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HIERARCHICAL CLASSIFICATION OF THE CLASS EOSYNAPSIDA+

HIERARCHICAL CLASSIFICATION OF THE CLASS EOSYNAPSIDA+[i]

EUKARYA>UNIKONTA>OPISTHOKONTA>ANIMALIA>BILATERIA>DEUTEROSTOMATA>VERTEBRATA>GNATHOSTOMATA>TETRAPODA>AMNIOTA>EOSYNAPSIDA
CLASS EOSYNAPSIDA LINKS

Description of Class

The following descriptions come from Benton (2005). The structure of the following system is also based, in part, on the phylogenetic treatment by Mikko’s Phylogeny Web (from the Finnish Museum of Natural History). Taxa in red are extinct.
  • CLASS EOSYNAPSIDA[i]- THE MAMMAL-LIKE REPTILES+
    • This group is paraphyetic because it includes the sister group to the mammals.
    • In this taxonomic treatment, I have retained the two traditional groups as orders even though the Pelycosauria is paraphyletic. The therapsids are monophyletic, but they include the sisters to the Mammals.
    • Benton (2005) called this class the basal synapsids. Eosynapsida is a name of my invention (see footnote).
  • ORDER PELYCOSAURIA
  • The temporal fenestra was relatively small. These taxa do not form a monophyletic group, but appear to form nested sister groups to the therapsids.
    • FAMILY EOTHYRIDIDAE
    • Mainly carnivores with long teeth; two canine-like teeth on each side of the upper jaw.
    • Lower Permian
    • Eothyris, Oedaleops, Bayloria
    • FAMILY CASEIDAE
    • These animals were herbivorous as evidenced by their teeth, and large rib cage. The skull, which was unusually small for their body size, had very large openings for the nares and the anterior end projected beyond the mouth. It had a somewhat reduced dentition of spatulate, rather than pointed teeth. There were no canine-like teeth. They had a very large pineal opening on the top of the skull.
    • Cotylorhynchus, Angelosaurus, Oromycter, Casea, Caseopsis, Caseoides, Ennatosaurus, Knoxosaurus
    • FAMILY VARANOPSEIDAE
    • Agile predators, the varanopids had long gracile limbs, a long low skull with jaws of sharp teeth.
    • Permian
    • Varanops, Varanodon, Aerosaurus, Elliotsmithia, Mesenosaurus, Mycterosaurus, Varanosaurus, Thrausomosaurus, Basicranodon, Archaeovenator
    • FAMILY OPHIACODONTIDAE
    • Mainly carnivores feeding on fishes and tetrapods. Have a high narrow skull with very strong legs. The orbit and temporal fenestra both were relatively small.
    • Middle Carboniferous to lower Permian
    • Ophiacodon, Paleothrys, Archaethyris, Baldwinonus, Clepsydrops, Stereophallodon, Stereorhachis
    • FAMILY EDAPHOSAURIDAE
    • These herbivores had elongated neural spines of the neck and back vertebrae that produced a sail-like structure on their backs. The skulls were relatively small and their teeth were peg-like and they had many palatal teeth (and similar ones below) to form a large crushing surface.
    • Lower Permian
    • Edaphosaurus, Ianthasaurus, Glaucosaurus
    • FAMILY SPHENACODONTIDAE
    • These carnivores had elongated neural spines just like those of the edaphosaurids. However, their skulls were relatively large with several pairs of fang-like teeth above and below. It had a small orbit and a high temporal fenestra.
    • Upper Carboniferous (Pennsylvanian) to Lower Permian
    • Dimetrodon, Ctenospondylus, Neosaurus, Secodontosaurus, Sphenacodon, Steppesaurus, Ctenorhachis
  • ORDER THERAPSIDA
  • The temporal fenestra was large, a reduction in palatal teeth, a jaw joint that was farther forward, and loss of the supratemporal bone. They also had an outward curvature at the back of the lower jaw.
    • SUBORDER UNNAMED
    • Intermediate between the sphenacodontids and therapsids, Tetraceratops has an enlarged temporal fenestra and fewer palatal teeth.
    • Lower Permian
    • Tetraceratops
    • SUBORDER BIARMOSUCHIA
    • Similar to the spenacodontids, these had had back-sloping temporal fenestra, but that had not supratemporal bone and they had only a single large canine on each side of the upper jaw. Fewer palatal teeth. The septomaxilla, a bone element in the nose of pelycosaurs, lies on the side of the nose opening.
    • Upper Permian
      • FAMILY BIARMOSUCHIDAE
      • Upper Permian
      • Biarmosuchus
      • FAMILY BURNETIIDAE
      • Permian
      • Bullacephalus, Burnetia, Lemurosaurus, Lobalopex, Niuksenitia, Paraburnetia, Proburnetia
      • FAMILY EOTITANOSUCHIDAE
      • Permian
      • Eotitanosuchus
      • FAMILY HIPPOSAURIDAE
      • Permian
      • Hipposaurus
      • FAMILY ICTIDORHINIDAE
      • FAMILY PHTHINOSUCHIDAE
      • Upper Permian
      • Phthinosuchus
      • FAMILY RUBIDGINIDAE
    • SUBORDER DINOCEPHALIA
    • A relatively large and diverse group that includes both carnivorous and herbivorous forms. Titanophoneus, a carnivorous form, had short limbs and a heavy skull. Similarly, Moschops, a herbivore, had massive legs, a stocky body, and a skull that was unusually thick at the top (for head-butting?). The hind legs were erect and pillar-like while the forelegs still retained the sprawling position.
    • Upper Permian
    • Titanophoneus, Moschops
    • SUBORDER DICYNODONTIA
    • These animals were herbivorous. They had a horny beak, a single pair of large upper canines and reduced post canine chewing teeth. They were pig-like in their proportions – stocky with a short tail.
    • Upper Permian to Triassic
    • Robertia, Kannemeyeria
