Egg paleopathology

Last updated
Hypselosaurus priscus egg. Hypselosaurus egg 2.jpg
Hypselosaurus priscus egg.

Egg paleopathology is the study of evidence for illness, injury, and deformity in fossilized eggs. A variety of pathological conditions afflicting eggs have been documented in the fossil record. Examples include eggshell of abnormal thickness and fossil eggs with multiple layers of eggshell. The identification of egg paleopathologies is complicated by the fact that even healthy eggs can be modified during or after fossilization. Paleontologists can use techniques like cathodoluminescence or thin sectioning to identify true paleopathologies in fossil eggs. Despite the diversity of paleopathologies known from fossil eggs, the vast majority of conditions known to afflict modern eggs have not yet been seen among fossils.

Contents

Identification

Differences in preservation and diagenesis make it difficult to confidently identify eggshell pathologies in the fossil record. Multilayered eggshell identification can be complicated if the secondary eggshell is disconnected from the original. When this happens the two shells can vary in structure and may even be mistaken as coming from different types of eggs. [1] Stacked eggshells from collapsed or compressed eggs may resemble multilayered eggshells but in thin cross-sections viewed under a microscope the stacked shell will have mammilae facing opposite directions whereas egg shells that developed in layers on top of each other will have mammilae facing the same direction. [2] Also, in stacked shells the layer separating the shells will resemble the sediment surrounding the fossil and in multilayer the layer between shells will resemble the primary shell more than the surrounding sediment. [3]

Cathodoluminescence can be used to distinguish pathological egg shell from egg shell that has been altered diagenetically. Rigid egg shells, except in turtles, are composed of calcite. Since calcitic and aragonitic structures do not luminesce, signs of luminescence point to alterations to the chemical composition or structure of the shell. Magnesium-Calcite is sometimes replaced by Manganese-Calcite, which luminesces bright red-orange or yellow orange. In multilayer eggs the shells will appear the same color, but the shell membrane between them tends to be replaced by manganese-calcite that luminseces bright orange. Manganese is the primary instigator of luminescence whereas iron hinders it. [4]

Known paleopathologies

Abnormal shell thickness

Abnormally thin eggshell can allow excessive evaporation to dehydrate the embryo or shell membrane. Dehydrated membranes have a negative impact on gas permeability of the egg. Eggshell pathology can cause the shell to be so thin that the egg collapses. [5] Abnormally thin eggshell has been attributed to Hypselosaurus priscus and some experts have speculated that this was the cause of the species' extinction, with vegetation changes, climatic change and overcrowding being the original impetus for the shell thinning. However, there are alternative explanations for the thin eggshell not dependent on pathology. Later researchers found resorption craters in the basal caps at the base of the columns, meaning that the eggs hatched. Some researchers postulated that the thinner " Hypselosaurus priscus " eggshells came from different taxa than the thicker eggshells, and subsequent researchers have come to support this idea. Another potential explanation for variation in eggshell thickness is that the thinner eggs were laid by younger individuals than older ones. There are also natural variations of eggshell thickness within a single species. [6] Stressful environmental conditions may have resulted in dinosaur egg shells thinning. This may have played a role in dinosaur extinction, but is a controversial subject. [7]

Multilayered shell

Multilayered eggs are known from the fossil record and were first reported from the Late Cretaceous of France and later India and Argentina. More recent discoveries of this pathology have been in Late Cretaceous strata in Montana and Late Jurassic strata in Utah. [7] Multilayered eggs can cause embryos to suffocate as the extraneous layer's pore canals won't line up well enough with the original layers' to allow gas to travel to the embryo. [8] Multilayered dinosaur eggs are known from, in order of discovery, France, Spain, Mongolia, India, Argentina, Canada, Montana, and Utah. [9] Most multilayered dinosaur eggs are of the megaloolithid oofamily with a discretispherulitic morphotype. Other types of fossil eggs with these pathologies are prismatic, filispherulitic, dendrospherulitic, and prolatospherulitic morphotypes. Multilayered fossil eggs resemble those of modern forms in sometimes having incomplete extra layers and pore canals that don't properly align. The shell membranes of these eggs have been either dissolved or been replaced with secondary calcite. In the pathological specimens from Spain and Montana the redundant shell layer is as thick as in the original. In the specimen from Alberta it is only three fourths of the thickness of the original. The Utah specimen's pathological layer is only half that of the original. The egg is split open but still connected at one side. Some aspects of this egg suggests it was still in its mother's body when it was buried. [10] The term ovum in ovo has been used for multilayered dinosaur eggs although this is inaccurate use of the term. Pathologies of eggshell are difficult to recognize in fossil specimens. [11] Multilayered eggs are most common in the discretispherulitic egg morphotype and less common in others. This type of egg is attributed to sauropods. The greater abundance may indicate that these eggs were more prone to such pathologies, but are most likely due to a larger sample size of them. The pathological egg still inside its mother from Utah is a unique occurrence. [12]

