About 50 species of birds on Earth today do not belong to the same group as the other 10,000 currently in existence. Known as the paleognaths, this small group features many of the largest and heaviest bird specimens on our planet, bringing together ostriches and their distant South American relatives the rheas, as well as emus and cassowaries. Ground-dwelling kiwis and tinamous complete the family. None of these birds can fly, except for the tinamous. Paleognath eggs are also somewhat distinct from the rest of the bird population, being larger and sporting thicker shells. Advanced genetic analyses in the 2000s have upended researchers’ understanding of in what sequence these birds have evolved, and how they are related to each other. The new family tree for paleognaths offers the opportunity to re-evaluate previous conclusions about the group, which could in turn clarify the evolution and lifestyle of flightless modern and extinct dinosaurs.
Flightless or poorly-flighted modern, and recently extinct, paleognath birds with their respective egg sizes. Lithornis, an ancient flying paleognath bird, is depicted and has an unknown egg size. Image credit: Hyunjoo Shin / CC BY 4.0.
Non-avian dinosaurs became extinct at the end of the Cretaceous period, but avian dinosaurs, e.g. birds, are still extant and flourishing today as the most speciose land-vertebrate lineage. For example, their eggshell sizes, shapes, and pigment colors and patterns show phenomenal diversity.
Palaeognathae is one of two major clades of modern birds. Living paleognaths are usually larger than other birds and are flightless except for the poorly flighted tinamous.
Proportional to their body size, absolute sizes of their eggs and eggshells are usually large and thick, respectively.
Furthermore, paleognath eggshells show distinctive microstructures compared to those of other birds.
Species diversity of Palaeognathae is much lower than that of other birds, but it is critical for avian egg research.
Previous family trees focused on the physical characteristics of paleognaths such as emus, ostriches, and rhea.
But the new genetic information paired with other modern tools may provide even more information about how and why these creatures evolved their unique characteristics.
“We wanted to know if the three types of eggshells seen in paleognaths were inherited from a common ancestor or evolved independently,” said Dr. Seung Choi, a postdoctoral researcher in the Department of Earth Sciences at Montana State University.
Paleognath, neognath, and non-avian maniraptoran eggshells. Image credit: Choi et al., doi: 10.7554/eLife.81092.
Dr. Choi and colleagues used X-ray crystallography to analyze the microstructure of eggshells from modern paleognath birds, including the ostrich, rhea, emu, cassowary, kiwi, elephant bird, and two types of moa.
They also analyzed the structure of fossilized eggshells from this group of birds and a selection of fossilized eggs from bird-like dinosaurs.
They compared them with the eggshells of a few modern flighted birds, including the common pheasant, northern goshawk, European green woodpecker, Japanese quail, and common murre.
The analysis revealed that the wedge-like microstructures that characterize rhea eggshells trace back to ancient ancestors of paleognath birds.
However, the prism-like microstructures of ostrich and tinamous eggs likely evolved independently later.
“The pattern of evolution in paleognath eggshells we revealed may help ornithologists decipher the trajectory of eggshell evolution among this group of flightless birds,” Dr. Choi said.
“Genetic and microstructural analysis of these living fossils’ eggshells may provide new insights on the evolution of birds and dinosaurs,” said Montana State University’s Professor David Varricchio.
“It may also help us better understand how early humans lived and interacted with these fascinating creatures.”
The findings appear in the journal eLife.
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Seung Choi et al. 2023. Microstructural and crystallographic evolution of palaeognath (Aves) eggshells. eLife 12: e81092; doi: 10.7554/eLife.81092
