Weird Backs Month #07 – Sphenacodon

A close relative of Dimetrodon (who’ll be featuring tomorrow), Sphenacodon lived during the Early Permian (~295-270 mya). Two species are known from New Mexico, USA – and one possible specimen from the United Kingdom – with the larger reaching up to 3m in length (9′10″).

Its elongated vertebrae formed a long low dorsal crest, the very base of which seems to have supported strong back muscles. These would have helped to stiffen the spine, reducing sideways flexion as Sphenacodon moved.

While sphenacodonts are often depicted in sprawling lizardlike postures, fossil trackways show a much more upright semi-erect gait similar to that of crocodilian “high walking”.

The wedge point tooth, Sphenacodon (1878)

Phylum : Chordata
Family : Sphenacodontidae
Subfamily : Sphenacodontinae
Genus : Sphenacodon
Species : S. ferox, S. feriocor, S. britannicus

  • Early Permian (299 - 270 Ma)
  • 3 m long and 45 kg (size)
  • New Mexico, United States (map)

The skull of Sphenacodon is very similar to that of Dimetrodon. It is narrow from side to side and vertically deep, with an indented notch at the front of the maxillary bone in the upper jaw. The upper and lower jaws are equipped with an array of powerful teeth, divided into sharp pointed “incisors”, large stabbing “canines”, and smaller slicing back teeth. The orbit is set high and far back with a single opening (temporal fenestra) behind and partly below the eye, a characteristic of synapsids.

Body proportions are also similar to Dimetrodon, with a very large head, short neck, robust trunk, relatively short front and hind limbs, and a tapering tail that makes up about half the animal’s entire length. However, the tops of the neural spines along the back bone are strikingly different in each genus. In Dimetrodon, the neural spines develop into long, narrow, cylindrical projections that support a tall vertical dorsal sail that ends near the base of the tail. In Sphenacodon, the neural spines are enlarged but retain a flat-tipped, blade-like shape along the back and tail, and form a crest rather than a tall sail. (The sphenacodontid genus Ctenospondylus also has blade-like neural spines, but its dorsal crest is taller than in Sphenacodon, although not as tall as the sail in Dimetrodon.)

There is evidence for strong epaxial muscles along the base of the raised neural spines in both Sphenacodon and Dimetrodon, likely helping to stiffen and strengthen the backbone for walking and for lunging at prey by restricting side-to-side flexing motion. A recent study  of the structure of the neural spines on Sphenacodon confirms that the upper parts were not encased in a thick muscular hump and instead protruded above a layer of muscle to form a low dorsal crest. Finds of sphenacodontid specimens in which postmortem distortion of the body caused the dorsal spines to overlap suggests that the spines were not connected by hard or particularly tough tissue. The possible function of a low, skin-covered crest in Sphenacodon is debated. A thermoregulatory role seems unlikely, although the taller crest in Sphenacodon ferocior is allometrically larger than in S. ferox. Recent research has favored a display role for the tall sails in Dimetrodon and Edaphosaurus.

Both Sphenacodon and Dimetrodon have been depicted with their short limbs splayed outward at 90 degrees from the body in a wide pushup position and with the tail (and even belly) dragging on the ground, similar to modern lizards and crocodiles. A sprawling stance is also typical for Sphenacodon and Dimetrodon skeletons as currently mounted in museums. However, trackways called Dimetropus (“Dimetrodon foot”) that match the foot configuration of large sphenacodontids show animals walking with their limbs brought under the body for a narrow, semi-erect gait without tail or belly drag marks. Such clear evidence for a more efficient upright posture suggests that important details about the anatomy and locomotion of Sphenacodon and Dimetrodon may not be fully understood.  Some well preserved narrow Dimetropus tracks found in parts of the Prehistoric Trackways National Monument in New Mexico match the smaller size of Sphenacodon, a genus known from skeletal fossils in the state, but could also come from a small Dimetrodon.

Sphenacodon skeleton

Weird Backs Month #08 – Dimetrodon

Named for the different-sized teeth in its jaws, the sphenacodontid Dimetrodon is by far the most famous of the synapsids, and is all too frequently mistaken for a dinosaur. Living during the Early Permian (~295-272 mya), at least 13 different species have been identified – mostly from the southwestern USA, as well as one from Canada and another from Germany. The smallest, D. teutonis, was only 60cm long (2′), while the largest, D. angelensis, reached 4.5m (15′).

This particular reconstruction is based on a medium-sized species (D. grandis) which was around 3m long (9′10″).

Although traditionally depicted with a fully skin-covered sail, the bone structure of Dimetrodon’s neural spines suggests differently. The base seems to have anchored strong back muscles, much like yesterday’s Sphenacodon, with soft-tissue webbing only extending partway up the bones. Meanwhile the upper portions of the “sail” were probably free-standing, and many fossils show crooked bends where the unsupported spines didn’t grow straight.