Abstract
The degree of sutural closure between bones generally allows for the classification of skeleton maturity in tetrapods. In mammals, the sutural closure of skull bones was previously used as proxy to evaluate the ontogenetic stage of single individuals. However, due to temporal variation, this process can be only applied among mammalian subclades, but not for all mammals in general. In contrast, the process of sutural closures in bird skulls could be a more reliable ontogenetic proxy for this clade as adult birds commonly show a generally high degree of bone fusion. To test this, we studied the process of sutural closure in ontogenetic series of 18 extant bird species regarding the presence of an ontogenetic signal and compared the results with changes in skull size and proportions. Univariate analyses indicate that bone fusion happens faster in altricial than in precocial birds. However, the use of PCoA and multivariate regressions reveal that the skull bone fusion follows a common pattern among birds and thus can be used as proxy to identify different ontogenetic stages. In general, the process of sutural closure spreads from posterior to anterior and from ventral to dorsal. In contrast, skull measurements reflect rather interspecific allometry than ontogeny. The used of bone fusion as proxy will help to better identify and compare different stages of maturation in birds, including historical material from osteological collections.
Highlights
Ontogeny, the developmental history of an organism from the time of fertilization to its death, is a key research subject in evolutionary biology
In order to test the relationship between skeletal maturation and size for each bird species, we expressed the values of the average of the sutural closure as ratio from the most mature specimen of each species (RASC) and applied an ordinary least squares regression (OLS) between RASC and relative size percentage (RSP), using the lm function in R package stats 4.0.3 (R Development Core Team, 2011)
Based on the ANCOVA, the slopes do not indicate any taxonomic distinction between most bird clades, except for Inopinaves, which are significantly different from Aequorlitornithes and Palaeognathae (Supplementary Table 9)
Summary
The developmental history of an organism from the time of fertilization to its death, is a key research subject in evolutionary biology. The postnatal development is only poorly documented so far The ontogenetic stage of a bird was determined in several case studies based on the bone textures (e.g., Tumarkin-Deratzian et al, 2006; Watanabe, 2018), degree of bone pneumatization (e.g., Winkler, 1979; Hogg, 1984), plumage generations (e.g., Dwight, 1900; Humphrey and Parkes, 1959; Köppl et al, 2005) and bone fusion (e.g., Hogg, 1977; Bailleul et al, 2016), but never compared and verified on an interspecific level
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