Episode 92 – Eggs

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Animals are many and varied, but one of the things nearly all of them share is that they start life off inside a small and special capsule complete with everything the growing embryo needs for protection and nutrition. In this episode, we discuss the diversity, evolution, and fossil history of Eggs.

In the news
Tiny reptile from Madagascar hints at the ancestry of dinosaurs and pterosaurs
New specimens provide a fresh look at the crested dinosaur Dilophosaurus
Possible evidence of a flight-related molting pattern in Microraptor
New extinct species of dolphin was a large apex predator

What’s an Egg?

Almost all animals start life in an egg, by which we mean a capsule that protects and nourishes the embryo (even before then, they start as an egg cell or ovum, the female gamete that becomes an embryo when combined with a male gamete). Eggs are about as diverse as the animals that lay them, but they tend to include one or more protective membranes and a yolk that provides food for the embryo. Eggs are often categorized by the amount of yolk inside.

Many invertebrate eggs only have a single membrane, the vitelline membrane. Insects eggs, like those pictured here, also have a protective shell known as the chorion. Image: Slashme [CC BY-SA 4.0]

Elasmobranch (sharks, rays, etc) eggs often come in leathery or rigid egg cases of a variety of strange shapes. Many are called “mermaid’s purses” due to their resemblance to a coin purse. Imae: horn shark (Heterodontus francisci) on the left, and swellshark (Cephaloscyllium ventriosum) on the right, by Jean [CC BY 2.0]

The eggs of frogs, pictured above, and many bony fish only have the vitelline membrane but are also surrounded by a gel-like layer for protection.  The gel layer dries out easily and thus these eggs must typically be laid in water. Image: Tarquin [CC BY-SA 3.0]

But the title of most complex and “advanced” eggs typically goes to amniotes, reptiles, birds, and mammals, whose namesake is the amniotic egg, consist of many different membranes that separate the interior of the egg into specialized compartments for food, waste, and embryo. These eggs also have a specialized shell that allows air to pass through but protects the embryo from drying out. In fact, this shell is considered one of the major innovations that allowed amniotes to spread across continental habitats.

Amniotic eggs can be soft-shelled like those of lizards and snakes or hard-shelled (mineralized with calcium carbonate) like those of crocodilians and birds. Bird eggs, especially, show a huge diversity of sizes and shapes. Recent research suggests that some of that variety is driven by how the birds fly!

Amniotic eggs act as an enclosed environment with ideal conditions for the growing embryo, including various structures for nutrition, respiration, and waste removal. Image by KDS4444 [CC BY-SA 4.0]

Eggs Through Time

Eggs have been around for a long time. And while some aspects of egg chemistry seem to evolve quickly, the overall shape and structure of eggs has remained pretty consistent. It works!

The fossil record of eggs goes back at least 600 million years, but it’s a mixed bag; hard-shelled eggs preserve very well, but soft-shelled eggs typically don’t. And it’s even rarer for eggs to preserve enough information to tell us what adult animal they belong to. But even with these challenges, we manage to learn quite a lot from fossil eggs.

Dinosaur eggs are among the most commonly-studied eggs in the fossil record. Their mineralized shells preserve quite well. Photo: Dinosaur eggs at the Kunming Natural History Museum of Zoology by Daderot [CC0]

If there’s an embryo inside a fossil egg, we can learn about developmental processes. Various studies on dinosaurs, including theropods, sauropodomorphs, and ornithischians have examined tooth development to reveal that different dinosaurs spent different amounts of time in their eggs.

Recent research has found chemical evidence of colored dinosaur eggshells, which suggests these eggs were laid in open nests and may have needed color for camouflage or to be better identified by the parent. Egg nests in general can tell us a lot about ancient parenting behaviors, and there have even been some exceptional cases where adult dinosaurs have been found associated with their own nests.

“Big Mama” is a famous specimen of Citipati found in what looks like a brooding position! Ironically, this dinosaur and other oviraptorids got their family name (ovi = egg; raptor = thief)  thanks to early paleontologists mistakenly thinking one of them was stealing eggs that were more likely its own! Ghedoghedo [CC0]

Recent research shows that early dinosaurs may have even laid soft shelled eggs, which might help explain why dinosaur eggs are only known from certain groups and certain time periods.

Many animals have famously stopped laying eggs and instead give birth to live young. This transition is surprisingly common; it appears to have evolved independently about 150 times across various groups of fish, reptiles, and mammals. Recent research into this transition includes studying living lizards that do both!

Extra Reading

Which came first: the chicken or the egg?
‘Baby Louie’, the dinosaur embryo from a giant egg
Has the importance of the amniote egg been overstated? (Technical paper)

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If you enjoyed this topic and want more like it, check out these related episodes:

• Episode 81 – Metamorphosis
• Episode 33 – Ontogeny
• Episode 37 – Evolution of Birds

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