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Included in these Supplementary Materials are geological setting, supplementary methods, description
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and analysis, and a review of putatively venomous fossil taxa.
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1. Geological Setting of Uatchitodon Localities
Uatchitodon is known from three localities, with the Tomahawk locality and the Placerias
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Quarry occurrences previously documented in the literature (Sues 1991, 1996; Sues et al. 1994) while
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the Moncure locality is a new site discussed in recent publications and abstracts (Heckert et al. 2008).
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The type locality of Uatchitodon kroehleri, the Tomahawk locality, is in the Upper Triassic Vinita
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Formation of the Richmond Basin (Sues 1991, Sues et al. 1994). Kaye and Padian (1994) and Sues
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(1996) assigned enigmatic teeth from the Upper Triassic Placerias Quarry of the Chinle Group of
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Arizona to Uatchitodon sp.
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The Moncure locality is similar to the other localities in its fine grain size, dark clays, possibly
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pedogenic carbonates, and seemingly palustrine depositional setting. However, the Moncure locality
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differs from the other sites in lacking many of the larger fossils present, and in having yielded such an
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immense number of fossils (>50,000, although the Placerias Quarry has a large number from a much
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larger area). Most of these fossils are either fish scales or indeterminate broken fossil fragments, but
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many (>1000) are identifiable teeth or bones. All three Uatchitodon-bearing sites also possess a very
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diverse vertebrate assemblage. A commonality between the sites is the predominance of synapsids, with
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the Placerias Quarry dominated by the dicynodont Placerias and the Tomahawk and the Moncure
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locality by the traversodontid Boreogomphodon and the chiniquodontoid Microconodon. All three have
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yielded more than 10 tetrapod genera. Depositional similarity also unites the localities, as the Placerias
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Quarry has been described as a palustrine environment (Fiorillo et al. 2000) and sedimentology from
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both the Tomahawk and Moncure localities support assignment to similar environments (Sues et al.
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1994). These sedimentological and faunal similarities strongly suggest similar depositional settings for
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all three sites, and may suggest that environmental, rather than geographic, provincialism was
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responsible for much of the biotic segregation in the Late Triassic, though much more analysis is
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necessary. A key aspect of microvertebrate sites is that they tend to be time-averaged, with specimens
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representing the evolution of local 'populations,' potentially over the time scale of several thousand
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years. This facet of microvertebrate taphonomy likely explains why, while there is a distinct
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morphological separation between the samples of Uatchitodon from the Tomahawk and Moncure
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localities, there is a morphological gradient in teeth from both sites.
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2. Supplementary Methods
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The analyses presented here are based primarily on the Moncure specimens, as they are both
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numerous and preserve the full range of variation between the deeply invaginated grooves of U.
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kroehleri and the fully enclosed, circular canals of Uatchitodon schneideri.
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Data from individual teeth were collected primarily from naturally broken surfaces, so in an
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effort to standardize position, we used measurements exclusively from the apical-most cross section
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below the aperture. To insure data consistency, a tooth (NCSM 24731) was sectioned twice at intervals
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of 0.5 mm and measurements were taken to determine variation within a single tooth. Further, we
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quantified the change in width with respect to distance from the tip by measuring the width of six teeth
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at 0.1 mm intervals from the tip, and performing an exponential regression to develop a model for
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predicting the distance an apical surface was from the tip of the tooth originally. Our model was
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restricted to the four posterior teeth from Moncure, and the one complete tooth from the Placerias
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Quarry (MNA V3680).
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The teeth were imaged with the backscatter detector of the FEI Quanta 200 Environmental
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Scanning Electron Microscope (SEM) housed at Appalachian State University, and collected images
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were manipulated in Adobe Photoshop CS5. The measurements shown in Figure S1 were taken using
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ImageJ, and this data is summarized in Tables S1-S2. In the regressions, the average (when only one
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canal was present, its value was considered the average), labial and lingual canal shapes were plotted
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separately, due to evidence that the labial canal was more circular, and that they varied in different
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ways through the tooth. We analyzed our data using the freely available statistical software, R v2.11.1,
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and utilized the scatterplot3d package. We report the results of the nonparametric Wilcoxon signed rank
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tests (sometimes called Mann-Whitney U tests) as our sample sizes were small and we sought to be
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conservative in our interpretations. We used a significance of p < 0.05.
