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



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
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.
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
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
Equation 1. Formula for determining tooth
replacement rate (); ffunctional tooth
dentine thickness; rreplacement tooth
dentine thickness; mean mean IVEL
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.
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.
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). ...
Full-text available
Dietary habits in extinct species cannot be directly observed; thus, in the absence of extraordinary evidence, they must be reconstructed with a combination of morphological proxies. Such proxies often include information on dental organization and function such as tooth formation time and tooth replacement rate. In extinct organisms, tooth formation times and tooth replacement rate are calculated, in part via extrapolation of the space between incremental lines in dental tissues representing daily growth (von Ebner Line Increment Width; VEIW). However, to date, little work has been conducted testing assumptions about the primary data underpinning these calculations, specifically, the potential impact of differential sampling and data extrapolation protocols. To address this, we tested a variety of intradental, intramandibular, and ontogentic sampling effects on calculations of mean VEIW, tooth formation times, and replacement rates using histological sections and CT reconstructions of a growth series of three specimens of the extant archosaurian Alligator mississippiensis. We find transect position within the tooth and transect orientation with respect to von Ebner lines to have the greatest impact on calculations of mean VEIW—a maximum number of VEIW measurements should be made as near to the central axis (CA) as possible. Measuring in regions away from the central axis can reduce mean VEIW by up to 36%, causing inflated calculations of tooth formation time. We find little demonstrable impact to calculations of mean VEIW from the practice of subsampling along a transect, or from using mean VEIW derived from one portion of the dentition to extrapolate for other regions of the dentition. Subsampling along transects contributes only minor variations in mean VEIW (
Full-text available
Rhabdodontidae is a successful clade of ornithopod dinosaurs, characteristic of Late Cretaceous continental faunas in Europe. A new rhabdodontid from the late Campanian, of southern France, Matheronodon provincialis gen. et sp. nov., is characterized by the extreme enlargement of both its maxillary and dentary teeth, correlated to a drastic reduction in the number of maxillary teeth (4 per generation in MMS/VBN-02-102). The interalveolar septa on the maxilla are alternately present or resorbed ventrally so as to be able to lodge such enlarged teeth. The rhabdodontid dentition and masticatory apparatus were adapted for producing a strict and powerful shearing action, resembling a pair of scissors. With their relatively simple dentition, contrasting with the sophisticated dental batteries in contemporary hadrosaurids, Matheronodon and other rhabdodontids are tentatively interpreted as specialized consumers of tough plant parts rich in sclerenchyma fibers, such as Sabalites and Pandanites.
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Tooth replacement rate can be calculated in extinct animals by counting incremental lines of deposition in tooth dentin. Calculating this rate in several taxa allows for the study of the evolution of tooth replacement rate. Sauropod dinosaurs, the largest terrestrial animals that ever evolved, exhibited a diversity of tooth sizes and shapes, but little is known about their tooth replacement rates. We present tooth replacement rate, formation time, crown volume, total dentition volume, and enamel thickness for two coexisting but distantly related and morphologically disparate sauropod dinosaurs Camarasaurus and Diplodocus. Individual tooth formation time was determined by counting daily incremental lines in dentin. Tooth replacement rate is calculated as the difference between the number of days recorded in successive replacement teeth. Each tooth family in Camarasaurus has a maximum of three replacement teeth, whereas each Diplodocus tooth family has up to five. Tooth formation times are about 1.7 times longer in Camarasaurus than in Diplodocus (315 vs. 185 days). Average tooth replacement rate in Camarasaurus is about one tooth every 62 days versus about one tooth every 35 days in Diplodocus. Despite slower tooth replacement rates in Camarasaurus, the volumetric rate of Camarasaurus tooth replacement is 10 times faster than in Diplodocus because of its substantially greater tooth volumes. A novel method to estimate replacement rate was developed and applied to several other sauropodomorphs that we were not able to thin section. Differences in tooth replacement rate among sauropodomorphs likely reflect disparate feeding strategies and/or food choices, which would have facilitated the coexistence of these gigantic herbivores in one ecosystem. Early neosauropods are characterized by high tooth replacement rates (despite their large tooth size), and derived titanosaurs and diplodocoids independently evolved the highest known tooth replacement rates among archosaurs.
Significance Little is known regarding nonavian dinosaur embryology. Embryological period relates to myriad aspects of development, life history, and evolution. In reptiles incubation is slow, whereas in birds it is remarkably rapid. Because birds are living dinosaurs, rapid incubation has been assumed for all dinosaurs. We discovered daily forming growth lines in teeth of embryonic nonavian dinosaurs revealing incubation times. These lines show slow reptilian-grade development spanning months. The rapid avian condition likely evolved within birds prior to the Cretaceous–Paleogene (K–Pg) mass extinction event. Prolonged incubation exposed nonavian dinosaur eggs and attending parents to destructive influences for long periods. Slow development may have affected their ability to compete with more rapidly generating populations of birds, reptiles, and mammals following the K–Pg cataclysm.
The tooth enamel microstructure of all the dinosaur taxa that are adequately represented in the American Museum of Natural History collections were analyzed using scanning electron microscopy. This study aims to determine whether or not better sampling within a major nonmammalian amniote (hereafter referred to descriptively as "reptile") clade will unearth phylogenetic patterns in enamel microstructure in addition to those dictated by tooth function. While interest in reptile enamel microstructure has increased in the past few years, intensive sampling focused on just one monophyletic reptile clade was not previously implemented. This study reveals that phylogenetic constraints play a larger role in shaping enamel microstructure in reptiles than previously thought. Within many monophyletic dinosaur clades the combination of enamel types and enamel features within a tooth-the schmelzmuster-is the same in all the taxa due to their common ancestry, and their schmelzmusters are diagnostic of their respective clades. While distantly related taxa with similar teeth and diets have similar schmelzmusters due to functional constraints, phylogenetic constraints keep those schmelzmusters distinct from one another. An interesting finding of this analysis is that the enamel complexity of a taxon does not necessarily coincide with the position of the taxon on a phylogenetic tree; more derived taxa do not necessarily have more derived enamel and more primitive taxa do not necessarily have more primitive enamel.