Minute theropod eggs and embryo from the Lower Cretaceous of Thailand and the dinosaur-bird transition

Abstract

We report on very small fossil eggs from the Lower Cretaceous of Thailand, one of them containing a theropod embryo, which display a remarkable mosaic of characters. While the surficial ornamentation is typical of non-avian saurischian dinosaurs, the three-layered prismatic structure of the eggshell is currently known only in extant and fossil eggs associated with birds. These eggs, about the size of a goldfinch's, mirror at the reproductive level the retention of small body size that was paramount in the transition from non-avian theropods to birds. The egg-layer may have been a small feathered theropod similar to those recently found in China.

Oology and the evolution of thermophysiology in saurischian dinosaurs: homeotherm and endotherm deinonychosaurians?

“…One pole is extremely rounded, which offers a greater mechanical stress resistance as it approximates the shape of a sphere (Bain, 1991). This asymmetry is far more pronounced than that of Clark et al, 2002 andBuffetaut et al, 2005, and oological analyses from Grellet-Tinner et al, (2006) and Grellet-Tinner and Makovicky (in press). Note the coeveal appearance of a change in the architecture of the nest, egg shape, and eggshell structure namely in oviraptorids and at the level of troodontids.…”

“…However, the thickness of layer 1 exceeds that of layer 2 (by a ratio close to 1.4) in these enantiornithines (Grellet-Tinner, 2005;Grellet-Tinner et al, 2006) and Phu Phok eggs (Buffetaut et al, 2005). This character is so far unique to these two groups and not recorded in any other positively identified non-avian theropod or modern birds (Buffetaut et al, 2005;Grellet-Tinner et al, 2006). Several eggs in Neuquén were discovered within a limited area but only a few were complete and the rest consists of the halves with a tapered pole (without air cell).…”

“…However, a high taphonomic level does not always correspond to a first level of taxonomic confidence. The identification of the skeletal remains could be uncertain, controversial (Horner, 1997;Manning et al, 1997;Mateus et al, 1997), rebutted (Chiappe et al, 2001b), or at a low taxonomic hierarchical level that does not allow an identification at the species or family level (Schweitzer et al, 2002, Buffetaut et al, 2005), leading to a lower level of taxonomic confidence. Although such occurrences do not by essence discriminate against their use for phylogenetic analyses or paleobiologic interpretations, the descriptions of their discoveries and identifications need to be reported.…”

“…Oological and reproductive characters known to be useful for phylogenic analyses (Grellet-Tinner, 2000;Makovicky and Grellet-Tinner, 2000;Buffetaut et al, 2005; Grellet-Tinner in press) also provide a source of new data for the paleobiology of saurischian dinosaurs . Investigation of the physiology in fossilized vertebrates seems a doubtful query, as it cannot be observed directly.…”

Dinosaur Integument

“…Reprinted from Reisz et al (2010). Courtesy AAAS Minute theropod eggs and embryos from the Lower Cretaceous of Thailand (Buffetaut et al 2005) show many important features that the authors propose were highly important in the dinosaur-bird transition. The shell thickness, without the surficial ornamentation, averages 354 lm, a value very similar to that of the much more voluminous Gallus gallus (domestic chicken) eggs and 30 % more than that of an ornithothoracine bird from the Late Cretaceous of Argentina (Schweitzer et al 2002).…”