A new study of venomous reptile fossils sheds light on the evolution of snake fangs.
By Hans-Dieter Sues
Venom is a highly effective means to subdue and kill prey before eating it, as well as a great defense against predators. Furthermore, studies have shown that some snake toxins can also help in breaking down proteins and thus may aid in digestion.
The use of venom among present-day land-dwelling vertebrates is largely restricted to colubroid snakes (which include all known venomous species of snakes), two species of lizards (the Gila Monster and the Beaded Lizard), and, among mammals, some species of shrews and the nearly extinct Caribbean insectivore Solenodon. However, a recent study has argued that some toxins may also be present in other lizards.
Up to 600 of the nearly 3,000 known species of present-day snakes are potentially harmful to humans, but only about 50 species are implicated in the majority of reported snakebites. Snakes have a variety of tooth structures for venom delivery. Some species have teeth in which an open groove extends along much of the height of the tooth crown and others have tubular fangs with a fully enclosed venom canal.
There is still little fossil evidence for the evolution of snake fangs. The oldest known fangs of venomous snakes, dating from the early Miocene Epoch (some 20 million years ago), already have fully enclosed canals.
Studies on the development of the fangs in present-day snakes have shown that these teeth change in structure during their development in the snake’s mouth. Early-stage teeth have a groove along the crown, but later the edges of the groove approach each other and, in late-stage teeth, fuse to enclose a canal. During this change, the pulp cavity is reduced to a narrow crescent when viewed in a cross-section of the tooth crown.
A study just published in the online edition of the scientific journal Naturwissenschaften by a team of American researchers (including the author of this blog entry) on teeth of ancient reptiles not related to snakes now suggests that the development of snake fangs mirrors the evolutionary change from grooves to canals for venom conduction in the extinct forms.
The teeth of the Late Triassic Uatchitodon (named for an ancient Egyptian cobra goddess) show that the developmental data from snake fangs match the evolutionary change inferred in at least one other reptilian lineage. Although known only from isolated teeth to date, Uatchitodon is probably related to the Archosauriformes–the reptilian group including crocodylians, dinosaurs, and pterosaurs–on the basis of its teeth being implanted in sockets in the jaw bone (which can be inferred from the tooth roots) and the presence of serrated cutting edges on the tooth crowns. The tooth crowns are tall and flattened from side to side. The serrations on the cutting edges each have in turn their own serrated edges, creating a very effective cutting tooth!
Above, two figures illustrate the evolution of venom conduction on the tooth crowns of the Late Triassic reptile Uatchitodon. The left figure is a drawing of a tooth of the geologically older form (from Virginia) with an open groove along the side of the crown. The right figure is a scanning electron microscope image of a tooth of the younger species (from Arizona) with a small opening for the otherwise enclosed venom canal near the tip of the crown, marked by an arrow.
The oldest known species of Uatchitodon, from a Late Triassic fossil locality near Richmond, Virginia, has deeply infolded but enamel-lined grooves that extend along much of both the outside and inside of each tooth crown. A slightly younger form, known from sites in North Carolina and Arizona, has fully enclosed canals on both sides of the tooth crowns, each terminating in a small opening near the tip of the tooth. On some teeth, a faint line on the enamel surface still shows where the canal became fully enclosed as the tooth developed.
Teeth of Uatchitodon differ from those of present-day venomous snakes in having serrated cutting edges. Several researchers have recently claimed that the Komodo dragon, the largest extant lizard, is venomous. The dragon has recurved tooth crowns with serrated cutting edges, but there is no trace of any groove.
The older species of Uatchitodon may have used venom in a manner similar to the Gila Monster and Beaded Lizard, which have open grooves on their lower teeth. These lizards introduce venom into their prey by repeatedly biting and mouthing it. The toxin is produced by a gland on the lower jaw and drawn into the grooves by capillary action.
By contrast, venom delivery in the younger species of Uatchitodon must have resembled the sophisticated mechanism in venomous snakes. In the latter, contractions of jaw musculature press the secretion from the venom-producing gland in the upper jaw through the tubular fangs into the prey once its skin has been pierced. The opening of the venom canal is near but not right on the tip of the tooth. The arrangement resembles the beveled end of a hypodermic needle.
There have been other reports of features for venom delivery in extinct vertebrates, including a small predatory dinosaur, but most of these examples have not stood up to closer scrutiny. Researchers have suggested that the grooves on the teeth of some mammals may assist in honing the cutting edges of opposing teeth or in more easily extracting teeth after a bite. Producing venoms is physiologically costly for an animal, and most vertebrates have evolved other means for acquiring prey and defense.
Hans-Dieter (Hans) Sues is a vertebrate paleontologist based at the National Museum of Natural History in Washington, D.C. He is interested in the evolutionary history and paleobiology of vertebrates, especially dinosaurs and their relatives, and the history of ecosystems through time. A former member of the National Geographic Committee for Research and Exploration, Hans has traveled widely in his quest for fossils and loves to share his passion for ancient life through lectures, writings, and blogging.
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