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In more rapidly swimming odontocetes, relatively elongated transverse processes of the
vertebrae constrain lateral bending of the vertebral column (Long et al., 1997). Shortened
rostrocaudal dimensions of cervical vertebrae and varying degrees of fusion are additional
limitations on mobility. These features simultaneously reduce vertebral mobility while
increasing stability. In oceanic odontocetes, these limitations significantly affect the anterior
torso, where transverse processes and centra are long (Rommel, 1990; Buchholtz, 2001). In
these respects, the anatomy of Inia departs. Figure 6 shows the lumbar vertebrae of Inia
geoffrensis compared to pelagic odontocetes. Inia has proportionally wider (transversely) and
longer (rostrocaudally) centra compared to the pelagic species. Also note the proportionally
shorter lateral projection of the transverse processes. These features presumably allow Inia to
retain lateral mobility in the torso region, while unfused cervical vertebrae (Buchholtz, 2001)
allow neck mobility.
Whether such features are retained from ancestors or secondarily evolved as an
adaptation to riverine environments is a crucial question. Although the ancestry of modern
river dolphins is much debated (de Muizon, 1994; Messenger, 1994), it seems highly
plausible that the morphological features enabling mobility of the anterior torso are primitive
features. This is the most undulatory axial region during dorsoventral swimming movements
in Inia (Buchholtz, 2001), as was probably the case for many fossil cetaceans (Buchholtz,
1998). Miocene delphinoids likely had the greatest amount of undulation in the anterior torso
as well, based on relative centrum lengths (Buchholtz, 2001).
However, there are ways in which Inia stand out among extant odontocetes. Inia have
uniformly high relative centrum length throughout the torso (Buchholtz, 2001). Since greater
centrum length augments mobility (Buchholtz, 1998), this probably accounts for the great
lateral flexibility in both anterior and posterior portions of the torso observed in the captive
Inia . A question that seems unresolved at present is whether the morphological features that
facilitate this nearly uniformly mobile torso are primitive or derived. If Inia and some of the
other river dolphins also derive from a delphinoid lineage, disparate trends for oceanic and
riverine dolphins would not be surprising. Vertebral morphology in some known fossil
delphinoids departs in significant ways from some extant delphinoids. For example, they do
not exhibit a relative increase in centrum length in the posterior torso (as seen especially in
the Delphinidae - Buchholtz, 2001). With more detailed knowledge of their ancestry, derived
features of the vertebral morphology of river dolphins may become apparent.
S UMMARY AND C ONCLUSSIONS
In this chapter, extreme lateral bending in cervical and torso axial regions are
documented. Further investigations must determine whether this mobility, and the
morphological features that permit it, are unique adaptations as opposed to primitive features
that characterized an ancestral condition. In either case, the ―flexible body design‖ (Fish,
2002) of Inia is apparent both behaviorally and in vertebral morphology. The evolutionary
trend for these dolphins includes the ―sacrifice (of) speed for maneuverability...‖ (Fish, 2002).
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