Description and Classification

Zapsalis is identified solely from isolated fossil teeth; no other skeletal remains have been definitively assigned to this genus. These teeth are typically small, blade-like, and curved backwards, characteristic of predatory dinosaurs. The specific features of these teeth have led most paleontologists to classify Zapsalis within the Dromaeosauridae family. Dromaeosaurids, often called “raptors,” were generally small to medium-sized, agile carnivores, famous for members like Velociraptor and Deinonychus, and known for their sickle-shaped claws on their feet, though such claws cannot be confirmed for Zapsalis.

The validity of Zapsalis as a distinct genus is a topic of debate among scientists. Some researchers suggest that the teeth attributed to Zapsalis may actually belong to other known dromaeosaurids from the same time and place, such as Saurornitholestes or Dromaeosaurus. When a dinosaur is known only from such fragmentary material, it is sometimes referred to as a “tooth taxon.” This can make it difficult to understand what the whole animal looked like or how it differed from its relatives. As a result, Zapsalis is often considered a nomen dubium, meaning a “doubtful name,” until more complete fossil evidence is discovered.

Distinguishing Features

The features that distinguish Zapsalis are based entirely on the morphology of its teeth:

• Teeth are relatively small and laterally compressed (flattened from side to side).
• A notable backward curve (recurvature) is present in the tooth crown.
• The serrations (denticles) along the edges of the teeth are distinctive. The posterior (back) edge typically has fine, numerous serrations.
• The anterior (front) edge often displays coarser, more hooked serrations, particularly towards the tip of the tooth. Some teeth may lack serrations at the base of the anterior edge.

These dental characteristics are similar to other dromaeosaurids but are considered by some to be unique enough to warrant a separate genus name.

Paleoenvironment and Diet

Fossil teeth of Zapsalis have been discovered in the Judith River Formation of Montana, USA, and the Dinosaur Park Formation of Alberta, Canada. During the Late Cretaceous period (specifically the Campanian age), these regions were part of a vast coastal plain on the western edge of the Western Interior Seaway, an ancient sea that divided North America. The environment was warm and humid, characterized by rivers, floodplains, swamps, and lush forests.

Zapsalis shared this rich ecosystem with a diverse array of other dinosaurs. These included large herbivores like the hadrosaur Parasaurolophus, ceratopsians such as Centrosaurus and Chasmosaurus, and armored ankylosaurs. Apex predators of the time included tyrannosaurids like Gorgosaurus. Given its dromaeosaurid-like teeth, Zapsalis was undoubtedly a carnivore. It likely preyed on smaller animals, such as juvenile dinosaurs, small mammals, lizards, and possibly fish, or scavenged carcasses. Its small size, inferred from its teeth, suggests it was not tackling large prey on its own.

Significance and Ongoing Research

Zapsalis holds historical significance as one of the numerous dinosaur taxa named by Edward Drinker Cope during the “Bone Wars” of the late 19th century, a period of intense fossil discovery and rivalry in American paleontology. The study of Zapsalis highlights the challenges paleontologists face when working with incomplete fossil remains, particularly “tooth taxa.”

The ongoing debate about whether Zapsalis represents a unique dinosaur or if its teeth belong to a previously known species underscores the meticulous process of scientific classification. Researchers continue to analyze theropod teeth from Late Cretaceous formations using advanced techniques like scanning electron microscopy to study fine details of tooth wear and serration patterns. These studies aim to better differentiate between similar-looking teeth and refine our understanding of dromaeosaurid diversity. Even if Zapsalis is eventually synonymized with another genus, its teeth still provide valuable information about the presence and characteristics of dromaeosaurid predators within its ancient ecosystem, contributing to a fuller picture of life during the Age of Dinosaurs.