Description and Classification

Enigmosaurus was a medium-sized member of the Therizinosauria group. Scientists estimate it grew to be about 5 meters (16 feet) long and may have weighed between 400 to 900 kilograms (880 to 1980 pounds). Like other therizinosaurs, Enigmosaurus likely had a small head on a long neck, a bulky body, and strong hind limbs for walking on two legs (bipedal). A key feature of therizinosaurs, which Enigmosaurus probably shared, was the presence of very large claws on its hands. These claws could have been used for pulling down branches to eat leaves, or for defense against predators. Although no skin impressions have been found for Enigmosaurus itself, many of its relatives, like Beipiaosaurus, are known to have possessed feathers, so it is likely Enigmosaurus was also feathered.

Enigmosaurus belongs to Therizinosauria, an unusual group of theropod dinosaurs that adapted to a plant-eating lifestyle. Therizinosaurs are classified as maniraptoran theropods, meaning they are relatively closely related to birds, as well as to other dinosaur groups like oviraptorosaurs (such as Oviraptor) and dromaeosaurs (such as Velociraptor). Within Therizinosauria, Enigmosaurus is placed in the family Therizinosauridae. Other well-known therizinosaurs include the giant-clawed Therizinosaurus, Segnosaurus, Erlikosaurus, and the North American Nothronychus. The exact position of Enigmosaurus within this family is still studied, as its fossils are incomplete.

Distinguishing Features

Enigmosaurus can be distinguished from other therizinosaurs primarily by the unique characteristics of its pelvis. These features were considered “enigmatic” at the time of its discovery:

• The pubis bone (part of the hip) was less swept back and more vertically oriented compared to the strongly backward-pointing pubis found in many other advanced therizinosaurids from the same time, such as Segnosaurus or Erlikosaurus.
• The ilium (the large, uppermost bone of the pelvis) had distinctive features that differed from those of other therizinosaurs.
• The ischium (the lower, rear-pointing bone of the pelvis) also showed unique characteristics.
• It possessed a generally robust build for a therizinosaur of its size.

Paleoenvironment and Diet

Fossils of Enigmosaurus were discovered in the Bayanshiree Formation in Mongolia. During the Late Cretaceous period, this area was likely a semi-arid floodplain environment with meandering rivers and distinct wet and dry seasons. This ecosystem supported a variety of dinosaurs. Enigmosaurus shared its habitat with other dinosaurs such as the hadrosauroid Gobihadros, the armored ankylosaur Talarurus, the large predatory theropod Achillobator, and even other therizinosaurs like Erlikosaurus and Segnosaurus. Turtles and crocodilians were also present in this ancient environment.

Like other therizinosaurs, Enigmosaurus is considered to have been an herbivore, meaning it ate plants. While teeth have not been found for Enigmosaurus, its close relatives had small, leaf-shaped teeth suited for stripping vegetation. Its wide pelvic structure suggests it had a large gut for digesting tough plant material, and its long neck would have allowed it to browse on leaves from trees and tall shrubs. The large claws on its hands were probably used to pull branches closer to its mouth or possibly for digging, rather than for hunting.

Significance and Ongoing Research

Enigmosaurus is significant because it was one of the earlier therizinosaurs to be described, contributing to the initial understanding of this peculiar group of dinosaurs. Its distinct pelvic anatomy helped to highlight the anatomical diversity within Therizinosauria and fueled scientific discussion about their evolutionary relationships and how they evolved from carnivorous ancestors to become plant-eaters. Enigmosaurus serves as an important example of the diverse adaptations that occurred within theropod dinosaurs.

Ongoing research continues to shed light on Enigmosaurus. The discovery of more complete fossil material, especially a skull or more limb bones, would greatly help scientists better understand its full anatomy and lifestyle. Paleontologists also continue to study its phylogenetic position, aiming to clarify its precise relationship to other therizinosaurs, particularly those like Erlikosaurus and Segnosaurus that were found in similar geological formations. Research into the paleoecology of the Bayanshiree Formation also helps scientists reconstruct the environment Enigmosaurus lived in and how it interacted with other species.