How did marine mammals evolve?
- Mammals evolved on land around 160 million years ago. Each taxonomic marine mammal group evolved from a different group of land mammals, whose ancestors separately ventured back into the ocean environment.
May 21, 2007 · Also, the two recently evolved marine mammals, the polar bear and sea otter, spend most of their time in the water feeding. This is no great surprise, because a great deal of mammalian evolution has been linked to changes in feeding ecology.
- Mark D. Uhen
Feb 01, 2019 · Mammals evolved on land around 160 million years ago. Each taxonomic marine mammal group evolved from a different group of land mammals, whose ancestors separately ventured back into the ocean environment. Despite these different origins, many marine mammals evolved similar features — streamlined bodies, paddle-like limbs and tails ...
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- Mammals vs. Reptiles
- Lifestyles of First Mammals
- Split in Family Tree
- Mammals Survive Extinction Event
Before discussing how the first mammals evolved, it's helpful to define what distinguishes mammals from other animals, especially reptiles. Female mammals possess milk-producing mammary glands with which they suckle their young. All mammals have hair or fur during at least some stage of their life cycles, and all are endowed with warm-blooded (endothermic) metabolisms. Regarding the fossil record, paleontologists can distinguish ancestral mammals from ancestral reptiles by the shape of their skull and neck bones, as well as the presence, in mammals, of two small bones in the inner ear (in reptiles, these bones constitute part of the jaw).
The most distinctive thing about the mammals of the Mesozoic Era is how small they were. Although some of their therapsidancestors attained respectable sizes. For example, the late Permian Biarmosuchus was about the size of a large dog. Very few early mammals were larger than mice, for a simple reason: dinosaurs had already become the dominant terrestrial animals on earth. The only ecological niches open to the first mammals entailed a) feeding on plants, insects and small lizards, b) hunting at night (when predatory dinosaurs were less active), and c) living high up in trees or underground, in burrows. Eomaia, from the early Cretaceous period, and Cimolestes, from the late Cretaceous period, were fairly typical in this regard.
Recently, paleontologists discovered conclusive fossil evidence for the first important split in the mammal family tree, the one between placental and marsupial mammals. Technically, the first, marsupial-like mammals of the late Triassic period are known as metatherians. From these evolved the eutherians, which later branched off into placental mammals. The type specimen of Juramaia, the "Jurassic mother," dates to about 160 million years ago, and demonstrates that the metatherian/eutherian split occurred at least 35 million years before scientists had previously estimated.
Ironically, the same characteristics that helped mammals maintain a low profile during the Mesozoic Era also allowed them to survive the K/T Extinction Event that doomed the dinosaurs. As we now know, that giant meteor impact 65 million years ago produced a kind of "nuclear winter," destroying most of the vegetation that sustained the herbivorous dinosaurs, which themselves sustained the carnivorous dinosaurs that preyed on them. Because of their tiny size, early mammals could survive on much less food, and their fur coats (and warm-bloodedmetabolisms) helped keep them warm in an age of plunging global temperatures.
- Bob Strauss
- Science Writer
Jan 17, 2018 · Marine Mammal Groups. The cetaceans have evolved to become the dominant group of marine mammals, with the highest levels of taxonomic and ecological diversity, as well as the widest geographic range. They evolved from a group of hoofed terrestrial ancestors within the order Artiodactyla more than 50 million years ago during the Eocene period.
The Mammal-like Reptiles, or Therapsids, first appeared about 285 million years ago – near the beginning of the Permian (which is well before the dinosaurs). They evolved quickly and many different groups arose. They were very successful until about the end of the Permian, about 245 million years ago.
Evolution of mammals. The evolution of mammals has passed through many stages since the first appearance of their synapsid ancestors in the Pennsylvanian sub-period of the late Carboniferous period. By the mid- Triassic, there were many synapsid species that looked like mammals. The lineage leading to today's mammals split up in the Jurassic ...