Description and Classification

Tawa was a bipedal carnivore, meaning it walked on two legs and ate meat. It was relatively small and agile, estimated to be about 2 to 4 meters (6.5 to 13 feet) long and likely weighed around 15 to 30 kilograms (33 to 66 pounds). It possessed a slender build with a long neck, a long tail for balance, and sharp, recurved teeth typical of predatory dinosaurs. Its hands had three functional fingers, useful for grasping prey.

In terms of classification, Tawa is considered a basal, or very early, theropod dinosaur. Its discovery in 2009 was significant because its skeleton shows a mix of features: some are primitive, similar to very early dinosaurs like Herrerasaurus, while others are more advanced and link it to the group called Neotheropoda, which includes dinosaurs like Coelophysis and most later theropods. This mosaic of traits makes Tawa a key species for understanding the evolutionary steps that led to the great diversity of theropods. Scientists place Tawa very close to the base of the Neotheropoda branch, suggesting it was a close relative of the common ancestor of many familiar carnivorous dinosaurs. One notable advanced feature is the presence of air sacs (pneumaticity) in its neck vertebrae, making the neck lighter, a trait that became widespread in later theropods and eventually birds.

Distinguishing Features

Tawa can be identified by several unique anatomical characteristics:

• A distinct kink or flexible joint in its upper jaw, between the maxilla and premaxilla bones, which might have helped in grasping prey.
• A relatively long and slender neck compared to some other early theropods.
• Air-filled pockets (pneumaticity) in its cervical (neck) vertebrae, a feature more common in later, more derived theropods.
• Specific characteristics in its ankle bones (astragalus and calcaneum) that are intermediate between those of very early theropods and more advanced neotheropods.
• The anatomy of its femur (thigh bone) also exhibits a blend of primitive and derived traits.
• Relatively long shin bones (tibia) compared to its thigh bone (femur), suggesting it was a fast runner.

Paleoenvironment and Diet

Tawa lived in what is now New Mexico during the Late Triassic period. The fossils were found at Ghost Ranch, in rocks of the Chinle Formation. During this time, the area was a warm, semi-arid floodplain with rivers and streams that experienced distinct wet and dry seasons. The ecosystem was rich with diverse flora, including conifers, cycads, and ferns, along with a variety of animal life.

Tawa shared its habitat with other early dinosaurs, such as the abundant small theropod Coelophysis and the herrerasaurid Chindesaurus. Other prominent animals included large crocodile-like phytosaurs, heavily armored aetosaurs, formidable predatory rauisuchians, and early relatives of mammals called cynodonts. As a carnivore, Tawa likely preyed upon smaller animals available in its environment. Its diet might have included small reptiles, early mammal relatives, and perhaps the young of other dinosaurs or larger herbivores. Its sharp teeth and agile build would have made it an effective hunter of small, quick-moving prey.

Significance and Ongoing Research

The discovery of Tawa has had a significant impact on our understanding of early dinosaur evolution. Its remarkably complete fossils provided crucial data that helped clarify the relationships among the earliest theropods. Tawa confirmed that dinosaurs like Herrerasaurus were indeed early members of the theropod lineage or very close relatives. It also helped demonstrate that the three main dinosaur groups – Theropoda (carnivores), Sauropodomorpha (long-necked herbivores like Plateosaurus), and Ornithischia (bird-hipped herbivores like Lesothosaurus) – had already diverged and were distinct by the Late Triassic.

Furthermore, the existence of Tawa in North America, with features linking it to dinosaurs from South America, supports theories about the geographic origins and dispersal of early dinosaur groups across the supercontinent Pangea. Research continues on the Tawa fossils, focusing on detailed anatomical studies to refine its growth patterns, biomechanics, and precise placement within the theropod family tree. Paleontologists also continue to explore localities like Ghost Ranch, hoping to unearth more fossils that can provide further insights into the dawn of the age of dinosaurs.