ZoomDinosaurs.com Dinosaur and Paleontology Dictionary

PLACERIAS (pronounced pluh-SEHR-ree-us) Placerias was a herding dicynodont (a so-called "mammal-like reptile") that lived during the late Triassic period, roughly 222 to 215 million years ago. This thick-set, hippo-like herbivore (plant-eater) ate low-lying plants, like ferns, club mosses, roots, etc. It had sturdy tusks and a beak that it may have used to root in the soil and obtain food on desert-like plains. Placerias was up to 12.5 ft (3.5 m) long and weighed about 1 ton. Fossils have been found in the USA, including over 40 P. gigas fossils found at the Petrified Forest National Park, near St Johns, Arizona [the site was first excavated in the 1920's by Charles Lewis Camp (1893-1975)].

PLACODONT (pronounced PLAK-uh-dont) Placodonts (meaning "plate-like or flat teeth") were chunky, relatively-sedentary marine reptiles that lived in shallow seas during the Triassic period, going extinct at the end of the Triassic. Many of these sauropterygian diapsids had turtle-like shells (dermal armor) and sprawling legs. Placodonts ate shellfish which were crushed between their strong, flat teeth. Placodonts included Placodus, Placochelys, the armored Henodus, and Claudiosaurus. They were not dinosaurs.

PLACODERM Placoderms (meaning "plated skin") were armored fish that evolved during the Silurian Period, about 420 million years ago. They diviersified and came to dominate the seas by the Devonian. They went extinct bout 355 million years ago. They had hinged bony armor on their head and thorax. They had no teeth, but did have bony ridges that acted like teeth. Some placoderms included the Antiarchi (like Bothriolepis), Dunkleosteus (the largest placoderm), Groenlandapsis, and Phyllolepis. Placoderms were early fish, not dinosaurs.

PLACODUS Placodus was a marine reptile (a placodont) that lived in shallow seas during the early to middle Triassic period. This air-breather had four legs, a short neck, a very large, wide head and two types of teeth (peg-like teeth in the front and large, flat teeth in the sides of the mouth). Its long tail was flattened and may have had a fin. Unlike Plesiosaurs and Ichthyosaurs, Placodus was not a very strong swimmer. It was about 7 feet (2.15 m) long. It ate shellfish, like oysters and clams, which were pried open with peg-like teeth and crushed between the flat teeth. Fossils have been found in Europe. Placodus was not a dinosaur.

PLANICOXA Planicoxa was a plant-eating dinosaur that lived during the early Cretaceous period. This iguanodintid (spiked-thumb plant-eater) was found in eastern Utah, USA. Planicoxa was named by DiCroce and Carpenter in 2001; the type species is P. venenica.

PLANT EATER Plant-eating animals are also called herbivores. Most dinosaurs were plant-eaters. Plant-eaters are also called primary consumers.

PLANTIGRADE Animals that are plantigrade walk in a flat-footed manner. This is a relatively slow way of walking since most of the foot comes in contact with the ground. Most reptiles (like alligators), amphibians (like frogs) and early mammals (plus people and bears) are plantigrade. (Compare with digitigrade.)

PLATECARPUS Platecarpus was a mosasaur, an extinct marine reptile that lived in the seas during the late Cretaceous period. They were about 14 feet (4.3 m) long. They swam by moving the long tail and used the short flippers to steer. They had sharp teeth and ate fish and squid. Fossils have been found in Europe. They were not dinosaurs.

PLATEOSAURIDS (pronounced PLAT-ee-oh-SAWR-ids) Plateosaurids (meaning "flat lizards") were prosauropod dinosaurs that lived from the late Triassic period to the early Jurassic period. These quadrupedal herbivores had a long neck, a small head, bulky body, a long tail and long snout. They ranged in length from 5-26 ft long (1.5-8 m). These dinosaurs ate leaves high in the trees (like conifers and cycads) with sharp, leaf-shaped teeth. Many plateosaurids lived in herds in what is now Europe, Africa, Asia, North America, and South America. Plateosaurids included Plateosaurus, Ammosaurus, Coloradisaurus, Euskelosaurus, Massospondylus, and Mussaurus.

