Washington - Long-necked plant-grazing dinosaurs that roamed the Earth 150 million years ago evolved a nifty way of fixing broken teeth. They just grew new ones, said a US study on Wednesday.Scientists analysed the fossils of two of the largest herbivores known to have lived in North America - Diplodocus and Camarasaurus - and found they grew fresh smiles every six weeks or so.They carried several spares, much like human adult teeth that descend after baby teeth fall out, said the research in the journal PLoS ONE."A nearly 100-foot-long [30m] sauropod would have had a fresh tooth in each position about every one to two months, sometimes less," said researcher Michael D'Emic from Stony Brook University in New York."Effectively, sauropods took a 'quantity over quality' approach to making teeth, opposite the approach taken by large animals - mammals - today."Scientists can discern tooth formation in long extinct creatures by studying the lines of deposition of tooth dentin, a layer beneath the enamel.Camarasaurus had as many as three "baby teeth" lined up in each tooth socket at any one time, and replaced its teeth every 62 days, the study said.The Diplodocus tooth sockets held up to five replacement teeth and one functional tooth. Each tooth was replaced every 35 days.Scientists say this ability came in handy because the lumbering dinosaurs ate huge amounts of food, and gnawing on trees and hard grasses all the time would have caused heavy wear and tear on their chompers.