Fossils of Lystrosaurus, a dicynodont therapsid, are most common in parts of mod
ID: 9951 • Letter: F
Question
Fossils of Lystrosaurus, a dicynodont therapsid, are most common in parts of modern-day South America, South Africa, Madagascar, India, South Australia, and Antarctica. It apparently lived in arid regions, and was mostly herbivorous. It originated during the mid-Permian period, survived the Permian extinction, and dwindled by the late Triassic, though there is evidence of a relict population in Australia during the Cretaceous. The dicynodonts had two large tusks, extending down from their upper jaws; the tusks were not used for food gathering, and in some species were limited to males. Food was gathered using an otherwise toothless beak. Judging from the fossil record, these pig-sized organisms were the most common mammal-like reptiles of the Permian.3. The observation that tusks were limited to males in several species, and were apparently not used in food-gathering, is evidence that the tusks probably
A) were insignificant to the survival and/or reproduction of dicynodonts
B) were used by males during the sex act
C) served as heat-dissipation structures
D) are homologous to claws
E) were maintained as the result of sexual selection
Explanation / Answer
Some people have suggested that the tusks might have been used for digging, but it’s most likely that they were used for fighting and display. A study of hundreds of fossils of the type of dicynodont called Diictodon has concluded that the best explanation for the tusks is that they were used by males to tussle with each other to win a mate. About 50% of the Diictodon fossils had no tusks and were probably females. This means the tusks can’t have been used to dig up food, or the females would have starved. Thus, answer is "were maintained as the result of sexual selection" in the choices given above.