PLATEOSAURUS (pronounced PLAT-ee-oh-SAWR-us) Plateosaurus (meaning "flat lizard") was a late Triassic quadrupedal herbivore with a long neck and long snout. This 23 feet (7 m) long dinosaur ate leaves high in the trees (like those of conifers and cycads) with sharp, leaf-shaped teeth. It lived in herds in a dry environment in what is now Europe.

Forward Backward PLATE TECTONICS Plate tectonics is the now-established theory that chunks of the Earth's crust (plates) float on the surface and change both position and size over time.

PLATYBELODON Platybelodon, commonly known as a "Shovel Tusker," was a huge, plant-eating mammal that lived in herds during the middle to late Miocene (roughly 25 million years ago). This elephant-like browser had a shovel-shaped, scoop-like lower jaw (mandible). This huge mouth had sharp teeth at the front edge; these teeth were probably used for cutting leaves to eat. Flat cheek teeth ground up the leaves. It also had 2 small, forward and downward-pointing tusks toward the front of the upper jaw, probably used for defense against predators. It lived in wet parts of prairies and ate soft plants (including water plants). It was 20 ft (6 m) long, was 9 ft (2.8 m) tall at the shoulder and weighed about 4.5 tons (4 tonnes). Fossils have been found in North America, eastern Europe, Africa, and northwestern China (these animals probaly crossed via the Siberian land bridge between the continents). Classification: Order Proboscidea, Suborder Elephantoidea, Family Gomphothere (closely related to Amebelodon).

PLATYHYSTRIX Platyhystrix (meaning "flat porcupine") was an ancient amphibian that had flat, blade-like spines on its back (it was not a dinosaur). It may have had a sail on its back, was anchored by spines; the sail may have been used to regulate its body temperature and/or for protection. This early land animal was about 3 feet (1 m) long. It walked on four short, sprawling legs and had a short tail. A meat-eater, it had conical teeth. Platyhystrix lived during the Permian period. Fossils have been found in Texas, USA. Classification: Class amphibian, Subclass Labyrinthodontidae, Order Temnospondyl, family Dissorophid, genus Platyhystrix.

PLATYPTERGIUS Platypterygius (meaning "flat wing") was an Ichthyosaur (not a dinosaur), a reptile with flipper-like limbs that lived in the water (but breathed air). It was 23 feet (7 m) long, was dolphin-like and had large eyes. It dates from the early Cretaceous period in Queensland, Australia.

PLATYRRHINI The platyrrhini are the living new world monkeys (living in South and Central America and southern Mexico), like the Howler Monkeys Squirrel Monkeys, Sakis, Uakaris, Capuchin Monkeys, Woolly Monkeys, Spider Monkeys & their relatives. They are arboreal (living in trees) and herbivorous (plant-eaters). (The platyrrhini include the Family Cebidae and Family Callithricidae.)

PLATYSPERMS Platysperms (meaning "flat seeds") are plants with flattened ovules. Platysperms include gingkos, conifers, glossopterids, and cordaites.

PLEISTOCENE The Pleistocene was an epoch of geologic time that lasted from 1.8 million to 11,000 years ago; it was a period of widespread glaciation and large Ice Age animals (caled Pleistocene Megafauna). The first humans (Homo sapiens) evolved during the Pleistocene. Mammoths, mastodons, saber-toothed cats, giant ground sloths, and other Ice Age mammals evolved. A mass extinction of large mammals and many birds occurred about 10,000 years ago, probably caused by Ice Ages.

PLESIADAPSIS Plesiadapis was a mammal that lived from the late Paleocene and the early Eocene (roughly 65 million years ago). It was 2.5 feet (80 cm) long. A quadruped (it walked on four legs), it had a long, bushy tail, long legs, and long, clawed fingers and toes. It may have lived in trees part of the time. Plesiadapis had long, large, rodent-like incisors (pointed teeth) at the front of the mouth and wide, flat grinding teeth (molars) at the rear of the jaws. The snout was long. It may have eaten insects. Fossils have been found in Cernay, France, and Colorado, USA. There is some debate about Plesiadapis' classification; it is either a primate or a close relative of the primates. Plesiadapis was named by Gervais in 1877.

PLESIOMORPHY Plesiomorphy (meaning "old form") is a primitive character of a group.

