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Systematic paleontology
Dinosauria Owen, 1842
Ornithischia Seeley, 1887
Ornithopoda Marsh, 1881
Iguanodontia Sereno, 1986
Methods
Prior to sampling, we molded and cast the specimen MM14-FS10. Following
Hwang (2005), we prepared the tooth for microscopy by embedding the
tooth in a clear epoxy resin (EPO-TEK 301), cut it along the longitudinal plane
(see Fig. 1) with a Buehler IsoMet 1000 Precision Saw, and polished on one
side with a Buehler MetaServ 250 Grinder Polisher using a series of abrasive
paper disks with decreasing grit sizes (400–1200). We made three
longitudinal sections from the tooth. We used a Nikon Eclipse Ci-POL
petrographic microscope to examine growth lines in dentin. To calculate
mean width of incremental von Ebner lines (IVELs) we analyzed taken images
in Adobe Photoshop. We used Avizo 3D analysis software with Micro CT data
obtained prior to this study for the dentary NCSM 29373 in order to
determine total dentine thickness at the tip of the tooth crowns of a pair of
unworn functional and replacement teeth in the dentary. Then we calculated
tooth replacement rate for the studied sequence with Equation 1.
Materials
A partial skeleton including dentary and isolated
teeth of an early diverging iguanodont
(NCSM 29373, named ‘Fortunate son’) was surface
collected from the Mussentuchit Member of the
Cedar Mountain Formation in Emery County, Utah
during the 2014 field seasons. To avoid damaging
the dentary, we sampled a complete isolated tooth
MM14-FS10.
Unusual tooth replacement in a new Cenomanian iguanodontian
from the Mussentuchit Member of the Cedar Mountain formation
Sokolskyi, T. 1, Kosch, J. 2, Zanno, L. 2
1 – North Carolina Museum of Natural Sciences, Raleigh, NC, United States of America; Department of Biology, Duke university, Durham, NC
2 – North Carolina Museum of Natural Sciences, Raleigh, NC, United States of America; North Carolina State University, Raleigh, NC
Fig. 1. Tooth MM14-FS10
sampled for sectioning
(dotted line is the cut line).
Scale 1 cm.
=f−r
mean
Equation 1. Formula for determining tooth
replacement rate (); f–functional tooth
dentine thickness; r–replacement tooth
dentine thickness; mean –mean IVEL
width.
Results
Mean IVEL width of the tooth MM14-FS10 is 17 μm.
With the microscopy data (fig. 2) we calculated
dentine thickness of this tooth of 6719 μm that
revealed formation time to be 395 days. Enamel
thickness obtained from microscopy is 89 μm for
the labial side and 60 μm for the lingual side.
Dentine thickness of functional and replacement
teeth in the dentary is 7070 and 3530 μm
respectively, as shown by CT data (fig. 3). Resulting
tooth formation times are 416 days for the
functional tooth and 208 days for the replacement
tooth with the replacement rate being 208 days in
this tooth pair. Obtained data is summarized in
Table 1.
IW, μm 17
DTMM14 −FS10, μm 6719
TFTMM14−FS10, days 395
ETlabial, μm 89
ETlingual, μm 60
f, μm 7070
r, μm 3530
TFTfunctional, days 416
TFT, days 208
TRR, days 208
Table 1. Data obtained for tooth MM14-FS10
and functional/replacement tooth pair in the
dentary. IW –mean IVEL width, ET –enamel
thickness, TFT –tooth formation time.
References
1. Erickson, G. M., Zelenitsky, D. K., Kay, D. I., & Norell, M. A. (2017). Dinosaur incubation periods directly determined from growth-line counts in embryonic teeth show reptilian-grade development. Proceedings of the National Academy
of Sciences,114(3), 540-545.
2. Godefroit, P., Garcia, G., Gomez, B., Stein, K., Cincotta, A., Lefèvre, U., & Valentin, X. (2017). Extreme tooth enlargement in a new Late Cretaceous rhabdodontid dinosaur from Southern France. Scientific reports,7(1), 13098.
3. Hwang, S. H. (2005). Phylogenetic patterns of enamel microstructure in dinosaur teeth. Journal of Morphology,266(2), 208-240.
4. D’Emic, M. D., Whitlock, J. A., Smith, K. M., Fisher, D. C., & Wilson, J. A. (2013). Evolution of high tooth replacement rates in sauropod dinosaurs. PLoS One,8(7), e69235.
Discussion
TRR of 208 days of Fortunate Son is much greater than any previously
recorded data for ornithischian dinosaurs –45-81 days for derived
hadrosaurs, 30 for Protoceratops and 83 for Triceratops (Erickson et al.,
2017). It is also much greater than average extant crocodyliform TRR of
approx. 100 days (D’Emic et al., 2013). In fact, Fortunate Son’s TRR is most
comparable to that of small theropods, such as Deinonychus (D’Emic et al.,
2013). This unusual value could possibly be related to trophic specialization
of this species for hard sclerenchyma-rich plant material. In fact, enamel
thickness of Fortunate Son is comparable to that of Rhabdodontids, for
which such trophic specialization is described (Godefroit et al., 2017). Great
TRR value could also be explained by Fortunate Son being a basal iguanodont
without a sophisticated tooth battery such as in hadrosaurs.
Fig. 3. CT scan of NCSM 29373 dentary visualized in Avizo 3D analysis
software. A –cross section of the dentary with functional (red) and
replacement teeth (green). B –lateral view of the dentary.
Fig. 2. Light microscopy images of MM14-FS10 cross section showing von
Ebner lines. A –section of the entire tooth, scalebar 5 mm. Abbreviations:
eling –lingual enamel, elab –labial enamel, d –dentine. B – 10x
magnification of a part of the tooth section showing ivels –incremental von
Ebner lines. Scalebar 100 µm.