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Heterodonty is well established within certain members of Archosauriformes, and this
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heterodonty is taken to different extremes (Currie et al. 1990; Farlow et al. 1991; Gomani 1997;
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Hungerbühler 2000; Smith 2005). These workers have shown that premaxillary teeth tend to have a
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lingually deflected mesial carina, and a lingual curvature of the tooth crown. We suggest that the angle
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formed by the two carinae can be used as a proxy for tooth position, with more sagittally aligned
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carinae (angle near 180°) representing teeth posterior (distal) to those with lower angles, which we
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interpret as derived from more mesial positions. However, as this method of positional assignment is
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still novel, the gross morphology of the putative anterior teeth was examined and it was determined that
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they were consistent with definitively premaxillary teeth in heterodont archosaurs, as described in the
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previously cited literature.
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Many of the values we report are ratios (lateral compression, canal shape, canal offset), and are
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thus unit-less, while our raw measurements are all in millimeters. For canal shape, a value of 1 would
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indicate a circular canal, and 0 would represent a perfectly linear canal, such that higher values
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represent greater circularity. For lateral compression, a value of 1 would indicate a circular cross-
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section, while 0 would again indicate a linear form, such that more blade-like teeth have lower values.
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For canal offset, a value of 1 would represent the canals in the center of the tooth, while 0 indicates
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canals on the distal edge of the carina, so that a lower value indicates a more distally offset tooth.
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3. Description and Statistical Analysis
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The teeth of Uatchitodon have very tall, serrated crowns, presumably thecodont implantation,
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and possess infoldings (either open channels or enclosed canals) on the labial and lingual crown
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surfaces. The tooth crowns from the Placerias Quarry and Moncure locality appear less recurved than
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those from the Tomahawk locality. Averages of all measurements made in this study are reported in
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Table S1.
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The fully enclosed canals of Uatchitodon schneideri terminate in an elongate apical aperture
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that resembles that of non-spitting cobras (Figure S2). The sample did not allow for a meaningful
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quantitative measure of orifice length to fang length (=crown height) as per Wüster and Thorpe (1992)
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because only two complete teeth are known (one each from the Placerias Quarry and Moncure
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locality). However, qualitative assessment shows that, in spitting cobras, the orifice is short, circular,
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and has a steepened apical margin, whereas the teeth of Uatchitodon have an elongate, oval aperture
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with a steep basal margin and shallow, tapering apical margin. These features are more consistent with
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an injection (rather than projectile) method of envenomation (Figure S2) (Wüster and Thorpe 1992).
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Similarly, the fact that all teeth of Uatchitodon apparently had two venom canals (with one facing
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lingually) renders spitting an improbable strategy.
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Based on thin-sections, Sues (1991) described the grooves on the teeth of Uatchitodon kroehleri
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as being enamel-lined, while the Moncure specimens of U. schneideri appear to have lost this enamel
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lining. The enamel lining may have been lost quickly when the channel was no longer exposed to the
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abrasion during feeding, especially considering the minimal thickness of Uatchitodon's enamel
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(<5µm). Sectioning revealed that growth lines in the dentine bend around the canals, rejecting the
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hypothesis that the channels are the result of wear, and, by extension, supporting the inference of
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functional role (Figure S3 A). Venom conduction is the only function served by hollow teeth in extant
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tetrapods, which shifts the null hypothesis for teeth with fully enclosed canals to venom conduction,
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per the comparative method described by Orr et al. (2007).
