Fossil Snakes on the Snake
Unlike many famous areas that boast about how big their fossils are, Hagerman Fossil Beds National Monument is famous for its small species. Many of the fossil localities within the Monument contain what paleontologists call microvertebrates, small animals like fish, frogs, shrews and mice. Often hard to spot on the ground while standing, these little fossils are collected by crawling around on your hands and knees with your nose inches away from the ground. Among the variety small animal fossils encountered this way are snakes.
Lacking arms and legs, snakes are mostly vertebrae and ribs so it is not surprising that vertebrae are the part most commonly preserved. The snake skulls and jaws are rarely found. The reason is that they tend to fall apart quickly after the animal dies. Since they are held together by muscles and ligaments, the skull is flexible enough to allow the snake to swallow its prey whole.
Snake vertebrae have a distinctive shape that allows them to be quickly identified. Each vertebra connects to the one before and behind it at the centrum, (main body of the vertebrae) via a ball and socket system. This type of connection provides the flexibility to the backbone that snakes use to slither along. While all snake vertebrae have this basic structure, fortunately the shape of the processes where muscles attach, and the proportions of the vertebrae of different snakes are sufficiently different that it is possible to tell the different types of snakes apart, even with only a single vertebrae.
Six different types of snakes are found as fossils at Hagerman Fossil Beds National Monument. Three of the fossil snakes: garter snake - Thamnophis, rattlesnake - Crotalus, and racer - Coluber are still found in the state. Today, Idaho has ten different types of snakes so the sample at Hagerman does not represent the same level of diversity present in the state. Changes in the environment through time have caused some snakes to disappear from the region, like the water snake - Nerodius, rat snake - Elaphe, and milk snake - Lampropeltis, Although the fossil wetlands found in the Fossil Beds may have been suitable to the water snake, the drying of the environment now makes the area unsuitable for this species. Yet, while this environmental change eliminated one type of snake, it made it possible for other types to enter the area. Likewise, snakes common in the area today, like the rattlesnake, which is often encountered on the Monument, may not have been as common in the past. Our fossil record for the rattlesnake is based on a single vertebra that was only recently found during excavations at the Horse Quarry.
Snakes like all animals and plants respond to environmental change and this is reflected in where they are found. An animal's distribution is not fixed but changes. It may expand as conditions become more favorable, or shrink during less favorable times. Every species, whether it is the fossil fauna at Hagerman, or the modern one, is the result of historical changes in the distribution of animals and is the result of the dynamic response of its animals to environmental changes.
Holman, J.A. 1968. Upper Pliocene snakes from Idaho. Copeia 1:152-158.
Mead,J. I., J.T. Sankey, and H.G. McDonald. 1998. Pliocene (Blancan) herpetofaunas from the Glenns Ferry Formation, Southern Idaho. Idaho Museum of Natural History Occasional Paper 36:94-109.
This article originally appeared in The Fossil Record, Summer 2000