North American Least Shrew - Evolutionary History

Evolutionary History

Evolutionary analysis seems to show the shrews evolved from the ancestor Crocidosorex in Europe and crossed over into the Nearctic, consisting of North and Central America, via the Bering Strait (above sea level at the time). The earliest fossils of shrews, Crocidosorex piveteaui, are from the Soricidae family and date back to the Oligocene epoch, but shrews are thought to have originated in the late Eocene (30-40 million years ago). It is debated whether four or five ancient subfamilies occurred, but only two are left today: Soricinae and Crocidurinae. Once the descendants of the Crocidosorex crossed into North America, this subfamily of Soricidae gave rise to the genus Antesorex during the Miocene. In the late Miocene, they split into Adeloblarina and Alluvisorex. From Adeloblarina, two genera, Blarina and Cryptotis, diverged during the Pliocene. The North American least shrew species, Cryptotis parva, then arose from the Cryptotis genus.

The primitive features of Cryptotis parva suggest its ancient origins. The zygomatic arches are missing in the skull, which are present in most mammals even of that size, such as rodents. The mandible also has a more primitive structure with a double articulating surface. The cerebral hemisphere is relatively small, which is necessary in manipulative abilities. However, the olfactory lobes are well developed, revealing the significance of the ability to smell in the shrew's survival. The reproductive and urinary systems are joined in one external opening, called the cloaca, which is a primitive character not found in higher mammals. Another trait uncommon to mammals is the testes are found inside the abdominal cavity.