ECOL 182h: The Great American Interchange Thylacosmilus , a South American borhyaenid, died out after the arrival of true sabertooths from the north. Also shown are a Glyptodont (Xenarthra) and toxodonts (Notoungulata). 1 Mammalian Evolution. Simplified amniote (reptiles, birds and mammals) phylogeny. Mammals arise in the Triassic and subsequently (late Jurassic) differentiate into pr o- therians (platypus, echidna) and therians. The latter split into metatherians (marsupials) and eutherians (placentals) early in the Cretaceous. 2 Mammalian Evolution – II. A more detailed view of mammalian evolution. Of the various Mes- ozoic mammals, only multituberculates survived the end- Cretaceous mass extinction. 3 Marsupial Dispersal. Marsupials are believed to have evolved in Asia from whence they dispersed into the Americas, Antarctica and Australia prior to the separation of North and South America. 4 History of South Amrica. • Connected to NA (directly or by islands) during most of the Mesozoic. • An island continent thereafter until late Tertiary. • During this period, a unique mammalian fauna evolved. 1. Marsupials including borhyaenid carnivores. 2. Placental herbivores including a. Xenarthrans (armadillos, sloths, anteaters) b. Indigenous ungulates (condylarths, litopterns, etc .) 3. Some spectacular examples of convergent evolution. • Following reconnection with SA, 1. Many northern species went south. 2. A smaller number (opossums, amadillos, sloths) went north. 3. Many indigenous pecies died out. 5 SA Old Timers (Marsupials & Xenarthrans). 6 SA Old Timers (Ungulates). 7 Litoptern Relations. Whereas Darwin imagined Macrauchenia , to be closely related to then living Guanaco (Camelidae), Litopterna and Artiodactyla (along with Perissodactyla and Cetacea, turn out to be sister texa. 8 Mammalian Dentition. Dentition of Juramaia sinensis, a late Jurassic eutherian. Note the differentiation of teeth into in- cisors, canines, premolars and molars. From Luo et al . (2011). 9 Carnivore Evolution and Convergence. Top. Evolution of eutherian Carnivora. Dental formulas refer to shearing teeth (carnassials). Bottom. Thylacosmilus (left ) and Smilodon (right ). 10 Thoatherium : A Horse-like Litoptern. Equus (left) and Thoatherium (right) compared. Note the absence of splints (vestigial 2 nd and 4 th toes) in the latter. 11 Terror Birds (Phorusrhacids). • Dominant South American predators during most of the Cenozoic. 1. 1-3 m in height. 2. One species, Titanis walleri, migrated north to FLA / TX during the Pleistocene. After the interchange. Terror birds drive Smilodon from a kill. Video at http://www.youtube.com/watch?v=-o5OvDJxM78. 12 After the Interchange. 13 After the Interchange – II. Results of the great American faunal exchange. Not all southe rners making it north are shown; some that did, e.g. , sloths, subsequently died out north of the filter. 14 .