Dinosaur egg colour had a single evolutionary origin

Abstract

Birds are the only living amniotes with coloured eggs1-4, which have long been considered to be an avian innovation1,3. A recent study has demonstrated the presence of both red-brown protoporphyrin IX and blue-green biliverdin5-the pigments responsible for all the variation in avian egg colour-in fossilized eggshell of a nonavian dinosaur6. This raises the fundamental question of whether modern birds inherited egg colour from their nonavian dinosaur ancestors, or whether egg colour evolved independently multiple times. Here we present a phylogenetic assessment of egg colour in nonavian dinosaurs. We applied high-resolution Raman microspectroscopy to eggshells that represent all of the major clades of dinosaurs, and found that egg colour pigments were preserved in all eumaniraptorans: egg colour had a single evolutionary origin in nonavian theropod dinosaurs. The absence of colour in ornithischian and sauropod eggs represents a true signal rather than a taphonomic artefact. Pigment surface maps revealed that nonavian eumaniraptoran eggs were spotted and speckled, and colour pattern diversity in these eggs approaches that in extant birds, which indicates that reproductive behaviours in nonavian dinosaurs were far more complex than previously known3. Depth profiles demonstrated identical mechanisms of pigment deposition in nonavian and avian dinosaur eggs. Birds were not the first amniotes to produce coloured eggs: as with many other characteristics7,8 this is an attribute that evolved deep within the dinosaur tree and long before the spectacular radiation of modern birds.

