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allowing an embryo to breathe through pores in the eggshell. Thinking more ecu-
menically, though, I like to consider dinosaur eggs as traces of the mothers that de-
veloped them. By extension, then, every dinosaur body is a trace fossil of its moth-
er, and ultimately of both parents mating. But such thoughts might be a little too
metaphysical, so let's just go back to thinking about eggs as body fossils.
Despite their lowly status as body fossils, dinosaur eggs provide valuable
clues about dinosaur reproduction, brooding, and their surrounding environments,
which all supplement information provided by the dinosaur fossils that really mat-
ter—namely,theirtracefossils.Infact,themerepatternshownbyanumberofeggs
laid by a dinosaur mother in one egg-laying episode (known as a clutch) constitutes
a trace fossil in itself, regardless of whether a nest structure is associated with the
clutch or not. Or if a dinosaur parent actively pressed an egg against the side of a
nest and thus made an external mold of that egg, that would count as a trace fossil
of the parent, too.
Vertebrates that lay eggs today, such as reptiles, birds, and monotremes (egg-
laying mammals, such as the platypus and echidnas), are all classified as
amniotes
.
Thismeanstheyshareacommonancestorthatevolvedan
amnion
(asaccontaining
a developing embryo), which was enclosed by an egg. This novel trait likely deve-
lopedinresponsetowidespreadaridconditions,whichnaturallyselectedamphibian
ancestorscapableofputtingwateryconditionsinsideanegg,regardlessofwhatever
climate might have been outside of them. This new reproductive strategy evolved
invertebrates byabout320to330
mya
,orabout100million yearsbeforedinosaurs
existed.
Later in their evolutionary history, amniotes evolved myriad reproductive
strategies, including live birth which cut out the “middle egg” (so to speak) by de-
veloping an embryo inside the mother until it's ready to emerge, instead of trust-
ing this to all take place in an egg. We currently have no reasonable evidence that
dinosaurs gave birth to live young, but this trait (
viviparity
) showed up in ichthy-
osaurs—marine reptiles unrelated to dinosaurs—by the Early Jurassic, around 190