Oligocene strata from West Germany have produced 27 multilayered gecko eggs. Another multilayered egg from West Germany is not a gecko egg. The shells of these eggs were broken in the typical fashion of hatching eggs, interpreted by the original describers as meaning that the pathological eggshell was not fatal to the developing embryos. In a 2001 survey of fossil eggshell pathologies, Karl F. Hirsch criticized these authors on the basis that multilayered eggs were "very unlikely" to allow embryos to hatch because even if the pores of the secondary shell layer were perfectly aligned with those of the primary layer the eggshell would still be too thick for the struggling neonate to break free. [10]

Extraspherulitic growth units are sometimes found in otherwise normal eggs. In Jurassic specimens from Colorado these are rare, but occupy almost the entire shell layer in specimens obtained from Late Cretaceous Montana. [13] Egg shell has been recovered from the Milk River area of Alberta which may have been partially dissolved and then had additional calcite deposited on it. [14]

Undocumented pathologies

Irregularities in the surface of an egg and unusual egg shapes can be caused by convulsions or contractions of the uterus. Eggs can be wrinkled, bulged, ridged, or have nodules. Eggs can be restricted, bound, or truncated. These pathological eggs often have shell units and internal microstructures that are not interlocked as tightly as those of healthy eggs. These phenomena have not been found in fossil eggs. [15] Ovum in ovo has low preservation potential and has not yet been observed in the fossil record. [7] The term ovum in ovo has been used for multilayered dinosaur eggs although this is inaccurate use of the term. Pathologies of eggshell are difficult to recognize in fossil specimens. [11] Hirsch concludes that the majority of pathological phenomena known to occur in modern eggs are not represented in the fossil record. [16]

See also

Footnotes

  1. "Multilayered and Stacked Eggshell," Hirsch (2001); page 386.
  2. "Multilayered and Stacked Eggshell," Hirsch (2001); pages 386–387.
  3. "Multilayered and Stacked Eggshell," Hirsch (2001); page 387.
  4. "Pathologically and Diagenetically Altered Eggshell," Hirsch (2001); page 387.
  5. "Thin Eggshell," Hirsch (2001); page 380.
  6. "Thin Eggshell," Hirsch (2001); page 381.
  7. 1 2 3 "Abstract," Hirsch (2001); page 378.
  8. "Thick Eggshell," Hirsch (2001); page 382.
  9. "Thick Eggshell," Hirsch (2001); pages 382–383.
  10. 1 2 "Thick Eggshell," Hirsch (2001); page 383.
  11. 1 2 "Thick Eggshell," Hirsch (2001); page 384.
  12. "Discussion," Hirsch (2001); page 389.
  13. "Extraspherulitic Growth Units," Hirsch (2001); page 385.
  14. "Extraspherulitic Growth Units," Hirsch (2001); pages 385–386.
  15. "External Abnormalities," Hirsch (2001); page 379.
  16. "Discussion," Hirsch (2001); pages 387–389.

Related Research Articles

<i>Hypselosaurus</i> Extinct genus of reptiles

Hypselosaurus is a dubious genus of titanosaurian sauropod that lived in southern France during the Late Cretaceous, approximately 70 million years ago in the early Maastrichtian. Hypselosaurus was first described in 1846, but was not formally named until 1869, when Phillip Matheron named it under the binomial Hypselosaurus priscus. The holotype specimen includes a partial hindlimb and a pair of caudal vertebrae, and two eggshell fragments were found alongside these bones. Because of the proximity of these eggshells to the fossil remains, many later authors, including Matheron and Paul Gervais, have assigned several eggs from the same region of France all to Hypselosaurus, although the variation and differences between these eggs suggest that they do not all belong to the same taxon. Hypselosaurus has been found in the same formation as the dromaeosaurids Variraptor and Pyroraptor, the ornithopod Rhabdodon, and the ankylosaurian Rhodanosaurus, as well as indeterminate bones from other groups.

<span class="mw-page-title-main">Dinosaur egg</span> Vessel for dinosaur embryo development

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.

<i>Elongatoolithus</i> Fossil dinosaur eggs

Elongatoolithus is an oogenus of dinosaur eggs found in the Late Cretaceous formations of China and Mongolia. Like other elongatoolithids, they were laid by small theropods, and were cared for and incubated by their parents until hatching. They are often found in nests arranged in multiple layers of concentric rings. As its name suggests, Elongatoolithus was a highly elongated form of egg. It is historically significant for being among the first fossil eggs given a parataxonomic name.

<i>Macroolithus</i> Oogenus of dinosaur egg

Macroolithus is an oogenus of dinosaur egg belonging to the oofamily Elongatoolithidae. The type oospecies, M. rugustus, was originally described under the now-defunct oogenus name Oolithes. Three other oospecies are known: M. yaotunensis, M. mutabilis, and M. lashuyuanensis. They are relatively large, elongated eggs with a two-layered eggshell. Their nests consist of large, concentric rings of paired eggs. There is evidence of blue-green pigmentation in its shell, which may have helped camouflage the nests.

