Huaxiagnathus

Huaxiagnathus Temporal range: Early Cretaceous, 125 Ma PreꞒ Ꞓ O S D C P T J K Pg N ↓
Huaxiagnathus orientalis fossil displayed in Hong Kong Science Museum
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Dinosauria
Clade:	Saurischia
Clade:	Theropoda
Family:	†Sinosauropterygidae
Genus:	†Huaxiagnathus Hwang et al., 2004
Species:	†H. orientalis
Binomial name
†Huaxiagnathus orientalis Hwang et al., 2004

Huaxiagnathus is an extinct genus of coelurosaurian theropod dinosaur from the Lower Cretaceous of China. Huaxiagnathus was larger than the related Sinosauropteryx, with the largest specimen about 1.8 meters (5.9 feet) in length.^[1]

The name Huaxiagnathus is derived from the Chinese Hua Xia, 華夏, a traditional word for "China", and from the Greek gnathos, Latinised into gnathus, meaning "jaw."

The holotype (CAGS-IG-02-301, Chinese Academy of Geological Sciences, Beijing) specimen was collected from the Yixian Formation (Jehol Group, Aptian) at Dabangou Village, Sihetun area, near Beipiao City, in western Liaoning Province. The holotype is a subadult and consists of an essentially complete skeleton, lacking only the end of the tail, preserved on five large slabs. Partially digested bones of an unidentified vertebrate were found within the holotype specimen.^[1] A larger, ontogenetically older specimen of Huaxiagnathus was discovered earlier in the Yixian Formation of the Sihetun area (NGMC 98-5-003, National Geological Museum of China, Beijing), but damage and mistakes made during its preparation rendered it unsuitable as a holotype.^[1]

Cladistic analysis indicates that Huaxiagnathus is the basal most known compsognathid, as indicated by its unspecialized forearm.

Hwang et al. (2004, pp. 14–15) diagnosed this genus as follows: differing from all other known compsognathids in having a very long posterior process of the premaxilla that overlaps the antorbital fossa, a manus equal to the combined lengths of the humerus and radius, large manual unguals I and II which are subequal in length and 167% the length of manual ungual III, a first metacarpal which has a smaller proximal transverse width than the second metacarpal, and the presence of a reduced olecranon process on the ulna.^[1]

In 2024, Andrea Cau published a study on the phylogenetics of compsognathids and immature theropods in general that called the assessment of Huaxiagnathus within Compsognathidae into question. The paper recovered the taxon, along with four other proposed compsognathids in a polytomy within basal Coelurosauria. This polytomy notably did not include Composognathus proper, which would make none of these species compsognathids.^[2]

Qiu et al. (2025) also argued against the monophyly of Compsognathidae and revised the definition of the monophyletic Sinosauropterygidae within Coelurosauria as a family containing all compsognathid-like theropods from the Jehol Biota of China: Sinosauropteryx, Huadanosaurus, Huaxiagnathus and Sinocalliopteryx. Their phylogenetic analyses are reproduced below:^[3]