Early Odontocetic Environmental Analysis

River dolphins have been an elusive group of cetaceans, taxonomically and biologically, since the time of their first noted observation in the wild. They are proven to have oceanic ancestors which invaded the riverine systems on at least three independent events and hence develop ecological extreme specialization to adapt to the freshwater environment. Although this hypothesis has been challenged, invasion of the freshwater systems might have insured their survival against either competitive exclusion by more recent and better adapted smaller cetaceans or changing physical parameters of the oceanic systems. (Cassens, 2000; Geisler, 2011) (reversal of this may happen due to deteriorating habitat?)
Direct habitat sharing combined with the degradation

of river systems and consequent shrinking and drying up of major river systems as a result of anthropogenic activities has endangered survival of the river dolphins (Paudel, 2015; Smith, 1993; Williams, 2016). Shortcomings in the taxonomical classification can lead to poor and ineffective conservation and management, hence, defining their taxonomy, lineage and evolutionarily significant units (ESU) is essential to planning and execution of effective conservation and management to avoid further endangering of the species’ (Braulik, 2015; Reeves, 2004)
Classifications and evolutionary studies until the late 1990’s were based solely on morphological observations but with the advancements in genetics and informatics, ancestral and fossil linkages could be included to critically analyze the evolutionary and morphological relationships shared by each species (Messenger & McGuire, 1998; Geisler, 2011)
Nuclear and mitochondrial DNA sequences are analyzed and paired with sequences from other species stored across a database, cross referencing these with a more recent common ancestor within the subject group. (Braulik, 2015; Moritz, 1994).
To avoid convergence and reversals across lineages while comparing sequences, are used irreversible and independent nature of the insertion of the short interspersed repetitive elements (SINEs) are used for searching common ancestry. Use of complete mitochondrial DNA is also essential while comparing since individual mtDNA genes may provide varying tree alignment layouts for different phylogenetic relationships (Arnason, 1992; Cao, 1994; Xu, 1996; Zhou, 2005)
Mysteceti and Odontoceti are known to have diverged in the late Oligocene period.

Evidence suggests that early Odontocetes invaded the freshwater river systems at least three times, diverging completely during the late Oligocene to early Miocene period. Although contentious, there is a possibility of Iniidae, Pontoporiidae and Lipotiidae being monophyletic and thus a yet older invasion of the riverine habitat than the very first divergence is suggested. However invasion of the riverine environment by the most recent common ancestor does not correlate the current distribution of species with that of tectonic plate movements. Hence, based on these observations and the phylogenetic analysis a minimum of three independent invasions of the freshwater habitat are suggested (Cassens, 2000).
-- Hamilton et al. expanded upon the hypothesis of separate, freshwater invasions by speculating that the ancestors of extant river dolphins remained in river systems after sea level regressed from its middle Miocene highs (Geisler, 2011; Hamilton, 2001)
-- it is possible that the intensity of competition, not the simple presence or absence of a single delphinoid species, could explain the absence of close relatives to Lipotes and Platanista in modern marine environments (Geisler,

2011).
(2000, Cassens) franciscana is mostly marine and diverged from the boto well before the radiation of delphinids, it is ambiguous whether the common ancestor of the two South American river dolphins was marine or riverine. If it was indeed riverine, then one must assume a recent ecological reversal, i.e., reinvasion of the marine coastal habitat. On the other hand, the ancestors of the franciscana might have escaped extinction, even though it had not invaded the riverine habitat per se, because it was ecologically specialized. The very small size of the extant species might then reflect coastal adaptations, which would explain its survival in the marine habitat.
-- Platanista has two possible different positions: one is that Platanista diverges after the divergence of sperm and beaked whales (in this order), and no sister relationship between the latter two (under the assumption of rate homogeneity among sites, data not shown); the other is that Platanista and beaked whales form a clade which diverged after sperm whales (Zhou, 2005)
Recent studies by Braulik et al.

hypothesize that the two species of Platinistidae emerged in the larger Ganges-Brahmaputra River system, moving towards the adjacent Indus River system around 0.5 - 0.6 mya, being reproductively isolated since 0.55 mya when they shared a common ancestor, at a time when the two systems were loosely connected by a few tributaries (Braulik, 2015). This is supported by studies that show from 5 mya to 0.3 mya, major tributaries of Ganges in Punjab were gradually re-routed into the Indus (Clift and Blusztajn, 2005).
Since the populations in these river systems are separated by hundreds of kilometers of landmass, dispersal between the river systems and contemporary genetic exchange is deemed very unlikely and although these dolphins are occasionally sighted in brackish water, their dispersal between river systems through the ocean would involve a highly improbable journey through exposed saline waters, of at least 4,600 km around the Indian peninsula (Braulik,

2015).