Embedded Files
Reptilian mammal ancestors (synapsids) dominated the Permian Period (upper four pictures in Figure 11-1); however, the Great Permian extinction wiped out the large synapsids. Tiny mammals evolved superior sensory systems and intelligence to survive in the world of archosaurs (third row in Figure 11-1). A few of the ancestors of modern mammals, tiny insectivores, survived the End Cretaceous extinction and became rodents, canines, felines, primates, grazing animals, and all the other mammals of the modern world (lower two pictures in Figure 11-1).
Figure 11‑1. Progression of mammal evolution over time. Credit: FunkMonk Dmitry Bogdanov Nobu Tamura John and Karen, retouched by Zwoenitzer Fir0002. Used here per CC BY-SA 4.0. Upper to lower, left to right (Permian: herbivorous pelycosaur, carnivorous pelycosaur, carnivorous therapsid, herbivorous therapsid, Moschops. Jurassic: cynodont (beaver-like), cynodont mammaliamorpha. Cretaceous - Present: eutherian tiger, monostreme echidna.
Figure 11‑2. Nodes of mammalian evolution. After Botha.
Jennifer Botha, The Cynodontia, National Museum in Bloemfontein, South Africa, Accessed in 2006 at http://www.nasmus.co.za/PALAEO/jbotha/the_cynodontia.htm
Figure 11-2 is a cladogram of mammal evolution, but it does not show the diversification of mammals in the Cenozoic Era.
The mammals first evolved within a group of reptiles, the synapsids. These mammal ancestors dominated the single large continent of Pangaea during the Permian (Section 11-2). The sequence in the Permian was pelycosaurs, therapsids, and cynodonts (Figure 11‑1).
The cynodonts survived the Great Permian Extinction (251 Ma), but the archosaurs (dinosaurs and crocodilians) began to dominate the world, and subsequent cynodont evolution in the Triassic took place in a world dominated by the archosaurs (Section 11-3).
Living in the dangerous world of the archosaurs led to the evolution of advanced mammalian intelligence and sensory systems. The first monotreme (Echidna and platypus) and eutherian (placental) mammals appeared in the Jurassic (Section 11-4). The first metatherian (marsupial) mammals appeared in the Cretaceous. It is likely that three insectivores were the ancestors of modern placental mammals: Zhalambdalestidae (rodents and primates), Zhelestidae (ungulates), and Cimolestes (carnivores).
The Chicxulub asteroid destroyed the archosaurs at the end of the Cretaceous Period (66 Ma). They story of the discovery of this asteroid is one of the more fascinating stories in science (Section 11-5). The End Cretaceous extinction was also caused by an enormous lava event in India, the Deccan Traps, that released sulfur dioxide into the air and also acidified the upper part of the water column (Section 11-6).
Scientists had long been puzzled about the End Cretaceous extinction. After the discovery of the Chicxulub asteroid and the Deccan Traps, there was a long debate about which disaster caused the extinction of the dinosaurs (Section 11-7) and the survival of mammals. Another subject of debate concerns the lack of placental mammal fossils in the Cretaceous and the rapid diversification of placental mammal orders after the End Cretaceous extinction (Section 11-8). Did each of the three insectivores suddenly become placental mammals after the extinction or was there only one ancestor of all placental mammals?
Banner
Banner
Reconstructed skeleton of Jurassic mammal Trechnotheria Zhangheotherium. This was an early ancestor of placental mammals. Credit: Ghedoghedo. Used here per CC BY-SA 3.0
Page updated
Report abuse