How mammals evolved from reptiles
Infobae giraffe neck
The first tetrapods must have lived as early as the early Middle Devonian, as evidenced by footprints found in marine sediments from Poland. The oldest known fossilized individuals of terrestrial vertebrates, Acanthostega, date back to the Cimmerian Devonian of East Greenland. The first tetrapods lived in the early Middle Devonian.
At this time there were also swampy habitats such as low marshes, coastal lagoons and brackish riverbanks, and there is ample evidence to suggest that this was the kind of environment in which tetrapods evolved.
The evolutionary specialization of the vertebrates of the Upper Devonian led to sarcopterygian fish such as Panderichthys having descendants such as Eusthenopteron that could wing air and lived on shallow banks; Tiktaalik whose duck-like fins could carry it ashore, and preceded the first tetrapod amphibians such as Acanthostega, whose feet had eight digits, and Ichthyostega, who developed strong limbs that allowed them to swim on land. For their part, lobe-finned fishes evolved into present-day forms such as the living fossil coelacanth.
Evolution of the whale
Reptiles gave rise to birds and mammals. In primitive birds, insulating feathers conserved body heat. Consequently, these animals could adapt to cold climates. Later, some of these birds developed larger feathers on their limbs which allowed them to move up trees or helped them to jump to reach an insect. From this point on, birds were able to evolve to flight.
The abundant hair on the primitive limbs also allowed them to withstand low temperatures. Unlike birds, which retained the reptilian habit of laying eggs, mammals were able to gestate inside the mother and she was able to feed her young with secretions from her mammary (milk-producing) glands.
Being able to keep the heat in their bodies led to the development of hair in mammals, also the first mammals were characterized by laying eggs as well as reptiles, when they had their young they fed on breast milk secreted by their mothers through glands in the skin of the mother.
Why dinosaurs were so big
Although many of the projects we carry out in the Terrestrial Vertebrate Ecology and Conservation lab are related to the diverse species of mammals that inhabit Mexico, and in some cases other parts of the world, we know very little about their origin, evolution and diversification around the world. In the laboratory we often ask questions such as when did these unique creatures arise, why do marsupials exist in the Americas and Australia but not in Africa, and how is it that groups as different as bats, whales and giraffes shared the same ancestor?
Figure 1. Skull modification through time and the evolution of mammals. (Image modified from :http://animaldiversity.org/collections/contributors/Grzimek_mammals/structure_function/v12_id3_con_jawearstr/)
As more fossils are discovered, and better genetic techniques are developed, we will be able to better understand how not only mammals, but other groups of animals, and other living things in general, evolved. However, this is only part of the story of how this interesting group of animals, to which we belong, arose and evolved. For now, we already know who were the first ancestors of mammals, and how long ago they appeared. Later we will be putting the puzzle together to discover the origin and diversification of all the current orders of mammals.
Why animals used to be bigger
The process that originated the Dinosauromorpha and the first true dinosaurs can be followed through the fossils of the first Archosauria such as the Proterosuchidae Erythrosuchidae and Euparkeria, whose fossils date from 250 Ma ago, through the archosaurs of the middle Triassic, such as Ticinosuchus from 232-236 Ma ago. Crocodiles are also descendants of archosaurs from the middle Triassic. The first confirmed dinosaur fossils comprise the saurischian (from 'lizard skin') Nyasasaurus from 232-236 Ma ago.
The earliest confirmed dinosaur fossils comprise the saurischian (from 'lizard skin') dinosaurs Nyasasaurus from hai 243 Ma, Saturnalia from hai 225-232 Ma, Herrerasaurus from hai 220-230 Ma, Staurikosaurus possibly from hai 225-230 Ma, Eoraptor from hai 220-230 Ma and Alwalkeria from hai 220-230 Ma. Saturnalia may be a saurischian or a prosauropod. The others are basal saurischians.
In this figure it is clear that the first saurischians were reminiscent of the first ornithischians, but not of modern crocodiles. The saurischians are distinguished from ornithischians by retaining the ancestral configuration of the bones of the skull. Another difference is in the skull, the upper skull of ornithischians is more solid and the joint connecting the lower jaw is more flexible; both are adaptations to herbivory and both can already be seen in Lesothosaurus.