Continuoolithus is an oogenus of dinosaur egg found in the late Cretaceous of North America. It is most commonly known from the late Campanian of Alberta and Montana, but specimens have also been found dating to the older Santonian and the younger Maastrichtian. It was laid by an unknown type of theropod. These small eggs are similar to the eggs of oviraptorid dinosaurs, but have a distinctive type of ornamentation.

Ageroolithus is an oogenus of dinosaur egg. It may have been laid by a theropod.

Dispersituberoolithus is an oogenus of fossil egg, which may have been laid by a bird or non-avian theropod.

Megaloolithidae is an oofamily of fossil egg of the Dinosauroid-spherulitic morphotype. They probably are the eggs of sauropods.

<span class="mw-page-title-main">Timeline of egg fossil research</span>

This timeline of egg fossils research is a chronologically ordered list of important discoveries, controversies of interpretation, taxonomic revisions, and cultural portrayals of egg fossils. Humans have encountered egg fossils for thousands of years. In Stone Age Mongolia, local peoples fashioned fossil dinosaur eggshell into jewelry. In the Americas, fossil eggs may have inspired Navajo creation myths about the human theft of a primordial water monster's egg. Nevertheless, the scientific study of fossil eggs began much later. As reptiles, dinosaurs were presumed to have laid eggs from the 1820s on, when their first scientifically documented remains were being described in England. 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, however he thought they were laid by giant birds.

<span class="mw-page-title-main">Egg taphonomy</span> Study of the decomposition and fossilization of eggs

Egg taphonomy is the study of the decomposition and fossilization of eggs. The processes of egg taphonomy begin when the egg either hatches or dies. Eggshell fragments are robust and can often travel great distances before burial. More complete egg specimens gradually begin to fill with sediment, which hardens as minerals precipitate out of water percolating through pores or cracks in the shell. Throughout the fossilization process the calcium carbonate composing the eggshell generally remains unchanged, allowing scientists to study its original structure. However, egg fossils buried under sediments at great depth can be subjected to heat, pressure and chemical processes that can alter the structure of its shell through a process called diagenesis.

Reptile egg fossils are the fossilized remains of eggs laid by reptiles. The fossil record of reptile eggs goes back at least as far as the Early Permian. However, since the earliest reptile eggs probably had soft shells with little preservation potential, reptilian eggs may go back significantly farther than their fossil record. Many ancient reptile groups are known from egg fossils including crocodilians, dinosaurs, and turtles. Some ancient reptiles are known to have given live birth and are therefore not anticipated to have left behind egg fossils.

<span class="mw-page-title-main">Elongatoolithidae</span> Oofamily of dinosaur eggs

Elongatoolithidae is an oofamily of fossil eggs, representing the eggs of oviraptorosaurs. They are known for their highly elongated shape. Elongatoolithids have been found in Europe, Asia, and both North and South America.

Guegoolithus is an oogenus of fossil egg from the early Cretaceous of Spain. It is classified in the oofamily Spheroolithidae, and was probably laid by an ornithopod dinosaur.

Undulatoolithus is an oogenus of Chinese fossil dinosaur egg belonging to Elongatoolithidae. It is very similar to Macroolithus, but has different ornamentation. Like other elongatoolithids, it was probably laid by oviraptorosaurs.

Dictyoolithidae is an oofamily of dinosaur eggs which have a distinctive reticulate organization of their eggshell units. They are so far known only from Cretaceous formations in China.

Triprismatoolithus is an oogenus of dinosaur egg native to Teton County, Montana. It is classified in the oofamily Arriagadoolithidae, the eggs of alvarezsaurs.

Nipponoolithus is an oogenus of fossil egg native to Japan. It is one of the smallest known dinosaur eggs, and was probably laid by some kind of non-avian maniraptor.

Incognitoolithus is an oogenus of medioolithid fossil bird egg. It is notable for bearing evidence of predation, possibly from a bird pecking the eggshell.

<i>Gobioolithus</i> Fossil bird egg native to Mongolia

Gobioolithus is an oogenus of fossil bird egg native to Mongolia. They are small, smooth-shelled, and elongated eggs that were first discovered in the 1960s and early 70s during a series of fossil-hunting expeditions in the Gobi desert. Two oospecies have been described: Gobioolithus minor and G. major. The eggs were probably laid in colonial nesting sites on the banks of rivers and lakes.

Pachycorioolithus is an oogenus of small, thin-shelled fossil egg from the early Cretaceous in China. It probably belongs to a bird, though there is a possibility the parent was a non-avian theropod. It was named in 2016, based on a single specimen found in Zhejiang.

References