Evolution and systematics

The timing of the origin of extant Insectivora species has been and continues to be a topic of great debate among sci entists. It is widely accepted, however, that Insectivora are the most primitive of the true placental mammals existing today and the ones from which present day mammals evolved. Most of the primitive eutherian (placental) mammals were insectivores. To date, common classification practice has been to include some of the primitive eutherians with all recent Insectivora members based on their similar dentition. The earliest insectivore fossils are believed to be those of Batodon and Paranyctoides. These remains date from the mid to late Cretaceous period, approximately 100 million years ago (mya). Remains of small insect eaters from Asia, zalamb-dalestids, along with kennalestid remains from Central America, are believed to date back to the Late Cretaceous.

Erinaceidae fossils date back to the Paleocene to early Pliocene in North America. Other fossils discovered in Africa date from the early Miocene to Recent, from late Paleocene to Recent in Europe, and from Eocene to Recent in Asia. The Solenodontidae dates from the late Mesozoic and early Ceno-zoic in North America and the Caribbean to Recent in Cuba and Hispaniola. Eight species of Nesophontidae are believed to have survived the Pleistocene period on into the 1900s. Lack of physical evidence of their ongoing survival has led scientists to conclude that they are now extinct. Most of the Tenrecidae began to evolve in Madagascar from the Pleistocene to Recent. Other records show that they existed during the middle Eocene to middle Oligocene in North America and from the Miocene to Recent in Africa. Chrysochloridae fossils dating back to the Miocene (Kenya) and Pleistocene (South Africa) and resembling Recent species were discovered in Africa. There are varying opinions regarding the evolution

A smoky shrew (Sorex fumeus) foraging in leaf litter. (Photo by Gary Meszaros. Bruce Coleman, Inc. Reproduced by permission.)

of the Soricidae. This is due to the rarity of soricid fossils. Some scientists believe that the Eocene period in Africa, Eurasia, northern South America, and North America marks the onset of this family's existence. Others believe that the Soricidae existed only in Europe and North America during this period and from Miocene to Recent in Africa and Asia, Pleistocene to Recent in northwest South America, and early Oligocene to Recent in Europe and Asia. We know of recent Talpidae fossils dating back to Miocene in Europe and recent protein studies show that they were present in North America during that period as well. Otherwise their geological range is also believed to be early Oligocene to Recent in North America, and late Miocene to Recent in Asia.

For a long time, Insectivora was the order into which scientists put species of questionable lineage or those mammals that were characterized by the lack of distinctive features possessed by other mammals. As phylogenetics (identifying and understanding relationships between life forms) and other scientific dating methods improve, and as additional fossils are discovered, Insectivora is becoming less of a catchall order but not necessarily less controversial.

There are many examples of the difficulties in classification surrounding Insectirora. Prior to 1972, for example, elephant shrews were part of Insectivora. Then one group of scientists decided that because these mammals possessed certain physical characteristics not typical of most other insectivores— moles, shrews, hedgehogs, and tenrecs—such as an auditory bulla, an entotympanic bone, a complete zygomatic arch and a large jugal bone, elephant shrews should be placed in a separate order, namely Macroscelidea. Other taxonomists believed that Insectivora should be divided into two suborders: Meno-typhla consisting of Macroscelididae (elephant shrews), Tupai-idae (tree shrews), and Lipotyphla with the living families Erinaceidae (gymnures and hedgehogs), Chrysochloridae

(golden moles), Tenrecidae (tenrecs), Solenodontidae (solen-odons), Soricidae (shrews), and Talpidae (moles, shrew moles, and desmans). Other scientists felt that tree shrews belonged with the primates and yet others believed that they should be placed in a separate order altogether, Scandentia. There are also those who believe that they have molecular evidence for the origins of Insectivora and for a new order, Afrosoricida, of endemic African mammals to include the families Chrysochloridae (golden moles) and Tenrecidae (tenrecs). Reallotment of these two families would decrease the number of Insectivora species by 51. The elephant shrews (Macroscelididae) would be included in this new order as well. The Cynocephalidae (colu-gos) has been moved around over the years between Insectivora and Dermoptera, where they now reside. With Insectivora, it would appear that as scientific techniques evolve, so families and species fragment, multiply, and/or relocate.

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