Forearm Range of Motion in Australovenator wintonensis (Theropoda, Megaraptoridae)

Abstract

The hypertrophied manual claws and modified manus of megaraptoran theropods represent an unusual morphological adaptation among carnivorous dinosaurs. The skeleton of Australovenator wintonensis from the Cenomanian of Australia is among the most complete of any megaraptorid. It presents the opportunity to examine the range of motion of its forearm and the function of its highly modified manus. This provides the basis for behavioural inferences, and comparison with other Gondwanan theropod groups. Digital models created from computed tomography scans of the holotype reveal a humerus range of motion that is much greater than Allosaurus, Acrocanthosaurus, Tyrannosaurus but similar to that of the dromaeosaurid Bambiraptor. During flexion, the radius was forced distally by the radial condyle of the humerus. This movement is here suggested as a mechanism that forced a medial movement of the wrist. The antebrachium possessed a range of motion that was close to dromaeosaurids; however, the unguals were capable of hyper-extension, in particular manual phalanx I-2, which is a primitive range of motion characteristic seen in allosaurids and Dilophosaurus. During flexion, digits I and II slightly converge and diverge when extended which is accentuated by hyperextension of the digits in particular the unguals. We envision that prey was dispatched by its hands and feet with manual phalanx I-2 playing a dominant role. The range of motion analysis neither confirms nor refutes current phylogenetic hypotheses with regards to the placement of Megaraptoridae; however, we note Australovenator possessed, not only a similar forearm range of motion to some maniraptorans and basal coelurosaurs, but also similarities with Tetanurans (Allosauroids and Dilophosaurus).

Fig 1. Australovenator wintonensis.

A reconstruction of Australovenator wintonensis grasping a small theropod with its arms in a flexed posture.

Fig 2. Determining range of motion from three dimensional meshes.

(A) An example of determining the ROM of manual phalanx I-1 around metacarpal I in extended and flexed positions. The guide meshes are visible projecting from the specimen meshes. (B) The specimen meshes removed leaving the created guides used for determining the range of motion. A line of neutral articulation was created along a horizontal plain which is used as the basal limit for extension and flexion measurements. Abbreviations: ex extension angle; fl flexion angle; h hinge joint; nal neutral articulation line.

Fig 3. Articulated right humerus and antebrachium.

(A) Right humerus and antebrachium flexed in medial view. (B) Right humerus and antebrachium flexed in lateral view. (C) Cranial view of radius and ulna in flexion displaying the distal movement of the radius indicated by the black arrow. (D) Distal articulation of the ulna and radius during flexion. (E) Right humerus and antebrachium extended in medial view. (F) Right humerus and antebrachium extended in lateral view. (G) Cranial view of radius and ulna in extended position with no distal displacement of the radius. (H) Distal articulation of the ulna and radius during extension. (I) Distal view of the humerus displaying the radial and ulna condyles. (J) Cranial view of the right humerus and antebrachium in extension displaying the position of the radial condyle which forces the distal displacement during flexion. Abbreviations: radial condyle (rc), ulna condyle (uc).

Fig 4. Right forearm and manus of Australovenator and Gallus forearm.

Right forearm in extended lateral view; (A) Gallus. (B) Australovenator. Right forearm extended in lateral view; (C) Gallus. (D) Australovenator. Right forearm flexed in lateral view; (E) Gallus. (F) Australovenator. Right forearm flexed in medial view; (G) Gallus. (H) Australovenator. Australovenator manus; (I) extended cranial view; (J) extended ventral view; (K) flexed cranial view; (L) flexed ventral view.

Fig 5. Articulated radius and radiale.

Radius and radiale in: (A) cranial view; (B) caudal view; (C) lateral view; (D) medial view; (E) radiale in distal view; (F) radiale in proximal view.

Fig 6. Distal carpal 1, metacarpal I and metacarpal II.

(A) Distal carpal 1 in ventral view displaying the ventral articular facet that buttresses metacarpal I. (B) The proximal articular facet of metacarpal I where distal carpal I articulates. (C) The lateral face of metacarpal I displaying the articulating morphology with metacarpal II. (D) The medial face of metacarpal II displaying the articulating morphology of metacarpal. Articulated distal carpal I, metacarpal I and metacarpal II in: (E) Cranial view; (F) Ventral view; (G) proximal view; (H) proximal outline. Abbreviations: afr articular facet ridge; dc1 distal carpal 1, McI metacarpal I, McII metacarpal II, McIaf metacarpal I articular facet, McIIaf metacarpal II articular facet.

Fig 7. Phalangeal range of motion.

(A) Cranial view of metacarpal I and manual phalanx I-1 displaying medial and lateral rotation. Extension and flexion in medial view of: (B) Metacarpal I and manual phalanx I-1; (C) Metacarpal II and McII-1; (D) Manual phalanx II-1 and II-2.

Fig 8. Manus ungual range of motion.

(A) Manual phalanx I-2 extended and flexed. (B) Manual phalanx II-3 extend and flexed. (C) Manual phalanx III-4 extended and flexed.

Fig 9. Phylogeny of theropods mentioned in the text.

Various forearm range of motion evolutionary trends are identified. The phylogeny was created with reference to other range of motion analysis and more recent phylogenetic analysis [6, 13, 15, 20, 49].