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Session Schedule & Abstracts

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Sunday 3rd July, 2016

Locomotion 5

Room: Salon F   9:30 am–10:45 am

Moderator(s): Bonnan MF, Klinkhamer A
LOC5-1  9:30 am  Digital musculoskeletal modelling of an Australian sauropod dinosaur. Klinkhamer A*, University of New England
Abstract: Since muscles are rarely fossilised, advanced computer modelling techniques are increasingly being used to reconstruct the musculature of extinct species to investigate muscle function and its role in locomotion. The gigantic size of sauropods posed many challenges for their musculoskeletal system in regards to supporting their body mass and moving on land. Three-dimensional models of the forelimbs and hind limbs and associated muscles of a well preserved Australian titanosaur from the Cretaceous, Diamantinasaurus matildae, were reconstructed to gather information on mobility, support and bipedal rearing ability. After digitising the fossil material, musculoskeletal modelling software (SIMM) was used to reconstruct the musculature of D. matildae using information from extant archosaurs (i.e. crocodiles and birds). Muscle moment arms and associated forces were analysed at multiple joint angles, as well as interactions between muscles to assess muscle activation during locomotion. Modelling musculoskeletal features of extinct taxa, like the appendicular skeleton of D. matildae, can provide insight into locomotor function and assist in developing our understanding of sauropod palaeobiology.

LOC5-2  9:45 am  How the largest known flying animal, the pterosaur Quetzalcoatlus, walked on land. Padian K., University of California, Berkeley; Cunningham J.R., Cunningham Engineering Associates; Langston W.A., University of Texas; Conway J.,; Manafzadeh A.*, University of California, Berkeley
Abstract: The giant pterosaur Quetzalcoatlus northropi (QN) lived at the very end of the Cretaceous Period, about 67 million years ago, in Texas. It was named on the basis of a few incomplete post-cranial bones that suggested a wingspan of 11-13 m; a morph about half this size is known from numerous bones and partial skeletons. In the air, like most large aerial animals, it mainly soared, but it could flap to some degree. We studied the functional morphology of the skeleton by manipulation of many dozens of bones and assessing range of movement, reconstructing posture and gait, and calculating the kinematics of walking and flying at each joint. On the ground, like all pterodactyloids, QN walked quadrupedally, but this was mainly because its metacarpals were so long that the manus could not avoid touching the ground. Pterosaurs were originally bipedal, like their ancestors. Manipulation of QN’s forelimb and hindlimb bones confirms that in a quadrupedal pose the humerus had limited rotation (about 25°) and the forearm and metacarpus could be slightly elevated and depressed at the elbow, but the forelimb had no significant retractory power. All joints of the hindlimb are hinges except the hip, a ball-and-socket offset by a neck oriented dorsally, medially, and posteriorly. The hindlimb thus had an erect stance and parasagittal gait, as in other ornithodirans. Pterodactyloids such as QN lifted their limbs unusually, because overstepping was not possible: the lift cycle was LM – LP – RM – RP, where M is manus and P is pes; however, the sequence of emplacement would have been LP – LM – RP – RM. The full step cycle was: LM lift – LP lift – LP place – LM place – RM lift – RP lift – RP place – RM place. Although technically quadrupedal, QN showed its bipedal heritage when it walked on land. Our analysis shows the importance of incorporating phylogenetic heritage into interpretations of functional morphology.

LOC5-3  10:00 am  Forelimb kinematics of rats using XROMM, with implications for small eutherians and their fossil relatives. Bonnan M.F.*, Stockton University; Shulman J., Stockton University; Horner A., California State University San Bernardino; Brainerd E., Brown University
Abstract: The earliest eutherian mammals were small-bodied locomotor generalists with a forelimb morphology that strongly resembles that of extant rats. Understanding forelimb bone kinematics in extant rats can inform and constrain hypotheses concerning typical posture and mobility in early eutherian forelimbs. The locomotion of Rattus norvegicus has been extensively studied, but the three-dimensional kinematics of the bones themselves remains under-explored. We used markerless XROMM (Scientific Rotoscoping) to explore three-dimensional long bone movements in Rattus norvegicus during walking. Our data show a basic kinematic profile that agrees with previous studies on rats and other small therians: a crouched forelimb posture is maintained throughout the step cycle, and the ulna is confined to flexion/extension in a parasagittal plane. However, our three-dimensional data illuminate long-axis rotation (LAR) movements for both the humerus and the radius. Medial LAR of the humerus throughout stance maintains an adducted elbow with a caudally-facing olecranon process, which in turn maintains a cranially-directed manus orientation (pronation). The radius also shows significant LAR correlated with manus pronation and supination. Moreover, we report that elbow flexion and manus orientation are correlated in R. norvegicus: as the elbow angle becomes more acute, manus supination increases. Our data also suggest that manus pronation and orientation in R. norvegicus rely on a divided system of labor between the ulna and radius, and radius LAR is necessary for manus pronation. We suggest that forelimb posture and kinematics in Juramaia, Eomaia, and other basal eutherians were grossly similar to those of rats, and that humerus and radius LAR may have always played a significant role in forelimb and manus posture in small eutherian mammals.

