Dinosaur eggs and nesting behaviors: A paleobiological investigation


Abstract

Although dinosaur eggs were first discovered and identified in the
late 1800s, limited attention was given to the scientific value of
oological fossils in contrast to observations based on skeletal
features. Here, we offer a review of Mesozoic saurischian egg
materials,
in comparison with extant crocodilians and avians, and their
paleobiological interpretation based either on the presence of
embryos in
ovo or brooding adults on egg clutches. Our study focuses on the
eggs of the oviraptorid Citipati osmolka (Mongolia), the troodontid
Troodon formosus (North America), the theropod oospecies
Macroelongatoolithus xixiaensis (China), the ornithothoracine bird
(Argentina), an indeterminate theropod (Thailand), and titanosaurs
(Argentina). Results show that (1) many oological characters and
reproductive behaviors associated with modern birds are rooted among
non-avian theropods, (2) there is a reproductive evolutionary
cline from crocodilians to modern birds with (3) a noticeable
pattern of coeval development between the accretion of eggshell
layers,
origination and size increased of larger air cells (inferred from
egg polar asymmetry), and brooding/incubating behaviors. Most of
these pre-adaptations are grouped in two main clades of the
saurischian cladogram: one at the level of Oviraptorosauridae and the
other at Troodontidae. Although undeniably these two theropod taxa
seem to represent two important phases for the evolution of avian
reproduction, the phylogenetic distance between these clades and
Titanosauria cannot be ignored. As such, the reproductive features
that appeared in concert in oviraptorids might have gradually
evolved across more basal theropod clades. Although Troodon formosus
by its egg shape and nesting behavior seems to be in this study the
precursors of modern avian reproduction, the importance of
smallbodied
theropods such as those who laid the Phu Phok eggs cannot be
dismissed and the eggs of such dinosaurs could suggest a closer
phylogenetic ties to Aves than troodontids. At a higher level of
inferences, there is a strong possibility that the evolution of these
reproductive features is concurrent with profound physiological and
metabolic changes that occurred in saurischian dinosaurs
throughout their evolution.