help... grammar SOS. Since then, over 100 tons of fossils, 1.5 million from vertebrates, 2.5 million from invertebrates, have been recovered, often in densely concentrated and tangled masses. The creatures found range from insects and birds to giant ground sloth's, but a total of 17 proboscides (animals with a proboscis or long nose) — including mastodons and Columbian mammoths — have been recovered, most of them from Pit 9, the deepest bone-bearing deposit, which was excavated in 1914. Most of the fossils date to between 40,000 and 10,000 years ago. 这句话的成分怎么划分呢？ 倒数第三行的里面的them指什么？求解， 请高手帮忙