Daliansaurus

Discovery and naming

Daliansaurus is known from a single nearly-complete skeleton preserved in three dimensions. 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. The head and neck of the fossil are curled backwards, while the tail itself is curved forwards; the left arm is extended while the right arm is tucked inwards. The specimen was found in Early Cretaceous (~126 Ma old, dating to the Barremian epoch) volcanic deposits of the Lujiatun Beds of the Yixian Formation, in the village of Lujiatun near Beipiao, Liaoning, China. It is stored in the Dalian Natural History Museum (DNHM) in Dalian, Liaoning, under the specimen number DNHM D2885.

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. They named the genus Daliansaurus after the city of Dalian, where the fossil is stored, and they named the type species after the province of Liaoning. 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. For this analysis, they destructively sampled thin sections from the midshaft portion of the bones.

Classification

A phylogenetic analysis was conducted by Shen and colleagues, who modified the Theropod Working Group dataset to add Daliansaurus and Sinusonasus. They used the analysis to identify traits that allowed Daliansaurus to be placed in the Troodontidae, concurring with previous assessments: 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. This group, which was named the Sinovenatorinae, was united by a synapomorphy not observable for Daliansaurus, the possession of a wide and flattened fourth metatarsal.

Within the Sinovenatorinae, Sinovenator, Sinusonasus, and Daliansaurus formed a subgroup to the exclusion of Mei. 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. Finally, the analysis united Daliansaurus and Sinusonasus to the exclusion of Sinovenator, with the two sharing the bifurcation of the chevrons at both ends near the end of the tail; and the fusion of the greater and lesser trochanters on the femur into a continuous trochanteric crest.

The results of the analysis conducted by Shen and colleagues are shown below.

Paleobiology

The long metatarsals present in Daliansaurus suggest that its running capabilities were better-developed than those of other troodontids. At the same time, the possible presence of uncinate processes on its ribs would have provided reinforcement for the rib cage, as well as an increase in the effectiveness of the muscles involved in breathing, as in modern birds.

Histology

Thin sections from the ulna and radius of the holotype specimen DNHM D2885 indicate that the bones are devoid of secondary osteons, or osteons formed through bone remodeling. Seasonal bone growth is apparent through the thin sections; three lines of arrested growth (LAGs) separate the cortical bone into four distinct zones within the radius, indicating that the individual was at least four years of age when it died. (There appear to only be two LAGs within the ulna.) These estimates were to considered to be lower bounds, because expansion of the medullary cavity within the bone could have obliterated additional LAGs. 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. 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.

Across the four zones, the neurovascular canals are denser in the first zone than in subsequent zones, indicating that growth was most intensive during the first year of life and rapidly decreased later on. Subsequent minor fluctuations in growth rate are reflected by alternating bands of osteons and avascular bone (i.e. bone lacking neurovascular canals). Thin peripheral growth bands are visible between the LAGs in the outer two zones of the radius, which constitute an external fundamental system (EFS). The presence of an EFS suggests minimal growth rates, indicating that the individual may have reached skeletal maturity at this size. 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. Overall, anatomical and histological evidence suggests that the individual grew quickly, but was only in the late subadult stage of life. The three-phase model of bone growth - rapid growth followed by slowed growth and formation of the EFS - is also seen in Stenonychosaurus.

Paleoecology

The Liaoning region of northeastern China has been one of the most productive fossil sites for understanding the evolution of feathered dinosaurs, including birds. All of the sinovenatorines - Sinovenator, Sinusonasus, Mei, and Daliansaurus - lived in the Lujiatun Beds of the Yixian Formation. Rather than strict coexistence, it is possible that they replaced each other over time, or were geographically and ecologically segregated. Shen and colleagues identified the sinovenatorines as a localized radiation of troodontids confined to Liaoning during the Early Cretaceous; a similar radiation occurred with the many microraptorine dromaeosaurids which lived at roughly the same place and time. Sinovenatorines also paralleled dromaeosaurids in the minute but taxonomically informative anatomical differences between different members of the group. However, it is possible that some of these differences may arise from individual or ontogenetic variation.

A wide variety of dinosaurs lived in the Lujiatun Beds alongside the sinovenatorines. These included the microraptorine Graciliraptor lujiatunensis; the oviraptorosaur Incisivosaurus gauthieri; the ornithomimosaurs Shenzhousaurus orientalis and Hexing qingyi; the proceratosaurid tyrannosauroid Dilong paradoxus; the titanosauriform sauropod Euhelopus sp., based on teeth; the ornithopod Jeholosaurus shangyuensis; and the ceratopsians Psittacosaurus lujiatunensis (including P. major and Hongshanosaurus houi) and Liaoceratops yanzigouensis. Psittacosaurus lujiatunensis was the most common, with thousands of specimens being known, followed by troodontids and Jeholosaurus shangyuensis. No fossils of birds are known from the Lujiatun Beds.

Mammals were also present in the Lujiatun Beds, including Acristatherium yanensis, Gobiconodon zofiae, Juchilestes liaoningensis, Maotherium asiaticus, Meemannodon lujiatunensis, Repenomamus giganticus, and R. robustus. The frogs Liaobatrachus zhaoi and Mesophryne beipiaoensis have also been found, along with the lizard Dalinghosaurus longidigitus. 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. Fossils of invertebrates and plants are absent from the Lujiatun Beds, but pollen from coniferous plants dominates the contemporary Jianshangou Beds. Mean annual air temperatures were cold in the region during the Barremian and Aptian epochs, being as cold as 10 °C (50 °F).