''Psephoderma'' (meaning "pebbly skin", from the Ancient Greek ''psepho'' (ψῆφος), "pebbly", and ''derma'' (δέρμα), "skin") is a genus of placodonts very similar to the related genera '' Placochelys'' and '' Cyamodus''. ''Psephoderma'' had a flattened skull and a narrow, straight rostrum, much narrower than that of its relatives. Inside this skull, embedded in the jaws, were rounded teeth specialized for crushing the

shellfish Shellfish is a colloquial and fisheries term for exoskeleton-bearing aquatic invertebrates used as food, including various species of molluscs, crustaceans, and echinoderms. Although most kinds of shellfish are harvested from saltwater envir ...

it ate. Unlike henodontid placodonts, ''Psephoderma'''s

carapace A carapace is a Dorsum (biology), dorsal (upper) section of the exoskeleton or shell in a number of animal groups, including arthropods, such as crustaceans and arachnids, as well as vertebrates, such as turtles and tortoises. In turtles and tor ...

was divided into two pieces, one on the shoulders and back, and another on the ventral end. ''Psephoderma'' grew to long, larger than many of its relatives, and lived in the Late Triassic ( Norian -

Rhaetian The Rhaetian is the latest age of the Triassic Period (in geochronology) or the uppermost stage of the Triassic System (in chronostratigraphy). It was preceded by the Norian and succeeded by the Hettangian (the lowermost stage or earliest age ...

), about 210 million years ago. It was one of the last placodonts to live. Fossils of ''Psephoderma'' have been found in the Rhaetian deposits in the Alps and in England, hence the specific names.

Description

''Psephoderma'' had a long, narrow rostrum, which was the main difference between its skull and those of its relatives. This rostrum bore paired grooves on the inside of the mouth, which led to the

internal nares The choanae (singular choana), posterior nasal apertures or internal nostrils are two openings found at the back of the nasal passage between the nasal cavity and the throat in tetrapods, including humans and other mammals (as well as crocod ...

and are a diagnostic feature for the genus. The anterior processes of the

maxillae The maxilla (plural: ''maxillae'' ) in vertebrates is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. The t ...

stretched forwards a long way along the rostrum, while the posterior processes of the

premaxillae The premaxilla (or praemaxilla) is one of a pair of small cranial bones at the very tip of the upper jaw of many animals, usually, but not always, bearing teeth. In humans, they are fused with the maxilla. The "premaxilla" of therian mammal has b ...

stretched a long way back up the rostrum and came between the

external nares A nostril (or naris , plural ''nares'' ) is either of the two orifices of the nose. They enable the entry and exit of air and other gasses through the nasal cavities. In birds and mammals, they contain branched bones or cartilages called turbi ...

and between the

nasal bones The nasal bones are two small oblong bones, varying in size and form in different individuals; they are placed side by side at the middle and upper part of the face and by their junction, form the bridge of the upper one third of the nose. Eac ...

. The nasal bones were very small, mere splinters of bone. The orbits were large and somewhat irregularly shaped, and indicate that the eyes were on top of the head looking upwards. Its pineal foramen was large, and anteriorly placed, with its anterior edge bordered by the

frontal bones The frontal bone is a bone in the human skull. The bone consists of two portions.''Gray's Anatomy'' (1918) These are the vertically oriented squamous part, and the horizontally oriented orbital part, making up the bony part of the forehead, part ...

. Unlike most of its relatives, its ectopterygoid is not known and may not be present, as in '' Protenodontosaurus.'' The ''P. alpinum'' species had decidedly elongated, posterior, tooth-bearing plates for crushing the

that formed its diet. No teeth were present along the premaxillae or the rostral area of the maxillae, but four crushing teeth were in each side of the palate, the first three in a triangle of small, rounded teeth and the fourth a short distance behind them. The fourth tooth was huge, far larger than the first three. The lack of teeth near the front of the mouth and rostrum indicate that the elongated rostrum was mainly used for rooting out or digging up shellfish, which would then be taken into the back of the mouth and crushed by the large teeth. ''Psephodermas

temporal fossae The temporal fossa is a fossa (shallow depression) on the side of the skull bounded by the temporal lines and terminating below the level of the zygomatic arch. Boundaries * Medial: frontal bone, parietal bone, temporal bone, and sphenoid bon ...

were narrow, but highly elongated. The posterior process of the postorbital bones reached far back into the temporal arches, underneath the temporal fossae. The whole skull was highly ventrodorsally compressed. It had temporal tubercles, but they were small, and restricted solely to the posterior ends of the

squamosal bones The squamosal is a skull bone found in most reptiles, amphibians, and birds. In fishes, it is also called the pterotic bone. In most tetrapods, the squamosal and quadratojugal bones form the cheek series of the skull. The bone forms an ancestral co ...

. Its occiput was deeply excavated and its entire

braincase In human anatomy, the neurocranium, also known as the braincase, brainpan, or brain-pan is the upper and back part of the skull, which forms a protective case around the brain. In the human skull, the neurocranium includes the calvaria or skul ...

very small. However, the braincase walls were quite thick and well armoured. The carapace of ''Psephoderma'' was made up of two parts, one of which covered the rib cage and thorax and the other of which covered the

pelvis The pelvis (plural pelves or pelvises) is the lower part of the trunk, between the abdomen and the thighs (sometimes also called pelvic region), together with its embedded skeleton (sometimes also called bony pelvis, or pelvic skeleton). The ...

. Both parts of the carapace were made of partially fused scutes, roughly hexagonal or circular in shape. Its tail was quite long, up to about 80 cm, and had scutes all the way down it. The limbs were relatively small and would have been used as paddles, but had individual toes rather than a flipper.

References

External links

Family: PLACOCHELYIDAE
paleofile.com Placodonts Late Triassic reptiles of Europe Fossil taxa described in 1858 Fossils of Italy Sauropterygian genera {{triassic-reptile-stub