Evolution of river dolphins

Abstract

The world's river dolphins (Inia, Pontoporia, Lipotes and Platanista) are among the least known and most endangered of all cetaceans. The four extant genera inhabit geographically disjunct river systems and exhibit highly modi¢ed morphologies, leading many cetologists to regard river dolphins as an unnatural group. Numerous arrangements have been proposed for their phylogenetic relationships to one another and to other odontocete cetaceans. These alternative views strongly a¡ect the biogeographical and evolutionary implications raised by the important, although limited, fossil record of river dolphins. We present a hypothesis of river dolphin relationships based on phylogenetic analysis of three mitochondrial genes for 29 cetacean species, concluding that the four genera represent three separate, ancient branches in odontocete evolution. Our molecular phylogeny corresponds well with the ¢rst fossil appearances of the primary lineages of modern odontocetes. Integrating relevant events in Tertiary palaeoceanography, we develop a scenario for river dolphin evolution during the globally high sea levels of the Middle Miocene. We suggest that ancestors of the four extant river dolphin lineages colonized the shallow epicontinental seas that inundated the Amazon, Parana¨, Yangtze and Indo-Gangetic river basins, subsequently remaining in these extensive waterways during their transition to freshwater with the Late Neogene trend of sea-level lowering.

Marine incursion into East Asia: a forgotten driving force of biodiversity

“…rspb.royalsocietypublishing.org Proc R Soc B 280: 20122892 may have facilitated ecological shifts of marine-derived taxa, such as molluscs, ostracods, fishes and dolphins [4,7,12,19]. Because marine incursion events also function as obvious dispersal and ecological barriers, particularly for resident terrestrial and freshwater organisms [9,40], congruent biogeographic patterns would reflect shared responses by different species to the same major vicariant event.…”

“…Consequently, this unique set of conditions Blue dots denote the terrestrial species P. japonica, red dots denote its sister species P. pacifica and black dots denote additional Platorchestia coastal species. Blue-shaded areas illustrate regions with marine ecosystems or regions into which episodic marine incursions occurred during the Miocene in East Asia (reconstructed from [7,21]). (b) Chronogram obtained using a relaxed clock model.…”

“…Palaeogeographic reconstruction of the fluvio-deltaic marine system and episodic marine incursions was adapted from [7,21].…”

“…There is little doubt that the combined effect of Andean uplift and multiple historical marine incursions have contributed significantly to the extraordinarily high biodiversity of the Neotropics [3,4,10]. Interestingly, despite increasingly available evidence of marine incursion for other continents, such as North America and Africa [11][12][13], few attempts have been made to document the impact of marine incursion on the East Asian continent [7].…”

“…These topographic changes also set the stage for marine incursions during the Early-to-Middle Miocene (23 -11 Ma), a time of high global sea levels [18]. Indeed, marine and brackish foraminifer, ostracod and gastropod fossils have been reported in eastern China [19,20], suggesting the occurrence of brief large-scale episodic marine incursions during the Miocene [7,21]. Surprisingly, very few investigations have been carried out to address the potential impact of such incursions on extant biodiversity; an effect on river dolphins is currently the sole documented case [7].…”