Friday 12 December 2014

A fossil Bird’s egg from the Late Cretaceous of São Paulo State, Brazil.

Birds evolved from Theropod Dinosaur ancestors in the Jurassic, and have a fairly extensive Mesozoic fossil record, with around 120 species described from around the world. The fossil record of Bird’s eggs is much more limited (unlike that of larger Dinosaurs), though fossil Bird eggs are known from the Mesozoic of Mongolia, China, Argentina and possibly Spain. The fossil record of Mesozoic Birds in Brazil, is very limited, comprising a few fragmentary specimens and feathers, not all of which are universally accepted as being of avian origin; no Bird eggs have been recorded from the Mesozoic of Brazil to date.

In a paper published in the journal Alcheringa on 11 June 2014, Júlio Marsola of the Laboratório de Paleontologia de Ribeirão Preto at the Universidade de SãoPaulo, Gerald Grellet-Tinner of the Centro Regional de Investigaciones La Rioja at the Consejo Nacional de Investigaciones Científicas y Técnicas, FelipeMontefeltro of the Departamento de Zoologia at the Universidade Estadual Paulista, Juliana Sayão of the Laboratório de Diversidade do Nordeste, Núcleo de Biologia at the Universidade Federal de Pernambuco and Annie Schmaltz Hsiou and Max Langer, also of the Laboratório de Paleontologia de Ribeirão Preto at the Universidade de São Paulo, report the discovery of a Bird’s egg from the Late Cretaceous Vale do Rio do Peixe Formation in São Paulo State, Brazil.

The egg is almost complete, but slightly compressed, which probably occurred during burial, and which is probably responsible for it not being an obvious egg shape. It is 31.4 mm long and 19.5 mm wide at its widest point.

A nearly complete eggfrom the Late Cretaceous Vale do Rio do Peixe Formation in São Paulo State, Brazil. Marsola et al. (2014).

The shell of the egg is 125 μm thick, with a very smooth surface and rounded pore openings. This shell is calcite, and appears to be a good representation of the original structure, though some re-crystallization has probably occurred. It is made up of three shell layers, L1, L2 and L3, with L1 averaging 38 μm thick, L2 42 μm thick and L3 45.5 μm thick. Layers L1 and L2 appear to have been laid down horizontally, while L3 (the outermost layer, which would have been laid down last) has crystals arranged in columns.

SEM of the shell in radial section. Black arrow points tothe rounded pore opening. Marsola et al. (2014).

The shell is very thin compared to that of modern Neognath Birds such as Ducks or Chickens, which have shells of about 500 μm in thickness, or Palaoegnath Birds such as Rheas, which can have shells as thick as 10 mm (small modern Passerine Birds have thinner shells, but Mesozoic Birds as small as small Passerines are unknown, and the eggs of small Passerines are themselves much smaller). Modern Palaeognath Birds typically have a two-layered shell, third and fourth layers may be present, but when they are are much thinner than the first two layers. Modern NeognathBirds typically produce a three-layered shell, but in these eggs the middle layer (L2) is much thicker than the other two layers. A previously described Bird’s egg from Brazil had a three-layered egg in which L1 was 92.9 μm thick, L2 58.7 μm thick and L3 14.4 μm thick, and Bird eggs have also been reported from Argentina in which L1 and L2 were much thicker than L3. While Theropod eggs of possible avian affinity from Spain had only two layers. This suggests that the Brazilian egg may come from a Bird group with no prior fossil eggs recorded, though there is no way of confirming this.

SEM of the shell in radial section. Arrow indicates a spherulite core. Note also the delimitation of the shell units and the gradual (prismatic) contact between L1 and L2, and the abrupt (aprismatic) contact between L2 and L3. White dashed lines separate the three structural layers, and a blackdashed line indicates the boundary of shell units. Marsola et al. (2014).

See also…

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The dominant group of Birds during much of the Cretaceous Period were the Enatiornithine, toothed Birds related, but not ancestral, to modern Birds. The largest known species of Enatiornithine from the Early Cretaceous is Pengornis houi, described from a...

The two surviving groups of Archosaurs, Crocodilians and Birds, show very different reproductive strategies.  Crocodilians have paired ovaries and produce large clutches of small eggs, which mature slowly prior to being laid, and make a relatively small investment in parental care. Birds have only a single ovary, a...

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