Placodont
Encyclopedia
Placodonts were a group of marine reptiles that lived during the Triassic
period, becoming extinct at the end of the period. It is believed that they were part of Sauropterygia
, the group that includes Plesiosaurs. Placodonts were generally between 1 to 2 m (3.3 to 6.6 ft) in length, with some of the largest measuring 3 metres (10 ft) long.
The first specimen was discovered in 1830, and they have since been discovered throughout central Europe
, North Africa, the Middle East
and China
.
The earliest forms like Placodus
, which lived in the early to middle Triassic
, resembled barrel-bodied lizards somewhat similar to the marine iguana
of today, but larger. But unlike the marine iguana, which feeds on algae
, the placodonts ate molluscs and so their teeth were flat and tough to crush their shells. In the earliest periods their size was probably enough to take away the top sea predators of the time: the shark
s. However, as time passed other kinds of carnivore reptiles began to colonize the seas, such as ichthyosaur
s and nothosaur
s, and later placodonts developed bony plates on their backs to protect their bodies while feeding. By the Late Triassic, these plates had grown so much that placodonts of the time such as Henodus
and Placochelys
resembled the sea turtle
s of modern day more than their ancestors without bony plates. Other placodonts like Psephoderma
developed plates as well, but in a different articulated manner that resembled the shells of horseshoe crab
s and trilobite
s more than those of sea turtles. All these adaptations can be counted as perfect examples of convergent evolution
, as placodonts were not related to any of these animals.
Because of their dense bone and heavy armour plating, these creatures would have been too heavy to float in the ocean and would have used a lot of energy to reach the water surface. For this reason and because of the type of sediment found accompanying fossils it is suggested they lived in shallow waters and not in deep oceans.
Their diet consisted of marine bivalves, brachiopod
s, and other invertebrates. They were notable for their large, flat, often protruding teeth which they used to crush molluscs and brachiopods, which they hunted on the sea bed (another way in which they were similar to walruses). The palate teeth were adapted for this durophagous
diet, being extremely thick and large enough to crush thick shell.
The clade Helveticosauroidea was previously considered to be a basal superfamily of placodonts with the sole member Helveticosaurus
. However, it is now thought that Helveticosaurus was not a placodont but possibly an unusual member of the Archosauromorpha
.
Triassic
The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...
period, becoming extinct at the end of the period. It is believed that they were part of Sauropterygia
Sauropterygia
Sauropterygia were a group of very successful aquatic reptiles that flourished during the Mesozoic before they became extinct at the end of the era. They were united by a radical adaptation of their shoulder, designed to support powerful flipper strokes...
, the group that includes Plesiosaurs. Placodonts were generally between 1 to 2 m (3.3 to 6.6 ft) in length, with some of the largest measuring 3 metres (10 ft) long.
The first specimen was discovered in 1830, and they have since been discovered throughout central Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
, North Africa, the Middle East
Middle East
The Middle East is a region that encompasses Western Asia and Northern Africa. It is often used as a synonym for Near East, in opposition to Far East...
and China
China
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...
.
Palaeobiology
Placodus
Placodus was a genus of marine reptiles, belonging to the order Placodontia, which swam in the shallow seas of the middle Triassic period...
, which lived in the early to middle Triassic
Triassic
The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...
, resembled barrel-bodied lizards somewhat similar to the marine iguana
Marine iguana
The Marine Iguana is an iguana found only on the Galápagos Islands that has the ability, unique among modern lizards, to live and forage in the sea, making it a marine reptile. The Iguana can dive over 30 ft into the water. It has spread to all the islands in the archipelago, and is...
of today, but larger. But unlike the marine iguana, which feeds on algae
Algae
Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...
, the placodonts ate molluscs and so their teeth were flat and tough to crush their shells. In the earliest periods their size was probably enough to take away the top sea predators of the time: the shark
Shark
Sharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago....
s. However, as time passed other kinds of carnivore reptiles began to colonize the seas, such as ichthyosaur
Ichthyosaur
Ichthyosaurs were giant marine reptiles that resembled fish and dolphins...
s and nothosaur
Nothosaur
Nothosaurs were Triassic marine sauropterygian reptiles that may have lived like seals of today, catching food in water but coming ashore on rocks and beaches. They averaged about in length, with a long body and tail. The feet were paddle-like, and are known to have been webbed in life, to help...
s, and later placodonts developed bony plates on their backs to protect their bodies while feeding. By the Late Triassic, these plates had grown so much that placodonts of the time such as Henodus
Henodus
Henodus chelyops was a placodont of the Late Triassic period during the early Carnian age. Fossils of Henodus chelyops were found in Tübingen, Germany. It was around in length....
and Placochelys
Placochelys
Placochelys is an extinct genus of reptile from the Triassic period of Germany.Placochelys looked remarkably similar to a sea turtle, and grew to about in length. It had a flat shell covered with knobbly plates, and a compact skull...
resembled the sea turtle
Sea turtle
Sea turtles are marine reptiles that inhabit all of the world's oceans except the Arctic.-Distribution:...
s of modern day more than their ancestors without bony plates. Other placodonts like Psephoderma
Psephoderma
Psephoderma is a genus of placodont that was very similar to its relatives Placochelys and Cyamodus. Psephoderma had a flattened skull and a narrow, straight rostrum. Inside this skull, embedded in the jaws, were rounded teeth specialized for crushing the shellfish it ate...
