Seymouria individuals were robustly-built animals, with a large head, short neck, stocky limbs, and broad feet. The sensory apparatus of the skull also deserves mention for an array of unique features. The
orbits (eye sockets) were about midway down the length of the skull, although they were a bit closer to the snout in juveniles. They were more rhomboidal than the circular orbits of other seymouriamorphs, with an acute front edge. The
palate (roof of the mouth) had some similarities with both amniote and non-amniote tetrapods. On the one hand, it retained a few isolated large fangs with maze-like internal enamel folding, as is characteristic for "labyrinthodont" amphibians. On the other hand, the
vomer bones at the front of the mouth were fairly narrow, and the adjacent
choanae (holes leading from the nasal cavity to the mouth) were large and close together, as in amniotes. The palate is generally solid bone, with only vestigial interpteryoid vacuities (a pair of holes adjacent to the midline) separated by a long and thin cultriform process (the front blade of the base of the braincase). Apart from the fangs, the palate is also covered with small denticles radiating out from the rear part of the
pterygoid bones.
Seymouria has a few amniote-like characteristics of the palate, such as the presence of a prong-like outer rear branch of the pterygoid (formally known as a transverse flange) as well as an
epipterygoid bone which is separate from the pterygoid. However, these characteristics have been observed in various non-amniote tetrapods, so they do not signify its status as an amniote.
Postcranial skeleton The
vertebral column is fairly short, with a total of 24
vertebrae between the hip and skull. In addition, later studies found that the atlas intercentrum was divided into a left and right portion, more like that of amphibian-grade tetrapods. Unlike almost all other Paleozoic tetrapods (amniote or otherwise),
Seymouria completely lacks any bony remnants of scales or scutes, not even the thin, circular belly scales of other seymouriamorphs. The
pectoral (shoulder) girdle has several reptile-like features. For example, the
scapula and
coracoid (bony plates which lie above and below the shoulder socket, respectively) are separate bones, rather than one large shoulder blade. Likewise, the
interclavicle was flat and mushroom-shaped, with a long and thin "stem". The
humerus (forearm bone) was shaped like a boxy and slightly twisted L, with large areas for muscle attachment. This form, which has been described as "tetrahedral", is plesiomorphic for tetrapods and contrasts with the slender hourglass-shaped humerus of amniotes. On the other hand, the lower part of the humerus also has a reptile-like adaptation: a hole known as an
entepicondylar foramen. The
radius was narrowest at mid-length. The
ulna is similar, but longer due to the possession of a pronounced
olecranon process, as is common in terrestrial tetrapods but rare in amphibious or aquatic ones. The
carpus (wrist) has ten bones, and the hand has five stout fingers. The carpal bones are fully developed and closely contact each other, another indication of terrestriality. The phalanges (finger bones) decrease in size towards the tip of the fingers, where they each end in a tiny, rounded segment, without a claw. The phalangeal formula (number of phalanges per finger, from thumb to little finger) is 2-3-4-4-3. Two sacral (hip) vertebrae were present, though only the first one possessed a large, robust rib which contacted the ilium (upper blade of the hip). Some studies have argued that there was only one sacral vertebra, with the supposed second sacral actually being the first caudal due to having a shorter, more curved rib than the first sacral. Each
ilium is low and teardrop-shaped when seen from the side, while the underside of the hip as a whole is formed by a single robust puboischiadic plate, which is rectangular when seen from below. Both the hip and shoulder sockets were directed at 45 degrees below the horizontal. The
femur is equally stout as the humerus, and the
tibia and
fibula are robust, hourglass-shaped bones similar to the radius and ulna. The
tarsus (ankle) incorporates 11 bones, intermediate between earlier tetrapods (which have 12) and amniotes (which have 8 or fewer). The five-toed feet are quite similar to the hands, with phalangeal formula 2-3-4-5-3. There were only about 20 caudal (tail) vertebrae at most. Past the base of the tail, the caudals start to acquire bony spines along their underside, known as
chevrons. These begin to appear in the vicinity of the third to sixth caudal, depending on the specimen. Ribs are only present within the first five or six caudals; they are long at the base of the tail but diminish soon afterwards and typically disappear around the same area the chevrons appear.
Differences between species Seymouria baylorensis and
Seymouria sanjuanensis can be distinguished from each other based on several differences in the shape and connections between the different bones of the skull. For example, the downturned flange of bone above the otic notch (sometimes termed the "tabular horn" or "otic process") is much more well-developed in
S. baylorensis than in
S. sanjuanensis. In the former species, it acquires a triangular shape (when seen from the side) as it extends downwards more extensively towards the rear of the skull. In
S. sanjuanensis, the
postfrontal bone contacts the
parietal bone by means of an obtuse, wedge-like suture, while the connection between the two bones is completely straight in
S. baylorensis. Some authors have argued that the
postparietals of
S. baylorensis were smaller than those of
S. sanjuanensis, but some specimens of
S. sanjuanensis (for example, the "Tambach lovers") also had small postparietals. In addition, the "Tambach lovers" have a
quadratojugal bone which is more similar to that of
S. baylorensis rather than
S. sanjuanensis. The combination of features from both species in these specimens may indicate that the two species are part of a continuous lineage, rather than two divergent evolutionary paths. Likewise, some differences relating to the proportions of the rear of the skull may be considered to be an artifact of the fact that most
S. sanjuanensis specimens were not fully grown prior to the discovery of the "Tambach lovers", which were adult members of the species. Nevertheless, several traits are still clearly differentiated between the two species. The
lacrimal bone, in front of the eyes, only occupies the front edge of the orbit in
S. baylorensis. Conversely, specimens of
S. sanjuanensis have a branch of the lacrimal which extends a small distance under the orbit. In
S. sanjuanensis, much of the rear edge of the orbit is formed by the chevron-shaped
postorbital bone, which is more rectangular in
S. baylorensis. The shape of the lacrimal and postorbital of
S. sanjuanensis closely corresponds to the condition in other seymouriamorphs, while the condition in
S. baylorensis is more unique and derived. The tooth-bearing maxilla bone, which forms the side of the snout, is also distinctively unique in
S. baylorensis. In
S. sanjuanensis, the maxilla was low, with many sharp, closely spaced teeth extending along its length. This condition is similar to other seymouriamorphs. However,
S. baylorensis has a taller snout, and its teeth are generally much larger, less numerous, and less homogenous in size. The palate is generally similar between the two species, although the
ectopterygoids are more triangular in
S. baylorensis and rectangular in
S. sanjuanensis. ==Paleobiology==