Xiphactinus audax Leidy
|Description: This predatorily primitive tarpon is represented at Big Brook by its teeth and occasionally by its vertebrae.
The upper anterior teeth are large and elongated. They range from 3 to 6 cm (about .75 to 2.25 in) in length. They have a slight curve posteriorly and are oval in cross section at the root becoming more lens shaped and laterally compressed towards the tip. The surface of these teeth is covered with a thin coat of glossy looking enamel and is only marked with very fine growth cracks that resemble striations. Sometimes the thin enamel has been worn off by stream action. There are two cutting edges that are on the sides of the tooth that runs most of the length of the tooth. The lower anterior teeth are very similar to the upper anterior teeth but are less curved, more laterally compressed and proportionally smaller. These teeth have a shallow conical indent on the bottom of the root for attachment to the jaws.
The vertebrae are large and range upwards to 4 to 5 cm (about 1.5 to 2 in) in diameter and around 3 to 4 cm (about 1.25 to 1.5 in) thick. The vertebrae are strongly concaved and are marked with many different sized longitudinal pits, some of which reach all the way down to the middle of the vertebrae
Commonality: The large dagger-like teeth Xiphactinus audax occurs uncommonly at Big Brook. The vertebrae are rare. Both are, on the other hand, reportedly more common at other Atlantic Coastal Plain localities and in the Mid-West.
Similar fossils: The teeth are sometimes confused with shark, Enchodus, and plesiosaur teeth but attention to a few details can clear up any problems. Tarpon teeth lack the general shape and the heavy enamel coating of shark teeth. Most shark teeth are also smaller. The upper lateral teeth of Enchodus are smaller while the teeth of the tarpon are larger. They also lack the conical shaped indent in the bottom of the root. Plesiosaur teeth lack the large cutting edges of tarpon teeth. The vertebrae are similar to shark's vertebrae but tarpon vertebrae are longer than the diameter. Shark's vertebrae usually have a diameter larger than the length.
Size: The fossil record of this predacious fish indicates that these tarpons could obtain incredible lengths, up to 13.3 meters (about 40 feet) long from some of the Mid-West localities. Based on fossil evidence from the teeth and vertebra from the Atlantic Coastal Plain, these fish were generally smaller, reaching only up to about 1 to 2 meters (about 3 to 6 feet) long.
Notes: Most of what paleontologists know about this fossil tarpon comes from comparing them to the modern tarpons. In life, they probably hunted other fish in the open surface waters of the Cretaceous seas. They are related to the bonefish.
a. Front / Back b. Side c. Cross section