developed plates as well, but in a different articulated manner that resembled the shells of horseshoe crab
Horseshoe crab
The Atlantic horseshoe crab, Limulus polyphemus, is a marine chelicerate arthropod. Despite its name, it is more closely related to spiders, ticks, and scorpions than to crabs. Horseshoe crabs are most commonly found in the Gulf of Mexico and along the northern Atlantic coast of North America...
s and trilobite
Trilobite
Trilobites are a well-known fossil group of extinct marine arthropods that form the class Trilobita. The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period , and they flourished throughout the lower Paleozoic era before...
s more than those of sea turtles. All these adaptations can be counted as perfect examples of convergent evolution
Convergent evolution
Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...
, as placodonts were not related to any of these animals.
Because of their dense bone and heavy armour plating, these creatures would have been too heavy to float in the ocean and would have used a lot of energy to reach the water surface. For this reason and because of the type of sediment found accompanying fossils it is suggested they lived in shallow waters and not in deep oceans.
Their diet consisted of marine bivalves, brachiopod
Brachiopod
Brachiopods are a phylum of marine animals that have hard "valves" on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, while the front can be opened for feeding or closed for protection...
s, and other invertebrates. They were notable for their large, flat, often protruding teeth which they used to crush molluscs and brachiopods, which they hunted on the sea bed (another way in which they were similar to walruses). The palate teeth were adapted for this durophagous
Durophagy
Durophagy is the term for describing the eating behavior of animals that consume hard-shelled or exoskeleton bearing organisms, such as corals, shelled mollusks, or crabs....
diet, being extremely thick and large enough to crush thick shell.
Classification
- Class SauropsidaSauropsidaSauropsida is a group of amniotes that includes all existing reptiles and birds and their fossil ancestors, including the dinosaurs, the immediate ancestors of birds...
- Superorder SauropterygiaSauropterygiaSauropterygia were a group of very successful aquatic reptiles that flourished during the Mesozoic before they became extinct at the end of the era. They were united by a radical adaptation of their shoulder, designed to support powerful flipper strokes...
- Order Placodontia
- Superfamily Placodontoidea
- Family Paraplacodontidae
- Genus ParaplacodusParaplacodusParaplacodus was a Placodont from the middle Triassic period, during the Anisian to Ladinian stages. The fossils were uncovered in Northern Italy and the species was named in 1931 by Bernhard Peyer. Paraplacodus means "Almost Placodus", and thus far only one species, P...
- Genus Paraplacodus
- Family Placodontidae
- Genus PlacodusPlacodusPlacodus was a genus of marine reptiles, belonging to the order Placodontia, which swam in the shallow seas of the middle Triassic period...
- Genus Placodus
- Family Paraplacodontidae
- Superfamily Cyamodontoidea
- Family Henodontidae
- Genus HenodusHenodusHenodus chelyops was a placodont of the Late Triassic period during the early Carnian age. Fossils of Henodus chelyops were found in Tübingen, Germany. It was around in length....
- Genus Henodus
- Family Cyamodontidae
- Genus CyamodusCyamodusCyamodus was a placodont, known from fossil remains discovered in Germany, in the early-to-mid-19th century and was named by Christian Erich Hermann von Meyer, in 1863. The fossils have been dated to the Triassic Period, from the Anisian to Ladinian stages...
- Genus Protenodontosaurus
- Genus Cyamodus
- Family Placochelyidae
- Genus PlacochelysPlacochelysPlacochelys is an extinct genus of reptile from the Triassic period of Germany.Placochelys looked remarkably similar to a sea turtle, and grew to about in length. It had a flat shell covered with knobbly plates, and a compact skull...
- Genus PsephodermaPsephodermaPsephoderma is a genus of placodont that was very similar to its relatives Placochelys and Cyamodus. Psephoderma had a flattened skull and a narrow, straight rostrum. Inside this skull, embedded in the jaws, were rounded teeth specialized for crushing the shellfish it ate...
- Genus Placochelys
- Family Henodontidae
- incertae sedisIncertae sedis, is a term used to define a taxonomic group where its broader relationships are unknown or undefined. Uncertainty at specific taxonomic levels is attributed by , , and similar terms.-Examples:*The fossil plant Paradinandra suecica could not be assigned to any...
- Genus Saurosphargis
- Superfamily Placodontoidea
- Order Placodontia
- Superorder Sauropterygia
The clade Helveticosauroidea was previously considered to be a basal superfamily of placodonts with the sole member Helveticosaurus
Helveticosaurus
Helveticosaurus is an unusual extinct genus of diapsid reptile. Fossils have been found from Monte San Giorgio, Switzerland, an area well known for its rich record of marine life during the Middle Triassic.-Description and paleobiology:...
. However, it is now thought that Helveticosaurus was not a placodont but possibly an unusual member of the Archosauromorpha
Archosauromorpha
Archosauromorpha is an infraclass of diapsid reptiles that first appeared during the late Permian and became more common during the Triassic. Included in this infraclass are the groups Rhynchosauria, Trilophosauridae, Prolacertiformes, Archosauriformes, and, tentatively, Choristodera...
.