Geoscience Reference
In-Depth Information
million years ago. Live birth in ichthyosaurs made sense evolutionarily, because it
kept ichthyosaur mothers in the seas instead of having to lumber up on land to dig
nests and lay eggs. Although that strategy has worked very well for sea turtles for
thepast100millionyears,itapparentlywasselectedoutofichthyosaursearlyonin
their evolutionary history. Viviparity also occurs in some modern species of lizards
and snakes, including sea snakes; the latter shows how other marine reptiles came
up with the same reproductive mode.
The oldest known dinosaur eggs come from Early Jurassic rocks, or about the
same time ichthyosaurs were using live birth. Dinosaurs, however, were very likely
laying eggs in the Late Triassic Period, too; but these may have been soft, leath-
ery eggs that were not as easily preserved as the Jurassic ones. The Early Jurassic
fossil eggs, which were preserved because of calcite in their shells, give us a min-
imumtimeforwhendinosaurshaddevelopedmineraizedeggs.Basically,thismade
eggshells that went from “squish” to “crunch” when another dinosaur stepped on
them. Hence, the eggs we are sofamiliar with inoureveryday lives, thanks tothose
mostfamousofmoderndinosaurs(chickens),gottheiroriginsrelativelyearlyinthe
evolutionary history of dinosaurs.
Despite the fact that dinosaur eggs are technically considered body fossils,
paleontologists were still motivated enough to devise a classification scheme that
helped distinguish different types of eggs. Paleontologists, using a system parallel
to a biological classification, came up with
ootaxonomy
, in which the “oo” prefix
refers to eggs. These categories, based on shell microstructure, arrangements of
the pores, and overall forms, were given linguistically daunting names such as
Spheroolithidae, Ovaloolithidae, Megaloolithidae, Dendroolithidae, Faveoloothid-
ae, and at least a half dozen others. Despite such names causing computer spell-
checkers to run and hide, they are essential for the small and dedicated group of
dinosaur-egg paleontologists to better communicate with one another.
Unfortunately, these egg categories do not always exactly match clades of di-
nosaurs. Although we can be reasonably sure that theropods, sauropods, ornitho-