PLESIOSAUR (pronounced plee-zee-oh-SAWRS) Plesiosaurs (meaning "near lizard") were flippered marine reptiles from the Mesozoic Era - they were not dinosaurs. Plesiosaurs are divided into two groups: the Plesiosauroids with long, snake-like necks, tiny heads, and wide bodies, and the Pliosauroids, which had large heads with very strong jaws and short necks. Some plesiosaurs included Plesiosaurus, Kronosaurus, Elasmosaurus, etc.

PLEISTOCENE (pronounced PLEES-toh-seen) The Pleistocene was an epoch in geologic time that lasted from 1.8 million years ago until about 10,000 years ago (it was at the beginning of the Quaternary period). During this epoch, the first humans (Homo sapiens) evolved. Mammoths, mastodons, saber-toothed cats, giant ground sloths, and other Pleistocene megafauna (huge animals) lived. A mass extinction of large mammals and many birds happened about 10,000 years ago (at the end of this epoch), probably caused by climate changes (the last Ice Age ended).

PLESIOSAURUS (pronounced plee-zee-oh-SAWR-us) Plesiosaurus was a genus of flippered marine reptiles from the early Jurassic period. It was roughly 7.6 feet (2.3 m) long with 4 wide, paddle-shaped flippers, and a tapered body. Plesiosaurus lived in what is now England and Germany. It as not a dinosaur, but a plesiosaur, a marine reptile that lives during the time of the dinosaurs. Plesiosaurus was first found by Mary Anning in 1821 and was named by H.T. De La Beche and William D. Conybeare later that year.

PLEUROCOELS Pleurocoels are openings in the sides of the vertebrae of some large theropod dinosaurs (like T. rex). These openings decrease the weight of the bone. They may have also been used for other purposes, such as an air-sac system that is used by modern birds in which the vertebrae contain a series of air-sacs which are connected to the lungs.

PLEUROCOELUS (pronounced PLOOR-oh-SEEL-us ) Pleurocoelus (meaning "hollow-sided (vertebrae)") was a small brachiosaurid sauropod dinosaur that lived during the early Cretaceous period, about 131-119 million years ago. This plant-eater had a long neck, long tail, bulky body, and a small, narrow head. The front legs were longer then the rear legs, giving it a giraffe-like stance. The fourth toe or each rear leg had a small claw (this is rare for sauropods). The vertebrae had long, deep cavities in them (hence the dinosaur's name). Pleurocoelus was about 30 ft (8.8 m) long. Fosils were found in USA (Maryland, Texas, and Utah). The type species is P. nanus. Pleurocoelus was named by paleontologist O. Marsh in 1888.

PLIOCENE (pronounced PLEE-oh-seen) The Pliocene was an epoch in geologic time that lasted from 5-1.8 million years ago (it was at the end of the Tertiary period). During this epoch, the first hominids (australopithecines) andmodern forms of whales appeared. The ancient shark megalodon swam the seas during this time.

PLIOPLATECARPUS (pronounced PLEE-oh-PLAT-ee-CAR-pus) Plioplatecarpus was a mosasaur, a marine reptile that swam in shallow seas during the late Cretaceous period. They were not dinosaurs. These powerful swimmers were about 20 feet (6 m) long. They had a big head, large brain, big eyes, four flippers, and a tail that ended in a fin. Carnivores, they had sharp teeth, which they used to catch fish and cephalopods. Fossils have been found in North America. One Plioplatecarpus fossils was found with a fossilized embryo inside it; this seems to indicate that Plioplatecarpus gave bith to live young. Plioplatecarpus was named by Dollo in 1884; the type species is Plioplatecarpus marshii (named to honor O. Marsh).

PLIOSAUR (pronounced PLY-oh-SAWR) Pliosaurs (meaning "greater lizard") were huge, short-necked plesiosaurs. These flippered marine reptiles were carnivores with sharp teeth in large, powerful jaws. They may have eaten ichthyosaurs and other plesiosaurs. They lived during the Jurassic and Cretaceous periods and they were not dinosaurs. Some Pliosaurs include Peloneustes, Macroplata, Kronosaurus, and Liopleurodon.

