg shells viewed in thin section via microscope, although new techniques such as electron backscatter diffraction have been used. There are three main categories of dinosaur eggs: spherulitic (sauropods and hadrosaurs), prismatic, and ornithoid (theropods, including modern birds).

Oogenera

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

* '' Ageroolithus''
* '' Apheloolithus''
* '' Boletuoolithus''
* '' Cairanoolithus''
* '' Continuoolithus''
* ''Dendroolithus''
* ''Dictyoolithus''
* '' Dispersituberoolithus''
* '' Ellipsoolithus''
* ''Elongatoolithus''
* '' Faveoolithus''
* ''Heishanoolithus''
* '' Laevisoolithus''
* ''Macroolithus''
* ''Macroelongatoolithus''
* '' Megaloolithus''
* '' Nanshiungoolithus''
* '' Oblongoolithus''
* ''Ovaloolithus

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