During the last 60,000 years, including the end of the last glacial period, approximately 51 genera of large mammals have become extinct in North America. Of these, many genera extinctions can be reliably attributed to a brief interval of 11,500 to 10,000 radiocarbon years before present, shortly following the arrival of the Clovis people in North America . Prominent paleontological sites include Mexico and Panama, the crossroads of the American Interchange. Most other extinctions are poorly constrained in time, though some definitely occurred outside of this narrow interval. In contrast, only about half a dozen small mammals disappeared during this time. Previous North American extinction pulses had occurred at the end of glaciations, but not with such an ecological imbalance between large mammals and small ones (Moreover, previous extinction pulses were not comparable to the Quaternary extinction event; they involved primarily species replacements within ecological niches, while the latter event resulted in many ecological niches being left unoccupied). Such include the last native North American terror bird ("Titanis"), rhinoceros ("Aphelops") and hyena ("Chasmaporthetes"). Human habitation commenced unequivocally approximately 22,000 BCE north of the glacier, and 13,500 BCE south, however disputed evidence of southern human habitation exists from 130,000 BCE and 17,000 BCE onwards, described from sites in California and Meadowcroft in Pennsylvania. North American extinctions (noted as herbivores (H) or carnivores (C)) included: