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SUMMARY:Periodic incremental markings in the enamel of cynodonts and mamma
 liaforms: the origin of mammalian growth patterns - Rachel O'Meara
DTSTART:20140725T153000Z
DTEND:20140725T160000Z
UID:TALK53556@talks.cam.ac.uk
CONTACT:30967
DESCRIPTION:The origin of determinate growth in mammals is associated with
  other important mammalian characters\, including diphyodont dentition\, l
 actation and endothermy. These characters likely evolved in the extinct cy
 nodont 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 asses
 s periodic enamel increments have not previously been applied to cynodonts
  and mammaliaforms\, and are likely to improve understanding of their dent
 al 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 assess
 ed measures relating to dental growth patterns\, including daily secretion
  rates of enamel\, and crown and root extension rates. Two orders of incre
 mental 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 m
 ammaliaform enamel than in crown mammal teeth of comparable size. Very hig
 h mean crown extension rates were observed\, both in diapsids and cynodont
 s (e.g. Crocodylus: 33.9µm/day\; Thrinaxodon: 104.2µm/day)\, with cynodo
 nt extension rates at least double those of crown mammals with comparably 
 sized teeth. Enamel thus extended extremely rapidly in thin ‘sheath’-l
 ike layers in diapsids and cynodonts. In contrast\, crown extension rate i
 n Morganucodon is much lower (7.8µm/day)\, more closely resembling the cr
 own 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 pa
 tterns in Morganucodon and of more indeterminate growth in cynodont specie
 s.
LOCATION:Tea Room\, Department of Zoology
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