Life history of a basal bird: morphometrics of the Early Cretaceous Confuciusornis

Abstract

Confuciusornis sanctus stands out among the remarkable diversity of Mesozoic birds recently unearthed from China. Not only is this primitive beaked pygostylian (birds with abbreviated caudal vertebrae fused into a pygostyle) much more abundant than other avian taxa of this age but differences in plumage between specimens-some having a pair of long stiff tail feathers-have been interpreted as evidence for the earliest example of sexual dimorphism in birds. We report the results of a multivariate morphometric study involving measurements of more than 100 skeletons of C. sanctus. Our analyses do not show any correlation between size distribution and the presence or absence of blade-like rectrices (tail feathers), thus implying, that if these feathers are sexual characters, they are not correlated with sexual size dimorphism. Our results also provide insights into the taxonomy and life history of confuciusornithids, suggesting that these birds may have retained ancestral dinosaurian growth patterns characterized by a midlife exponential growth stage.

Preservation of ovarian follicles reveals early evolution of avian reproductive behaviour

“…The elongate paired tail feathers present in some enantiornithines strongly resemble those of Confuciusornis, which have been interpreted as a sexually dimorphic trait present only in males; this interpretation is controversial as the feathers do not co-vary with size as expected 29 . Feathers are only preserved in STM29-8 (Fig 2A).…”

Sexual size dimorphism is the most consistent explanation for the body size spectrum of Confuciusornis sanctus

“…Chiappe et al (2008) reported a 'bimodal size distribution' in C. sanctus and hypothesized that it reflected 'a mid-development phase of exponential growth that separates earlier and later phases of slower growth' (figure 1a). Avian allometry suggests a body mass of approximately 700 g for the larger animals (size class sIII; Peters & Peters 2009) which, according to the hypothesis, were the most mature ones.…”

“…The SSD hypothesis is the most parsimonious explanation for the observed size spectrum; it also happens to be compatible with bird-like growth. Therefore, as explained by Peters & Peters (2009), the SSD hypothesis resolves the apparent conflict between the size spectrum and bird-like growth that Chiappe et al (2008) had discussed. This resolution, however, is a consequence, not a premise, of the SSD hypothesis.…”

“…Mature female birds of this weight produce hatchlings of approximately 30 g, which is an order of magnitude below the estimated body mass of the smaller cohort (sII) in the 'bimodal distribution'. This gap between hatchlings and sII (figure 1a) was unaccounted for by Chiappe et al (2008), who did not consider the egg or neonate masses implied by their hypothesis.The gap in the hypothesis has no equivalent in the fossil record. Some specimens fall right between the estimated hatchling mass and sII (Peters & Peters 2009), corroborating independently that hatchlings must have been much smaller than sII.…”

“…Mature female birds of this weight produce hatchlings of approximately 30 g, which is an order of magnitude below the estimated body mass of the smaller cohort (sII) in the 'bimodal distribution'. This gap between hatchlings and sII (figure 1a) was unaccounted for by Chiappe et al (2008), who did not consider the egg or neonate masses implied by their hypothesis.…”