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LETTER https://doi.org/10.1038/s41586-018-0646-5
Dinosaur egg colour had a single evolutionary origin
Jasmina Wiemann1*, Tzu-Ruei Yang2 & Mark A. Norell3
Birds are the only living amniotes with coloured eggs
1–4
, which have
long been considered to be an avian innovation
1,3
. A recent study
has demonstrated the presence of both red-brown protoporphyrin
IX and blue-green biliverdin5—the pigments responsible for
all the variation in avian egg colour—in fossilized eggshell of
a nonavian dinosaur6. This raises the fundamental question of
whether modern birds inherited egg colour from their nonavian
dinosaur ancestors, or whether egg colour evolved independently
multiple times. Here we present a phylogenetic assessment of egg
colour in nonavian dinosaurs. We applied high-resolution Raman
microspectroscopy to eggshells that represent all of the major clades
of dinosaurs, and found that egg colour pigments were preserved
in all eumaniraptorans: egg colour had a single evolutionary
origin in nonavian theropod dinosaurs. The absence of colour in
ornithischian and sauropod eggs represents a true signal rather
than a taphonomic artefact. Pigment surface maps revealed that
nonavian eumaniraptoran eggs were spotted and speckled, and
colour pattern diversity in these eggs approaches that in extant birds,
which indicates that reproductive behaviours in nonavian dinosaurs
were far more complex than previously known3. Depth profiles
demonstrated identical mechanisms of pigment deposition in
nonavian and avian dinosaur eggs. Birds were not the first amniotes
to produce coloured eggs: as with many other characteristics7,8 this
is an attribute that evolved deep within the dinosaur tree and long
before the spectacular radiation of modern birds.
The huge diversity of avian egg colour
9
has previously been attributed
to the exploration of empty ecological niches after the extinction of
nonavian dinosaurs at the terminal Cretaceous event1. Different nesting
environments, as well as nesting behaviours, are thought to influence
egg colour10–12. Egg colour may reflect selective pressure as a result of
an ecological interaction between the egg producer and an egg pred-
ator (camouflage) or parasite (egg recognition). Avian egg colour has
previously been shown to react in a plastic fashion to changes in the
incubation strategy or climate, or even in mating behaviour
1,10,12–18
.
However, all previously proposed selective factors rely on the fact that
the eggs are exposed to the environment
10,11
and, with scant exception,
not buried or covered. More-recent research suggests that egg colour
may have co-evolved with (partially) open nesting habits in nonavian
dinosaurs6 but offers only a single data point of egg colour outside
crown birds, in open-nesting oviraptorid dinosaurs. Information on
eggshell pigments in a larger sample of nonavian dinosaurs is required
to understand the evolution of egg colour.
Both eggshell pigments—biliverdin and protoporphyrin IX—are
tetrapyrroles with minor structural differences that affect their chemi-
cal properties and their distribution across the eggshell
19–22
. In contrast
to the more hydrophilic biliverdin, which extends deep into the pris-
matic zone of the eggshell, the more hydrophobic protoporphyrin—
which causes spots and speckles—is restricted to the waxy cuticle21,23.
The different solubility properties of biliverdin and protoporphyrin
appear to be key to their preservation potential
6,21,22,24
. Protoporphyrin
is more resilient to elution than biliverdin but both pigments are
preserved in detectable trace amounts
6,24
. Eggshell pigments appear to
be restricted, if not bound, to the proteinaceous scaffold of the eggshell
matrix25.
Proteins transform during diagenesis into pyrrole-, pyridine- and
imidazole-rich polymers through oxidative crosslinking
26
; the resulting
protein fossilization products (PFPs) appear similar to biliverdin and
protoporphyrin IX in their chemical composition. Raman spectros-
copy distinguishes between true egg-colour pigments and pigment-like
PFPs
26
(Extended Data Fig.1), and identifies and maps out pigments
over eggshell surfaces and across vertical egg sections to characterize
colour patterns and pigment deposition in fossil eggs. Placing this
information in a phylogenetic context offers insights into whether
egg colour evolved once within nonavian dinosaurs or multiple times
independently, and might help to identify selective factors.
In our sample of nineteen archosaur eggshells, egg colour pigments
are absent in eggshells of Alligator mississipiensis, the North American
hadrosaurid Maiasaura peeblesorum, the South American saltasaurid,
the French titanosaurid and the North American troodontid (Fig.1,
Extended Data Figs.2, 3). Egg colour pigments are preserved in
eggshells from the oviraptorid Heyuannia huangi, Mongolian micro
-
troodontids, the Chinese and Mongolian troodontids, the dromaeo-
saurid Deinonychus antirrhopus, the Mongolian enantiornithine,
Psammornis rothschildi, Rhea americana, the North American ratite,
Dromaius novaehollandiae and Gallus domesticus (Fig.1, Extended
Data Figs.2, 3).
Only biliverdin was detected in D. novaehollandiae, whereas only
protoporphyrin IX was present in the eggshells of the Mongolian
microtroodontid (MAE 14-40(specimen codes in parentheses)), the
Chinese and Mongolian troodontids, the Mongolian enantiornithine,
P. rothschildi and G. domesticus. Both egg colour pigments were
detected in eggshells from H. huangi, the Mongolian microtroodon-
tid (IGM 100/1323) and macrotroodontid (AMNH FARB 6631),
D. antirrhopus, R. americana and the North American ratite. The
presence of eggshell pigments corresponds to (partially) open nesting
habits (Fig.1).
All eggshell and associated sediment samples were plotted on a whole
spectra-based principal component analysis (PCA) (Extended Data
Fig.4 and its Source Data). Principal component 1 (PC1, 57.118%)
represents variability in pigment type, concentration and mode of egg
-
shell alteration, whereas principal component 2 (PC2, 23.841%) sep-
arates samples into unpigmented and pigmented eggshells (Extended
Data Fig.5). The PCA (Extended Data Fig.4a) revealed that eggshell
biomolecules are distinct from organic material in the sediment, with
both clusters separating across PC1 (73.116%). Within the eggshell
cluster, extant and fossil materials are separated across PC2 (10.977%)
(Extended Data Fig.4a). A separate PCA (Extended Data Fig.4b) based
on the spectral fingerprint region of biliverdin and protoporphyrin IX
(1,500cm
−1
–1,650cm
−1
± 2cm
−1
) included all fossil eggshell sam-
ples: the resulting chemo-space identified a characteristic cluster of
pigmented eggshells, distinct from a separate cluster of unpigmented
eggshells. Mapping protoporphyrin IX on the eggshell surface (Fig.2a)
demonstrated that the eggs of H. huangi were spotted, as were those
of the Mongolian microtroodontids and troodontids, D. antirrhopus,
and the Mongolian enantiornithine. Reconstructions of the egg colours
are shown in Fig.2a.
Depth profiles (Fig.2b) across vertical eggshell sections show that
pigments are absent in all layers of the A. mississipiensis eggshell as
1Department of Geology & Geophysics, Yale University, New Haven, CT, USA. 2Steinmann Institute for Geology, Mineralogy, and Paleontology, University of Bonn, Bonn, Germany. 3Division of
Vertebrate Paleontology, American Museum of Natural History, New York, NY, USA. *e-mail: jasmina.wiemann@yale.edu
22 NOVEMBER 2018 | VOL 563 | NATURE | 555
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