Studies On The Morphology And Osteohistology Of A New Specimen Of The Early Cretaceous Ornithuromorph In Western Liaoning

The Archaeorhynchus is a small to medium-sized avian species with a horny bill,belonging to the group of Ornithuromorpha within the western Liaoning Jehol Biota.Currently,it includes only one species,the Archaeorhynchus spathula.This article describes a new specimen of Ornithuromorpha from the Jiufotang Formation in the Lamaodong area of Jianchang,western Liaoning.The specimen is missing the skull,but the rest of the skeleton is well-preserved.The new specimen exhibits typical features of Ornithuromorpha,such as the development of the premaxillary bone before the premaxillary process,the presence of a carina on the entire sternum,and a prominent proximal surface of the humerus.Compared to other Ornithuromorpha,the new specimen shares similarities with the Archaeorhynchus,such as a relatively narrow and U-shaped furcula without a furcular process,a wide sternum with a V-shaped central process in the posterior region,an elongated posterior lateral process with a fan-shaped expanded distal end,and a slightly shorter and inwardly curved posterior medial process,as well as longer forelimbs compared to hind limbs.Due to the absence of the skull in the new specimen,key diagnostic features for comparison with the Archaeorhynchus,such as the presence of teeth in the upper and lower jaws and an enlarged spoon-shaped anterior end of the dentary bone,cannot be examined.Additionally,the new specimen also exhibits some features distinct from the Archaeorhynchus,such as the absence of sternal keels and ventral tubercles on the humerus.Therefore,the new specimen is temporarily classified as an indeterminate species of Archaeorhynchus.In order to understand the developmental stage and characteristics of the forelimbs and hind limbs in the new specimen,microscopic bone tissue sectioning was employed in this study to prepare and observe bone tissue sections of various parts of the forelimbs and hind limbs,including humerus,ulna,radius,femur,tibia and tarsal bones.Observation of the bone tissue sections revealed the presence of endosteal lamellar bone and secondary osteons in all parts of the forelimbs and hind limbs of the new specimen.However,the presence of periosteal lamellar bone and growth arrest lines was not evident,indicating that the new specimen had entered a slow-growth developmental phase and represented a nearly adult individual.The degree of vascularity in the forelimbs of the new specimen was higher than that in the hind limbs,indicating faster growth in the forelimbs and suggesting a clear example of differential growth during individual development.Currently,comprehensive bone tissue studies on the forelimbs and hind limbs of small to medium-sized prehistoric birds are lacking.This study serves as a preliminary attempt to reveal the developmental characteristics of the forelimbs and hind limbs in ancestral-type Archaeorhynchus.