Permian Reptile Coursework Materials

PERMIAN REPTILE The chapter examines the evolution of reptiles during the Permian period and covers the largest mass extinction in Earth's history. The Permian, from 280 to 250 million years ago, was named for a sequence of well-exposed terrestrial marine rocks and redbeds on the western side of the Ural Mountains in Russia's Perm district. Rocks from the Permian period differ in western North America, particularly Texas, Nevada, and Utah (Fig. 119). Important reserves of oil and natural gas are in the Permian Basin of Texas and Oklahoma. Extensive coal deposits from the Permian period existed in Asia, Africa, Australia and North America. During the Permian, all major continents were combined into the supercontinent Pangaea, where vast mountain buildings and extensive volcanism were prevalent. The interior of Pangea is mostly desert, causing a decline in amphibians in favor of reptiles. By the end of the Permian, perhaps the largest extinction Earth had ever known wiped out more than 95 percent of all species, paving the way for the rise of dinosaurs. THE ERA OF REPTILES The age of reptiles, which began in the Permian and lasted 200 million years, witnessed the evolution of about 20 orders of the reptile family. Amphibians, which were prominent in the Carboniferous, declined dramatically in the Permian due to the preference for living in water. As Carboniferous swamps dried up and were largely replaced with deserts, amphibians gave way to reptiles, which adapted well to drier climates. In the late part of the Permian, reptiles replaced amphibians and became the dominant land animal in the Mesozoic era. The generally warm Mesozoic climate was favorable for reptiles and helped them colonize the land. The increase in the number of fossil footprints of reptiles in Carboniferous and Permian sediments indicates the emergence of reptiles at the expense of amphibians. The advantages of reptiles are largely due to their more efficient mode of locomotion. Even at an early age, 290 million years ago, small reptiles were bipedal and ran on two legs (Fig. 120), the fastest way to travel. They need this speed not only to hit prey but also to escape from various meateating reptiles, including dimetrodon, a ferocious Permian carnivore. The most convincing evidence for bipedalism is that the length of the hind limbs is much longer than the forelimbs and therefore would make walking on four legs awkward. It is a member of the diapsid group, one of the most primitive reptile lineages. It gave rise to the most living dinosaurs, birds, and reptiles, including crocodiles, lizards, and snakes. Reptiles are also better suited for full-time life on dry land. In contrast, amphibians rely on local water sources to moisturize their skin and for reproduction. Reptile feet are a major improvement compared to amphibians, with changes in the shape of digits, the addition of a thumb-like fifth digit, and the appearance of claws. In some reptiles, the trail narrows and the stride lengthens. Others maintain a four-legged gait and run on their hind legs. Although most reptiles walked or ran on all fours, by the end of the Permian, some smaller reptiles often stood on their hind legs when they needed speed and agility. The body rotates at the hips, and the long tail compensates for the almost upright trunk. This attitude frees the front legs to attack small prey and complete other useful tasks. Reptiles have scales that retain animal body fluids. In contrast, amphibians have permeable skin that must be moistened frequently. Another major advance over amphibians is the mode of reptile reproduction. Like fish, amphibians lay eggs in water. After hatching, the young fend for themselves, often falling prey to predators. Reptile eggs have a hard, waterproof shell so they can be laid on dry ground. Reptiles belong to a group known as amniots, which also includes birds and mammals. Amniotes are animals with complex eggs that evolved from amphibians. Reptile parents protect their offspring, which gives them a better chance of survival, contributes to the great success of reptiles in populating the soil. Like fish and amphibians, reptiles are cold-blooded, a misleading term because they draw heat from the environment. Therefore, the blood of reptiles basking on rocks can actually be warmer than the blood of warm-blooded mammals. High body temperature is just as important for reptiles as mammals to achieve maximum metabolic efficiency. On cold mornings, reptiles are sluggish and prone to predators. They bask in the sun until their bodies are warm and their metabolism can operate at peak performance. Reptiles only need about one-tenth the amount of food that mammals need to survive because mammals use most of their