Uatchitodon possesses complex denticles, with some larger, primary denticles containing up to
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five secondary denticles (H-DS and JSM pers. obs.; Figure S3 B). This secondary denticulation is
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variable, with the larger Placerias Quarry specimens bearing more extreme secondary denticulation
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than the specimens known from Moncure (Figure S3 B) or the Tomahawk specimens (Sues et al.,
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1994). The Moncure specimens have 8 denticles/mm, consistent with the reported value for Tomahawk
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locality specimens (Sues 1991) but less than the 14/mm on the Placerias Quarry specimens, although
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the high degree of secondary denticulation in the Placerias Quarry specimens of U. schneideri makes
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discerning the limits of (and thus counting) ‘true’ denticles difficult.
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The tooth NCSM 24731 was transversely sectioned twice, and the top and bottom of each
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section was imaged for a total of four measurable points (shown in Figure S3 C, with the measurements
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summarized in Table S2). The middle of the tooth appears to have more medial canals, while the apical
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and basal regions of the tooth have more distally offset canals. Paired data from other tooth fragments
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shows that the apical portion of the canals tends to be offset more than the basal portion (one-tailed
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paired Wilcoxon test of apical and basal surfaces of teeth, N=15, p: 0.009). Lateral compression and the
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circularity of the lingual canal are both found to be nearly constant. Paired data from other teeth
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(lingual canals basal and apically) weakly support the consistency of shape of the lingual canal by
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failing to find a difference (two-tailed paired t-Test, N=10, p: 0.11, one-tailed with the apical canals
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being more rounded, p: 0.055), but there are too few paired samples of the labial canal (N=5) to even
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attempt a test.
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Our interpretation of venom conduction in Uatchitodon is supported by dentine growth lines
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that indicate that the canals are not a product of wear and by the fact that Uatchitodon schneideri
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possesses fully enclosed canals (Fig. S3A). Further paleobiological inferences can be made through
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statistical analysis, and so we analyzed canal shape with respect to size, lateral compression, distal
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offset and carina angle. Also, because Uatchitodon is the only known taxon to have two venom canals
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per tooth, we analyzed the differences between the canals to look for trends.
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We tested the hypothesis that ontogeny may play a role in canal-shape variation, using size
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(fore-aft crown length, or mesiodistal length) as a proxy. We found support for a negative correlation of
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size with respect to average apical canal shape, with more circular canals in smaller teeth (N=20, rho: -
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0.69, p: <0.001). A correlation persists even when size and lingual canal shape are compared (N=19,
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rho: -0.66, p: 0.002). To try to remove the confounding variable of position within the tooth (which has
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an obvious effect on mesiodistal length, and which the paired data suggest impacts canal shape), the
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mesiodistal width (fore-aft length) of six tooth fragments that still preserved their tips were measured at
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0.1mm intervals below the tip. These data form the basis of a model predicting the distance a fragment
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was from its tip based on the mesiodistal width (one carina to the other along the apical-most
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measurable surface, similar to fore-aft basal length of Currie et al., 1990) with the resulting equation
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being: [Distance from tip]=1.607*[Fore-aft Length]1.7432. Although it was nearly complete, NCSM
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24753 was excluded from the analysis due to its more anterior jaw position. This equation was then
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used to estimate the distance from the tip of many of the teeth and the results were evaluated with a
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regression analysis (where lingual canal shape was the dependent variable, and distance from the tip
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and mesiodistal width the independents). This analysis revealed that whereas canal shape varied
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significantly with distance from the tip and length separately (N=25, R2-adj: 0.131 & 0.133, p: 0.045 &
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0.046), it did not vary significantly when the covariance of the two variables was compensated for
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(N=25, R2-adj: 0.09, p: 0.140) nor with either individually when compensated for one another (distance
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from tip’s p: 0.946 and mesiodistal length’s p: 0.811) allowing us to tentatively reject size variation (a
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proxy for ontogeny) as the primary source of canal shape variation.