PLIOSAURUS (pronounced PLY-oh-SAWR-us) Pliosaurus (meaning "greater lizard") was a huge, short-necked plesiosaur that lived during the middle to late Jurassic period. This carnivore was a flippered marine reptile (a pliosaur and a plesiosaur, but not a dinosaur) with many sharp teeth in large, powerful jaws resembling those of a crocodile; the teeth were three-sided in cross-section. It was 33-40 ft (10-12 m) long and the skull was 5-6.6 ft (1.5-2 m) long. It may have eaten ichthyosaurs and other plesiosaurs. Fossils have been found in England and South America. Pliosaurus was named by paleontologist Richard Owen in 1841; the type species is Pliosaurus brachydeirus. It may be the same as Liopleurodon.

PLOTOSAURUS (pronounced PLOAT-oh-SAWR-us) Plotosaurus was a mosasaur, a marine reptile that swam in shallow seas during the late Cretaceous period. These powerful swimmers were about 33 feet (10 m) long. They had four flippers and a tail that ended in a fin. These carnivores had sharp teeth and ate fish, squid, and perhaps shellfish. Fossils have been found in North America. They were not dinosaurs.

PODOKESAURUS (pronounced POH-doh-kuh-SAWR-us) Podokesaurus (meaning "swift-footed lizard" in Greek) was a small, bipedal, meat-eating dinosaur from the early Jurassic period, about 200-188 million years ago. This fast-moving theropod was about 3.5 feet (1 m) long. This lightly-built dinosaur had a long neck, short arms, bird-like feet (with three toes and a dewclaw), and four-fingered hands . A single fossil was found in Massachusetts, USA; that specimen was destroyed in a fire but a cast of it remains. The type species is P. holyokensis, named for Mount Holyoke College, where the fossil was found. Podokesaurus was named by US paleontologist Mignon Talbot in 1911. The dinosaur Grallator, known only from its footprints, may be Podokesaurus. This dinosaur may be the same as Coelophysis or perhaps an immature Rioarribasaurus.

POEKILOPLEURON (pronounced POH-key-loh-PLURE-on) Poekilopleuron (meaning "varied rib or side" in Greek) was a small, bipedal, meat-eating dinosaur with powerful arms. It dates from the early to middle Jurassic period about 166-163 million years ago. This fast-moving megalosaurid theropod was about 26 feet (8 m) long. The type species is P. bucklandii, but the type specimen was destroyed in World War II. Poekilopleuron was named by paleontologist Eudes-Deslongchamps in 1838. Fossils were found in northern France. 8 to 10 stones (perhaps gastroliths) were found within the ribs of Poekilopleuron in 1835. Poekilopleuron may be the same as Megalosaurus; it also resembled Torvosaurus.

POIKILOTHERMS Poikilotherms are animals whose internal temperature changes depending on the environment. Reptiles are poikilothermic.

POLACANTHUS (pronounced pol-ah-CAN-thus) Polacanthus (which might be the same as Hylaeosaurus) was a primitive ankylosaur with spines. It was about 15 feet (4.6 m) long and lived during the early Cretaceous period (135-119 million years ago) in Europe. It was discovered by Rev. Wm. Fox in 1865 and named by Sir Richard Owen in 1867.

POLYONYAX (pronounced pol-ee-OH-nax) Polyonax (meaning "master over many") was a plant-eating dinosaur from the late Cretaceous period. This ceratopsian was about 23 ft (7 m) long. Polyonax may be a juvenile Triceratops. Fragmentary fossils have been found in Colorado, USA. Polyonax was named by Cope in 1874. The type species is P. mortuarius. This genus is a nomen dubium.

POLYPHYLETIC A polyphyletic group consists of organisms but not their common ancestors. This is an artificial group which is based primarily on physical characteristics rather than on evolutionary relationships. An example is "flying vertebrates" which includes birds, pteranodons and bats.

POSTOSUCHUS (pronounced POST-oh-SOOK-us) Postosuchus (meaning "post crocodile" named for the Post Quarry in Texas, USA) was a fast-running archosaur that lived during the late Triassic period, about 222 to 215 million years ago. This carnivore (meat-eater) had a long tail, a large, wide skull, and a narrow snout with many large, dagger-like teeth. It was a unusual quadrupedal reptile whose legs were columnar (and not sprawling out to the sides); the rear legs were longer than the front legs. It had five-toed, clawed feet. Postosuchus was about 20 feet (6 m) long and about 6.5 ft (2 m) tall. Fossils have been found in the Post Quarry in Texas and the Petrified Forest in Arizona, USA, North America. Postosuchus was not a dinosaur; it was a reptile that was ancestral to the crocodiles.

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