Saturday, 25 January 2014

New species of river dolphin

The Araguaian boto (C) Nicole Dutra
The boto, Inia geoffrensis, popularly known as the pink dolphin, is widely distributed in the rivers of the Amazon basin. It is also found in the Araguaia-Tocantins river basin. Since the two basins became disconnected at the Pliocene-Pleistocene boundary, Tomas Hrbek and colleagues (here) asked if this really was the same species. They analysed morphological and molecular data (including mitochondrial genes), concluded that the Araguaian boto was a distinct species and named it Inia araguaiaensis. In addition they proposed that the boto found in the Bolivian sub-basin be raised from subspecies to species status as Inia boliviensis.     

Distribution map of species and subspecies of Inia
From Hrbek et al. PLoS ONE 2014: 9: e83623
(c) Hrbek et al. (Creative Commons)
Some time ago Dr. Vera da Silva, a co-author of the new study, afforded me the opportunity to study placentation in the boto and also the tucuxi (Sotalia fluviatilis), another dolphin found in the Amazon (here).

Placenta of the boto
From da Silva et al. RBE 2007; 5: 26
For more about these creatures including their place in folklore I recommend Journey of the Pink Dolphins: An Amazon Quest by Sy Montgomery.

Wednesday, 22 January 2014

More debate on the origin of placentals

The first placental mammal
From O'Leary et al. Science 2013; 339: 662-7
Reprinted with permission from AAAS
Controversy over the timing of the mammalian tree (previous post) continues. The latest contribution, by Mario dos Reis et al. (here), boldly states, "O'Leary et al. seek to reignite a controversy over the age of the placental ancestor that otherwise has been settled." The italics are mine. The references that support this statement are a paper by the same authors and one by Meredith et al. No false modesty here.

Alternative calibrations of the same tree
From dos Reis et al. Biol Lett 2014; 10: 20131003
(c) The authors (Creative Commons)
The problem arises because of different approaches to calibration of the molecular clock. The figure shows trees with identical topology but dated with different sets of fossils. In the first three, many extant orders of mammal can trace their origin to the Late Cretaceous. The problem is that all known fossils of placentals date from after the mass extinction event at the Cretaceous-Paleogene boundary. A literal interpretation of the fossil record, on the other hand, has all orders and even placentals themselves postdating the boundary.
It is not easy to choose appropriate fossils for calibration. Dos Reis and colleagues criticise O'Leary et al. for placing too much weight on Protungulatum donnae. They point out that the fossils of the genus may be found in the Cretaceous, yet the authors they cite (here) are unsure whether or not Protungulatum is a placental.

For a sober discussion of the difficulties in fossil calibration I highly recommend a recent paper by Bibi (here) on the evolution of ruminants. En passant Bibi observes, "The problem stems in part from the opacity of paleontological literature to non-specialists."

The O'Leary paper is co-authored by some of the most eminent specialists in fossil mammals. Arraigned gainst them we have some of the most competent molecular phylogeneticists. There seems little chance of compromise to judge from the acrimonious tone in this latest contribution. Surely the reviewers and editor of Biology Letters could asked the authors to damp things down?

Friday, 3 January 2014

Was Homo erectus the fourth hominin?

Skull of Homo erectus (Wikimedia Commons)
The current issue of Nature has a comprehensive analysis of the genome of Neanderthals and comparisons with modern humans and Denisovans (here). The supplementary material alone fills 16 Mb but there is an excellent summary (here) entitled "Four makes a party." As it says, "it does seem that Eurasia during the Late Pleistocene was an interesting place to be a hominin, with individuals of at least four quite divergent groups living, meeting and occasionally having sex."

The fourth hominin was alluded to in a previous post and is inferred from analysis of the Denisovan genome (previous post). The current study estimates that this putative hominin's ancestor diverged from other hominins 0.9-1.4 million years ago. This date is compatible with the unknown hominin being Homo erectus a hominin well known from the fossil record.