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

Abstract

Evolutionary developmental biology Invited replySexual size dimorphism is the most consistent explanation for the body size spectrum of Confuciusornis sanctusRecently, we suggested that the size spectrum of Confuciusornis sanctus is best explained by sexual size dimorphism (SSD) and is compatible with a bird-like life history (Peters & Peters 2009). We find the criticism of Chiappe et al. (2010) interesting, but irrelevant to our main argument.Let us follow this argument from the beginning. 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. 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. To close this gap in the original hypothesis, one may postulate that C. sanctus had a biphasic growth curve (figure 1b). Alternatively, the size spectrum may be interpreted as resulting from two monophasic growth curves with different asymptotes, as in species exhibiting SSD (figure 1c). Both hypotheses explain the size spectrum, but the former (figure 1b) postulates a novel, unique type of growth curve, whereas the latter (figure 1c) works with growth curves commonly observed. Consequently, we accepted the SSD hypothesis (figure 1c), precisely because it does not rely on 'questionable ideas about growth patterns' (Chiappe et al. 2010).The above is the logical backbone of our argumentation (Peters & Peters 2009). Chiappe et al. (2010) do not comment on it. Instead, they state that we interpreted the evidence 'through the lens of considering the growth pattern of C. sanctus as comparable to that of modern birds'. This is incorrect; our argument (figure 1) requires no assumption of this type. 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. The accompanying comment can be viewed at http://dx.doi.org/

The primary feather lengths of early birds with respect to avian wing shape evolution

“…, 2008, 2010). Earlier workers have suggested that the skeletal proportions and the presence of tail feathers in this highly abundant Chinese bird are consistent with sexual size dimorphism within the known specimen sample (Peters & Peters, 2009, 2010).…”

“…, 1995; Chiappe et al. , 1999, 2008, 2009; Peters & Peters, 2009, 2010). One biomechanical analysis of the strength of their narrow primary feather rachises (Nudds & Dyke, 2010) and recent anatomical studies indicating restricted upstroke capabilities (Senter, 2006; Zhou & Zhang, 2007) suggest that Confuciusornis was not capable of vigorous flapping flight.…”

Homology and Potential Cellular and Molecular Mechanisms for the Development of Unique Feather Morphologies in Early Birds

“…Interpretations of the similar feathers present in confuciusornithiforms have instead been dominated by suggestions that they were used for sexual display—their variable presence in the known specimens suggestive of sexual dimorphism [39,51,60,61]. However, recent morphometric studies have not been able to support or reject this hypothesis [45,51] and the function of these feathers remains controversial [62]. The variation in length and morphology observed among the tail feathers of enantiornithines may suggest they are also related to display, species recognition [63], or other forms of visual communication [64].…”

A combined MR and CT study for precise quantitative analysis of the avian brain

“…lifestyle specific and seasonal fluctuations 1 2 ) to overlay the intrinsic mosaic evolution of avian 9 and mammalian 29 brain development. As an extension, dimorphic segregation might also be discernible amongst the restored brains of fossil avians, and could be used to discriminate disputed sexual morphs accepting the constraints of adequate diagenetic retro-deformation and sample size 30 31 32 33 . More importantly, exact quantitative measure of different brain regions, especially the proportional enlargement of the forebrain, which has been qualitatively contested as an indicator of enhanced somatosensory integration and the capacity for flight 19 20 , requires further investigation.…”