Australia: The Land Where Time Began

A biography of the Australian continent 

Dinosaur Ancestors

The  monophyly of the Ornithodira, a group that includes the Pterosauria, Lagerpeton, and Dinosauriformes, and their most recent common ancestor, has been studied in detail by some workers (Gauthier, 1986; Sereno, 1991a). Evidence has been found that Pterosauria were outgroups of a clade comprised on Marasuchus plus Dinosauria (Sereno & Novas, 1990; Sereno, 1991a; Novas, 1992a). The phylogenetic placement of 2 taxa was also clarified, Lagerpeton (Arcucci, 1986, 1997; Sereno & Arcucci, 1993)  Sereno, 1991a), now considered a sister taxon of the Dinosauriformes (Sereno & Novas, 1990; Sereno. 1991a) and Pseudolagosuchus, that was formerly in the "Lagosuchidae"; Arcucci, 1987) was interpreted more recently as the immediate sister group of Dinosauria (Novas, 1992a). It is now considered that Pterosauria and Lagerpeton are successive ornithodiran outgroups of Dinosauriformes (e.g., Sereno, 1991a).

Dinosauriformes, a clade (Novas, 1992a) was defined to include the common ancestor of Marasuchus and Dinosauria as well as all the descendants of the common ancestor. Synapomorphies of neck anatomy and hindlimb morphology have been recognised as supporting the hypothesis of Dinosauriformes being monophyletic. There is an incipient sigmoid neck curvature in Marasuchus, though it's neck doesn't have the significant S-shape curvature seen in dinosaurs, including birds (Bonaparte, 1975; Gauthier, 1986).

Marasuchus has hindlimbs that show notable similarity to those of primitive dinosaurs, that include on the proximal articular surface of the femoral head, a trochanteric fossa that slid over a prominent pelvic antitrochanter on the posterior part of the acetabulum. There is also an anterior trochanter on the femur that conforms to a subvertical prominence that is continuous distally with a subhorizontal ridge, the trochanteric shelf, in both Marasuchus and dinosaurs. The trochanteric shelf is a prominent ridge on the lateroproximal surface of the femur, that is oriented posterodistally, that has been documented only among dinosauriform archosaurs, such as Marasuchus, Pseudolagosuchus, Silesaurus, Herrerasaurus, Syntarsus.

The tibia shape is also similar in early dinosaurs and Marasuchus. A feature that the author1 says is important. A cnemial crest is present on the proximal end of the tibia that in side view has a square shape. The distal end of the tibia is quadrangular and notched laterally by a groove running longitudinally along the distal part of the shaft. In distal aspect the groove is expressed as a U-shaped notch bounded by the facet for the ascending process of the astragalus and the posterior descending process of the tibia. This notch on the tibia of dinosaurs correlates with the differentiation of a more complex dorsal surface of the astragalus.

Features that the author1 considers closer to the condition seen in dinosaurs than seen in Marasuchus, are present in some Dinosauriformes such as Pseudolagosuchus from the Middle Triassic of Argentina and the Silesaurus from the Late Triassic of Poland (Dzik, 2003). An example is an elongate pubis that is equidistant for 70% of the length of the femur in dinosaurs and Pseudolagosuchus and Silesaurus, while in more basal ornithodirans, such as Pterosauria, Lagerpeton, and Marasuchus, that have a shorter pubis, being almost as long as the ischium, about 50 % of the femoral length.

Articular anatomy also changed in both Dinosauria and Pseudolagosuchus, such as the development of a pyramid-shaped ascending process and a caudal subvertical facet and behind the process, an elliptical depression. In ancestral archosaurs the ascending process of the astragalus constitutes a ridge, that is oriented craniocaudally, separating the tibia and fibular facets (Novas, 1989a). Lagerpeton and Marasuchus retained this condition. The ascending process of Pseudolagosuchus is pyramid shaped, that results from the development of a surface that is posterior, subvertical and non-articular. On the proximal face of the astragalus there is an elliptical depression behind this process.

A feature that distinguishes dinosaurs from other dinosauriforms is the articulation of the posterior surface of the ascending process with the posterior process of the tibia. The posterior process of the tibia is broader transversally than is seen in basal Dinosauriformes. The elliptical depression on the dorsal astragalus surface is concealed by the posterior process of the tibia.

The ascending process of the astragalus displays important morphological variation in Dinosauria - in ornithischians, with the exception of Pisanosaurus (Bonaparte, 1976; Novas, 1989a) the ascending process is reduced dorsoventrally and flattened anterioposteriorly, as in Scutellosaurus, and Dryosaurus, (Colbert, 1981; Galton, 1981). In the tetanurine theropods the ascending process forms a tall, broad wedge that is flattened anterioposteriorly (Gauthier, 1986). A caudal facet is present on the ascending process that articulates with the cranial surface (the astragalar surface) of the distal tibia.


Sources & Further reading

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





Author: M. H. Monroe
Last Updated 17/02/2012 




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