Enigmosaurus (meaning "enigma lizard" or "enigmatic lizard") is a genus of therizinosauroid that lived in Asia during the Late Cretaceous period. It was a medium-sized, ground-dwelling, bipedal herbivore that represents the third therizinosaur taxon from the Bayan Shireh Formation, although it is known from the lower part. The genus is monotypic, including only the type species E. mongoliensis, known from a well-preserved pelvis and other tentative body remains.
Discovery and naming
thumb|left|280px|Fossil localities in Mongolia. Locality of Enigmosaurus in Khara Khutul, at Area D
The holotype, IGM 100/84, was discovered at the Khara Khutul locality in the Bayan Shireh Formation (sometimes called Baynshire Formation or the Baynshirenskaya Svita), southeastern Mongolia, dating from the Late Cretaceous period, and first reported in 1979 on a pelvic comparison with other theropod dinosaurs. At the time, little was known about therizinosaurs. In 1980, it was mentioned again, this time in the new infraorder created by the Mongolian paleontologists Rinchen Barsbold and Altangerel Perle: Segnosauria. Nicknamed as the "Dinosaur from Khara Khutul", it was shortly described and included into the Segnosauria.
thumb|Schematic reconstruction of the pelvis
Three years later, the type species, Enigmosaurus mongoliensis, was named and described in 1983 by Barsbold. The preserved elements consist of a partial skeleton, lacking the skull, that includes a well preserved pelvic girdle with other postcrania. The generic name, Enigmosaurus, was stated to be derived from Greek ' (, meaning enigma) and (, meaning lizard), in reference to the aberratic and unusual shape of its pelvis. The specific name, mongoliensis, refers to the country of its discovery, Mongolia. Some therizinosaur fossil findings at the Iren Dabasu Formation were first thought to be referable to Enigmosaurus. Although it is commonly known from the pelvis, several remains not mentioned in the original description of the holotype were labelled under the same specimen number (IGM 100/84). These elements are in poor condition compared to the pelvis, and they were not measured and illustrated: proximal end of a femur; large femoral shaft, might be a tibial shaft; some ribs; distal end of a humerus; a tentative radius and the proximal end of an ulna. The femur, however, was not found in association with the holotype individual, therefore it should be assigned to Therizinosauria incertae sedis; the other remains might pertain to the holotype. A very large left femur, measuring long, was labelled with the same specimen number too; however, it was not associated with the holotype due to its large size (larger than the pelvis itself). Nevertheless, it seems to approach the size of the closer Segnosaurus.
Possible synonymy with Erlikosaurus
Some paleontologists proposed that Enigmosaurus was likely the same animal as Erlikosaurus, since both were found in the same geologic formation, Enigmosaurus is known from pelvic remains, whereas the pelvis of Erlikosaurus is unknown and distinguishing traits between them were absent; if proven, this would make Enigmosaurus a junior synonym of Erlikosaurus. However, the Enigmosaurus pelvis doesn't resemble that of Segnosaurus as would be expected for the connection with Erlikosaurus, and there is a huge size difference. In addition, both genera are known from different strata (lower and upper), and they are considered to be distinct animals by most authors.
Description
thumb|left|Diagram of the sacrum, in A ventral and B dorsal views
Enigmosaurus was a relatively large-bodied therizinosaur, with an estimated length of and a weight between . As noted by Barsbold in the original description of Enigmosaurus, it can recognised by the following characteristics: the pubis and ischium are short; elongated margin in the anterior presymphyseal region of the distal pubis. Being a therizinosaur, it was likely a slow herbivore and/or omnivore, as suggested by most authors all over the time. The biodiversity across the formation was characterized by therizinosaurs, as seen on the remains of Enigmosaurus, Erlikosaurus and Segnosaurus. The holotype location, Khara Khutul, has also yielded the contemporary Segnosaurus Most of the remaining paleofauna from this formation is known from upper strata, whereas Enigmosaurus is known from lower strata. Numerous fossil fruit findings at the locality reflect the large presence of angiosperm plants on the formation. The unearthed fruits bear some resemblance with Abelmoschus esculentus; however, definitive taxonomic affinities are quite unclear.
See also
- Timeline of therizinosaur research
- Rinchen Barsbold
- Glossary of dinosaur anatomy
- Anatomical terms of location