Evidence for the existence of sharks dates from 450–420 million years ago, before land vertebrates existed and before many plants had colonized the continents. Only scales have been recovered from the first sharks and not all paleontologists agree that these are from true sharks. The oldest generally accepted shark scales are from about 420 million years ago. The first sharks looked very different from modern sharks. The majority of modern sharks can be traced back to around 100 million years ago. Most fossils are of teeth, often in large numbers. Partial skeletons and even complete fossilized remains have been discovered. Estimates suggest that sharks grow tens of thousands of teeth over a lifetime, which explains the abundant fossils. The oldest white shark teeth date from 60 to 66 million years ago, around the time of the extinction of the dinosaurs. These sharks attained gigantic proportions and include the extinct megatoothed shark, Carcharias megalodon. This giant shark reached a total length of more than 16 meters (52 ft). It may have approached a maximum of 20.3 meters (67 ft) in total length and 103 metric tons (114 short tons) in mass.