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Unusual tooth replacement in a new Cenomanian iguanodontian from the Mussentuchit Member of the Cedar Mountain formation

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Abstract

A partial skeleton including both dentaries and multiple isolated teeth of an early diverging iguanodontian (NCSM 29373) was excavated from the Cenomanian-aged Mussentuchit Member of the Cedar Mountain Formation in Emery County, Utah during the 2014-2016 field seasons. Due to the presence of jaw sections with unerupted teeth, we were able to calculate tooth replacement rates of NCSM 29373 by counting incremental von Ebner lines – dentine growth lines homologous to the lines in extant amniote teeth that represent daily dentine deposition. To avoid damaging the intact dentary, we made thin sections of a complete isolated tooth and using light microscopy calculated the mean width of von Ebner lines across several increments where they were preserved – 17 μm (n=11, range = 13.4-23.2 μm). A reconstructed crown height of 6719 μm, yields an estimated tooth formation time of 395 days. Enamel thickness was determined to be greatest at the central ridge on the labial side of the tooth – 159 μm compared to 88 μm in between the ridges. Micro CT scan of the dentary revealed two teeth in each alveolus - one functional tooth and one replacement tooth. Using this data we determined crown height in two successive teeth (7.07 mm for the functional tooth and 3.53 mm for replacement tooth) and derived formation time – specifically, 416 days for the functional tooth and 208 days for the replacement tooth and an estimated tooth replacement rate of 208 days. Tooth replacement rate in the Mussentuchit iguanodontian (NCSM 29373) is 2-4 times slower than calculated for hadrosaurs with specialized tooth batteries (e.g., Edmontosaurus and Prosaurolophus average 50 and 81 days respectively). In fact, although slightly faster, tooth replacement rate in NCSM 29373 is most comparable to that of theropods, perhaps reflecting a plesiomorphic condition – a slower rate is expected for early-diverging ornithopods not yet exhibiting a sophisticated tooth battery. Alternatively slower tooth formation times and replacement rates may be a dietary specialization of NCSM 29373; further data among non-hadrosaurian ornithopods is necessary to test amongst these competing hypotheses.
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 (4001200). 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 (); ffunctional tooth
dentine thickness; rreplacement 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.
... We found no smaller incremental lines even between the von Ebner line with the widest distance. As the incremental lines observed by us conform to the width of daily deposited von Ebner lines reported from other archosaurs (Allison et al., 2019;D'Emic et al., 2013Erickson, 1992Erickson, , 1996aErickson, , 1996bGren & Lindgren, 2013;Ricart et al., 2019;Sokolskyi, Zanno & Kosch, 2019) we conclude that we did not observe intradian (Smith, 2004) or ultradian (Ohtsuka & Shinoda, 1995;Klevezal, 1996) lines that might occur as bands subdividing daily lines. The range of incremental line width observed in our sample also falls in similar range to the VEIW observed in the evergrowing incisors of rodents (Rosenberg & Simmons, 1980;Ohtsuka & Shinoda, 1995;Klevezal, 1996;Smith, 2004). ...
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