LOC5-4  10:15 am  Ontogenetic changes in effective mechanical advantage in the Eastern cottontail rabbit (Sylvilagus floridanus). Foster AD*, NEOMED; Butcher MT, Youngstown State University; Smith GA, Kent State University at Stark; Young JW, NEOMED
Abstract: Juvenile animals must compete and survive in the same environment as adults, despite a smaller body size and musculoskeletal immaturity. One way to overcome these disadvantages is to increase effective mechanical advantage (EMA). EMA is defined as the quotient of extensor muscle lever arm length (r) and the length of the external load arm of the ground reaction force that those muscles must resist (R). This value is directly proportional to both the mass-specific muscle force needed to maintain posture as well as the magnitude of the output force resulting from a given muscle input force. Therefore, we predicted that to overcome absolutely lower muscle mass and muscle force capacity, juvenile animals should exhibit higher extensor muscle mechanical advantage either through growth-related changes in r or postural adjustments of R. We tested this hypothesis using a dataset on growth and locomotor development in Eastern cottontail rabbits (Sylvilagus floridanus) as a model system. We found that relative to body mass, hindlimb muscle lever arm length (r; averaged across the hip, knee, and ankle joints) scales with negative allometry, whereas hindlimb load arm (R; average across hindlimb joints) scales with positive allometry. Although the 95% confidence intervals of both variables overlapped the isometric expectation of proportional growth relative to body mass, predicted hindlimb EMA nevertheless decreases throughout ontogeny, likely giving juvenile rabbits an advantage in output force production. Given that ontogenetic declines in mechanical advantage appear to be common across mammals, these allometric patterns may represent a “pathway of least resistance” by which juveniles are able to overcome a deficit in muscle strength and still achieve adult-like levels of locomotor performance. Supported by NSF IOS-1146916 and NEOMED.

LOC5-5  10:30 am  Jumping performance in the Longshanks mouse. Bradley MM, University of Calgary; Hou L*, University of Calgary; Sparrow LM, University of Calgary; Rolian C   
Abstract: There is an evolutionary trend among jumping mammals to have long hind-limbs relative to body size. This convergence on long and gracile hind-limbs is hypothesized increase jumping performance by increasing the distance and time over which the limb muscles generate force, which, all else being equal, increases push-off velocity and jumping distance. Here, we used Longshanks, a unique long-limbed mouse produced over 19 generations of selective breeding for increased tibia length relative to body mass, to test this hypothesis. Specifically, we tested the prediction that the Longshanks mice, whose hind limb length is on average ~15% greater, can produce larger push-off velocities and jump higher than weight- and age-matched random-bred controls with normal limb length. Mice were first trained to jump onto a raised platform in an enclosure, using a positive reinforcement training schedule (operant conditioning). Jumps were then recorded using high-speed video (250 fps) to obtain kinematic data (angular velocities and accelerations about hind-limb joints, push-off durations and linear velocities). Contrary to our prediction, maximal jump height in Longshanks mice was lower than in Controls: average maximal jump height of the Longshanks mice was 20cm (n=6), while in Control mice it was 22.5cm (n=6). In a preliminary kinematic analysis (n=4 LS, n=2 C), at lower jump heights (h<17cm), push-off duration is longer in the Longshanks line. They also exhibited higher average angular accelerations and velocities about the hind limb joints, and produced greater push-off velocities. At lower heights, the Longshanks mice can thus generate greater push-off velocities and are in theory able to achieve greater jump heights than controls. Our kinematic data lend support to the hypothesis that elongated hind limbs improve jumping performance, suggesting that the lower maximal jump height in Longshanks is due to other factors such as behavior (e.g., motivation).

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