Australia: The Land Where Time Began

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Dinosaurs - Characteristics distinguishing dinosaurs from other archosaurs

Diagnostic features of dinosaurs, those derived features that evolved in the common ancestor or Ornithischia and Saurischia, but are not present in their immediate sister groups, included a number of anatomical changes in the head, neck and pelvic girdles.

Pseudotemporal muscles in the back of the skull spread beyond the edges the supratemporal openings to partially invade the dorsal surface of the frontal bones. The author1 suggests the force of jaw closure may have been increased by this modification. A more pronounced inclination of the articular surfaces of the cervical centra gave the neck a distinctive sigmoid curve to bring their neck curve closer to the condition found in birds. Epipophyses above the postzygapophyses developed in at least some of the cervical vertebrae.

Most early dinosaurs, with the exception of the Staurikosaurus, Guaibasaurus, and Saturnalia, in which the sacrum was apparently comprised of 2 sacrals, increased the number of sacrals to 3 by incorporating a vertebra from the presacral series (e.g., Welles, 1984; Novas, 1996a). Craniocaudal shortening was involved in the incorporation of a new sacral (from the dorsal vertebrae) of the last dorsal vertebra and original sacral 1 and 2. The last dorsal vertebra was placed behind the tip of the pubic pedicle of the ilium as a result of this position change. As a result of the first sacral ribs accompanying the craniocaudal shortening of its corresponding centrum articulating in a more caudal position than in the ancestral position. This led to the last trunk vertebra being incorporated into the sacrum to become the first dorsosacral, allowing the articulation of the dorsosacral ribs to the ilium by the perpendicular orientation of the primordial sacral ribs. The most important process that made the incorporation of more vertebrae into the sacrum possible was probably the axial shortening of the last dorsal and primordial sacral vertebrae, at least during the early stages of dinosaur evolution. Among the Theropods, Ornithischia, and Sauropodomorpha, the number of sacrals increased independently, all having a sacrum with at least 5 vertebrae.

On the humerus of dinosaurs there is a deltopectoral crest, the distal corner being positioned down the shaft away from the humerus head at a greater distance than 30 % of the maximum length of the humerus (e.g., Bakker & Galton, 1974; Sereno & Novas, 1992; Sereno, Forster, Rogers & Monetta, 1993; Sereno, 1994).

The perforated acetabulum, resulting from opening of the medial acetabular walls of the ilium, pubis and ischium, is a feature that distinguishes the dinosaurs from the other dinosauriforms. In basal dinosaurs such as Guaibasaurus, Saturnalia, Staurikosaurus, Herrerasaurus, and  Lesothosaurus, the acetabulum opening is relatively small and in the more advanced dinosaurs such as sauropodomorphs and theropods it is larger. A brevis shelf, a distinct, prominent shelf on the posterolateral margins of the iliac blade, flaring laterally above the posteroventral iliac margin. A ventral fossa, that is bound by the brevis shelf and the posteroventral iliac margin, cradled the caudofemoralis muscle, that was a femoral retractor. A well-developed brevis shelf and brevis fossa that is transversally wide and dorsoventrally deep is present in most theropods, ornithischians and sauropodomorphs. Among the ornithodirans only the Dinosauria have a brevis shelf and fossa, the postacetabular part of the iliac blade is compressed transversely, with the posteroventral margin running from the ischiadic peduncle towards the posterior end of the blade in the basal forms such as Pterosauria, Lagerpeton, and Marasuchus (Novas, 1992a, 1996a).

The ischium of dinosaurs has a slender shaft and a ventral keel (obturator process) limited to the proximal third of the bone. The ischium is in the form of a plate-like structure distal to the acetabular region in basal archosaurs. The ancestral Ornithodira retained this condition, as it is present in Pterosauria, Lagerpeton, and Marasuchus, where the obturator process of the ischium that projects ventrally is expanded proximodistally. In theropods, sauropodomorphs and ornithischians there is a ischiadic shaft that is elongated, the obturator process being limited almost entirely to the proximal third of the bone.