    • SUBORDER GORGONOPSIA
    • The gorgonopsids were carnivores with a pair of large canines on the upper and lower jaws and the ability to open the jaws at about a 90-degree angle (similar to the saber-toothed cats that appeared much later). They were the dominant predators of the upper Permian.
    • Upper Permian
    • Lycaenops, Arctognathus, Leontocephalus
    • SUBORDER THEROCEPHALIA
    • Mainly carnivores but does include a few Triassic herbivores. Some had the mammalian hard palate and a reduction and specialization of the dentition also reminiscent of the mammals. Some even lost the bar of bone between the orbit and the temporal fenestra.
    • Upper Permian to lower Triassic
    • Theriognathus, Bauria
    • SUBORDER CYNODONTIA+
    • This is the sister group to the Mammals. Indeed, as a clade, it includes the mammals.
      • FAMILY PROCYNOSUCHIDAE
      • Very mammalian in appearance. The skull had a long snout, a saggital crest, and a large flaring zygomatic arch. The dentary made up more that 2/3 of the lower jaw and a rudimentary secondary palate.
      • Upper Permian
      • Procynosuchus, Dvinia
      • FAMILY THRINAXODONTIDAE
      • A well-defined sagittal crest; a broad zygomatic arch, a dentary that makes up most of the lower jaw and has a coronoid process that extends into the zygomatic arch. A reduced number of incisors (4 above, 3 below). Also, it has double occipital condyles. The dorsal vertebrae are clearly delineated into thoracic and lumbar. Their legs adopted a more erect stance. The gluteal muscle became the major retractor for the hind limb.
      • Lower Triassic
      • Thrinaxodon
      • FAMILY CYNOGNATHIDAE
      • Carnivores. The dentary makes up more than 90% of the lower jaw. The nondentary bones form a rod that lies in a groove on the dentary. The jaw joint has become more mammal-like. The teeth are seated within sockets in the jaw.
      • Middle Triassic
      • Cynognathus
      • FAMILY DIADEMODONTIDAE
      • Herbivores. The cheek teeth show mammal-like occlusions.
      • Lower to middle Triassic.
      • Diademodon
      • FAMILY CHINIQUODONTIDAE
      • Carnivores. They lost the parietal foramen and had a longer secondary palate. The had large heads, a long trunk and tail. The limbs are short and adapted to running. The hind limbs lock into place as in mammals. The femur could swing back and forward in a plane parallel to the midline of the body (parasagittal plane). The forelimbs still retained a more sprawling gait. They likely had the same physique and agility as a weasel. Some had a double jaw joint.
      • Middle and Upper Triassic
      • Probelesodon, Probainognathus
      • FAMILY TRAVERSODONTIDAE
      • Herbivores. Non-dentary bones in the lower jaw were modified to acoustic function and away from articulation. The stance was similar to Probelesodon in that the hind legs were erect and the forelegs sprawling. They had reduced ribs in the lumbar region, an indication of the development of a diaphragm.
      • Middle Triassic
      • Massetognathus, Scalenodon
      • FAMILY TRITYLODONTIDAE
      • Herbivores with specialized dentition and skull resembling that of rodents. Their cheek teeth occlude. They had some characters in common with mammals, particularly a complete orbital wall, a stapedial process on the quadrate, and an anteriorly inclined ilium.
      • Upper Triassic to Lower Cretaceous
      • Tritylodon, Kayentatherium
      • FAMILY TRITHELEDONTIDAE
      • These are known from fragmentary evidence. Their mammalian features included the loss of a postorbital bar, a slender zygomatic arch, large nasal cavity, a bony wall between eyes and a ridge on the upper cheek tooth bearing bone. Jaw joint almost entirely between the dentary and squamosal, the other bones modified to transmit sound. Two halved of the lower jaw not fused but held together with connective tissue. They had a flexing backbone and limbs with the range of motion similar to that of mammals.
      • Upper Triassic and lower Jurassic
      • Diathrognathus, Pachygenethus
[i] Eosynapsida is a name that literally means the dawn synapsids. I chose to keep them separate rather than create even more problems by lumping them into the Mammalia.
LITERATURE CITED

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Battail, B. and M. V. Surkov. 2000. Mammal-like reptiles from Russia. In: M. J. Benton, M. A. Shishkin, D. M. Unwin, and E. N. Kurochkin, eds. The Age of Dinosaurs in Russia and Mongolia. Cambridge Univ. Press, pp 86-119.

Benton, M. J. 2005. Vertebrate Paleontology. Third Edition. Blackwell Publishing, Malden, MA.

Bramwell, C. D. and P. P. Fellgett. 1973. Thermal regulation in sail lizards. Nature. 242: 203–205.
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Modesto, S. P. and N. Rybczynski. 2000. The amniote faunas of the Russian Permian: implications for Late Permian terrestrial vertebrate biogeography. In: M. J. Benton, M. A. Shishkin, D. M. Unwin, and E. N. Kurochkin, eds. The Age of Dinosaurs in Russia and Mongolia. Cambridge Univ. Press. pp. 17-34.

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Surkov, M. V., N. N. Kalandadze, and M. J. Benton. 2005. Lystrosaurus georgi, a dicynodont from the Lower Triassic of Russia. Journal of Vertebrtae Paleontology. 25: 402-413.
By Jack R. Holt. Last revised: 01/20/2013
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