Forearm bone histology of the small theropod Daliansaurus liaoningensis (Paraves: Troodontidae) from the Yixian Formation, Liaoning, China

We present the first histological analysis of forelimb bones in a troodontid dinosaur, Daliansaurus liaoningensis, from the Early Cretaceous of China using osteohistological thin-sectioning and high-resolution synchrotron-based imaging. The thin wall compacta consists of primary bone, and three lines of arrested growth (LAG) in the radius (R) and two in the ulna (U) divide these into successive zones. Results show that the new fossil has four distinct bone depositional rates: (1) fastest deposition in inner zones R1 and U1 (fibro-lamellar bone with a plexiform-like vasculature); (2) slowed deposition in outer zones R1 and U1 + U2 (loss of vascular density and plexiform component); (3) fluctuating rates of deposition in zones R2 + R3 and in the inner zone U3 (alternating bands of circumferentially organised primary osteons and avascular bone); and (4) slowest deposition in zone R4 and the outer zone U3 (lamellar bone constituting the external fundamental system). Collectively, these growth characteristics suggest that the fossil is an individual that passed the exponential growth phase by the first year, and perished three years later. We conclude that the histology is consistent with an interpretation of this specimen as a late maturing individual that had not yet attained maximum somatic size.