Palaeos Vertebrates Synapsida: Sphenacodontia

Reflected lamina & retroarticular process

Sphenacodontia: (= Sphenacodontoidea = Sphenacodontida) Haptodus. Definition: Dimetrodon + Marilyn Monroe.

Characters: maxillary supracanine buttress (thickening of the maxilla on internal surface, above caniniforms); $ maxilla enlarged, separating lacrimal from narial rim; [BS00]; zygomatic process of quadratojugal lost & replaced by process of squamosal [BS00$] [HB86]; supraoccipital larger than in edaphosaurids [RP40]; paroccipital process elongated & directed ventrolaterally [BS00$] [HB86]; prootic forms anterolateral wall of otic capsule [M95]; dorsum sellae thick and plate-like [M95]; dentary deeper than in Edaphosaurus [RP40]; reflected lamina of angular present [BS00$] [HB86]; retroarticular process of articular oriented ventrally [BS00$] [HB86] [the combination of reflected lamina and ventral retroarticular process suggests early evolution of the mammalian middle ear; that is, that the angular (= tympanic), articular (= malleus), and stapes were used for hearing. For details, see Jaws and Ears]; Premaxillary teeth in deep sockets; enlarged caniniform maxillary teeth, transversely compressed, with anterior & posterior cutting edges [BS00$] [HB86]; dorsal centra relatively longer than in ophiacodonts [RP40]; iliac blade expanded [RP40].

Haptodus garnettensis Currie 1977. Length: about 60 cm. Mass: about 3 kg. Duration Kasimovian (Late Carboniferous). Fossil remains: skeleton and skull elements. Notes The most primitive known member of the Sphenacodontian lineage.

Haptodus longicaudatus (Credner 1888) (= Palaeohatteria). Length: about 60 cm (juveniles). Mass: about 3 kg. Duration: Sakmarian (Early Permian). Fossil remains: numerous remains, mostly of juveniles, from Niederhässlich, near Dresden, Germany.

Haptodus saxonicus (vonHuene 1925) (= Pantelosaurus). Adult Length: upto 140 cm. Adult Mass: about 30 kg. Duration: Asselian (Early Permian) Fossil remains: six nearly complete remains, from the Döhlen Basin, near Dresden, Germany (Cuseler Stufe, lower Rotliegende).

Introduction: Haptodus is a relatively small pelycosaur (60-150 cm and perhaps 3-30 kg) which lived in equatorial Pangea, within the "Edaphosaurid- Nectridean Empire." It was a medium-sized terrestrial predator which probably constiututed the basal or primitive ancestral type of the Sphenacodontia. It shares many structural features of the skull and skeleton with the more specialised sphenacodontids, indicating they are closely related. Haptodus is known from the Late Pennsylvanian (Latest Carboniferous) and Early Permian of North America and Europe. Both small and medium sized individuals are known. These animals were clearly effective predators, like contemporary large tropical lizards, feeding on both arthropods and small vertebrates. Haptodus lacked the spectacular sail that characterised the bigger Sphenacodontids like Dimetrodon, Ctenospondylus, and Secodontosaurus. It is also more common in Europe (central equatorial Pangea), whilst the other forms are primarily American (west equatorial Pangea). (MAK 000718).

Sphenacodontidae: Ctenospondylus, Dimetrodon, Secodontosaurus, Sphenacodon. Big sail-back carnivorous pelycosaurs and relatives. The dominant carnivores of the Early Permian.

Introduction: The sphenacodontids include the large carnivores Dimetrodon, Sphenacodon, Ctenospondylus, and Secodontosaurus. Reisz et al. (1992). These animals were the dominant predators of their time, and their fossil record extends from the Latest Carboniferous through to the early Middle Permian.

All the sphenacodonts had a deep, narrow skull with massive jaws and a formidable array of teeth -- long canines, daggerlike incisors and small, cutting cheek teeth. The name of the most well-known sphenacodontid, Dimetrodon, actually refers to this adaptation, it means "two measure teeth". The sphenacodonts were the first animals to develop such a specialized set of teeth, and were the first large terrestrial carnivores to evolve (all earlier large carnivores were either fully or semi- aquatic).

Many of these forms (Dimetrodon, Sphenacodon, Secodontosaurus, and Ctenospondylus) are all large to very large predators that have tall neural spines. These, in life, doubtless supported a large "sail" or fin on the animal's back. This strange structure most certainly served as a thermoregulatory device. In the cool morning the creature would turn side on to the morning rays, thus soaking up heat and becoming more active before its rivals or prey did. During the middle of the day, or whenever the creature was in danger of overheating, it could turn head on to the sun, thus shading the fin and allowing excess heat to dissipate.

These animals had long limbs, making them relatively agile, fast moving animals, especially when compared to their slower, bulkier herbivorous relatives. The later and larger types like Dimetrodon, Sphenacodon, and Ctenospondylus all have similarly constructed, massive skulls with extremely large anterior incisors and canines.

