Evolution What the Fossils Say and Why it Matters

Bossies and Blowholes 349

their close relationship. There are many details of the skull and jaw, and even of the teeth,
that closely resemble the early mastodonts. In addition, both living species of sirenian have
the same unique mode of tooth replacement that elephants have. Instead of pushing out
baby teeth from below, as most mammals do, elephants and sirenians have horizontal tooth
replacement. Their tooth row consists of a long “conveyer belt” of molars, with new teeth
erupting from the back of the jaw, then pushing the rest of the tooth row forward until the
older worn teeth are dropped off the front edge of the mouth. This unique condition occurs

American

mastodon

Stegodon

Zygolophodon

Deinotherium

Tetralophodon

Anancus

Mammoth

Asian

elephant Africanelephant

Stegotetrabelodon

Shovel-jawed

mastodonts

Gomphotherium

Palaeomastodon

Arsinotheres

Barytherium

Moeritherium

Numidotherium

Phiomia

Manatees and du

gon

gs

EOCENE

OLIGOCENE

MIOCENE

PLI

Q

5

23

34

55

Phosphatherium

FIGURE 14.19. Evolutionary history of the elephants and their kin (Proboscidea), starting with pygmy hippo-
like forms like Moeritherium with no trunk or tusks, through mastodonts with short trunks and tusks, and
concluding with the huge mammoths and the two living species. Early in their history, the other tethytheres
branched off from the Proboscidea. These include the manatees, order Sirenia, the extinct desmostylians, and
the extinct horned arsinotheres. (Drawing by C. R. Prothero; from Prothero 1994b)

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