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Lateral compression did not vary significantly with any other value, but instead held relatively
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constant with a mean of 0.55+/-0.07 for the measured apical sections (N=26; Fig. S4A). Despite this
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lack of trends, the teeth of U. kroehleri are more labiolingually flattened than those of U. schneideri
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(one-tailed Wilcoxon: N=14 U. kroehleri, N=26 U. schneideri; p-value:<0.01). A one-tailed paired
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Wilcoxon test between the basal and apical lateral flattening (N=16) shows a significant difference (p:
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0.02) with the basal portion more rounded (less labiolingually compressed) than the more apical
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portions. The Tomahawk specimens, however, showed a lateral compression with a mean of 0.45+/-
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0.06 for N=8 (Fig. S4B).
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We explored how the two canals related to one another by comparing their shape, heights and
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width. As Uatchitodon bears two venom grooves on each tooth, and venom is widely presumed to be
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metabolically “expensive”, it might be hypothesized that one canal would be “favored”. We did find a
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significant difference between the canal shapes, but this may be a function of tooth geometry given our
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findings of consistent canal width. The total range of variation in canal shape (width divided by height)
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is from 0.59 to 1, with the labial canal being significantly more rounded (one-tailed paired Wilcoxon
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test, N=22, p: 0.015). This variation possibly has less to do with a difference in canal width between
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sides (two-tailed paired Wilcoxon test, N=22, p: 0.081) but rather a greater height in the lingual canal
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(one-tailed paired Wilcoxon test, N=22, p: 0.019). These data are consistent with the data from NCSM
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24731, in which the height was the main canal metric that varied through the tooth. We also compared
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the average shape of the canals apically and basally, and found a significant difference, with basal
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canals more rounded than apical canals, but we were limited by low sample size (one-tailed paired
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Wilcoxon test, N=8, p: 0.039). The lingual canal showed nearly significant difference apically and
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basally (one-tailed [basal less rounded than apical] paired Wilcoxon test, N=10, p: 0.055), but the labial
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canal lacked enough paired data points to make any meaningful comparison (N=5).
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4. Critical Assessment of Purportedly Venomous Fossil Taxa
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Venomous non-squamate reptiles and mammals are rare, but they have been described and we
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critically evaluate the evidence supporting inferences of venom use. The Middle Jurassic
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sphenodontian Sphenovipera was diagnosed and described as potentially venomous based on its
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possession of enlarged anterior teeth with mesial grooves (putative fangs), the capacity for a wide gape,
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and a mandibular vacuity (Reynoso, 2005). These features in combination support the inference of
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venom use, though a more detailed analysis of the teeth to evaluate alternative hypotheses, such as
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wear, is warranted. The Late Permian therocephalian synapsid Euchambersia features grooved fangs
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and vacuities below each putative fang, as well as a distinct groove that leads from the vacuity to the
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maxillary fangs (Nopcsa 1933, Mendrez 1975, Folinsbee et al. 2007) and thus was almost certainly
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venomous. Some conodonts (if they are indeed vertebrates) have recently been reported as featuring
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fangs with a long and narrow groove that has been established as being controlled by conodont growth
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(as opposed to wear) (Szaniawski 2009). A dromaeosaurid theropod, Sinornithosaurus, was also
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recently proposed to have had oral venom conduction, based on grooved and “enlarged” maxillary and
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dentary teeth, as well as a maxillary vacuity (subfenestral fossa) and putative channels for venom
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transport (Gong et al. 2009). However, given the strutting and webbed nature described and illustrated
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(Gong et al. 2009, fig. 2) for the subfenestral fossa, it could easily be a pneumatic opening, as is
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common in theropods (especially maniraptoran coelurosaurs; Gianechini et al., in press). The putative
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venom grooves on the teeth resemble the depressions on the basal portion of the tooth crown in other
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theropods (H-DS, pers. obs.). Also, unlike Euchambersia and extant venomous forms, the hypothesized
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venom-conducting channel (“supradental groove”) lies external to the bone, and the relationship of the
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foramina of the supradental groove with the dental grooves has yet to be established. Grooved teeth are
