Reptile egg fossil
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Reptile egg fossils are the
fossilized A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved in ...
remains of eggs laid by reptiles. The fossil record of reptile eggs goes back at least as far as the
Early Permian 01 or '01 may refer to: * The year 2001, or any year ending with 01 * The month of January * 1 (number) Music * '01 (Richard Müller album), 01'' (Richard Müller album), 2001 * 01 (Son of Dave album), ''01'' (Son of Dave album), 2000 * 01 (Urban ...
. 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
crocodilia Crocodilia (or Crocodylia, both ) is an order of mostly large, predatory, semiaquatic reptiles, known as crocodilians. They first appeared 95 million years ago in the Late Cretaceous period ( Cenomanian stage) and are the closest living ...
ns,
dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...
s, and turtles. Some ancient reptiles are known to have given live birth and are therefore not anticipated to have left behind egg fossils.


Evolution and diversity

The earliest reptile eggshells probably had leathery membranes instead of hard shells. Eggs like this decay so quickly that fossilization is very unlikely. Therefore the fossil record is too incomplete for scientists to determine what kinds of eggshell most fossil reptile groups had. A 5.9 cm by 3.79 cm fossil from the Lower Permian was described in 1939 by Alfred Romer and Lewellyn Price as the oldest hard-shelled fossil egg. However, in a 1979 paper Karl Hirsch disputed Romer and Price's claim, since the fossil in question didn't show evidence for a
calcite Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...
shell. Hirsch found enough
phosphorus Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ear ...
in the object's outer layer to propose that the fossil was actually an egg with a leathery shell. If the contentious fossil really is a reptile egg, it's the oldest known.


Reptile groups with documented fossil eggs


Crocodilians

The earliest known fossil rigid eggs were laid by
South Africa South Africa, officially the Republic of South Africa (RSA), is the southernmost country in Africa. It is bounded to the south by of coastline that stretch along the South Atlantic and Indian Oceans; to the north by the neighbouring countri ...
n crocodilians during the
Late Triassic The Late Triassic is the third and final epoch (geology), epoch of the Triassic geologic time scale, Period in the geologic time scale, spanning the time between annum, Ma and Ma (million years ago). It is preceded by the Middle Triassic Epoch ...
or Early Jurassic; however, the rigid eggshell itself was probably much older than these specific fossils. The oldest known fossil crocodilian eggs are from Early Cretaceous rocks of Galve, Spain. These eggs had microstructures identical to those of modern crocodiles. This suggests that once the crocodilian's rigid eggshell first evolved it changed very little over time. Crocodilian eggshell is regarded as primitive because it only contains one type of calcite crystal and these crystals are arranged in "tight columns".


Dinosaurs

The eggs of the earliest dinosaurs are poorly known.
Dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...
eggshell was so resilient that it is the most common kind of egg fossils. Some theropod dinosaurs' eggshell microstructure are so similar to modern birds that its almost impossible to distinguish the two. These very bird-like eggs are only referred to non-avian dinosaurs due to their size and age. Bird eggshell microstructure is complex and more similar to theropod dinosaurs than to crocodilians. The commonality in eggshell structure is more evidence that birds evolved from theropods.


Turtles

The oldest known semi-rigid fossil eggshells date to the
Middle Jurassic The Middle Jurassic is the second epoch of the Jurassic Period. It lasted from about 174.1 to 163.5 million years ago. Fossils of land-dwelling animals, such as dinosaurs, from the Middle Jurassic are relatively rare, but geological formations co ...
of England. The egg specimen was originally called '' Oolithes bathonicae'', but renamed '' Testudoflexoolithus''. There are also semi-rigid turtle eggshells known from the Late Jurassic
Morrison Formation The Morrison Formation is a distinctive sequence of Late Jurassic, Upper Jurassic sedimentary rock found in the western United States which has been the most fertile source of dinosaur fossils in North America. It is composed of mudstone, sandsto ...
of the western US. Fully rigid turtle eggshell is known from Late Jurassic strata in Portugal. Fossil turtle eggs are known from the marine White Limestone,
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, and
Mooreville Chalk The Mooreville Chalk is a geological formation in North America, within the U.S. states of Alabama and Mississippi, which were part of the subcontinent of Appalachia. The strata date back to the early Santonian to the early Campanian stage of ...
formations. No modern reptile is known to lay their eggs on the seafloor, so these eggs likely originated in a different environmental setting. Ken Carpenter notes that while it's not possible to be completely sure how these eggs ended up on the seabed, some possibilities include carcasses of dead
gravid In biology and human medicine, gravidity and parity are the number of times a woman is or has been pregnant (gravidity) and carried the pregnancies to a viable gestational age (parity). These terms are usually coupled, sometimes with additional te ...
females being washed out to sea and releasing its eggs when by splitting open, carried out to sea by floods, or drifting out on mats of vegetation. At least two clutches of hard-shelled turtle eggs preserved in volcanic ash are known from the
Canary Islands The Canary Islands (; es, Canarias, ), also known informally as the Canaries, are a Spanish autonomous community and archipelago in the Atlantic Ocean, in Macaronesia. At their closest point to the African mainland, they are west of Morocc ...
. These eggs were likely buried in the debris by large tortoises.


See also

*
Cephalopod egg fossil Cephalopod egg fossils are the fossilized remains of eggs laid by cephalopods. The fossil record of cephalopod eggs is scant since their soft, gelatinous eggs decompose quickly and have little chance to fossilize. Eggs laid by ammonoids are the be ...
*
Fish egg fossil Fish egg fossils are the fossilized remains of fish eggs. Fossil fish eggs have an extensive record going at least as far back as the Devonian and spanning into the Cenozoic era. The eggs of many different fish taxa have contributed to this reco ...
* Timeline of egg fossil research *


Footnotes


References

* Carpenter, Kenneth (1999). ''Eggs, Nests, and Baby Dinosaurs: A Look at Dinosaur Reproduction (Life of the Past)'', Indiana University Press; . {{Eggs .