| SYSTEMATIC BIOLOGY |
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| CLASS EOSYNAPSIDA+[i] |
Eosynapsida (e-o-sin-AP-si-da) is formed from three Greek roots that mean "dawn animals with a single arch" [dawn- eos (έως); with- syn (συν); arch- apsida (αψίδα)]. The term is derived from a single temporal fenestra, also a characteristic of mammals.
The early synapsids formed two distinctive groups: pelycosaurs and therapsids. The pelycosaurs (from two Greek roots meaning "pelvis lizards") were among the earliest of the amniotes, animals that produced eggs surrounded by an amnion. The amnion, a special membrane surrounding the egg, allowed for the exchange of gasses (mainly carbon dioxide and oxygen) while retaining water. Thus, amniotes could lay eggs in environments that were terrestrial. The selective pressure for such a character may have been the reduction in predation. Eggs in the water were subject to predation by fish and aquatic tetrapods while those on land were exposed to a lower density of predators. So, the amniotic egg with a supporting shell may have preceded a fully terrestrial existence.
The pelycosaurs appeared in the lower Pennsylvanian and diversified into herbivores and carnivores that occupied terrestrial and semi-aquatic environments. In general, they were low with splayed legs. The most notable ones were the various sail-backed forms like Dimetrodon and Edaphosaurus.
The therapsids (from two Greek roots meaning "beast arches", a reference to the mammal-like zygomatic arch) were more mammal-like than the pelycosaurs. They had more pillar-like legs and a skull that had the long snout, saggital crest, single sets of canines, and larger zygomatic arches. The therapsids included both herbivores and carnivores and persisted from the lower Permian to the lower Cretaceous.
The separation of the two group is artificial from a cladistic perspective because the pelycosaurs are nested in the clade that includes the therapsids and the mammals. However, to make this group consistent from a cladistic perspective would require a series of nested taxa with the mammals, a sister group to the cynodonts (a suborder in this system).
| HIERARCHICAL TAXONOMY OF THE CLASS EOSYNAPSIDA+. 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. |
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Dimetrodon (Left), a sail-backed pelycosaur, and Cynognathus (Right), a therapsid that was very mammal-like. Image
of Dimetrodon from: www.ucmp.berkeley.edu/ Drawing
of Cynognathus from: www.oni.escuelas.edu.ar/.../ CLASS EOSYNAPSIDA[i]- THE MAMMAL-LIKE REPTILES+ These
likely were the earliest of the Amniota, animals that produced eggs that
included an amnion. The amnion is a
membrane that allows for the exchange of gasses with minimal loss of water;
thus, the eggs could be laid on land. Likely,
these animals did not go through a larval stage typical of the preamniotic
tetrapods. All synapsids have a
single temporal fenestra, a character shared with the Mammalia.
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). 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 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 FAMILY
VARANOPSEIDAE Agile
predators, the varanopids had long gracile limbs, a long low skull with jaws of
sharp teeth. Permian Varanops 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 FAMILY
EDAPHOSAURIDAE These
herbivores had elongate 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 FAMILY
SPHENACODONTIDAE These
carnivores had elongate 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 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 Biarmosuchus 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, Mochops, 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, Mochops 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 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. |
