theropod
|
publication ID |
https://doi.org/ 10.5281/zenodo.3678101 |
|
DOI |
https://doi.org/10.5281/zenodo.3682311 |
|
persistent identifier |
https://treatment.plazi.org/id/03FAE251-1E39-FFCF-FF77-F8F1FEA9FD7D |
|
treatment provided by |
Jeremy |
|
scientific name |
theropod |
| status |
|
The morphology of theropod locomotion can be derived from that of pseudosuchians. The change from a facultative biped (thecodont) to an obligate theropod biped is understandable in terms of efficiency. Bipedal locomotion is more energy efficient than is a reptilian method of quad rupedal locomotion ( Hotton 1980). The change to obligate bipedalism necessitates an overhaul in pseudosuchian morphology. The legs had to be brought under the body in theropods in order to support the weight of the body at less energy cost to the musculature. This change in stance brings the movement at all joints in the hindlimb in the same plane of motion. The result is an increase in torque to the joints and an increase in stride length ( Hildebrand 1974). In order to achieve this posture the crocodilian ankle joint must be modified. What apparently has occured is that the fibular condyle of the calcaneum has been reduced and the fibula has shifted back onto the dorsal surface of the calcaneal tuber (Tarsitano, in prep.). This condition is seen at least in the theropods, prosauropods and ornithopods. Through this modification the calcaneum ceases to move and the mesotarsal joint is established ( fig. 1d View Fig. 1 ). The change in function of the calcaneal tuber also changes its effect on the foot extensors thereby allowing the more medial placement of these muscles on the foot. With the development of the mesotarsal joint, the metatarsals would no longer need to overlap and the functionally symme trical “tridactyl” foot could be evolved. As the legs were brought under the body the torsion of the femur disappeared and the femoral head ex panded inward to form a roller. These adaptations lead to a more fore-aft swinging of the limb and a natural bipedal posture.
To understand the positioning of the vertebral column one must first understand the musculature of the crocodilian hindlimb. For the sake of brevity I will only refer to the crocodilian muscles which play key roles in locomotion. Full descriptions can be found in Gadow (1882), Romer (1923), Tarsitano (Ph.D. thesis) and Brinkman (1980b). Of the protrac tors, the M. puboischiofemoralis internus parts 1 and 2 and the anterior- most portions of the M. iliotibialis are most important. The M. puboischio femoralis internus part 1 originates in all crocodilians on the first sacral vertebra and the corresponding internal surface of the ilium ( fig. 4 View Fig. 4 ). The insertion is on the anterior surface of the fourth trochanter. The second part of this muscle originates from the last five presacral vertebrae. The insertion lies on the lateral surface of the femur just distal to the femoral head ( fig. 5 View Fig. 5 ). Thus, the M. puboischiofemoralis internus is pri marily responsible for protracting and lifting the femur. The M. pubo ischiofemoralis externus parts 1 and 2 originate on the broad surface of the pubis. They converge on the upper medial surface of the femur to insert with the third part of this muscle on the back of the femur below its head ( fig. 6 View Fig. 6 ). The major action of this muscle is to rotate the femur out ward, solving the femur’s “knocking on pubes” problem ( Charig 1972). This rotation of the femur is concordant with the movement of the hip roller joint of theropods as proposed by Hotton (1980). The M. puboischio femoralis externus parts 1 and 2 also serves to protract the femur. The M. ambiens ( fig. 4 View Fig. 4 ) originates at the junction of the ilium and pubis. Only part 1 of this muscle is significant for the present discussion. It crosses laterally over the knee joint between the layers of the extensor tendon formed by the M. femorotibialis ventrally and the M. iliotibialis dorsally ( Tarsitano, Ph.D. thesis), to run down the shank in the fascia of the M. gastrocnemius to insert on the calcaneal tuber and fifth metatarsus ( fig. 3 View Fig. 3 ). The M. ambiens protracts the femur and stabilizes the outward rotation of the femur; it is also a shank flexor and pedal extensor. Finally, the M. iliotibialis originates on the dorsal rim of the ilium and inserts onto the proximal anterior surface of the tibia, forming part of the extensor tendon of the knee. This muscle can act to lift and sligthly protract the thigh. The retractors of the femur are mainly the M. caudofemoralis lon gus and brevis (M. coccygeofemoralis). The longus originates from the third to the thirteenth caudal vertebrae ( Romer 1923). It inserts into the fourth trochanter and sends a long tendon to the M. gastrocnemius ( figs. 3 View Fig. 3 , 4 View Fig. 4 ). The brevis originates from the internal surface of the postaceta bular ilium and the last sacral vertebra. It also inserts into the fourth trochanter. The M. iliofemoralis may also aid in the retraction of the femur due to its insertion of the postero-lateral surface of the femur ( fig. 5 View Fig. 5 ).
The positioning of the vertebral column can now be understood in functional terms. If the vertebral column is oriented at about 50 degrees above the horizontal, the M. puboischiofemoralis internus will bring the femur upwards and not forward. The result is a high, inefficient “march ing-in-place” gait. In order to stand with the vertebral column at such an angle the M. caudofemoralis would have to be almost fully contracted. Thus at such a high angle, the vertebral column makes bipedal locomo tion impossible. If the vertebral column is held horizontally there are also problems in locomotion. The M. puboischiofemoralis internus may bring the femur only partially forward but can hardly lift the femur. The postures giving theropods a horizontal vertebral column and having the femur protracted to the level of the vertebral column are biomechanically and physiologically impossible since the femur would be dislocated from the hip (tearing the ligamentum teres) and the protractor muscles would have to contract more (by as much as three times) than is physiologically possible. When crocodilians run bipedally, the presacral region is lifted in order that the M. puboischiofemoralis internus can lift as well as pro tract the thigh. A horizontal vertebral column limits the protraction and retraction of the femur. This would allow theropods to walk but inhibit their ability to run. This may be explainable in terms of length tension curves of muscle contraction ( Ramsey 1960; Abbott and Wilkie 1953; Gans and Bock 1965). The lifting of the presacral region acts to stretch the protractor muscles loaded by the weight of the hindlimb. According to Wilson (1979), this would permit a faster shortening velocity of these muscles and would allow them to produce more work. If the vertebral column of theropods were held horizontally then both protractors and retractors would be either short (reducing the excursion of their inser tion points) or their contraction would produce less tension (due to the slackness of the muscles). For these reasons, extension of the vertebral column is essential to reptilian bipedal locomotion. The same is true for the retractor function of the caudofemoralis. Extension of the tail renders the same benefits to the retractor musculature. The first few caudal ver- tebrae of theropods never have elongated ossified postzygopophyses for this reason. Thus the vertebral column in theropods should have been held at an angle of about 20 degrees above the horizontal ( fig. 7 View Fig. 7 ). Attempts at giving theropods ratite avian postures can do so only by neglecting the large differences in osteology and musculature, as well as method of balance and locomotion that clearly exists between theropod dinosaurs and birds.
No known copyright restrictions apply. See Agosti, D., Egloff, W., 2009. Taxonomic information exchange and copyright: the Plazi approach. BMC Research Notes 2009, 2:53 for further explanation.