Thursday, April 23, 2015

The skull and teeth of Herrerasaurus

Herrerasaurus is one of the earliest dinosaurs. It lived about 230 million years ago, during the Late Triassic Period. Specimens are found only in northwest Argentina.

This rather small dinosaur was 3 to 6 m long (10 to 20 feet) and weighed around 210 to 350 kgm (460 to 770 pounds).  It was a lightly built bipedal (ran on two legs) carnivore with a long tail and relatively small head. Its long, narrow skull had large serrated teeth for eating meat. For some informative sketches as to the shape of this dinosaur, just "Google" its genus name.

Dinosaurs are classified as one of the several groups of diapsid reptiles (having one set of temporal fenestrae openings on both sides of the skull and another set on the top of the skull). Please note that "fenestra" is the singular form, and "fenestrae" is the plural form of this useful word.

The purpose of this post is show how the skull of one of the earliest dinosaurs differs from the skull of Dimetrodon, an early synapsid (having only one set of temporal fenestrae on both sides of the skull). I illustrated and discussed the skull of Dimetrodon in my last post.

The side view of a morphologically accurate Herrerasaurus skull shows an anorbital fenestra in front of the orbit, an irregular (rectangular) large temporal opening (called a lateral temporal fenestra) along the back of the skull, and a mandibular fenestra in the lower jaw. Herrerasaurus has a long rectangular skull that flat on top. Its teeth are numerous, closely spaced, and turned inward toward the back of the jaw. 

Side view of a replica of an Herrerasaurus ischigualastensis skull, 30 cm long.

The top view of the skull of the same replica of an Herrerasuarus skull shows two supratemporal fenestrae.

Top view of the same replica on an H. ischigualastensis skull, 30 cm long, 8 cm at its widest point.

Dimetrodon does not have an anorbital fenestra, nor does it have a mandibular fenestra. It also differs from Herrerasaurus by having only a simple, round temporal opening near the back of its skull. 
Dimetrodon has a strongly arched skull. Its teeth are not as solid looking as those of Herrerasaurus, less closely spaced, and not turned inward. Also, Dimetrodon shows development of distinct canine-like teeth and has throat teeth. In addition, the top of the Dimetrodon skull is solid and without any supratemporal fenestrae.

In summary, Herrerasaurus has the attributes of an early dinosaur. Dimetrodon, although referred by some toy manufacturers as a "dinosaur," is definitely not one. Instead, it is a synapsid.  

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