Cretaceous Period

Cretaceous Period (145.5-65.5 million years ago).* The Cretaceous Period was the last and longest segment of the Mesozoic Era. In the early Cretaceous, the continents were in very different positions than they are today. Sections of the supercontinents were drifting apart. By the middle of the period, ocean levels were much higher, most of the landmasses we are familiar with were underwater. By the end of the period, the continents were much closer to modern configuration. The Cretaceous Period saw the development of flowering plants. As diverse flower forms lured insects to pollinate them, insects adapted to differing ways of gathering nectar and moving pollen thus setting up the intricate co-evolutionary systems we are familiar with today. By the end of the Jurassic, the giant grazing dinosaurs were becoming extinct. They were being replaced by large herds of smaller herbivorous dinosaurs like Stegosaurus. Tyrannosaurus rex, continued as the apex predator until the end of the Cretaceous. Visit a tropical reef today and you’ll see coral. If you visited during the Cretaceous Period, you might have seen reefs built by molluscs, rudist clams. The rudistids were asymmetrical bivalves with one shell shaped as a cone anchored to a hard foundation) while the other shell was essentially a small hinged lid. Giant stalked crinoids occupied the seafloor during the Jurassic, but largely disappeared by the Early Cretaceous. The Cretaceous seas were also home to the inoceramids, huge bivalves with shells shaped like dinner plates that often grew in excess of 2 meters across. Symbiotic fish and other communities lived in their shells. In the Cretaceous skies, certain species had acquired wing spans of 7.5 meters. The great massive pterosaur Quetzalcoatlus had a wing span of 11 to 12 meters. Pterosaurs possibly hunted by gliding across the Cretaceous seaways plucking belemnites and fish from the surface of the water. Birds slowly replaced the pterosaurs in the air. Small gliding reptiles evolved feathers to stay warm, and the rest is all prehistory. Rudists died out about 65 million years ago, the same time dinosaurs and giant marine reptiles became extinct. The end came every suddenly, a 10 km (6.2 mi) asteroid hit the earth on the Yucatan Peninsula in modern Mexico. Temperatures would have reached levels high enough to cause flash fires in many locations and a tsunami inundated much of the coastline. A massive plume of ash blocked out the light and saw the food web break down. Nearly half of Earth’s species disappeared — including almost 75 percent of ocean species. Organisms doomed to extinction included many species of microplankton, brachiopods, fish, and land plants. All of the ammonites disappeared (only the chambered Nautilus survives today). Rudistids became extinct but scleractinian corals and coralline algae returned to dominate Cenozoic reef communities. Crocodiles and turtles still roamed the now relatively empty seas. 12 Flying Dinosaurs – the survival of BIRDS. One lineage of dinosaurs did survive the asteroid impact, the small flying lizards we now call birds. Some species continued to evolve, and they are so familiar to us now that we don’t realise how ancient they are. Shorebirds are the most common in the fossil record, as they tend to be more quickly buried after death. The anseriform lineage, which today includes ducks, has been found in a Late Cretaceous fossil. Modern divers – Gaviiformes is also ancient and is still represented in four living species. Charadriiformes, look a lot like modern auks, avocets, coursers, curlews, gulls, murres, oystercatchers, plovers, puffins, sandpipers, skimmers, stilts, terns, and woodcock among others. The fourth modern lineage are the Procellariiformes – includes the 92 living species of albatrosses, shearwaters, and petrels. The support for this seems to rest largely upon fragmentary wishbones from New Zealand and Mongolia. Additional Cretaceous fossils may include cormorants, pelicans, tropicbirds, flamingos, and relatives of chickens and turkeys, as well as others. But, we can’t be sure. Some fossils are only tiny. Musk duck (Biziura lobata) One duck that still has a feathery lizard-like look is the musk duck. The duck is native to southern Australia. This animal derives its common name from the musky odour it emanates during the breeding season. Musk ducks are moderately common in the wetter, fertile areas in the south of the continent: the southwest corner of Western Australia, Victoria, and Tasmania. This species prefers deep, still lakes and wetlands with areas of both open water and reed beds. They seldom emerge from the water and are awkward on dry land. They rarely fly, take off with difficulty, and landing is a clumsy plop onto the water. However, if they need to, musk ducks can fly swiftly and for long distances. In the water, musk ducks are agile and remain in the water all day long. They stay on the water at night, sleeping well out from land with the head tucked into the body or under a wing. Musk ducks are very much at home below the surface, staying submerged for as long as a minute at a time. They dive to escape predators or unwanted company, and to search for food, typically in fairly deep water. They can descend to at least 6 metres (about 20 feet). The primary diet items are water beetles, yabbies, water snails, freshwater shellfish, supplemented with a variety of aquatic plants and a few fish. When not breeding, adults are generally solitary. Adult males hold and defend territories, excluding other males and quite often females too. Younger birds form flocks on larger bodies of water at some times of year. It is not known at what age they reach sexual maturity in the wild, but it may take several years. They are a long-lived species and are still capable of breeding when 20 years old or more. 13 Nautilus Shells Nautiloids are a large group of marine cephalopods belonging to the subclass Nautiloidea that began in the Late Cambrian and are represented today by only a few living species. Nautiloids were the main predatory animals at one time and developed an extraordinary diversity of shell shapes and forms. Some 2,500 species of fossil nautiloids are known. They are loosely related to the now extinct ammonites and belemnites which looked similar and once dominated prehistoric seas. Much of what is known about the extinct nautiloids is based on what we know about modern nautiluses, such as the chambered nautilus, which is found in the southwest Pacific Ocean and in the Indian Ocean off the coast of Australia. It is not usually found in waters less than 100 meters deep and may be found as far down as 500 to 700 meters. Nautili are free swimming animals that possess a head with two simple lens-free eyes and arms (or tentacles). They have a smooth shell over a large body chamber, which is divided into subchambers filled with an inert gas making the animal neutrally buoyant in the water. As many as 90 tentacles are arranged in two circles around the mouth. The animal is predatory, and has jaws which are horny and beak-like, allowing it to feed on crustaceans. Nautili propel themselves by jet propulsion, modern nautili do not have an ink sac. Only a single nautiloid suborder continued throughout the Mesozoic, where they co-existed quite happily with their more specialised ammonoid cousins. They had a resurgence when the ammonoids became extinct in the Cretaceous. With the global cooling of later eras affected their geographic distribution and they declined in diversity. Today there are only six living species, all belonging to two genera, Nautilus (the pearly nautilus), and Allonautilus.