Compared to the more basal dinosauromorphs the early dinosaurs have a number of modifications of the bones of the forelimbs. In crurotarsan archosaurs and basal ornithischians, such as Lagerpeton, Marasuchus, and Pseudolagosuchus, the proximal femur has a well-developed tuberosity on the caudal surface of the head. Externally, the groove for a ligament attached to the femoral head is bounded by this tuberosity, and has a rounded prominence that is transversely wide in basal archosaurs. The tuberosity is located almost halfway between the medial and lateral margins of the proximal end of the femoral head. The tuberosity is greatly reduced and had a medially placed slight prominence in the Dinosauria. In the proximal end of the femur of dinosaurs another modification is related to the development of a prominent anterior trochanter that projects proximally for the attachment of femoral protractors.

In the basal dinosaurs there are modifications that the author1 describes as interesting such as in the articulation of the tibia-astragalus. The tibia articulates with the proximal surface of the astragalus, though it doesn't ascend laterally past the ascending process of the astragalus in basal archosaurs. As a result the distinction between the posterior process of the tibia and the rest of the articular surface of the bone is not clear. Ancestral Dinosauriformes retained this condition, as it is present in Marasuchus and Pseudolagosuchus, the distal end of the tibia being convex transversely and inclined posterodistally. In dinosaurs the tibia covers the ascending process posteriorly, the result being that the articulation of the latter beneath  the craniolateral corner of distal articular surface of the tibia, though the tibia in basal dinosaurs such as Herrerasaurus, and Pisanosaurus is quadrangular (Novas, 1989a, 1994). It is possible to distinguish an anterolateral facet for the ascending process of the astragalus and a posterior process that projects ventrally, as the distal tibial articular surface becomes topographically complex. The ventrally projected posterior process becomes covers the ascending process of the astragalus posteriorly and its dorsal surface behind the ascending process. The dorsal surface of the astragalus that is posterior to the ascending process enlarges towards the rear, the posterior bone margin becoming straight in dorsal view. This part of the astragalus is deeply notched in dorsal view in Pseudolagosuchus, and is a non-articular surface. Also, a deep elliptical basin is present immediately posterior to the ascending process in the dinosaur astragalus.

In basal dinosaurs there were 2 more modifications in the tarsal bones. A tighter articulation with the distal end of the fibula is created by the proximal surface of the calcaneum becoming concave. There is a hemicylindrical proximal condyle for articulation with the fibula on the calcaneum of Marasuchus and Pseudolagosuchus. Also, in dinosaurs the distal tarsal 4 is flattened proximodistally and a contour that is triangular-shaped in the proximal view, that differs from the distal tarsal 4 of basal ornithodirans that is more block-like.

According to the author1 the available evidence suggests the early evolution of the Ornithodira saw locamotory abilities improving in a sustained manner and acquisition of a digitigrade posture by the species of ancestral Ornithodirans (Gauthier, 1986) that was inherited by the dinosauromorphs, including the Dinosauria ancestrally. Among the Ornithodira (Sereno, 1991a) many of the synapomorphies are associated with the hindlimb. Such adaptations that involved the transformations in the skeleton of the pelvis and hindlimbs, also related to locomotion. Features of the tarsus and pes constitute about 70 % of the apomorphies that are diagnostic of Dinosauromorpha, according to the results of studies by Sereno, 1991a; Sereno & Arcucci, 1994).

A strong sigmoid curvature of the neck is included among the synapomorphies diagnostic of Dinosauriformes, though most of the new evolutionary features are concerned with the apparatus for locomotion. Some of the outstanding skeletal modifications among early dinosauriforms include remodeling of the head of the femur with the development of a trochanteric fossa, a characteristic the the author1 suggests probably restricted the movement of the femur in the acetabular socket when compared to the movement allowed in the ancestral Archosauria. Greater development of, or greater differentiation of both the femoral protractors and retractors is suggested by the presence of the anterior trochanter and trochanteric shelf. Interesting modifications are also found in both the proximal and distal ends of the tibia, a cnemial crest being present on the proximal tibia, suggesting differentiation and enlargement of the protractive (extensor) muscles of the lower leg, and flexor muscles of the ankle.

