This stylised bird skeleton highlights the uncinate processes.
"The uncinate processes of the ribs are extensions of bone that in birds project caudally from the vertical segment of each rib. (Uncinate means hooked from Latin uncinatus, from uncinus, barb, from uncus, hook.) These processes help to strengthen the rib cage of birds by overlapping with the rib behind them. They are also shown to have a role in respiration by increasing the effectiveness of muscles involved in inspiration including the appendicocostal muscles.".
"The sternal ribs of well preserved examples of Rhamphorhynchus and Pteranodon bear elaborate dorsal and ventral processes that we term sternocostapophyses (Figs. 2d,f, S1c–e). These projections likely functioned as levers that increased the moment arm for the intercostal muscles, conferring an enhanced capacity for moving the sternal ribs during lung ventilation. Sternocostapophyses are analogous in function to the uncinate processes of birds and maniraptoran theropods –. However, the mechanical advantage (leverage) provided by the sternocostapophyses likely differed from that conferred by the uncinate processes of maniraptoran theropods and birds. The sternocostapophyses are located on the sternal ribs rather than the vertebral ribs and, generally, there are multiple sternocostapophyseal projections per sternal rib, rather than a singular (uncinate) process as found in birds. Similar to the uncinate processes of extant birds, the leverage provided by the sternocostapophyseal projections of pterosaurs likely lowered the work of breathing of the intercostal musculature, and resulted in costal and sternal displacement. However, in pterosaurs, the greatest mechanical advantage would have been provided in the ventral rather than dorsal thoracic region."
Sternal ribs are typically simple and rod-shaped, but those of pterosaurs possess dorsal and ventral processes that Claessens et al. (2009) term sternocostapophyses
The ribcage of pterosaurs, including those of the earliest known forms such as the Late TriassicEudimorphodon ranzii , consists of a large ossified sternum and distinct vertebral and sternal ribs (Fig. 2a–b, d–f).
Cladogram illustrating the relationship of birds with major groups of non-avian coelurosaurian theropods. The numbers in circles at each branching node indicate the first appearance of feathers and other key morphological characters. 1, unbranched feathers; 2, uncinate processes on ribs; 3, true branched feathers; 4, retroverted pubis; 5, reversed hallux; 6, asymmetrical flight feathers; 7, pygostyle; 8, horny beak; 9, alula (bastard wing); 10, large, keeled sternum. Taxa indicated with an asterisk are known to have possessed either protofeathers or true feathers (From Zhou et al. 2003).
Functional significance of the uncinate processes in birds -- Uncinate processes are bony projections that extend from the vertebral ribs of most extant birds. In 1935, Zimmer (1935) postulated that the uncinate processes played some role during inspiration. Other hypotheses have linked these processes with stiffening or strengthening the rib cage or providing attachment sites for muscles stabilizing the shoulder. Recent electromyographic studies of Giant Canada Geese confirmed Zimmer's hypothesis by demonstrating that these processes are integral component of the ventilatory mechanics of birds being involved in both inspiration and expiration (Codd et al. 2005). The processes are associated with fleshy parts of the Mm. intercostales externi, the Mm. appendicocostales that originates from the proximal edge of the uncinate and inserts onto the following vertebral rib. The Mm. appendicocostales is active during inspiration in Giant Canada Geese, suggesting the processes facilitate the craniad movement of the ribs, which would in turn move the sternum ventrally. The base of the uncinateprocesses serves as a brace for the insertions of the `finger-like' projections of the M. externus obliquus abdominus that pull the sternum dorsally during expiration. Given that the processes provide attachment sites for these important respiratory muscles, any change in uncinate morphology may have a significant effect on ventilation.