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Age of Animals


Cretaceous Period, 145.0 - 66.0 MYA

Cretaceous Period
  • FA flowering plants (angiosperms) - The modern types of flowering plant became common some time in the middle of the Cretaceous. Hardwood trees slowly replaced the conifers as the dominant trees of the forests (see Figure 09a). These new plants provided fruits, flowers and nectar as new sources of food, causing great changes in all life on land.
  • Dinosaurs continued to dominate the land. But the fauna was very different from that of the late Jurassic. Large sauropods were rare, and the medium-sized coelurosaurs had been replaced by the ostrich-like ornithomimids, which included the duck-billed hadrosaurs and the armoured ankylosaurs and ceratopsians. The 30 feet long sea monsters called mosasaurs were the descendants of nothosaurs.
  • Carnivorous mammals such as Repenomamus were beginning to come out of the shadows. A complete fossil of this animal was found at the base of the Yixian Formation in northeastern China. On its left side, under the ribs where a mammal's stomach might well have been, lies a fragmentary and disarticulated skeleton of a young dinosaur about 14 cm long. The devourer of this little dinosaur was more than a meter long, and is estimated to have weighed 4 - 6 kg.

Figure 09a Cretaceous Period [view large image]


  • The smallest North American dinosaur fossil (50 cm high, weighed 2 kg) has been identified to be predator of small animals. The clawed dinosaur was slight, ran on two legs and had dagger-like teeth. It had an enlarged sickle-shaped claw on its second toe. This cat-sized dinosaur is called Hesperonychus (western claw).
  • Cat-like Crocodile Another cat-like fossil by the name of Pakasuchus was uncovered in East Africa. It seems that this creature had embarked on a failed attempt to evolve from crocodile to cat around 100 million years ago. It has nasal openings on the front of their skulls implying land living. By contrast, surviving crocodilians all have nares on the tops of their heads for breathing while partially submerged. The specimen had a short, broad, cat-like skull but the bone arrangement is otherwise distinctively crocodilian. It had mammalian teeth but retained the scaly armour on its tail. At about 50 cm long from nose to tail, these reptiles would have been active, agile hunters of insects and other small preys. This kind of creature disappeared around the same time as the

    Figure 09b Cat-like Crocodile [view large image]

    extinction of dinosaurs. But the ancestors of modern crocodiles were able to weather the storm and prosper ever since.

  • A cretaceous dinosaur fossil (Tianyulong Confuciusi) with long feather-like structures sticking up from its body has been discovered in Liaoning, China (Figure 09c). Based on the bones present, it looks like it was small, active, agile, and probably eating a mix of insects, small vertebrates and plants. This one is special because it is from the lineage of which is ornithischian,
    Tianyulong Dinosaur Skin Characteristics thought to become extinct at the end of the Triassic Period, and the feathered dinosaurs belong exclusively to the saurischian lineage (Figure 09d). It is suggested that if these are protofeathers, then they might not related in any way to flight. The fact that the filaments over the tail are so long and stiff, points to a possible display function. Figure 09d shows the progression of dinosaur skin characteristics from scaly skin to

    Figure 09c Tianyulong Confuciusi [view large image]

    Figure 09d Dinosaur Skin Characteristics

    filamentous proto-feathers and onto pennaceous feathers. This surprising discovery raises fresh questions about the evolution of feathers.
  • There was another mass extinction of marine and land life forms at the end of the Cretaceous period. Principal casualties are the dinosaurs and ammonites. A 10 km diameter asteroid hit the north coast of the present-day Yucatan (the impact created a sharp sediment boundary between the Cretaceous and the Tertiary periods called the K-T boundary). It caused about 75% of the previously existing plant and animal species to disappear. No species of land animal weighing more than about 25 kg survived into the Tertiary.
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