Angiosperms appeared during the mid-Cretaceous and steadily prospered. Unlike gymnosperms, which bear naked seeds, angiosperm seeds are enclosed within an ovary. This protects them from drying up, fungal infection and insects. By the end of the Cretaceous (‘chalky’) about fifty (out of about 500) families had appeared, including beech, birch, magnolia, oak, walnut and willow. Today there are at least 250,000 species of angiosperm compared to only about 550 of conifers.
The earliest known pollen grain is Clavatipollenites from the Early Cretaceous. Today, the principal pollinators of flowering plants are Hymenoptera (‘membrane wing’) – ants, bees, sawflies and wasps. Sawflies date back to the Triassic; wasps and bees to the Carboniferous.
Flowers became more and more adapted to one kind of pollinator during the Late Cretaceous and the Tertiary. The appearance of termites in the Early Cretaceous and of bees and ants in the Late Cretaceous marked the arrival of the first colony-based insects.
New predators roamed the seafloor. Neogastropods (‘new gastropods’), crabs and teleost fishes either actively sought prey or grazed on colonial animals (e.g. coral). The effect on the existing seabed life was dramatic. Brachiopods and crinoids went into decline. Some bivalves hid by burying themselves deep in sediment, while others grew massive shells in order to deter attackers; Inoceramus (‘strong pot’) grew to about 1.8 metres (6 ft) in diameter.
Above the seabed the ichthyosaurs were replaced as the top predators by the plesiosaurs, sharks, and by teleosts up to four metres (13 ft) long. The turtle Archelon (‘ruling turtle’) was over three metres (10 ft) long, but the most powerful marine reptiles were the mosasaurs (‘Meuse River lizards’), which grew up to ten metres (33 ft) long.
The earliest known gliding vertebrate is the Late Permian reptile Coelurosauravus (‘hollow-tailed flying lizard’). It was about 30 cm (1 ft) in length and its ribs supported an aerofoil-shaped skin membrane about 30 cm (1 ft) across. Sharovipteryx (‘Sharov’s wing’) a reptile from the Early Triassic, had a membrane between its long hind legs and its long tail.
The pterosaurs (‘winged lizards’), often referred to as pterodactyls (‘winged fingers’), were proficient flapping flyers. Their wings were formed by a membrane stretching from the thorax to a dramatically lengthened fourth finger. The most likely ancestor to the pterosaurs is the Late Triassic archosaur Scleromochlus (‘hard fulcrum’), a lightly built long-legged runner that shares many characteristics with early pterosaurs.
There have been two main types of pterosaur, rhamphorhynchoids (‘beak snouts’) that emerged in the Late Triassic, replaced in the Late Jurassic by pterodactyloids that flourished until the end of the Cretaceous. Pterodactylus itself had a wingspan of up to 2.4 metres (8 ft); Pteranodon (‘wing toothless’) had a wingspan of about nine metres (30 ft) and like modern birds and unlike earlier pterosaurs it had a toothless beak; Quetzalcoatlus (‘big bird from Big Bend’) had a wingspan close to 11 metres (35 ft).
The earliest known bird is the Late Jurassic Archaeopteryx (‘ancient wing’). Similar in size and shape to a European magpie, Archaeopteryx could grow to about half a metre (1.6 ft) in length. In the details of its anatomy Archaeopteryx shows a strong resemblance to theropods. Feathers in living birds originate in a skin layer under the outer layer that forms scales. It is then likely that feathers evolved under and between reptile scales, not as modified scales.
After Archaeopteryx the evolution of birds was rapid. From the Early Cretaceous the sparrow-sized Sinornis (‘Chinese bird’) had features better related to flight and perching than those of Archaeopteryx, though in the Late Cretaceous both the gull-like Ichthyornis (‘fish bird’) and the flightless Hesperornis (‘western bird’) still had teeth.
At the end of the Cretaceous about 66 mya there was a mass extinction of animals and plants, known as the K-T event – from Kreidezeit, German for Cretaceous, and the Tertiary, the period following the Cretaceous. The dinosaurs, mosasaurs, plesiosaurs, pterosaurs and many species of plant became extinct. Most of the mammals and birds survived.
Theories on what caused the K-T extinctions have centred on the possibility of a catastrophic event such as a meteor impact or increased volcanic activity. Such geological events may have reduced sunlight and hindered photosynthesis. This could be an explanation for the extinction of plants and phytoplankton and the organisms dependent on them, the predatory as well the large herbivore animals. Small creatures whose food chains were based on detritus could still have had a reasonable chance of survival.
The K-T extinctions are associated with a geological feature known as the K-T boundary, a thin band of sediment found around the world. In 1979 a team from the University of California (f.1868) was studying a K-T boundary sequence at Gubbio in Italy. They found that in a clay layer right on the boundary, the levels of the rare metal iridium were a hundred times higher than usual. As the iridium is present only in the boundary rocks it is described as a spike – a very short event. Since 1979, iridium spikes have been found in over a hundred K-T boundary sections all over the world.
Iridium is rare on Earth but it is found in meteorites. A possible explanation for the spike is that iridium was scattered worldwide as debris from a meteorite when it struck somewhere on Earth. In 1992 the most likely site for the K-T crater was identified as a geological structure buried under sediments near the town of Chicxubub in the northwest Yucatan.
Occasionally, a mass of hot magma rises from Earth’s interior and travels towards the surface as a plume. As it nears the surface it develops a head that can be a thousand metres (3300 ft) across and a hundred metres (330 ft) deep. When it breaks the surface the plume generates volcanic eruptions that pour basalt, i.e. flood basalts, over the surface. If the plume erupts through a continent it blasts material into the atmosphere as well. It is argued that the sequences of volcanism would be quite similar to those from an asteroid impact. One of these rare plume eruptions took place at a K-T boundary in what is now the Deccan plateau of central-west India. Known as the Deccan Traps (the term ‘trap’ or ‘stairs’ refers to the step-like hills forming the landscape of the region), it consists of multiple layers of solidified flood basalt that altogether are two kilometres (6600 ft) thick and cover an area of 500,000 km2 (193,000 square miles).
Leave a Reply