University of Cambridge > > Zoology Graduate Seminars > Periodic incremental markings in the enamel of cynodonts and mammaliaforms: the origin of mammalian growth patterns

Periodic incremental markings in the enamel of cynodonts and mammaliaforms: the origin of mammalian growth patterns

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The origin of determinate growth in mammals is associated with other important mammalian characters, including diphyodont dentition, lactation and endothermy. These characters likely evolved in the extinct cynodont and basal mammaliaform lineages of the Triassic and early Jurassic, with Morganucodon hypothesised to be the most basal mammaliaform to have both determinate growth and diphyodonty. Histological techniques to assess periodic enamel increments have not previously been applied to cynodonts and mammaliaforms, and are likely to improve understanding of their dental replacement and growth patterns. We prepared histological thin sections of postcanines of three cynodonts (Thrinaxodon, Scalenodon, Diademodon), molars of Morganucodon, and teeth from crown mammals and diapsids (e.g. Crocodylus, Canis, Tupaia). Using polarized light microscopy we assessed measures relating to dental growth patterns, including daily secretion rates of enamel, and crown and root extension rates. Two orders of incremental lines, analogous to daily prism cross-striations and longer period striae of Retzius of mammalian enamel, were observed in all cynodont and mammaliaform specimens. Daily secretion rate was lower in cynodont and mammaliaform enamel than in crown mammal teeth of comparable size. Very high mean crown extension rates were observed, both in diapsids and cynodonts (e.g. Crocodylus: 33.9µm/day; Thrinaxodon: 104.2µm/day), with cynodont extension rates at least double those of crown mammals with comparably sized teeth. Enamel thus extended extremely rapidly in thin ‘sheath’-like layers in diapsids and cynodonts. In contrast, crown extension rate in Morganucodon is much lower (7.8µm/day), more closely resembling the crown mammals used for comparison. Low extension- and daily secretion rates suggest lower differentiation rates and secretory function of ameloblasts in Morganucodon. If this is coupled with reduced function of osteoblasts, it may suggest reduced rates of bone growth during molar formation. These patterns would be consistent with the hypothesis of determinate growth patterns in Morganucodon and of more indeterminate growth in cynodont species.

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