Archosaurs

 Archosaurs had their early beginnings in the Permian Period and belonged to the reptile group called diapsids. These early archosaurs are called Archosauromorphs since they were “archosaur-like”. The first true Archosaur emerged in the Early Triassic and included groups such as the dinosaurs, pterosaurs and crurotarsi to name a few, all of which were successful during various times throughout the Mesozoic.

The contemporaries of the Archosaurs, the Therapsids (mammal-like reptiles), were very successful during the earlier stages of the Triassic and maintained the proverbial upper hand throughout most of the period. Based on the fossil record the archosaurs were more common in the northern continents while the therapsids enjoyed the more southerly climes of the continent of Gondwana. But as the Triassic waned, it was the Archosaurs who assumed dominance, a dominance that would not be relinquished for over 150 million years.

The following sections describe some of the unique physical characteristics developed by the archosaurs that made them extremely successful in the Triassic world.

Socketed Teeth and Holes in the Head

A characteristic feature of archosaurs was the development of socketed teeth which meant that the teeth, unlike those of reptiles, were secured in sockets in the jawbone, giving them greater strength thereby allowing the archosaur to grip more firmly and tear more fiercely than their competitors. 

A change in terminology.  The term Thecodont which means “socketed teeth” in ancient Greek was used at one time to describe the archosaurs. However this term is no longer used now that classifications of lineages and relationships had changed in paleobiology. The appellation Thecodont was too all-inclusive and didn’t accurately reflect the cladistic relationships between the various types of archosaurs and their subsequent development.

Antorbital Fenestra

As members of the diapsid group of reptiles, the archosaur skulls had two openings in the skull behind the eye socket (see top view of skull in diagram); but an additional opening was added between the eye and the nostrils. This is called the “antorbital fenestra” meaning the “window in front of the eye”. It is thought that this served to lessen the weight of the skull while at the same time providing more room for muscles and tendons to improve strength and flexibility. This proved useful for those archosaurs who took to the air (pterosaurs) and those archosaurs (dinosaurs) whose skulls grew larger and heavier in succeeding periods. Modern birds have retained this ancient feature although crocodiles have not.

Further lightening the load was another opening in the lower jaw called the mandibular fenestra, once again providing additional space for stronger jaw muscles, essential in battle and hunting.

Hip Joints and Postures

Reptiles were built with their legs sprawled to the side, giving them the familiar swaying gait seen in reptiles today. The early archosaurs were built in the same manner, but they soon developed two additional hip structures: erect and pillar erect.

• Erect

This posture was used by bipedal archosaurs and is used by humans today. The sockets on the hips open to the side. Fitting into this open socket are the top knobs of the leg bone (the femur) which has a large knob of bone perpendicular to the bone shaft and which fits into the side-socket of the hip, creating a smooth pendulum swing.

• Pillar-Erect

In this construction, the femurs are completely straight.  The femur "knob" sat atop the femur, not at a right angle as in the erect posture.  With the knob on top of the femur, the leg was able to fit up into the the hip sockets which faced down. This posture was used by some groups of Crurotarsans which gave them the ability to walk on more erect limbs improving their freedom of movement.  Some crurotarsans may even have been bipedal when desired.  It is thought that the bipedal option was only chosen for times when speed was needed. In this construction, the hip sockets face down.)

Respiration

In 2009 researchers from the University of Utah discovered that modern alligators have the same unidirectional respiratory system used by modern birds as well as ancient pterosaurs and some dinosaurs.  This discovery has now led scientists to believe that the development of this very efficient respiratory system occurred earlier in archosaur development than previously thought, further solidifying the reasons for their success.   For additional information on this issue see the articles on the crurotarsi and pterosaurs.

Ornithodira and Crurotarsi

Around 245 million years ago, the fossil record shows two main groups of archosaurs:

• Ornithodira (dinosaurs, pterosaurs )
• Crurotarsi (ancestors of the crocodiles and alligators)

The actual ancestry of Ornithodirans is still quite controversial. Some scientists believe that they originated from the Ornithodiran line while others feel it was much earlier, about the time when Archosauriformes split from the Prolacertiformes. See the cladogram above indicating some of the major distinguishing characteristics of each branch of archosaurs. (The numbers accompanying the cladogram indicate important innovations in archosaur physiology that produced the emergence of each new line of archosaurs.)  For a diagram of the dinosaur groups, go to the Dinosaur Beginnings page.

It would be the Crurotarsi that would rise to an early terrestrial supremacy while the Ornithodira, being late bloomers, didn’t come on strong until the Late Triassic.  In the early Jurassic, they soon supplanted the Crurotarsi decimated by the Triassic Extinction. The Jurassic and the Cretaceous would see the pinnacle of Ornithodiran primacy while the Crurotarsi occupied rivers, swamps, and in some cases, the sea.