Chapter 8 – Basic Information about Dinosaur Eggs and Nests
The Importance of Dinosaur Eggs and
Nests
Dinosaurs were oviparous, that is,
they reproduced through eggs formed within the body of a mother and laid on
land to hatch
Just like all birds, most reptiles,
and a few mammals
As opposed to being viviparous, or
giving live birth to offspring, typical of most mammals and a few reptiles
Developed slightly more than 300
million years ago and allowed amniotes to migrate within continental interiors
without depending on nearby water bodies
Dinosaurs eggs have been known
since the second half of the 19th Century, and Dinosaur nests since
the 1920’s
Mostly neglected until the late
1970’s
Renewed interest, originating from more
than 200 sites worldwide (mostly in Montana, France, Mongolia, China and
Argentina) with eggs and nests
Have identified dinosaur embryos
Have documented evidence for parental
care of hatchlings and juveniles
Dinosaur eggshells are valuable paleobiological
and paleoenvironmental indicators and have biogeochemical significance
Based on eggshell structure
Contain amino acids, oxygen and carbon
isotopes, trace elements
Dinosaur nests point toward
convergent behaviors among dinosaurs, crocodilians and birds
Dinosaur eggs and nests provide
information on dinosaurs in the absence or paucity of skeletal or track data
Dinosaur Sex
Dinosaur sexuality the subject of
much debate on the basis of very little scientific evidence
Sexual selection prior to mating
may be indicated for hadrosaurids (skull crests) and ceratopsians (horns and
frills) as well as for other groups that have bumps, knobs and hornlets on
their skulls
Presence of a pair of eggs in Sinosauropteryx and pairing of eggs in Troodon nests strongly suggest that theropods laid
eggs through paired oviducts
Arrangement of bones in pelvic
region may relate to muscles associated with reproductive organs or to
facilitate copulation
Dinosaur Eggs
Egg Definitions
Amniote Egg – enclosed yet
porous mineralized (calcium carbonate as calcite or aragonite) or organic
structure that contained or contains an AMNION (a fluid filled sac) surrounding
a developing embryo
Keeps nutrients in a restricted space
while allowing inflow of oxygen and exit of waste products through its pores
Eggs are body fossils; Nests are
trace fossils
Egg formation and development
Amnion forms around the embryo early
in development and its enclosing fluid suspends and protects the embryo from
concussion and drying
Eggshell is then secreted around the
embryo and a second sac (allantois) that serves as a respiratory organ for the
embryo develops between the seggshell and amnion
A yolk provides a food supply for the
embryo
Once a sufficiently protective shell
is secreted the egg is laid and further embryo development occurs outside the
mother’s body
Calcium for bone biomineralization is supplied by the eggshell
Dinosaur egg shapes (see Figure
8.4)
Dinosaur egg textures
Consequence of outward growth of calcite
or aragonite crystals perpendicular to the shell membrane surface (see figure
8.5)
Pores develop between crystals
Dinosaur egg pores are larger than
those of bird eggs and allow greater exchange of gases across the shell
Crystals in column layer often form ridges that channel gases along
the long axis of vertically oriented eggs
Probably reflects the fact that dinosaur eggs were covered with
vegetation or sediment
Dinosaur egg assemblages
Laid in clutches – a clutch is
one egg-laying episode
Two to as many as 35 eggs
Comparable to birds, but
not modern reptiles
Dinosaur egg brooding
Most birds remain near their clutches
and sit on the eggs for protection and insulation
Pretty strong evidence that Oviraptor and Troodon did and perhaps also Maiasaura
Some crocodilians stay near the nest
and help excavate any buried eggs, but most reptiles abandon the nest after the
eggs are laid
Egg Taphonomy
Pre-burial factors in egg
preservation
Egg structure - more mineralization
generally increases preservation potential
Biological condition of the egg -
infertile or unhatched eggs will remain whole
Nest type and surrounding environment
- covered eggs have higher preservation potential; the environment might be
erosional or depositional
Shell abnormalities - thin or weakened
eggs won’t preserve as well
Egg predation, scavenging, or stomping
- won’t be preserved