calories to maintain high body temperatures. The total energy consumption of mammals is 10 to 30 times that and the oxygen intake is about 20 times that of reptiles of the same weight. As a result, reptiles can live in deserts and other remote places and thrive on small amounts of food that would quickly starve mammals of similar size. The generally warm Mesozoic climate was very favorable for reptiles and helped them colonize the land. In contrast, amphibians, which avoid direct sunlight and are relatively cool and slow-moving, are at a disadvantage. Many early reptiles evolved into some of the most bizarre creatures. Perhaps the strangest reptile that ever lived was Tanystropheus, nicknamed the "saurian giraffeneck." The animal measures 15 feet from head to tail and is famous for its extremely long neck, which is more than twice the length of its trunk. As an adult, its neck grows at a much faster rate than the rest of its body. Apparently, reptiles are aquatic because it is impossible to support the weight of such a long neck while on land. Tanystropheus probably stretched its neck downward to scavenge bottom sediment for food. Phytosaurs were large, highly armored predatory reptiles with sharp teeth. They resemble crocodiles with an elongated muzzle, short legs, and a long tail but are not closely related to them. They evolved from thecodonts, which also gave rise to crocodiles and dinosaurs. They flourished in the late Triassic, developed fairly quickly, but did not seem to survive beyond the end of the period. Near the close of the Triassic, when reptiles were the main form of animal life, occupying land, sea, and air, an extraordinary group of reptiles called crocodiles appeared in the fossil record. Crocodiles come from Gondwana. They consist of crocodiles with blunt heads, crocodiles with elongated heads, and gavials with very narrow heads. Members of this group are adapted to life on dry land, semiaquatic life, or fully aquatic life with shark-like tails, slender heads, and legs reshaped into swimming paddles. Like many land animals that lived in the Cretaceous about 100 million years ago, crocodiles grew to gigantic size as shown by a 40-foot-long fossil found in southern Brazil.A fossil gavial-like monster from the lower Cretaceous in Niger, West Africa, measured about 35 feet long. A monstrous three-ton, 30-foot crocodile terrorized Cretaceous marshes and may have preyed on medium-sized dinosaurs. Crocodiles diversified over the past 200 million years. They spread to all corners of the globe and adapted to various habitats. Crocodile fossils found in the high latitudes of North America (Figure 121) indicate a warm climate during the Mesozoic. They belong to the subclass Archosauria, which literally means "ruling reptiles," along with dinosaurs and pterosaurs. They are the only surviving members to escape great death at the end of the Cretaceous. Reptiles that returned to the sea to compete with fish for abundant food supplies included plesiosaurs such as sea snakes, placodonts such as sea cows, and mixosaurus such as dolphins. Pachycostasaurus, meaning "thickly ribbed lizard," was a carnivorous marine reptile 9 feet or more long. They resembled plesiosaurs, with thick, heavy ribs that were probably used to house very large lungs and used for ballast to hunt at great depths. Shark-like ichthyosaurs (Figure 122), whose name is Greek for "fish lizard," were fast-swimming, shell-crushing marine predators that appeared to prey on ammon. The ichthyosaur would first pierce the shell from the victim's blind side, causing it to fill with water and sink to the bottom, where the attack could be carried out directly. The puncture marks on the ammonite shell fossils are spaced the same distance as the ichthyosaur tooth fossils, suggesting these highly aggressive predators may have accelerated the extinction of most ammonite species by the end of the Mesozoic. Placodonts are a group of short, fat marine reptiles with large, flat teeth. They may feed mainly on bivalves and other mollusks. Several other reptile species also roamed the sea, including lizards and turtles that were quite primitive. Many modern giant tortoises are descendants of such marine reptiles. However, only the smallest turtles survived extinction at the end of the Cretaceous. Turtles are the closest relatives of crocodiles, another very successful aquatic reptile. Crocodiles, which passed through extinction practically unscathed, are closer to turtles than lizards, snakes and birds. Tortoises, which are relics from an ancient group called anapsids that do not have holes in the sides of their skulls, were once considered outsiders among modern reptiles because living reptiles and birds called diapsids have two holes on the sides of their skulls.