Myology of the forelimb of Majungasaurus crenatissimus (Theropoda, Abelisauridae) and the morphological consequences of extreme limb reduction

Abstract

Forelimb reduction occurred independently in multiple lineages of theropod dinosaurs. Although tyrannosaurs are renowned for their tiny, two-fingered forelimbs, the degree of their reduction in length is surpassed by abelisaurids, which possess an unusual morphology distinct from that of other theropods. The forelimbs of abelisaurids are short but robust and exhibit numerous crests, tubercles, and scars that allow for inferences of muscle attachment sites. Phylogenetically based reconstructions of the musculature were used in combination with close examination of the osteology in the Malagasy abelisaurid Majungasaurus to create detailed muscle maps of the forelimbs, and patterns of the muscular and bony morphology were compared with those of extant tetrapods with reduced or vestigial limbs. The lever arms of muscles crossing the glenohumeral joint are shortened relative to the basal condition, reducing the torque of these muscles but increasing the excursion of the humerus. Fusion of the antebrachial muscles into a set of flexors and extensors is common in other tetrapods and occurred to some extent in Majungasaurus. However, the presence of tubercles on the antebrachial and manual elements of abelisaurids indicates that many of the individual distal muscles acting on the wrist and digits were retained. Majungasaurus shows some signs of the advanced stages of forelimb reduction preceding limb loss, while also exhibiting features suggesting that the forelimb was not completely functionless. The conformation of abelisaurid forelimb musculature was unique among theropods and further emphasizes the unusual morphology of the forelimbs in this clade.

Ceratosaur palaeobiology: new insights on evolution and ecology of the southern rulers

“…According to Agnolin and Chiarelli 40 , abelisaurs probably also lacked forearm mobility. However, recent analyses on Majungasaurus musculature suggest that, although much reduced, abelisaurids did not lose full mobility of the forelimb, and may have used it for intraspecific display 41 . Some taxa such as Aucasaurus , Majungasaurus and Carnotaurus may have lost the ungual of the digits I and IV 31 , 40 , 42 whereas the ceratosaurid Eoabelisaurus has strongly reduced the manual unguals 12 .…”

Forelimb muscle and joint actions in Archosauria: insights fromCrocodylus johnstoni(Pseudosuchia) andMussaurus patagonicus(Sauropodomorpha)

“…8 ). We found complex actions like these for SU, FU, AR, EDL, ECR and ECU, which were elbow extensors in Crocodylus but had mixed actions in Mussaurus (see Meers, 2003 ; Burch, 2014 , 2017 for different interpretations), or FDL, which had a mixed action in Crocodylus but was purely a flexor in Mussaurus . The differences in ROM values between both taxa seemingly did not affect the actions of these muscles ( Table 3 ; Table S8 ).…”

“…Remes (2008) inferred that sauropodomorphs lacked M. extensor carpi ulnaris (ECU), but this is contradicted by evidence from crocodylian myology (dozens of specimens studied by Meers (2003) and Allen et al (2014) so our models incorporated the ECU for Crocodylus and Mussaurus (also see discussion in Burch (2014 , 2017) and Klinkhamer et al (2017) . The archosaurian ECR (M. extensor carpi radialis longus sensu Meers, 2003 ) muscle inserts onto the radiale.…”

“…BB, HR and BR) or caudal (i.e. triceps group) to the elbow joint also showed unambiguous actions in Crocodylus and Mussaurus , and more generally in Archosauria ( Meers, 2003 ; Jasinoski, Russell & Currie, 2006 ; Burch, 2014 , 2017 ; Klinkhamer et al, 2017 ). The PT muscle was estimated to act as an elbow extensor in Crocodylus but an elbow flexor in Mussaurus ( Table 3 ; Fig.…”

Developmental patterns and variation among early theropods

“…Unlike in Tawa, which possesses a groove, in M. rhodesiensis this is a raised ridge; however, because this ridge is situated in a similar position to that reconstructed at the insertion of the m. latissimus dorsi for T. hallae (Burch, 2014: fig. 3) and Majungasaurus cranatissimus (Burch, 2017: fig. 3), I hypothesize that this scar is the osteological correlate for the insertion of the m. latissimus dorsi in M. rhodesiensis.…”