Nematoda, the phylum above the class Secernentea, has left very few fossil remains. The earliest fossils that contained nematode structures were found in Eocene strata (the era from about 55-38 million years ago). More authenticated fossils are of nematodes preserved in amber within such items as fossilized shark muscles and mammals frozen in permafrost. The fossil record is too fragmented to explain much about nematode origins, so most conclusions about phylogeny are based on observations of living species. It is hypothesized that nematodes originated during the Precambrian era, what was the Proterozoic period (about one billion years ago).
Earlier in the classification process Chitwood separated hematodes into two main classes, the Phasmidia (now Secer-nentea) and Aphasmidia (now Adenophorea). Controversies still exist, but for the most part, scientists, such as A. R. Maggenti, who helped to develop the classifications under this system, treat nematodes as a separate phylum with two classes, Adenophorea and Secernentea, which have been divided based on molecular and morphological characteristics. These two classes are primarily separated (along with other important criteria) with respect to whether they possess phasmids (as in Secernentea) or do not possess phasmids (as in Adenophorea). The total number of species of secernenteans is estimated at about 8,000 worldwide, with scientific surveys suggesting that an enormous number of species has yet-to-be discovered. There are six orders and the number of families ranges from 60 to 89.
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