 An argument against herding is that juvenile remains are very uncommon at this site , unlike modern cases with herd mortality . They more likely were the periodic victims of flash floods whose carcasses accumulated in a lake or marshy setting . The <unk> find , however , with its greater span of individual ages , more even mix of Dollodon or Mantellisaurus to Iguanodon bernissartensis , and confined geographic nature , may record mortality of herding animals migrating through rivers . 
