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Growth and life habits of the Triassic cynodont Trirachodon, inferred from bone histology

100%
Botha J., Chinsamy A.
Acta Palaeontologica Polonica
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2004
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tom 49
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nr 4
Growth pattern and lifestyle habits of the Triassic non−mammalian cynodont Trirachodon are deduced from bone histology and cross−sectional geometry. Several skeletal elements of Trirachodon were examined in order to document histological changes during ontogeny, as well as histovariability in the skeleton. The bone histology of all the elements consists of a moderately vascularized, periodically interrupted, fibro−lamellar bone tissue. This suggests that the overall growth of Trirachodon was probably rapid during the favourable season, but decreased or ceased during the unfavourable season. As the environment is thought to have been semi−arid with seasonal rainfall, it is possible that Trirachodon was sensitive to such environmental fluctuations. Some inter−elemental histovariability was noted where the number and prominence of growth rings varied. Limb bone cross−sectional geometry revealed a relatively thick bone wall and supports earlier proposals that Trirachodon was fossorial.
2
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Bone microstructure and growth patterns of early mammals

100%
Chinsamy A., Hurum J. H.
Acta Palaeontologica Polonica
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2006
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tom 51
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nr 2
We present results of the first studies of the bone microstructure of early mammals, based on the Early Jurassic Morganucodon, the Late Cretaceous multituberculates, Kryptobaatar and Nemegtbaatar, and the Late Cretaceous eutherians Zalambdalestes and Barunlestes. Our results show that the two eutherian taxa grew relatively slowly with periodic pauses in growth indicated by the presence of rest lines, while the multituberculates and Morganucodon had a faster rate of bone formation that suggests an overall rapid growth rate that slowed down later in ontogeny. Comparisons of the early mammalian bone microstructure with that of non−mammalian cynodonts, extant monotremes, and placentals are also made, and significant differences in the rate of osteogenesis in the various groups are documented. Our findings suggest differences in the growth rate between the multituberculates and the Mesozoic eutherians, and moreover, both groups appear to have slower growth rates as compared to modern monotremes and placentals. Our results further suggest that the determinate growth strategy typical of extant mammals evolved early in the evolution of the non−mammalian therapsids. We speculate that the sustained, uninterrupted bone formation among the multituberculates may have been an adaptive attribute prior to the K−T event, but that the flexible growth strategy of the early eutherians was more advantageous thereafter.
3

Variation of the outer circumferential layer in the limb bones of birds

61%
Ponton F., Elzanowski A., Castanet J., Chinsamy A., de Margerie E., de Ricqles A., Cubo J.
Acta Ornithologica
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2004
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tom 39
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nr 2
The core of the limb bone cortex of mammals and birds is made of rapidly deposited, fibro-lamellar bone tissue (also present in non-avian theropods), which is usually surrounded by an avascular outer circumferential layer (OCL) of slowly deposited parallel-fibered bone. We present the first comparative allometric study of the relative OCL thickness (expressed as a fraction of the diaphyseal radius) in modern birds. Body size explains 79% of the OCL variation in thickness, which is inversely correlated with size, that is, shows negative allometry (slope -0.799). This may explain the apparent absence of OCL in the ratites. Since the OCL is deposited at the end of growth, we propose that its relative thickness probably correlates with the amount of slow, residual growth, which our results suggest to be on the average larger in small birds.
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