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illustrated on the dentary (see Gong et al. 2009, Fig. 2), although the dentary lacks a vacuity similar to
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that of the maxilla, thus calling into question their association. The enlargement of the maxillary teeth
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of Sinornithosaurus is interpreted as evidence of their use in the envenomation of bird prey, yet the
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authors note “[i]nterestingly, much of the effective erupted length of the teeth is composed of the tooth
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root,” which raises the distinct possibility that the “enlargement” is actually the result of the teeth
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dropping out of their sockets after death (Gianechini et al., in press). Given the documented variation in
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theropod dentition (e.g., Smith et al., 2005), the known cranial pneumaticity of archosaurs, especially
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theropod dinosaurs (e.g., O'Connor and Claessens 2005), postulated pneumatic function of the
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antorbital fenestra (Witmer 1997), the presence of nutrient foramina on most amniote jaws, and the
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known problems of inferring venom conduction from a shallow, open groove (Orr et al. 2007), we
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argue that venom conduction in Sinornithosaurus is questionable. The last known extinct venomous
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non-squamate is Uatchitodon (Sues 1991). Known only from isolated teeth to date, it lacks many of the
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argued venom-related adaptations seen in the aforementioned taxa, but it represents the only known
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tetrapod other than certain snakes and Solenodon to have teeth with fully enclosed canals (Kaye and
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Padian 1994; Sues 1996), which we show cannot be explained any hypothesis other than venom-
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conduction.
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Table S1: Average measurements for the Moncure locality Uatchitodon teeth, see Figure S1 for
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measurement protocols. Lateral compression was found by dividing the Greatest Width by the sum of
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the Mesial Carina to Canals and the Distal Carina to Canals. Canal shape was found by dividing the
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Canal Width by the Canal Height. Distal offset was found by dividing the Distal Carina to Canals by
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the Mesial Carina to Canals. The path of the Mesial Carina to Canals and Distal Carina to Canals
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measures is the same path used to measure the angle of the carina.
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Table S2: Measurements for NCSM 24731. Measurements for the ‘base’ and ‘through the apertures’
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sections were taken from SEM images captured before embedding the tooth in epoxy resin.
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Figure S1: Measurements taken from the Moncure Uatchitodon specimens. The mesial carina is at top,
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and the lingual surface is to the right. See Table S1 for results.
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Figure S2: Diagram showing gross morphology of venomous teeth of cobras (A-B) to Uatchitodon
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schneideri (C-E). A-B modified from Wüster and Thorpe (1992, Fig. 2) showing the aperture
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morphology of a spitting cobra fang (A) and a non-spitting cobra fang (B). C shows the Placerias
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Quarry specimen (MNA V3680) and D shows a putative premaxillary specimen from Moncure (NCSM
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24753). E shows NCSM 24732, with arrows denoting the extent of the aperture. Note that all of the
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specimens of Uatchitodon schneideri show an oval aperture with a shallow apical margin, reminiscent
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of the non-spitting cobra. Further, specimens with tips show a distinct mesiolabial curvature of the
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tooth apical to the aperture, similar to what is seen in many extant venomous snakes.
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Figure S3: Details of the morphology of Uatchitodon teeth. A, Sectioned Uatchitodon tooth (NCSM
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24731), with the surface of the tooth to the bottom right, and a portion of the canal in the upper right,
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note the curvature of the incremental growth lines of dentine, suggesting a developmental origin for the
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canal. B, A close-up of the serrations on NCSM 25252 (1) and MNA V3680 (2) from the Placerias
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Quarry; scale bars = 0.1 mm. C, The sections of NCSM 24731 with apical to the right, and the base of
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the tooth (1), the bottom of the first section (2), the top of the first section (3) and the bottom of the
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second section (4) all scaled to 0.5 mm.
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Figure S4: Graphs showing the Uatchitodon lateral compression data (A-B), and size-related data (C-
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D). Lateral compression of A) Uatchitodon schneideri and B) Uatchitodon kroehleri. C) Plot of the ln
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of the distance from the tip as a function of the ln of tooth width (least-squares regression yields an R2-
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ad: 0.97, p: <0.001), and D) canal-shape as a function of the fore-aft length and distance from the tip.
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