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.  

Skull and teeth of Dimetrodon

Inexpensive replicas of Dimetrodon, the so-called "sail-fin reptile" or "finback reptile," are very eye-catching because they show the  fin supported by elongated spines projecting from along the backbone. In terms of the skull and teeth of Dimetrodon, however, these replicas, are commonly not morphologically accurate, and this post will attempt to shed some light on these subjects. 

Dimetrodon lived during the Early Permian, approximately 295 to 275 million years ago. Most specimens have been found in red beds in Texas and Oklahoma. At least a dozen or so species have been described. Dimetrodon was a carnivore that had a maximum body length of approximately 2 to 5 m (5 to 15 ft.) and a maximum weight of 250 kg (500 lbs). Its fin was used for thermal regulation and certainly not for "sailing." 

In earlier (outdated) classification systems, Dimetrodon would have been called a "pelycosaur" or a "mammal-like reptile." Dimetrodon is now classified as a synapsid mammal because it has a temporal opening, located relatively low on the back of each side of the skull. Synapsids show remarkable transitions from reptile-like to dog-like forms, and Dimetrodon was an early synapsid that had reptilian-like features.



The side view of a skull of Dimetrodon shown below is morphologically correct. The temporal opening (also called the infratemporal fenestra), which was used to accommodate bulging jaw muscles, is an important evolutionary feature used to link Dimetrodon with mammals. Notice that the size of the teeth of Dimetrodon differ considerably. Also there are several teeth at the opening of the mouth that are located higher (i.e, the maxillary step) than are the other teeth. The name "Dimetrodon" means "two shapes of teeth." Its teeth show some differentiation, and this is a mammalian trait. 

Side (lateral) view of a replica of a Dimetrodon skull, length 16 inches, height 10.5 inches.

The ventral view, shown below, of the lower jaw clearly shows how the tooth size varies and how the teeth at the front of the jaw are constricted. The red arrow points to a row of throat teeth (pharyngeal teeth), which were used to pin struggling prey.

Dorsal view of the interior of the lower jaw of the same replica.

Pharyngeal teeth are rarely mentioned in articles dealing with Dimetrodon. An interesting and well-illustrated article about its throat teeth can be found on the web, and the link is HERE

The last picture (see below) shows the top view of the skull, which is solid (i.e., no openings) and unlike the skulls of dinosaurs. In a future post, I shall illustrate how a dinosaur skull differs from that of synapsids, like Dimetrodon.

Top (dorsal) view of the replica of the skull of Dimetrodon.