Dinosaur origin of egg color: oviraptors laid blue-green eggs

Abstract

Protoporphyrin (PP) and biliverdin (BV) give rise to the enormous diversity in avian egg coloration. Egg color serves several ecological purposes, including post-mating signaling and camouflage. Egg camouflage represents a major character of open-nesting birds which accomplish protection of their unhatched offspring against visually oriented predators by cryptic egg coloration. Cryptic coloration evolved to match the predominant shades of color found in the nesting environment. Such a selection pressure for the evolution of colored or cryptic eggs should be present in all open nesting birds and relatives. Many birds are open-nesting, but protect their eggs by continuous brooding, and thus exhibit no or minimal eggshell pigmentation. Their closest extant relatives, crocodiles, protect their eggs by burial and have unpigmented eggs. This phylogenetic pattern led to the assumption that colored eggs evolved within crown birds. The mosaic evolution of supposedly avian traits in non-avian theropod dinosaurs, however, such as the supposed evolution of partially open nesting behavior in oviraptorids, argues against this long-established theory. Using a double-checking liquid chromatography ESI-Q-TOF mass spectrometry routine, we traced the origin of colored eggs to their non-avian dinosaur ancestors by providing the first record of the avian eggshell pigments protoporphyrin and biliverdin in the eggshells of Late Cretaceous oviraptorid dinosaurs. The eggshell parataxon Macroolithus yaotunensis can be assigned to the oviraptor Heyuannia huangi based on exceptionally preserved, late developmental stage embryo remains. The analyzed eggshells are from three Late Cretaceous fluvial deposits ranging from eastern to southernmost China. Reevaluation of these taphonomic settings, and a consideration of patterns in the porosity of completely preserved eggs support an at least partially open nesting behavior for oviraptorosaurs. Such a nest arrangement corresponds with our reconstruction of blue-green eggs for oviraptors. According to the sexual signaling hypothesis, the reconstructed blue-green eggs support the origin of previously hypothesized avian paternal care in oviraptorid dinosaurs. Preserved dinosaur egg color not only pushes the current limits of the vertebrate molecular and associated soft tissue fossil record, but also provides a perspective on the potential application of this unexplored paleontological resource.

Peer Review #3 of "Fossil eggshell cuticle elucidates dinosaur nesting ecology (v0.1)"

“…As noted above, the discovery of protoporphyrin (Wiemann et al, 2017) in oviraptorid 407 dinosaur eggshells from all three localities where the eggshells for this study were collected 408 (Table 1) is consistent with the cuticle preservation. Although Wilson et al (2017) indicated that 409 pigment deposition and cuticle formation are not codependent, the preserved protoporphyrin 410 pigments was probably from the cuticle layer.…”

Non-Avian Dinosaur Eggshell Calcite Contains Ancient, Endogenous Amino Acids

“…Sedimentological descriptions noted that those eggs contacted a sandstone layer below them while entombed by mudstone, indicating that they were laid on the surface of sandy depressions and subsequently buried by flooding (Chiappe et al 2003. Non-avian dinosaur eggshell is known to contain endogenous biomolecules such as stable porphyrin pigments (Wiemann et al 2017). More extreme claims of biomolecular preservation have been proposed in Auca Mahuevo eggshells, where immunochemistry was used as evidence for intact protein or protein-derived organics across the eggshell cross-section, including inter-crystalline regions considered to be outside of the closed system calcite crystals (Schweitzer et al 2005).…”

Heme‐Derived Bilins

“…Many non-mammalian animals accumulate heme-derived bilins that impart color, and a large fraction are biliverdins, mostly the biliverdin IXa form. [10] The bluish or greenish colors of avian and dinosaur egg shells, [10][11] fish, [12] and lizards, [13] and frogs [14] are attributed to biliverdin. Other biliverdin isomers and bilins occur in insects, [15] helminths, [16] and marine snails.…”