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A new tritylodontid from the Upper Jurassic of Xinjiang, China

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Hu Y., Meng J., Clark J. M.
Acta Palaeontologica Polonica
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2009
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tom 54
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nr 3
385-391
A new genus and species of Tritylodontidae, Yuanotherium minor, is described and compared with other known tritylodontids. The new taxon is represented by a partially preserved upper jaw with three postcanines, collected from the upper part of the Shishugou Formation (Oxfordian, Late Jurassic) in the Wucaiwan area of the Junggar Basin, northwestern Xinjiang, China. Like other tritylodontids its maxillary teeth have three rows of blade−like trenchant cusps separated by deep furrows. The new species differs from other tritylodontids mainly in having posteriormost two cusps of the median row on upper postcanines closely placed. The new tritylodontid may have been omnivorous rather than herbivorous, as previously suggested for tritylodontids in general.
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Functional morphology and biomechanics of the cynodont Trucidocynodon riograndensis from the Triassic of Southern Brazil: Pectoral girdle and forelimb

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de Oliveira T. V., Schultz C. L.
Acta Palaeontologica Polonica
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2016
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tom 61
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nr 2
Non-mammalian cynodonts provide insights on several points about mammalian evolution, such as the postural change and locomotory advances within the group. Unfortunately, complete skeletons of Triassic cynodonts are rather uncommon and where more complete specimens are found they can offer a global vision on some traits not available from partial specimens. This is the case of the cynodont Trucidocynodon riograndensis, from the Triassic of Brazil, that has preserved its forelimbs providing some insights into locomotory properties. The movements between interclavicle and clavicle must have been limited, as such as those occurring between the latter and the scapulocoracoid although the long acromion process of this should have permitted a greater degree of freedom. Some of the more significant movements were those on the shoulder joint, in which the maximum adduction should have been ca. 35º relative to the parasagittal plane and the greater abduction ca. 55º. The maximum adduction occurred when the humerus was in the more retracted position during stride and the variation in the adduction/abduction should have been significant to the limb posture during its recovery stroke. The long olecranon and the distal overlapping between radius and ulna suggest the predominance of simple flexion/extension on the forearm without significant pronation/supination. The poorly preserved hand suggests that Trucidocynodon could have evolved a slight semidigitigrad condition in its forelimbs. All these features give to this cynodont an important role in the evolution of the mammalian locomotory properties indicating that some features, such as the possibility of greater humeral adduction, evolved early in cynodont lineage.
3
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Bone microstructure and growth patterns of early mammals

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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.
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