thecodonts

Tarsitano, Samuel, 1983, Stance and gait in theropod dinosaurs, Acta Palaeontologica Polonica 28, pp. 251-264 : 252-253

Although progress has been made in understanding the phylogeny of thecodonts (Charig and Reig 1970; Bonaparte 1975; Sill 1974), they are still largely represented by grades instead of clades. This is due to the poor fossil record of thecodonts and the incomplete preservation of those specimens which are known. This is certainly true of the small upland and/or arboreal forms that must have existed, as evidenced by Longisquama . The present paper uses the term Pseudosuchia as, at least, the ancestors of theropods ( Broom 1913; Huene 1921; Walker 1964). This relationship is based on the synapomorphies of the skull, tarsus and ischia ( Broom 1913; Walker 1964; Tarsitano, Ph.D. thesis). Ancestors of other dinosaurian groups as well as birds may also be relegated to the Pseudosuchia as they become known ( Heilmann 1926). It should be understood however, that the avian ancestor would not belong to the same group of pseudosuchians which were ancestral to any dinosaur taxon ( Tarsitano and Hecht 1980).

The locomotory morphology of pseudosuchian thecodonts is essentially crocodilian in nature ( Krebs 1963; Ostrom 1976). Both crocodilians and pseudosuchians are mainly quadrupedal. This type of locomotion is correlated to, or a consequence of, a sprawling gait and is tied to the structure of the tarsus, overlapping metatarsals, femur, hip joint and pelvic and hindlimb musculature ( Schaeffer 1941; Brinkman 1980a, 1980b; Tarsitano, Ph.D. thesis; Hecht and Tarsitano, in press). Crocodilians and pseudosuchians have a crocodilian tarsus ( Krebs 1963) or a variation of this ankle type termed the “crocodilian reversed tarsus” ( Cruickshank 1979; Thulborn 1980). In the crocodilian and pseudosuchian tarsus the proximal tarsal elements play a key role in locomotion ( fig. 1a ). The astragalus is bound to the tibia while the calcaneum moves with the pes ( Schaeffer 1941). Thus there exists an intratarsal joint of a complex nature between the two proximal tarsals. The important features of this joint will be described here for convenience. A comprehensive description can be found in Hecht and Tarsitano (in press). The medial element, the astragalus, bears a peg-like structure on its lateral surface which articul­ ates with a socket on the medial surface of the calcaneum. This articul­ ation comprises the primary joint between the calcaneum and astragalus. The secondary joint occurs between the astragalar trochlear found on the posterior surface of the astragalus and the tongue of the calcaneum ( fig. 1b ). The tongue process lies directly posterior to the calcaneal socket and projects medially to glide over the trochlea of the astragalus. The calcaneum ( fig. 1a, b, c .) is also moveable against the fibula. The calcaneum bears proximally a condyle (fibular condyle) which is free to rotate under a ventrally cupshaped cartilage ventral to the fibula ( fig. 1c ). The weight of the fibula is born by the fibula facet of the astragalus. The calcaneum bears posteriolaterally a tuber which serves to change the direction of pull of the foot extensors and tendons of the M. flexor tibialis externus and M. ambiens as they make their way to metatarsal V ( Schaeffer 1941; Gadow 1882; Brinkman 1980b; Tarsitano, Ph.D. thesis). The femur of pseudosuchians and crocodilians is also very similar. The head of the femur is not medially extended to form a roller surface ( Hotton 1980, and pers. comm.). Instead, the head is anterioposteriorly directed. There is also a lateral torsion in the femur so that the shaft of the bone does not lie in the same plane as the head. In this regard, the lateral femoral condyle is larger than its medial counterpart. Finally, although the acetabulum may be perforate, an overhanging shelf forming the dorsal boundary of the acetabulum which is essential to a hip roller joint and upright stance does not exist in pseudosuchians.

