Evolution and systematics

The class Trematoda has about 6,000 species, although the number given varies considerably among different researchers. The class is commonly divided into two subclasses, Aspidogastrea and Digenea. Both are sometimes elevated to class level or dropped to order status; however, they will be treated in this text as subclasses of the class Trematoda. At one time, the class Monogenea was considered a subclass or order of the class Trematoda.

Recent studies of the evolution of trematodes indicate that the class may be paraphyletic—that is, it may represent only some of the descendants of a common ancestor; and that most of the species actually should be grouped together with the mollusks and annelids in the taxon Lophotrochozoa. Part of the difficulty in defining the placement of trematodes is that their fossil record is so sparse. Other than some fossil eggs dating from the Pleistocene epoch (1.8 million-8,000 years ago), historical evidence of trematodes consists mostly of trace fossils. Trace fossils are signs thought to have been left by an organism. For example, a trace fossil from a trematode might be a slight indentation on a fossil snail shell.

Despite the poor fossil record, scientists believe that As-pidogastrea is an ancient group, because many of its species, including those in the families Multicalycidae, Rugogastridae, and Stichocotylidae, use cartilaginous fishes as hosts. These fishes, which are in the class Chondrichthyes, evolved at an earlier point in time than the mammals and teleosts (bony fishes) generally used as hosts by digenetic trematodes. Species within the digenetic genera Nagmia and Probolitrema are exceptions to the rule, and will invade cartilaginous fishes.

A particularly noticeable synapomorphy of the two subclasses is the presence of a posterior sucker, which is manifested as the large, ventral disk in the subclass Aspidogastrea. In addition, both subclasses have life cycles that involve mollusks and vertebrates. Scientists still disagree about whether the former or the latter were the original hosts evolutionarily.

The subclass Aspidogastrea has four families with about 80 species and is often split into four orders. The subclass Dige-nea has about 6,000 species that are generally split into 10 major orders and numerous smaller orders. Frequently, taxa are switched from family to order status, so a total number of families and orders in this large subclass is difficult to ascertain.

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