A SIMPLIFIED KEY TO IDENTIFYING ISOLATED FOSSIL TEETH FROM LATE TRIASSIC ROCKS IN PETRIFIED FOREST NATIONAL PARK
Adrian P. Hunt
Department of Geology
University of Colorado at Denver
Denver, Colorado 80217
Spencer G. Lucas
New Mexico Museum of Natural History and Science
1801 Mountain Road N.W.
Albuquerque, New Mexico 87104
Vincent L. Santucci
Department of Parks and Recreation
Slippery Rock University
Slippery Rock, Pennsylvania 16057
Isolated teeth and jaw fragments are some of the most common fossils found in Upper Triassic rocks at Petrified Forest National Park. All these specimens can be identified to at least the family level using an identification key.
Isolated teeth or jaw fragments are among the most common remains of fossil vertebrates found at Petrified Forest National Park. These fossils occur in the Petrified Forest and Owl Rock Formations of the Chinle Group that are Late Triassic (about 210-230 million years old) in age. All well-preserved teeth can be identified to at least the generic level, and the purpose of this key is to provide a nontechnical means by which National Park Service personnel can identify them. We hope that this key will also be of use to students and amateur paleontologist working with Late Triassic vertebrates in other areas.
A FEW BASICS
Why so many teeth? Isolated teeth (teeth not associated with jaws) are common in Late Triassic rocks for several reasons:
Most lower vertebrate (reptile, amphibian, fish) teeth consist of an upper part, the crown and a lower part, root. The crown is covered by enameloid, similar to the enamel of human teeth, and the enameloid shiny if it is well preserved. The crown is often cone-shaped and has serrations on its edges. The crown is the "working part" of the tooth that is exposed in the mouth.
The root is the part of the tooth that fits in the jaw and it is usually cylindrical and not shiny, because it lacks an enameloid covering. When a reptile is about to replace a tooth it resorbs (dissolves and absorbs) the root of the tooth. Therefore, if you find a tooth with a large root you know that it came from a dead animal and was not a tooth that was replaced during life.
The teeth of carnivorous reptiles have small square serrations that are at right angles to the edge of the tooth (Fig. 1A). In contrast, the teeth of herbivorous reptiles have larger serrations that are blunt and point towards the top of the tooth (Fig. 1B).
THE CAST OF CHARACTERS
More than 90% of isolated teeth at Petrified Forest National Park represent 7 types of reptiles and amphibians. We have included here one type of tooth (prosauropod) which is not known from the park but which may be found in the future.
The most common Late Triassic vertebrates are phytosaurs, which are superficially crocodile-like reptiles with elongate snouts. Metoposaurs are one of the last of the groups of giant primitive amphibians that had large, flat heads shaped like shovels. The most common herbivores were aetosaurs which were large (up to 5 meters long) armadillo-like reptiles with heavy armor on their backs. The largest predators were rauisuchians which were quadrupedal animals with large heads. Rarer carnivores were the small and slightly built sphenosuchians, which were the ancestors of modern crocodiles.
Three groups of dinosaurs are found in the Late Triassic. All carnivorous dinosaurs were bipedal saurischians which were usually much shorter than a human in height. Similarly sized plant-eating dinosaurs are ornithischians, which were also bipedal. The largest Late Triassic dinosaurs were prosauropod which look like emaciated versions of their descendants, the sauropods (e.g., Diplodocus, Camarasaurus, Apatosaurus [Brontosaurus]).
Note that phytosaurs have several different kinds of teeth in the same jaw and so these appear at different places in the key. This condition is referred to as heterodonty.
Figure 1. The main kinds of isolated teeth and jaw fragments found in Upper Triassic rocks at Petrified Forest National Park. A, Saurischian tooth in side view with a close-up of the "carnivorous serrations"; Prosauropod tooth in side view with close-up of "herbivorous serrations"; C, Sphenosuchian tooth (stylized) in side view; D, Phytosaur tooth in side and top views; E, Aetosaur tooth in side view; F, Ornithischian tooth (stylized) in side and top views; G, Phytosaur tooth in side and bottom views; Metoposaur tooth in side and top views; I, Lungfish toothplate in top view; J, Three-pronged shark teeth; K, Button-like shark tooth; L-M, Conical fish teeth; N, Sphenodont upper jaw in side view with five triangular teeth; O, Trilphosaur teeth in top view.
This study was partially funded by the Petrified Forest Museum Association. Rany Pence drew the figures which are adapted from various scientific papers including those of Peter Galton and Phillip Murry.