The elements of the locomotory system of crocodilians and pseudo­ suchian thecodonts correspond to a mainly quadrupedal level of organiz­ ation. Their hindlimb morphology can now be explained in functional terms. In order for the intratarsal joint to function, the calcaneum must be free to rotate. This means that the pes must first be lifted from the lateral side. The foot extensors, the M. gastrocnemius (tibial and fibular heads), M. peroneus, M. flexor tibialis externus, M. ambiens and M. caudo- femoralis (by way of the M. gastrocnemius) all are directed to the lateral side of the foot ( figs. 2 , 3 ) in particular to the fifth metatarsal ( Brinkman 1980b; Schaeffer 1941; Tarsitano, Ph.D. thesis). Thus the muscular function coincides with that of the ankle. Since the pes is first lifted laterally, the metatarsals must overlap to brace the inside digits of the pes which supply the support and convey the applied force of the foot extensors to the ground. Furthermore, since the pes must be lifted from the lateral Side, the femur cannot be brought under the body and must be held at an angle to the vertical, hence the femoral torsion. All of the above stated morphology is part of one functional complex and is a level of organiz­ ation and not a clade. It is apparent that all saurischian dinosaurs have evolved from a pseudosuchian ancestry since the remmants of the croco­ dilian tarsus is to be seen in theropods, sauropods and prosauropods. The ischia and pubes of pseudosuchians are decidedly saurischian and not crocodilian. While episodes of bipedalism are not unknown in crocodilians, the normal mode of locomotion is quadrupedal. A bipedal posture is possible when enough momentum has been attained in order that the presacral region may be lifted (the vertebral column extended). Thus it is likely that pseudosuchians were also able to run bipedally in such fashion but this type of bipedalism should not be confused with that of theropods. The primitive method of balance in thecodontian and croco­ dilian bipedal progression is that of a cantilever system. In this system the downward torque of the presacral region is balanced by the down­ ward torque in the opposite direction produced by the tail. This system of balance is also used by bipedal lizards ( Snyder 1949, 1952, 1954) and bipedal dinosaurs ( Tarsitano, Ph.D. thesis). Birds have adopted another system of bipedalism. The tail is not used as a counterbalance but is instead reduced for aerodynamic reasons. With the reduction of the tail in birds (including Archaeopteryx ) the pubes had to grow posteriorly in order that the viscera could be shifted under the pelvis thereby reducing the presacral downward torque. This adaptation would shift the center of gravity posteriorly. The shortening of the femur and the elongation of the tibiotarsus coincided with the posterior shift of the center of gravity under the pelvis. The result of these modifications of the pelvis and hindlimb in birds is that the tibiotarsus bone-muscle complex is the primary system of locomotion. In thecodonts, crocodilians and dinosaurs it is the tail-femoral-bone-muscle complex which is most important in locomotion. Thus, in order to interpret the osteology and muscle scars of theropods, it is better to compare theropods to crocodilians which have the same morphology as the pseudosuchian predecessors of theropods. I have found that of the muscles which leave scars on the pelvis and femur ( Tarsitano, Ph.D. thesis), there is a one-to-one correspondence between crocodilian muscle scars and the muscle scars found in well preserved theropods. In contrast, the avian pelvis and system of balance has been so modified as to be unreliable in the interpretation of theropod morphology.

Figures

Fig. 1. Various tarsal elements of archosaurs: a distal view of the right tarsus of a eusuchian crocodilian; b proximal view of the right calcaneum of the pseudosuchian Prestosuchus; c lateral view of the left calcaneum of a eusuchian crocodilian; d lateral view of the left calcaneum of a theropod dinosaur AC articular cartilage; AH anterior hollow of the astragulus; CT calcaneal tuber; DR distal roller of the astragulus; DT distal tarsal four facet of the calcaneum; F fibula; FC fibular condyle of the calcaneum; P peg of the astragalus; S socket of the calcaneum; T tongue of the calcaneum; TI tibia.

Fig. 2. Superficial shank muscles of a eusuchian crocodilian. FTE tendon of the M. flexor tibialis externus; GF fibular head of the M. gastrocnemius; PLA plantar aponeurosis; T tibia.

Fig. 3. Lateral view of the left shank and pes of a eusuchian crocodilian. AMB M. ambiens; C calcaneum; FTE M. flexor tibialis externus; G M. gas­ trocnemius, fibular head; PA M. pero­ neus anterior; PP peroneus posterior; TCF tendon of the M. caudofemoralis attaching to the fibula; TCFM tendon of the M. caudofemoralis, forming the main origin for the fibular head of the M. gastrocnemius; TE tendon of the M. caudofemoralis to the extensor tendon of the knee; TFTE tendon of the M. flexor tibialis externus to the fifth metatarsus.