FOSSIL RECORD AND THE EVOLUTIONARY HISTORY OF INIODEA - Biology, Evolution and Conservation of River Dolphins Within South America and Asia - page 209

Early Pliocene deposits of North Carolina (Domning, 1982). It seems that some kind of
connection existed during Late Miocene and/or Early Pliocene between the South American
freshwater and Mexican Gulf faunas.

The Lipotoidea and their Relationships with Inioidea

The taxonomic status of the genus Parapontoporia , which has been assigned to the
Pontoporiidae (Barnes 1985) or the Lipotidae (Muizon 1988c), is controversial. All the
species of this genus are found in the northern hemisphere along the eastern margin of the
Pacific, a location biogeographically intermediate between the present geographic
distributions of Lipotes and Pontoporia. No other pontoporiids were found in this region.
The Chinese river dolphin, Lipotes vexillifer , historically has been linked to the South
American dolphins. In the original description, Miller (1918) suggested an overall
morphology of Lipotes similar to Inia and that their shared fluvial habitat was a basis for a
close relationship. In an influential summary of cetacean biology, Kellogg (1928) followed
Miller (1918) in grouping Lipotes together with Inia in the Iniidae. Barnes described the
genus Parapontoporia (1978, 1985) as morphologically and biogeographically intermediate
between Pontoporia and Lipotes, which he classified as sister taxa recognizing three
subfamilies in Pontoporiidae: Pontoporiinae, Parapontoporiinae and Lipotinae. However,
more recent morphological analyses (Muizon, 1988a; Messenger & McGuire, 1998) and
multiple molecular studies (Cassens et al., 2000; Hamilton et al., 2001; Nikaido et al., 2001)
do not support either of these views. While Inia and Pontoporia are now established as sister
taxa, their relationship to Lipotes remains uncertain.
Comparative nucleic acid sequence analysis of both mitochondrial and nuclear genes
failed to resolve the exact position of Lipotes (Cassens et al., 2000; Hamilton et al., 2001).
Two SINE insertions that are shared by Inia and Pontoporia were not found in Lipotes
(Nikaido et al., 2001). While multiple independent retroposon insertions are convincing
support for the monophyly of Inia + Pontoporia , the current SINE data demonstrate only the
monophyly of the clade ( Lipotes ( Inia + Pontoporia ) Delphinoidea).
However, only two possible evolutionary histories are plausible for Lipotes and allies
given the available data. Either Lipotes and allies are an independent early offshoot of the
stem leading to (Brachydelphidae (Iniiidae+Pontoporiiidae))+Delphinoidea, or Lipotidae,
Brachydelphidae, Iniidae , and Pontoporiidae share a unique common ancestry. There is some
evidence

supporting a monophyletic clade of (Lipotidae (Brachydelphidae

(Iniidae+Pontoporiidae))).
Heyning (1989), in a detailed phylogenetic analysis of odontocete facial anatomy,
grouped the three living genera of those families together in a single clade sister to the
Delphinoidea. However, within this clade remained an unresolved trichotomy. As noted
above, most molecular genetic studies do not statistically support a unique shared ancestry
between Lipotes and the Inia + Pontoporia clade. One exception is the recent study by
Nikaido et al. (2001). In one portion of this study, the authors analyzed almost 3 kb of nuclear
sequences that flanked retroposon insertions in 14 cetaceans, including these three river
dolphins. Their results find strong support for a monophyletic Lippotes + ( Inia + Pontoporia ).
The morphological and molecular data, although inconclusive, seems to favor a
monophyletic grouping of (Lipotoidae (Brachydelphidae (Iniidae+Pontoporiidae))) as sister