During the early evolution of dinosauriforms modifications to the pelvic elements for the bones of the hindlimbs were delayed, the author1 suggests it is assumed that no major modifications in the placement of origin of the pelvic musculature, as the morphology of the pelvic bones were retained from the condition in the ancestral archosaurs, such as an ilium that was short in the craniocaudal direction and a pubis and ilium that were short with no ventral elongation. Bone morphology, mainly the acetabular-femoral and the tibiofibular-tarsal articulations, though not muscular rearrangement in the area of origin, is implied to have been a major restraint on the movements of the hindlimbs in a parasagittal plane in the early stages of evolution of ornithodirans. At insertion sites of the protractive and retractive muscles of the femur are sites where modifications occurred, which the author1 says probably allowed the hindlimbs to gain an increase in stroke power. The pubis and ischium lengthened, with consequent enlargement of the origin area for the femoral protractors that occurred later in dinosauriform evolution, such as the common ancestor of Pseudolagosuchus and Dinosauria.

The skull, neck, vertebrae, forelimbs and hindlimbs, and pelvic girdle are where the diagnostic features of dinosaurs are found. Pertaining to the skull the characteristic features are equivocal synapomorphies of Dinosauria, as in the immediate sister taxa of Dinosauria - Pseudolagosuchus, Marasuchus and Lagerpeton, the skull is not known. 

Of the postcranial adaptive innovations that were hypothesised for the common dinosaurian ancestor 75 % related to pelvic girdle and hindlimb.  These include articulation strengthening between the pelvis and the vertebral column, with the addition of a new sacral vertebra, the development of prominences, such as trochanters and shelves, and to allow for muscular attachment, the widening of the pelvic bone surfaces. 40 % of the adaptations present in the hindlimbs pertain to the proximal femur, the remaining 60 % to the articulation of the distal tibial and tarsals. In dinosaurs the lateral surface of the iliac blade is apparently wider than in the early dinosauromorphs such as Lagerpeton and Marasuchus. In Pseudolagosuchus the pelvis is not completely known, so the ilium size increase is not certain to have occurred concomitantly with the lengthening of the pubis and ischium. In the ancestral dinosaurs the ilium retained the brachyiliac condition. Apart from the development of the brevis shelf and fossa.

In the dinosaur iliac musculature the main modification expected was a volume increase. The ancestral dinosaurs modified the pelvis much more than the other ornithodiran ancestors. No major modification of hindlimb anatomy occurred after the differentiation of the dinosaurs. Among the saurischians 9 unequivocal synapomorphies are used in the diagnosis (Novas, 1994), of which none pertain to the hindlimb, and 1 to the pelvis, such as a rod-like ischiadic shaft with a triangular cross-section. The ornithischians have been described by the author1 as more "progressive than the saurischians, referring to the morphology of the pelvic girdle and hindlimbs that show a few transformations of the anatomy of the hindlimbs. In the femur and pes 3 synapomorphies have been recognised by Sereno (1986), the pelvis being highly modified with 11 features that were derived (Sereno, 1986).

In the Ornithodira, Dinosauromorpha, Dinosauriformes, and Dinosauria most of the synapomorphies that are diagnostic involve anatomy of the hindlimb. According to the author1 it is reasonable to conclude that early Ornithodira evolution was characterised by improved capabilities of locomotion, such as erect hindlimbs, bipedalism and digitigrady. Though it may be shown by new discoveries that the hindlimb and pelvis were not the only areas of the skeleton to change extensively during ornithodiran early evolution, it is clear, according to the author1, that they improved locomotion. Modification in the anatomy of the pelvic girdle and the hindlimbs were more profound than any that occurred in their scapular girdle and forelimbs. Pterosaurs and theropods were the only forms to manifest extensive transformation of the skeletal elements of the scapular girdle and forelimbs (Gauthier, 1986; Sereno, 1991a; Novas, 1994).

Sources & Further reading

  1. Novas, Fernando E., 2009, The Age of Dinosaurs in South America, Indiana University Press.



Author:Author: M. H. Monroe
Last Updated 15/02/2012 




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