FOSSIL RECORD AND THE EVOLUTIONARY HISTORY OF INIODEA - Biology, Evolution and Conservation of River Dolphins Within South America and Asia - page 194

century). He placed Platanista and Inia into separate monotypic families, Platanistidae and
Iniidae, respectively. Pontoporia , however, was placed by Gray in the family Delphinidae.
The placement of Pontoporia (= Stenodelphis ) was debated since then, and many specialist
considered it as a distinct group within the true marine dolphins (Kellogg, 1928; Miller,
1923). Flower (1867), in a paper describing a complete skeleton of Inia and a skull of
Pontoporia , proposed a systematic scheme for the ―river dolphins‖ dividing the family
Platanistidae into two subfamilies, the Platanistinae containing only Platanista , and the
Iniinae, containing Inia and provisionally Pontoporia . This is the first and only reference until
Muizon (1985) considered Inia and Pontoporia as closely related groups.
Lipotes, the fourth genus of extant river dolphins, was described by Miller in 1918, and
referred to as Iniidae. Miller (1923) followed Gray in recognizing separate families,
Platanistidae and Iniidae (now consisting of Inia and Lipotes ), and in including Pontoporia
into the Delphinidae. Since Miller's 1918 description, the taxonomic relatedness between Inia
and Lipotes has been generally supported. Fraser & Purves (1960) examined multiple
characters of the outer and middle ear in 37 species of cetaceans, including the four genera of
river dolphins. They found the presence or absence of 32 features of the cetacean ear to be
identical in Inia and Lipotes . Kasuya (1973), based on a comparative analysis of the tympano-
periotic bone from 313 individual cetaceans in 30 genera, also recognized Inia and Lipotes as
the two modern members of the Iniidae. While the overall resemblance (particularly in skull
morphology) and shared ecological habit encouraged a grouping of Inia + Lipotes , the rarity
of specimens and paucity of scientific attention left considerable room for revision.
Slijper (1936) argued at length that the three nominate river dolphin subfamilies
Platanistinae, Iniinae ( Inia + Lipotes ) and Stenodelphininae (= Pontoporia ) belonged together
in the monophyletic Platanistidae. Simpson, in his classification of mammals (1945),
followed Slijper's placement of the four genera in three subfamilies (still the Iniinae consisted
of Inia + Lipotes ) within the single family Platanistidae, but established the superfamily
Platanistoidea to recognize the overall uniqueness of the group. However, Simpson (1945)
openly questioned river dolphin monophyly, noting the association was "recognized [ sic ] as a
habitus character that may have risen in sharply distinct lines of descent". Since Simpson, no
other systematist has argued that Pontoporia belongs with the Delphinidae.
Early attempts to put them in a cladistic framework showed that not a single
sinapomorphy was shared by the Platanistoidea (Zhou, 1982). The paraphyletic (or, more
properly, polyphyletic) nature of the group was evidenced by Muizon (1985; 1988b), who
also pointed out the sister group relationship between Inia and Pontoporia and both with the
Delphinida (sensu Muizon, 1988a).
This phylogenetic scheme, with relatively minor modifications, was supported by more
recent morphological and molecular studies (Heyning, 1989; Heyning & Mead 1990;
Fordyce, 1994; Messenger, 1994; Messenger & McGuire 1998; Cassens et al., 2000,
Hamilton et al., 2001; Nikaido et al., 2001; Verma et al., 2004; Yang et al., 2002; Yan et al.,
2005).
The only recent work supporting the monophyly of the traditional river dolphins group
was the one by Geisler & Sanders (2003). They recovered a monophyletic Platanistoidea,
including all the living genera and some selected fossil ones in a very detailed and
comprehensive osteological analysis. However, the main focus of this work was the basal,
archaic and poorly known Mysticeti and Odontoceti; so, it is not surprising that some of the
crown groups may appear somewhat distorted.