LPRP USP-0359, a near-complete fossilized egg of a Mesozoic bird. Image credit: Júlio Cesar. de A. Marsola et al.

The Mesozoic fossil record includes more than 120 species of birds found worldwide.
Over the last three decades, paleontologists have made important discoveries of fossil bird eggs including those of Gobipteryx and possible neognathid birds from Mongolia, ornithothoraces from Argentina, enantiornithines from China and bird-like theropods from Spain.
In contrast, the Brazilian record of Mesozoic birds is restricted to skeletal fragments of enantiornithine birds from the Upper Cretaceous Bauru Group of Minas Gerais and Sao Paulo, and two possible specimens from the Lower Cretaceous Crato Member of the Araripe Basin.
Now, Dr Julio Cesar de A. Marsola of the University of Sao Paulo and his colleagues have discovered a fossilized egg of a bird that lived in what is today Brazil during the Mesozoic era, between 252 and 66 million years ago.
The egg, labeled LPRP USP-0359, was recovered from Upper Cretaceous deposits of the Vale do Rio do Peixe Formation in the Bauru Group of Sao Paulo.
Although no remains were found inside it, the scientists’ extensive analysis revealed important information about both the egg itself and its wider context.
Their observations suggest that the egg is, in fact, one of the smallest and thinnest shelled Mesozoic bird eggs ever found.
It is slightly compressed, with its main axes measuring 3.14 cm x 1.95 cm.
The shell is 125.5 μm thick and externally smooth with rounded pore openings.
Similarities between the egg and specimens from Argentina suggest an affinity between them as Ornithothoraces, a group of birds that includes all enantiornithines and modern birds.
Given further similarities in where and how the eggs were found, the team suggests that the two birds may also have preferred the same types of breeding and nesting habitats – important clues that will help paleontologists build up a more detailed picture of South America’s Mesozoic past.

The discovery is detailed in a paper published online in the Alcheringa: an Australasian Journal of Palaeontology.

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