Abstract
The hindlimb of theropod dinosaurs changed appreciably in the lineage leading to extant birds, becoming more ‘crouched’ in association with changes to body shape and gait dynamics. This postural evolution included anatomical changes of the foot and ankle, altering the moment arms and control of the muscles that manipulated the tarsometatarsus and digits, but the timing of these changes is unknown. Here, we report cellular-level preservation of tendon- and cartilage-like tissues from the lower hindlimb of Early Cretaceous Confuciusornis. The digital flexor tendons passed through cartilages, cartilaginous cristae and ridges on the plantar side of the distal tibiotarsus and proximal tarsometatarsus, as in extant birds. In particular, fibrocartilaginous and cartilaginous structures on the plantar surface of the ankle joint of Confuciusornis may indicate a more crouched hindlimb posture. Recognition of these specialized soft tissues in Confuciusornis is enabled by our combination of imaging and chemical analyses applied to an exceptionally preserved fossil.
Highlights
The ridge resembles an intermediate state of the hypotarsus in the ontogeny of extant birds, in which the hypotarsus remains cartilaginous until the latest stages or after hatching, when it ossifies from a separate centre located on its distal medial corner[35,36]. Overall, this specimen of Confuciusornis reveals the remarkable preservation of residual tendons/ligaments belonging to or associated with the digital flexor muscles passing through two musculoskeletal structures comparable to two novel structures in the lower legs of crown-group birds: the tibial cartilage and hypotarsus (Fig. 5)
It may seem implausible that these specializations could have been overlooked in so many other fossil birds, but they are small and subtly expressed, like those in the late ontogeny of extant birds
We infer that there was a transition between the ancestral state in reptiles, in which digital flexor tendons contribute to the complex plantar aponeurosis[4,38], and an intermediate state in early ornithuromorph birds, involving an incipient osseous hypotarsus and tibial cartilage as interpreted here (Fig. 5)[4,31]
Summary
MES-NJU 57002 was identified by microCT scanning using a 2010 GE phoenix v|tome|x s240 system. MicroCT scan data are available[41]. The specimen was compared with others (Supplementary Table 1) to confirm its identity and adult status. Polished cross-sections and thin-sections were prepared for observation and imaged using a polarizedlight microscope and scanning electron microscope equipped with back-scattered electron (BSE) detector and EDX detector. SEM analysis used LEO1530 VP, which is an environmental s.e.m. capable of operating in variable pressure mode. Additional s.e.m. image data are available[42]
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