The Middle Jurassic is the second epoch of the Jurassic Period . It lasted from about 174.1 to 161.5 million years ago . Fossils of land-dwelling animals, such as dinosaurs, from the Middle Jurassic are relatively rare, but geological formations containing land animal fossils include the Forest Marble Formation in England, the Kilmaluag Formation in Scotland , the Calcaire de Caen of France , the Daohugou Beds in China, the Itat Formation in Russia , the Tiouraren Formation of Niger , and the Isalo III Formation of western Madagascar .
10-649: The Aalenian ( / ɑː ˈ l iː n i ə n / ) is a subdivision of the Middle Jurassic Epoch / Series of the geologic timescale that extends from about 174.7 ±0.8 Ma to about 170.9 ±0.8 Ma (million years ago). It was preceded by the Toarcian and succeeded by the Bajocian . The Aalenian takes its name from the town of Aalen , some 70 km east of Stuttgart in Germany . The town lies at
20-405: A few reptile -like anatomical traits. Tritheledontids were mainly carnivorous or insectivorous , though some species may have developed omnivory . Their skeletons show that they had a close relationship to mammals. Tritheledontids or their closest relatives may have given rise to the mammaliaforms . The tritheledontids were one of the longest lived non-mammalian therapsid lineages, living from
30-580: The Paleo-Tethys Ocean . A subduction zone on the coast of western North America continued to create the Ancestral Rocky Mountains .Significant subduction zones were active along practically all of the continental edges surrounding Pangea, as well as in southern Tibet, southeastern Europe, and other locations, to allow the formation of fresh seabed in the proto-Atlantic Ocean. Plate tectonic activity in subduction zones caused
40-826: The Aalenian (the base of the Bajocian) is at the first appearance of ammonite genus Hyperlioceras . In the Tethys domain , the Aalenian contains four ammonite biozones : Middle Jurassic During the Middle Jurassic Epoch, Pangaea began to separate into Laurasia and Gondwana , and the Atlantic Ocean formed. Eastern Laurasia was tectonically active as the Cimmerian plate continued to collide with Laurasia's southern coast, completely closing
50-528: The Middle Jurassic, (including cetiosaurs , brachiosaurs , megalosaurs and primitive ornithopods ). Descendants of the therapsids , the cynodonts , were still flourishing along with the dinosaurs. These included the tritylodonts and mammals. Mammals remained quite small, but were diverse and numerous in faunas from around the world. Tritylodonts were larger, and also had an almost global distribution. The first crown-group mammals appeared in
60-577: The construction of north-south mountain ranges such as the Rocky Mountains and the Andes all along the west coast of North, Central, and South America. The Middle Jurassic is one of the key periods in the history of life on Earth. Many groups, including dinosaurs and mammals , diversified during this time. During this time, marine life (including ammonites and bivalves ) flourished. Ichthyosaurs , although common, are reduced in diversity;
70-609: The late Triassic to the Jurassic period. Tritheledontids became extinct in the Jurassic period, possibly due to competition with prehistoric mammals such as the eutriconodonts . They are known from finds in South America and South Africa , indicating that they may have lived only on the supercontinent of Gondwana . The family Tritheledontidae was named by South African paleontologist Robert Broom in 1912. The family
80-538: The late Early Jurassic. A group of cynodonts, the trithelodonts , were becoming rare and eventually became extinct at the end of this epoch. Conifers were dominant in the Middle Jurassic. Other plants , such as ginkgoes , cycads , and ferns were also common. Trithelodont Tritheledontidae , the tritheledontids or ictidosaurs , is an extinct family of small to medium-sized (about 10 to 20 cm long ) cynodonts . They were highly mammal -like, specialized cynodonts, although they still retained
90-553: The northeastern end of the Swabian Jura . The name Aalenian was introduced in scientific literature by Swiss geologist Karl Mayer-Eymar in 1864. The base of the Aalenian is defined as the place in the stratigraphic column where the ammonite genus Leioceras first appears. The global reference profile ( GSSP ) is located 500 meters north of the village of Fuentelsaz in the Spanish province of Guadalajara . The top of
100-411: The top marine predators, the pliosaurs , grew to the size of killer whales and larger (e.g. Pliosaurus , Liopleurodon ). Plesiosaurs became common at this time, and metriorhynchids first appeared. In the Jurassic seas, a wide range of animals swam. Cartilaginous and bony fish were plentiful. Large fish and marine reptiles were plentiful. Many of the major groups of dinosaurs emerged during
#203796