Search

The bone histology of non‐avian theropods such as Troodon, early pygostylians such as Confuciusornis, and neornithines, is characterized by the post‐hatching formation of fibrolamellar complex. In contrast, the cortex of enantiornithine birds, like Concornis and Iberomesornis, is made of poorly vascularized parallel‐fibred tissue. The cortex of metatarsals of Concornis lacustris MCCM‐LH21006 is composed of a thin layer of lamellar endosteal bone and a thick layer of almost avascular parallel‐fibred periosteal bone organized into an inner part containing large spherical osteocyte lacunae and a few secondary osteons, and an outer part containing flattened osteocyte lacunae and lines of arrested growth. The cortex of metatarsals of Iberomesornis sp. MCCM‐LH09681 is similar to the inner layer described in Concornis. Endosteal bone, and the outer layer of periosteal bone containing flattened osteocytes and lines of arrested growth, are missing in Iberomesornis, suggesting that this specimen is a sub‐adult. Here we test statistically a hypothesis suggesting that the absence of post‐hatching formation of fibrolamellar bone in Enantiornithes may be a side effect of their low resting metabolic rates (RMR). For this, we construct a palaeobiological inference model including osteocyte lacunar density (explaining a significant fraction of variation in RMR) and phylogenetic eigenvectors as predictor variables. RMR inferred for Concornis and Iberomesornis are significantly higher than those measured in extant ectotherms, but they are not significantly different from the RMR measured in a sample of extant arboreal perching Neornithes of similar body size. We conclude that the 'intermediate physiological condition' hypothesis is rejected. [ABSTRACT FROM AUTHOR]

Apateon, a key genus among Branchiosauridae from the Carboniferous—Permian of Europe, is often considered closely related to salamanders on the basis of developmental similarities, anatomical features, and life history. The current work deals with recognition of heterochronies among three “time-averaged populations” of Apateon based on inference from histological features already studied in extant urodeles. Our study is performed on the long bones of 22 specimens of Apateon pedestris and Apateon caducus. Histological observations show that diaphyseal and epiphyseal ossification patterns of Apateon are similar to those of urodeles. From skeletochronological analysis, the identification of the age of sexual maturity allows us to discriminate juveniles from adults and to confirm the previous hypothesis of a paedomorphic condition based on anatomical data among these species. The current study also suggests a paedomorphic condition of each “population” at the histological level. This heterochrony may have been linked to peculiar ecological conditions such as hypoxic and fresh water environment. Functional reasons may also be invoked to explain differences of ossification between fore- and hindlimbs of the “populations” from Odernheim and Niederkirchen because paleoecological conditions are very different from one locality to another. This study illustrates the role that the acquisition of heterochronic features plays at the microevolutionary scale. [ABSTRACT FROM AUTHOR]

The biological features observed in every living organism are the outcome of three sets of factors: historical (inherited by homology), functional (biological adaptation), and structural (properties inherent to the materials with which organs are constructed, and the morphogenetic rules by which they grow). Integrating them should bring satisfactory causal explanations of empirical data. However, little progress has been accomplished in practice toward this goal, because a methodologically efficient tool was lacking. Here we use a new statistical method of variation partitioning to analyze bone growth in amniotes. (1) Historical component. The variation of bone growth rates contains a significant phylogenetic signal, suggesting that the observed patterns are partly the outcome of shared ancestry. (2) Functional causation. High growth rates, although energy costly, may be adaptive (i.e., they may increase survival rates) in taxa showing short growth periods (e.g., birds). In ectothermic amniotes, low resting metabolic rates may limit the maximum possible growth rates. (3) Structural constraint. Whereas soft tissues grow through a multiplicative process, growth of mineralized tissues is accretionary (additive, i.e., mineralization fronts occur only at free surfaces). Bone growth of many amniotes partially circumvents this constraint: it is achieved not only at the external surface of the bone shaft, but also within cavities included in the bone cortex as it grows centrifugally. Our approach contributes to the unification of historicism, functionalism, and structuralism toward a more integrated evolutionary biology. [ABSTRACT FROM AUTHOR]