Bossies and Blowholes 351in no other group of mammals and is strong evidence for a clade of Proboscidea plus Sirenia
that McKenna (1975) called the “Tethytheria.”
Since McKenna’s original daring hypothesis, the Tethytheria has been supported by
many additional anatomical analyses, so the relationship of manatees and elephants is one
of the best established in all of science. Every molecular system that has been examined, from
the proteins to the mitochondrial DNA to the nuclear DNA always clusters these two groups
together, so there is a complete convergence of evidence. In addition to the sirenians, sev-
eral other previously mysterious groups of fossil mammals now appear to cluster within the
Tethytheria. They include the huge two-horned elephant-sized beasts from the Oligocene of
Egypt known as arsinoitheres (fig. 14.19). These had once been a complete zoological mystery,
placed in their own order Embrithopoda for lack of a better place. But McKenna and Manning
(1977) linked them to the Paleocene Mongolian mystery fossil known as Phenacolophus, and
since then other arsinoitheres have been found in Eocene beds of Turkey and Romania. Yet
another mysterious group was the desmostylians, a hippo-like marine mammal that is found
only in the Oligocene and Miocene of the North Pacific. These too were placed in their own
order for lack of a better hypothesis, until Domning, Ray, and McKenna (1986) described very
primitive fossils of desmostylians called Behemotops and showed that they had the character-
istic tethythere configuration of the jaws and teeth. Once again, the fossil record has yielded a
transitional form that links a previously isolated group with other groups of mammals.
Finally, let’s return to the manatees, peacefully sleeping in the shallow warm waters of the
tropics and munching sea grasses. According to some historians, the legend of the mermaids
may have come from sailors who saw manatees floating upright, feeding their babies at their
paired breasts (a configuration also found in humans and elephants) and possibly with sea-
weed draped over them that resembled hair, and imagined that they were mermaids. Close up,
of course, they are so plug-ugly that they could never be mistaken for beautiful half-women/
half-fish, but never underestimate what months at sea can do for homesick and horny sailors!
But this myth, and the legend of the sirens trying to lure Odysseus’s sailors to their doom with
their beauty and seductive songs, is the basis for the name of the order, Sirenia.
When we look at manatees up close, they have many remarkable specializations. Their
skulls exhibit many unique features, especially in the way the upper bones are modified into
a snout. They have horizontal tooth replacement, and some also have short tusks as well.
Their ribs are unique among mammals in that they are extremely dense and heavy (pachy-
ostotic). These act as diving ballast and help keep the manatee floating at the proper depth.
Last but not least, the front limbs are modified into a flipper (different from the detailed
bone configuration found in whale or ichthyosaur flippers), the hind limbs have vanished
completely, and the tail is a broad flat horizontal fluke like that found in whales. Its shape is
rounded in manatees, but with pointed lobes in dugongs.
Sirenian fossils are well known, although they consist mostly of their distinctively dense
and heavy rib fragments, plus a few decent skulls that show their evolution (Domning 1981,
1982). But in 2001, another remarkable transitional form was discovered that clearly catches
the sirenians in the act of evolving from land mammals. Known as Pezosiren portelli (literally,
“Portell’s walking sirenian”) (fig. 14.21), it is a nearly complete skeleton from the Eocene of
Jamaica that was described by Daryl Domning (2001). The skull is much like many other
primitive sirenians, with all the hallmarks in the skull bones and teeth, and the ribs are thick
and heavy, showing that it too was mostly aquatic. But instead of flippers it has four per-
fectly good walking limbs, with strong shoulder girdles, hip bones, and even well-developed