Tracing the Origin & Evolution of Bird Reproduction in Dinosaurs

What really did come first--the chicken or the egg? Birds' reproductive biology is dramatically different from that of any other living vertebrates, and ornithologists and paleontologists have long wondered how and when the unique features of bird reproduction originated. A new Review in The Auk: Ornithological Advances examines answers from three sources--modern birds, fossils of primitive birds, and fossils of the dinosaurs from which birds are descended--to shed new light on the subject.

All modern birds share certain reproductive features, such as a single functional ovary and the practice of incubating their eggs through direct contact. Analysis of the bird family tree also suggests that early birds built simple, open nests on the ground and that their young were "precocial," meaning they were well-developed and almost ready to fend for themselves when they hatched. Those dinosaurs close to the ancestry of birds shared some of these traits, but they had two functional reproductive tracts, and their eggs were smaller relative to their body size and more elongated than those of modern birds.

Fossils of primitive birds and eggs from the Mesozoic era place them midway between their dinosaur ancestors and their modern descendants, with eggs between those of pre-avian dinosaurs and modern birds in term of size and shape. In this way, David Varricchio and Frankie Jackson of the Montana State University are able to trace the evolution of bird reproduction through a series of distinct stages, from pre-avian dinosaurs to the birds of today.

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Determining Dinosaur Endothermy Based on the Temperature the Egg Shells Formed

Were dinosaurs really fast, aggressive hunters like the ones depicted in the movie "Jurassic World"? Or did they have lower metabolic rates that made them move more like today's alligators and crocodiles? For 150 years, scientists have debated the nature of dinosaurs' body temperatures and how those temperatures influenced their activity levels.

New research by UCLA scientists indicates that some dinosaurs, at least, had the capacity to elevate their body temperature using heat sources in the environment, such as the sun. They also believe the animals were probably more active than modern-day alligators and crocodiles, which can be active and energetic, but only for brief spurts.

The researchers also found evidence that other dinosaurs they studied had lower body temperatures than modern birds, their only living relatives, and were probably less active.

The research is published today in the journal Nature Communications.

Led by Robert Eagle, a researcher in the department of earth, planetary and space sciences in the UCLA College, the scientists examined fossilized dinosaur eggshells from Argentina and Mongolia. Analyzing the shells' chemistry allowed them to determine the temperature at which the eggshells formed -- information that had not been previously known.

"This technique tells you about the internal body temperature of the female dinosaur when she was ovulating," said Aradhna Tripati, a co-author of the study and a UCLA assistant professor of geology, geochemistry and geobiology. "This presents the first the direct measurements of theropod body temperatures."

The Argentine eggshells, which are approximately 80 million years old, are from large, long-necked titanosaur sauropods, members of a family that include the largest animals to ever roam the Earth. The shells from Mongolia's Gobi desert, 71 million to 75 million years old, are from oviraptorid theropods, much smaller dinosaurs that were closely related to Tyrannosaurus rex and birds.

Sauropods' body temperatures were warm -- approximately 100 degrees Fahrenheit, according to the study. The smaller dinosaurs had substantially lower temperatures, probably below 90 degrees.

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Nidophis insularis: a Small Maastrichtian Cretaceous Snake Found in a Dinosaur Nest, but not Eating Eggs (hint, something small and toothy was eating it)

A Late Cretaceous madtsoiid snake from Romania associated with a megaloolithid egg nest – Paleoecological inferences

Authors:

Venczel et al

Abstract:

Here we report on the taphonomy and paleoecological implications of the first record of a small madtsoiid snake (Nidophis insularis) closely associated with a megaloolithid dinosaur egg nest. Taphonomic and sedimentologic evidence suggest that the snake was buried autochthonously within or nearby the egg nest, with at least partially articulated skeleton. Count of growth rings on the vertebral zygapophyses indicates that the holotype of Nidophis belonged to an adult individual approaching the limit of its maximum body size of about 1 m length. The presence of layers of arrested growth on the zygapophyses, together with other independent data (e.g., paleomagnetic data, sedimentology, paleosol development stage, stable isotope geochemistry) indicates that Nidophis lived under a semi-arid, seasonally variable subtropical climate, having alternative periods of active feeding. The trunk vertebrae with relatively low neural spines and without prezygapophyseal accessory processes indicate a relatively heavy-bodied, slowly-moving animal, one that probably had a semifossorial habit and was an active forager, but definitively not a dinosaur nest raider as suggested for certain large madtsoiid snakes (the Indian Sanajeh). Potential prey items, available around the dinosaur nesting area, probably ranged from small squamate eggs to various small vertebrates. Finally, one anterior trunk vertebra of the holotype displays distinct bite marks left by a small-sized and pointed-toothed predator, most probably a crocodyliform or a theropod, thus documenting that madtsoiids were also preyed upon.

Did mommy bring home a snack?

Albian Cretaceous Filter Feeding Pterosaur Pterodaustro guinazui Appears to Have Converged With Flamingoes Extensively

The first pterosaur 3-D egg: Implications for Pterodaustro guinazui nesting strategies, an Albian filter feeder pterosaur from central Argentina

Authors:

Grellet-Tinner et al

Abstract:

The increasing number of fossil pterosaur eggs sheds light on nesting environments and breeding behaviors of these extinct flying reptiles. Here we report the first partial three-dimensional egg of the pterosaur, Pterodaustro guinazui, from central Argentina. The specimen was discovered from the same Albian deposits as the exceptional P. guinazui embryo described in 2004. Microscopic characterizations indicate a pristine preservation of the 50 μm thick calcium carbonate, which differs significantly from the soft shell of Chinese pterosaur eggs. Estimate of the eggshell conductance implies that the nest had a minimum moisture content of 75%. This moisture estimate, combined with geological and taphonomical data, suggests that P. guinazui may have adopted a nesting strategy similar to those of grebes and flamingos rather than being buried on land, as previously hypothesized. Moreover, our results demonstrate that the nesting paleoenvironment of this pterosaur species was closely linked to a mesohaline lacustrine ecosystem in a basin governed by regional tectonic subsidence, a setting characteristic for the feeding and reproduction of modern flamingos.