Grooves to tubes: evolution of the venom delivery system in a Late Triassic “reptile”

Abstract

Venom delivery systems occur in a wide range of extant and fossil vertebrates and are primarily based on oral adaptations. Teeth range from unmodified (Komodo dragons) to highly specialized fangs similar to hypodermic needles (protero- and solenoglyphous snakes). Developmental biologists have documented evidence for an infolding pathway of fang evolution, where the groove folds over to create the more derived condition. However, the oldest known members of venomous clades retain the same condition as their extant relatives, resulting in no fossil evidence for the transition. Based on a comparison of previously known specimens with newly discovered teeth from North Carolina, we describe a new species of the Late Triassic archosauriform Uatchitodon and provide detailed analyses that provide evidence for both venom conduction and document a complete structural series from shallow grooves to fully enclosed tubular canals. While known only from teeth, Uatchitodon is highly diagnostic in possessing compound serrations and for having two venom canals on each tooth in the dentition. Further, although not a snake, Uatchitodon sheds light on the evolutionary trajectory of venom delivery systems in amniotes and provide solid evidence for venom conduction in archosaur-line diapsids.

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Acknowledgments

We would like to acknowledge two anonymous reviewers for their helpful critiques and comments, as well as the North Carolina Museum of Natural Sciences field crew for excavating the Moncure locality, Liz Sues, Pete Kroehler, and Ken Pitt for assistance in the excavation of the Tomahawk locality and the Appalachian State University Department of Geology and Office of Student Research for travel and research grants.

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Correspondence to Jonathan S. Mitchell.

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Figure S1
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Measurements taken from the Moncure Uatchitodon specimens. The mesial carina is at top, and the lingual surface is to the right. See Table S1 for results. (JPEG 1209 kb)

Figure S2
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Diagram showing gross morphology of venomous teeth of cobras (ab) to U. schneideri (ce). a, b Modified from Wüster and Thorpe (1992; Fig. 2) showing the aperture morphology of a spitting cobra fang (a) and a non-spitting cobra fang (b). c shows the Placerias Quarry specimen (MNA V3680) and d shows a putative premaxillary specimen from Moncure (NCSM 24753). e shows NCSM 24732, with arrows denoting the extent of the aperture. Note that all of the specimens of U. schneideri show an oval aperture with a shallow apical margin, reminiscent of the non-spitting cobra. Further, specimens with tips show a distinct mesiolabial curvature of the tooth apical to the aperture, similar to what is seen in many extant venomous snakes. The voucher specimens to the upper right of each diagram in Figure 2 are: A2-USNM542520, A3-USNM542523, A4-USNM542521, A5-NCSM 25238, A6-NCSM 25241, B1-USNM542524, B2-USNM542519, B3-USNM542518, B4-NCSM24753, B5-MNA3680. (JPEG 1330 kb)

Figure S3
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Details of the morphology of Uatchitodon teeth. a Sectioned Uatchitodon tooth (NCSM 24731), with the surface of the tooth to the bottom right and a portion of the canal in the upper right, note the curvature of the incremental growth lines of dentine, suggesting a developmental origin for the canal. b A close-up of the serrations on NCSM 25252 (1) and MNA V3680 (2) from the Placerias Quarry; scale bars = 0.1 mm. c The sections of NCSM 24731 with apical to the right and the base of the tooth (1), the bottom of the first section (2), the top of the first section (3), and the bottom of the second section (4) all scaled to 0.5 mm. (JPEG 1734 kb)

Figure S4d
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ESM 1

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Figure S4a

Graphs showing the Uatchitodon lateral compression data (a, b) and size-related data (c, d). Lateral compression of a U. schneideri and b U. kroehleri. c Plot of the ln of the distance from the tip as a function of the ln of tooth width (least-squares regression yields an R 2-ad = 0.97, p < 0.001) and d canal shape as a function of the fore-aft length and distance from the tip. (PDF 3 kb)

Figure S4b

(PDF 2 kb)

Figure S4c

(PDF 10 kb)

Table S3

(XLS 36 kb)

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Mitchell, J.S., Heckert, A.B. & Sues, H. Grooves to tubes: evolution of the venom delivery system in a Late Triassic “reptile”. Naturwissenschaften 97, 1117–1121 (2010). https://doi.org/10.1007/s00114-010-0729-0

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Keywords

  • Venom
  • Uatchitodon
  • Uatchitodon schneideri
  • Evolutionary trajectory
  • Triassic