Not only were these animals of great ecological significance during their long reign, but they also have an important position on the evolutionary tree, being close to the ancestry of the primitive therapsids, and ultimately to mammlas and man. (MAK 000718)

Characters: Deep, massive skull; premaxilla slopes anteroventrally ("hook-nose"); maxilla dorsoventrally broad to support fangs & excludes lacrimals from nares; $ ventral narial process of nasal forming posterior margin of nares; $ anterior process of frontal longer than posterior process; pineal foramen on parietal with distinct ridge; extensive postorbital - squamosal contact; paroccipital process having only terminal contact with squamosal [RP40];paroccipital process dorsal border is sharp ridge [RP40]; jaw articulation well below level of tooth row [shape and position of jaw articulation allowed mandibular adductors attached to back of skull to pull more efficiently, i.e. edge of surangular and coronoid almost perpendicular to muscle fibers]; articular enlarged; angular with "reflected lamina;" teeth with cutting edges formed from ridges extending only half-way tip towards base [R+97]; teeth robust & almost square in cross-section at base [R+97]; enlarged maxillary caniniform teeth which bulge into choana; specialized "incisors;" dorsal centra markedly taller than wide [RP40]; neural spines elongated into sail (probably thermoregulatory device, suggesting * were ectothermic); spines smooth, without crossbars; no sign of sexual dimorphism in sail; sail area heavily vascularized; relatively long, gracile limbs; supraglenoid foramen on scapular blade [RP40]; possibly first terrestrial animals to prey on similar-sized prey [RS01].

Sphenacodon ferox. 177 cm and 52 kg. From the Sakmarian of the Abo/Cutler Formation, New Mexico, USA. Distinguished from the contemporary S. ferocior by smaller size, more slender build, and less neural spine development.

Sphenacodon ferocior. 225 cm and 129 kg. From the Sakmarian of the Abo/Cutler Formation, New Mexico, USA. A larger and less common contemporary of S. ferox, distinguished by more robust proportions and greater neural spine development.

Introduction: Sphenacodon has been recovered from the Early Permian of Euramerica. Like most Sphenacodontids, it was a large predator, 150-250 cm in total length. The vertebral spines of Sphenacodon's backbone were long, and probably acted as attachment points for massive back muscles, allowing the animal to lunge powerfully at its prey. Sphenacodon did not have the extremely elongate spines and distinctive "sail" of more derived sphenacodontids. Two species are known. The more common S. ferox is smaller, more slender, and less development of the neural spines. The aptly named S. ferocior is a contemporary, but shows the greater size and greater spine development typical of the more derived Sphenacodontids. (adapted from MAK 000718)

Characters: head deep & narrow; massive jaws; regional specialization of teeth, with some differentiation of premaxillary incisor-like, canine and smaller maxillary dentition; elongate neural spines.

Introduction: Dimetrodon means "two-measures tooth," referring to the fact that unlike reptiles, the teeth differ in size. This was an enormous animal for its time, with an adult length of up to three metres or more and perhaps 150 kg, depending on the species. Dimetrodon is found in Early to Middle Permian deposits of a localised area in western equatorial Euramerica, now Oklahoma and Texas, in a part of the "West Edaphosaurid- Nectridean Empire" which we may call the "Dimetrodon sub-province." Dimetrodon is an advanced member of the Sphenacodontid family, and may have evolved from an early Sphenacodon-like form (e.g. Sphenacodon ferox) during the latest Carboniferous or earliest Permian (Gzhelian-Asselian) time.

Perhaps the most well-known prehistoric non-dinosaur, Dimetrodon was a common large carnivorous pelycosaur of the early Permian of North America, immediately recognisable by the "fin" or "sail" of elongate vertebral spines running along its back, upto a metre in length, and in life covered with a layer of skin and blood vessels and serving as a heat-exchange mechanism. In size and build it is comparable to a modern-day alligator, except that it was a fully terrestrial (land-living) creature. Dimetrodon was the dominant predator in its environment for some twenty-five million years, during which time it evolved through about a dozen species, become steadily larger as time progressed (an example of "Cope's Law"). Dimetrodon legs, although strong, were short, so it may have hunted through ambushing its prey, the sail on the back also no doubt serving to help camoflague the creature when it hid among stands of bamboo-like Calamite plants.

The largest, most specialised and most spectacular of the pelycosaur carnivores, Dimetrodon remained the dominant carnivore in its environment for some twenty-five million years, before being ousted by the up and coming eotitanosuchian and dinocephalian theraspids during the Ufimian age. Curiously, despite its obvious adaptions, Dimetrodon remained confined to a limited geographical region (fossils are known only from the Witchita, Clear Fork and San Angelo beds of the Texas-Oklahoma region, and some fragments of a small early species from New Mexico). This limited area (the actual extent being unknown due to the fact that only in a few places are there fossil deposits) is here termed the "Dimetrodon sub-province".

In skull and general body form, Dimetrodon was very similar to contemporary types like Sphenacodon and Ctenospondylus. However, these latter two have blade-like (narrow or flattened in cross- section) neural spines supporting the sail, whereas Dimetrodon has greatly elongated spines that are rounded in cross- section section.

Dimetrodon is such a well-known creature it's even featured in most popular books on dinosaurs (even though it is not a dinosaur, and not even related). There are also several webpages dedicated to it. (MAK 000718)

Introduction: Secodontosaurus is an unusual form that, although similiar in body, differs in the shape of the head from other sphenacodonts. The skull is low and narrow, although the neural spine morphology (of the "sail" backbone) is similiar to that of Dimetrodon. The cranial modifications of Secodontosaurus indicate an adaptation to some specialized feeding strategy, perhaps preying upon burrowing animals, or perhaps aquatic feeding habits. (MAK 000718)

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Synapsida: Sphenacodontia

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