Synapsids from Louis Marx

Synapsids from Louis Marx

• Sphenacodon • Moschops • Dimetrodon • Cynognathus •

Synapsids (class Reptilia, subclass Synapsida), the "mammal-like reptiles," are placed on the evolutionary line leading, more or less, to mammals. The classification is determined mainly by dentition and the holes on the sides of the skull. The synapsids had specialized teeth, allowing cutting, grinding, and (in some cases) chewing, clear advances over normal reptilian dentition. The synapsid skull had a single lower temporal fenestra (or one hole below, and aft of, each orbit), allowing room for nice big jaw muscles.

Classification based on an obvious feature, such as a hole in a skull, is obviously useful, as judgement calls don't enter into it - if a reasonably intact specimen of a skull is available, it either has holes where it should, or not. But it's not a particularly functional distinction, in the sense that it doesn't really pin down what made these animals different from their neighbors in their normal day-to-day activities (which of course consisted mainly of eating things). This is reflected in the taxonomic names. Synapsida refers to the holes in the skull. An earlier classification scheme called the order Theromorpha, a reference to their mammalian characteristics. Nowadays a compromise is used - the order is Synapsida, but they are called in English the "mammal-like reptiles." I liked Theromorpha, myself.

Although synapsid teeth and skulls were showing evolutionary progress, lower jaws were still basically reptilian. As evolution ground along its tedious course, most of the multiple bones comprising the reptilian jaw would gradually shrink and migrate, eventually becoming the tiny ossicles characteristic of the mammalian middle ear. (Unlikely though this may seem, the fossil evidence for this progression is relatively clear).

There are several orders in the Synapsida. The two of major concern to Rubber Dino afficionados (some classification schemes have only these two orders) are the pelycosaurs and the therapsids.

A half-dozen families are included in the Order Pelycosauria. The one of interest here is the sphenacodonts. The first large terrestrial carnivores were sphenocodonts. All members of the family had tall neural spines, as typified in the genera Dimetrodon Cope (extremely tall spines) and Sphenacodon Marsh (not so tall). Colbert (in The Age of Reptiles, 1965) points out that during the Lower Permian, a seaway covered western Texas, isolating New Mexico from eastern Texas. Dimetrodon is found in east Texas and Oklahoma, and Sphencodon appears on the other side of the seaway, in New Mexico. These two were both relatively large carnivores (some 3 meters or 10 feet long) sitting at the tops of their local food chains, and are anatomically similar, but obviously Sphenacodon never evolved a sail on his back.

Despite their new-fangled mammal-style teeth and skulls, the other details of sphenacodont anatomy and physiology were probably more similar to reptiles than mammals. The sphenacodonts, along with the other pelycosaurs, had disappeared by the mid-Permian.

Dimetrodon

Although the sail - with the tallest spines over three feet long - is the most obvious feature of Dimetrodon's anatomy, his most progressive feature was perhaps his teeth. He had two specialized types (hence his name, "two kinds of teeth") - formidable canines, and well developed shearing teeth. Without those, reptiles could only eat prey which they could swallow whole. Dimetrodon may have been the first large terrestrial carnivore who could kill and eat animals his own size. There are several known Dimetrodon species, varying in length from 3 to 10 feet (1 to 3 meters).

Both molds (those for the "small" mold group and the "revised" mold group) which produced the Marx Dimetrodon were three-segment ones (the usual left-right-ventral) - see the whole story in tedious detail here. The Marx figure doesn't show much in the way of teeth.
Dimetrodon markings - DIMETRODON (left side of tail), 7' LONG (right side of tail) - markings on (a), (b), and (c) versions are identical.

Sphenacodon

Sphenacodon had even fancier teeth than Dimetrodon, adding proper incisors to the lineup. He may have been the first large terrestrial predator. The prominent (though relatively short, at least compared to his cousin Dimetrodon) dorsal spines seem to have been attachment points for hefty muscles.

I always liked the Marx Sphenacodon, perhaps partially due to an air of mystery - I had no idea what he was. Marx managed to give him decent teeth, even though he was molded in the standard Marx-style three-cavity mold. He was part of the "small" mold group (along with two of the three Marx Dimetrodons), and so was initially available only in gray or green. He was reissued by Marx in the early 1970s.

Sphenacodon markings - SPHENACODON (left side of tail), 8' LONG (right side of tail).

Another order of synapsids, the therapsids, may have diverged from the sphenacodonts in the Lower Permian. The therapsids didn't have the elongate neural spines characteristic of the pelycosaurs, and therapsid dentition is a bit fancier. These were the most mammal-like of the mammal-like reptiles, and they dominated the Upper Permian and Lower Triassic faunas. There are several sub-orders and families.

Moschops

The therapsid family Tapinocephalidae includes Moschops. Despite his overall pugnacious aspect Moschops was a threat only to vegetation. He is well-known from fairly complete skeletons found in South Africa. Size seems to have run about 8 feet (2.5 meters) long, and about 5 feet high at the shoulder.

The Marx version came from a three-segment mold, and is especially noteworthy for its elegantly understated teeth. This figure was part of the "second series" mold group, so was in production from approximately 1960 to 1964, and made in green, gray, brown, and tan. He was also included in the reissued Play Sets of the early 1970s.

Moschops markings - MOSCHOPS 7' LONG (ventral surface)