Daliansaurus (meaning "Dalian reptile") is a genus of small troodontid theropod dinosaur, measuring approximately 1 metre (3 ft 3 in) long, from the Early Cretaceous of China.
In the Lujiatun Beds of the Yixian Formation, a volcanically-influenced region with a cold climate, Daliansaurus lived alongside its closest relatives - Sinovenator, Sinusonasus, and Mei, with which it forms the group Sinovenatorinae.
Daliansaurus is a small, lightly-built, and most likely feathered troodontid measuring approximately 1 metre (3 ft 3 in) in length,[1] similar in size to Sinovenator and Sinusonasus.
The ribs of two of the dorsal vertebrae unusually appear to bear uncinate processes, in the form of curved and rod-like projections similar to those of Velociraptor,[6] Linheraptor,[7] and Heyuannia.
Parts of the specimen had been reconstructed and do not represent genuine fossil material, including the back of the skull and the rear portion of the lower jaw.
Furthermore, the shoulder girdles are missing from the fossil, as is the right hindlimb; the left humerus, most of the pelvis, and the last few caudal vertebrae are buried in the surrounding matrix.
[1] In 2017, DNHM D2885 was described in a research paper published in Acta Geologica Sinica and authored by Shen Caizhi, Lü Junchang, Liu Sizhao, Martin Kundrát, Stephen Brusatte, and Gao Hailong.
[1] Later, Shen, Lü, Gao, and Kundrát published a histological analysis of the ulna and radius of DNHM D2885, along with Masato Hoshino and Kentaro Uesugi, in the journal Historical Biology.
They used the analysis to identify traits that allowed Daliansaurus to be placed in the Troodontidae, concurring with previous assessments:[10] the teeth are numerous and closely packed; the bar of the premaxilla separating the nostrils is flattened; there is a groove on the side of the dentary in the lower jaw; the neural spines are replaced by shallow grooves towards the end of the tail; and the metatarsals are asymmetrical, with the fourth being more robust than the second.
The analysis also found that Daliansaurus formed a distinct evolutionary radiation along with other troodontids in the Yixian Formation: Mei, Sinovenator, and Sinusonasus.
Several characteristics distinguish this subgroup, but were used by Shen and colleagues to diagnose the wider Sinovenatorinae: the premaxilla is only 10% the length of the maxilla at the bottom of the jaw; there is a contact between the premaxilla and the nasal bone below the level of the nostril; there are no serrations on the front edges of some teeth; the articular processes known as zygapophyses are unfused in the sacrals; and the process known as the antitrochanter is located behind and above the hip socket, or acetabulum.
The seasonal growth may have been triggered by periods of low temperature; a similar phenomenon is observed among modern animals living in cold climates, including even endothermic mammals.
[11] However, the individual probably died during a warm period, based on the tightly spaced deposition and the presence of primary osteons near the outer surface of the bone.
Thin peripheral growth bands are visible between the LAGs in the outer two zones of the radius, which constitute an external fundamental system (EFS).
However, the presence of some neurovascular canals near the surface periosteal layer, as well as some ill-defined osteons in the third zone, provide evidence that growth had still not finalized at the time of death.
These included the microraptorine Graciliraptor lujiatunensis; the oviraptorosaur Incisivosaurus gauthieri; the ornithomimosaurs Shenzhousaurus orientalis[16] and Hexing qingyi;[17] the proceratosaurid tyrannosauroid Dilong paradoxus;[16] the titanosauriform sauropod Euhelopus sp., based on teeth;[18] the ornithopod Jeholosaurus shangyuensis; and the ceratopsians Psittacosaurus lujiatunensis (including P. major and Hongshanosaurus houi[19]) and Liaoceratops yanzigouensis.
[16] Mammals were also present in the Lujiatun Beds, including Acristatherium yanensis,[20] Gobiconodon zofiae, Juchilestes liaoningensis, Maotherium asiaticus, Meemannodon lujiatunensis, Repenomamus giganticus, and R.
[26] Many of the Lujiatun fossils exhibit exceptional three-dimensional preservation; the presence of tuffaceous sandstone and conglomerate debris flow deposits suggests that the animals were killed suddenly and catastrophically by lahar from a nearby shield volcano.