Weathering and erosion of eggshells
– slightly acidic rain or groundwater will dissolve eggshell carbonate
Burial factors in egg preservation
Buried by the mother or by sediment
from the associated environment
Type of sediment enclosing and filling
the egg
Fine grain sediment can compact and the egg will collapse, or expanding clay fills will expand
and fracture the egg from the inside
Sandy sediment is ideal for shape retention
Post-burial factors in egg
preservation
Compaction; Diagenesis of the egg and
cementation of adjacent sediment
Mineralized eggs have the greatest preservation potential generally, except in acidic environments
Molds might form
Dinosaur eggs occur in a narrow
stratigraphic range
Late Cretaceous horizons are the most
prolific
Early Cretaceous stata next most
abundant
Late Jurassic strata follow
Middle and Early Jurassic eggs are
extremely rare
Late Triassic – only one nest with
eggs from Argentina
Probably reflects increasing
mineralization over time
Might reflect paleontologists overlooking or misidentifying Late
Triassic, Early Jurassic
and Middle Jurassic eggs
Some dinosaur eggs or fragments are
index fossils in some areas of the world, and allow correlation of Late
Cretaceous strata of France and China
Classification of Egg Types
See Figure 8.6 and Table 8.1
Based on egg microstructure and egg
morphology
Major dinosaur clades are associated
with different egg taxa
Eggs containing embryos allow a egg to
be definitely assigned to a species or clade
Maiasaura, Hypacrosaurus, Troodon and Oviraptor
Sauropods, probably
titanosaurids, and
probable therizinosaurs
Oviraptor eggs
originally assigned to Protoceratops until discovery of clearly brooding Oviraptor on nest and embryos in eggs (see
display on 1st floor)
Troodon eggs
originally assigned to Orodromeus until discovery of embryos in eggs
Dinosaur Egg Biogeochemistry
Calcite maybe recrystallized
Stable oxygen and carbon isotopes
Oxygen isotope (O16 and O18)
ratios should reflect the temperature of the water from which the egg
precipitated
Such ratios in recrystallized eggs give the temperature of the water
in which recrystallization took place
Carbon isotope (C12 and C13)
ratios should reflect the diet of the mother dinosaur
Recrystallization complicates the interpretation of such ratios
Trace elements (elements other than
calcium, carbon, oxygen, hydrogen, nitrogen and phosphorus) present in minute
quantities
Excess and deficient amounts,
particularly in eggs approaching the Cretaceous-Tertiary boundary, have been
interpreted as evidence for the laying of unhealthy eggs
Recrystallization and diagenesis complicates the interpretation of
trace element data
Amino acids and proteins in eggs
Discovered in dinosaur eggshells in
1968
Quantities and quality similar to
those in chicken eggs
Potential contamination from bacteria, fungi, insects
or other organisms in the original egg or introduced from modern sources
Dinosaur Nests
Nest Definitions
A biogenic structure typically
containing a clutch, or any semicircular depression with a raised rim that was
originally used to hold eggs or young
Dinosaur nests so far were constructed
on the ground, not in trees
Parameters in describing nests
Types of eggs; distribution,
orientation and spatial relationships of eggs; geologic setting, including
stratigraphic unit and location and facies and paleoenvironments; type, shape
and size of the nests; and spatial relationships of nests to one another in
space and time
Nest types (see figure 8.8)
Decay of vegetation provides heat for
incubation in mound type
Mothers often moved eggs after laying
them as show by vertical orientation and patterns (concentric circles, spirals,
parallel rows and clusters)
Nest taphonomy
Preservation factors
Near water, Nesting colonies (large
numbers of nests in a small area) and site fidelity (same nesting site used
year after year)
Maiasaura and Hypacrosaurus nesting colonies preserved in Montana
Nests can be hard to detect if eggs
or juvenile bones are absent
Altricial vs Precocial
Maiasaura and Hypacrosaurus hatchlings are altricial and stay in the nest;
juvenile bones may get preserved,
but eggs are pulverized
Troodon
hatchlings are precocial and leave the nest early; juvenile
bones are not preserved, but
eggs are
Nests can be detected as
bioturbation structures that contrast with surrounding sediment