''Psephoderma'' (meaning "pebbly skin", from the

Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Dark Ages (), the Archaic peri ...

''psepho'' (ψῆφος), "pebbly", and ''derma'' (δέρμα), "skin") is a genus of

placodont Placodonts (" Tablet teeth") are an extinct order of marine reptiles that lived during the Triassic period, becoming extinct at the end of the period. They were part of Sauropterygia, the group that includes plesiosaurs. Placodonts were genera ...

s very similar to the related genera ''

Placochelys ''Placochelys'' ('flat-plate turtle") is an extinct genus of placodont reptiles erected by Otto Jaekel in 1902. Fossil records Fossils of ''Placochelys'' dates back to the Triassic period (age range: 221.5 to 205.6 million years ago). They have ...

'' and ''

Cyamodus ''Cyamodus'' (pron.: SIE-ah-MO-dus) is a genus of placodonts known from several species described from Middle-Late Triassic of Europe and China. The genus was described by Christian Erich Hermann von Meyer in 1863, based on specimens found in Germ ...

''. ''Psephoderma'' had a flattened

skull The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible. In humans, the ...

and a narrow, straight

rostrum Rostrum may refer to: * Any kind of a platform for a speaker: **dais **pulpit * Rostrum (anatomy), a beak, or anatomical structure resembling a beak, as in the mouthparts of many sucking insects * Rostrum (ship), a form of bow on naval ships * Ros ...

, much narrower than that of its relatives. Inside this

, 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

s, ''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 The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest period ...

( 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

s to live. Fossils of ''Psephoderma'' have been found in the Rhaetian deposits in the

Alps The Alps () ; german: Alpen ; it, Alpi ; rm, Alps ; sl, Alpe . are the highest and most extensive mountain range system that lies entirely in Europe, stretching approximately across seven Alpine countries (from west to east): France, Sw ...

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 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 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 In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as a p ...

were large and somewhat irregularly shaped, and indicate that the

eyes Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision. Eyes detect light and conve ...

were on top of the head looking upwards. Its

pineal foramen A parietal eye, also known as a third eye or pineal eye, is a part of the epithalamus present in some vertebrates. The eye is located at the top of the head, is photoreceptive and is associated with the pineal gland, regulating circadian rhyth ...

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, pa ...

. Unlike most of its relatives, its ectopterygoid is not known and may not be present, as in ''

Protenodontosaurus ''Protenodontosaurus'' is an extinct genus of placodont Placodonts ("Tablet teeth") are an extinct order of marine reptiles that lived during the Triassic period, becoming extinct at the end of the period. They were part of Sauropterygia, th ...

.'' 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 A tooth ( : teeth) is a hard, calcified structure found in the jaws (or mouths) of many vertebrates and used to break down food. Some animals, particularly carnivores and omnivores, also use teeth to help with capturing or wounding prey, tear ...

were in each side of the

palate The palate () is the roof of the mouth in humans and other mammals. It separates the oral cavity from the nasal cavity. A similar structure is found in crocodilians, but in most other tetrapods, the oral and nasal cavities are not truly sepa ...

, 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 were narrow, but highly elongated. The posterior process of the

postorbital bone The ''postorbital'' is one of the bones in vertebrate skulls which forms a portion of the dermal skull roof and, sometimes, a ring about the orbit. Generally, it is located behind the postfrontal and posteriorly to the orbital fenestra. In some ve ...

s 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 ...

. Its

occiput The occipital bone () is a cranial dermal bone and the main bone of the occiput (back and lower part of the skull). It is trapezoidal in shape and curved on itself like a shallow dish. The occipital bone overlies the occipital lobes of the cereb ...

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 The rib cage, as an enclosure that comprises the ribs, vertebral column and sternum in the thorax of most vertebrates, protects vital organs such as the heart, lungs and great vessels. The sternum, together known as the thoracic cage, is a semi- ...

and

thorax The thorax or chest is a part of the anatomy of humans, mammals, and other tetrapod animals located between the neck and the abdomen. In insects, crustaceans, and the extinct trilobites, the thorax is one of the three main divisions of the cre ...

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 A scute or scutum (Latin: ''scutum''; plural: ''scuta'' "shield") is a bony external plate or scale overlaid with horn, as on the shell of a turtle, the skin of crocodilians, and the feet of birds. The term is also used to describe the anterior po ...

, 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