Resting metabolic rates in Concornis and Iberomesornis (1 Viewer)

[Fred Ruhe]

Jorge Cubo, Angela D. Buscaluini, Lucas J. Legendre, Estelle Bourdon, Jose L. Sanz & Antonio de Riqles 2021

Palaeohistological inferences of resting metabolic rates in Concornis and Iberomesornis (Enantiornithes, Ornithothoraces) from the Lower Cretaceous of Las Hoyas (Spain)

Palaeontology. in press.
doi:10.1111/pala.12583

Abstract: https://onlinelibrary.wiley.com/doi/10.1111/pala.12583

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.

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Fred

 

[Fred Ruhe]

FIG. 1. Cross-sectional, mid-diaphyseal (A–B, D–G), or distal metaphyseal (C), thin sections of metatarsals and tarsometatarsi of a sample of diapsids. A–D, metatarsals of: A, Varanus griseus; B, Crocodylus sp.; C, Iberomesornis sp.; D, Concornis lacustris.E–G, tarsometatarsi of: E, Troglodytes troglodytes; F, Ficedula hypoleuca; G, Parus major. Scale bars represent: 1 mm (A, B); 0.5 mm (C–G).

FIG. 2. Cross-sectional thin sections at the diaphyseal level observed under linearly polarized light unless otherwise specified. A, metatarsal IV of Varanus griseus. B, metatarsal IV of Crocodylus sp. C–D, metatarsal IV of Iberomesornis sp. at the level of the proximal metaphysis observed under linearly (C) and cross-polarized light with lambda compensator (D). E, metatarsal IV of Concornis lacustris. F, tarsometatrsus of Alectura lathami. Black arrow (E), endosteal resorption line; black arrow-heads (B, F), primary osteons; black vertical bar (B): zone of woven bone including primary osteons; white arrow (D), parallel-fibred bone penetrated by extrinsic fibre bundles; white arrow-heads (C, F), secondary osteons; white vertical bar (B), layer of parallel-fibred bone; white [ (F), outer circumferential layer. Scale bar represents 0.5 m .