An article was published in Nature in 2007:
Whales originated from aquatic artiodactyls in the Eocene epoch of IndiaJ. G. M. Thewissen, Lisa Noelle Cooper, Mark T. Clementz, Sunil Bajpai & B. N. Tiwari
Nature 450, 1190–1194 (20 December 2007)
doi:10.1038/nature06343
Download Citation
Received:
26 June 2007
Accepted:
03 October 2007
Published online:
20 December 2007
Here are the opening sections:
Abstract
Although the first ten million years of whale evolution are documented by a remarkable series of fossil skeletons, the link to the ancestor of cetaceans has been missing. It was known that whales are related to even-toed ungulates (artiodactyls), but until now no artiodactyls were morphologically close to early whales. Here we show that the Eocene south Asian raoellid artiodactyls are the sister group to whales. The raoellid Indohyus is similar to whales, and unlike other artiodactyls, in the structure of its ears and premolars, in the density of its limb bones and in the stable-oxygen-isotope composition of its teeth. We also show that a major dietary change occurred during the transition from artiodactyls to whales and that raoellids were aquatic waders. This indicates that aquatic life in this lineage occurred before the origin of the order Cetacea.
Main
Phylogenetic analyses of molecular data on extant animals strongly support the notion that hippopotamids are the closest relatives of cetaceans (whales, dolphins and porpoises)1,2,3. In spite of this, it is unlikely that the two groups are closely related when extant and extinct artiodactyls are analysed, for the simple reason that cetaceans originated about 50 million years (Myr) ago in south Asia, whereas the family Hippopotamidae is only 15 Myr old, and the first hippopotamids to be recorded in Asia are only 6 Myr old4. However, analyses of fossil clades have not resolved the issue of cetacean relations. Proposed sister groups ranged from the entire artiodactyl order5,6, to the extinct early ungulates mesonychians7, to an anthracotheroid clade8 (which included hippopotamids), to weakly supporting hippopotamids (to the exclusion of anthracotheres9,10).
The middle Eocene artiodactyl family Raoellidae11,12,13,14 is broadly coeval with the earliest cetaceans, and both are endemic to south Asia. Raoellids, as a composite consisting of several genera, have been added to some phylogenetic analyses5,10, but no close relation to whales was found because raoellid fossils were essentially limited to dental material11,12,13,14. We studied new dental, cranial and postcranial material for Indohyus, a middle Eocene raoellid artiodactyl from Kashmir, India (Fig. 1). All fossils of Indohyus were collected at a middle Eocene bone bed extending for about 50 m at the locality Sindkhatudi in the Kalakot region of Kashmir on the Indian side of the Line of Control. Our analysis identifies raoellids as the sister group to cetaceans and bridges the morphological divide that separated early cetaceans from artiodacyls. This has profound implications for the character transformations near the origin of cetaceans and the cladistic definition of Cetacea, and identifies the habitat in which whales originated. Taken together, our findings lead us to propose a new hypothesis for the origin of whales.
But that is just the suggestion of one group of palaeontologists. Others have other ideas:
Impact of increased character sampling on the phylogeny of Cetartiodactyla (Mammalia): combined analysis including fossilsAuthors
Maureen A. O'Leary,
John Gatesy
First published: 16 November 2007Full publication history
DOI: 10.1111/j.1096-0031.2007.00187.
Abstract:
The phylogenetic position of Cetacea (whales, dolphins and porpoises) is an important exemplar problem for combined data parsimony analyses because the clade is ancient and includes many well-known and relatively complete fossil species. We combined data for 71 terminal taxa (43 extinct/28 extant) to test where Cetacea fits within Cetartiodactyla, and where various fossil hoofed mammals (e.g., †entelodonts, “†anthracotheriids” and †mesonychians) are positioned. We scored 635 phenotypic characters (osteology, dentition, soft tissue, behavior), approximately three times the number of characters in the last major analysis of this clade, and combined these with > 40 000 molecular characters, including new data from 10 genes. The analysis supported a topology consistent with the majority of recently published molecular studies. Cetacea was the extant sister taxon of Hippopotamidae, followed successively by Ruminantia, Suina and Camelidae. Several extinct taxa were phylogenetically unstable, upsetting resolution of the strict consensus and limiting branch support, but the positions of several key fossils were consistently resolved. The wholly extinct †Mesonychia was more closely related to Cetacea than was any “artiodactylan.”“†Anthracotheriids” were paraphyletic, and, with the exception of one species, were more closely related to Hippopotamidae than to any other living taxon. The total evidence analysis overturned a highly nested position for Moschus supported by molecular data alone. The character partition that could be scored for the fossil taxa (osteological and dental characters) included more informative characters than most molecular partitions in our analysis, and had the fewest missing data. The osteological–dental data alone, however, did not support inclusion of cetaceans within crown “Artiodactyla.” Recently discovered ankle bones from fossil whales reinforced the monophyly of Cetartiodactyla but provided no particular evidence of derived similarities between hippopotamids and fossil cetaceans that were not shared with other “artiodactylans”.
Conclusions:
Our combined matrix supported a phylogenetic hypothesis that contrasted strikingly with recent cladistic analyses of cetartiodactylan phylogeny (Geisler, 2001; Thewissen et al., 2001; Geisler and Uhen, 2003, 2005; Boisserie et al., 2005a; Theodor and Foss, 2005). In particular, the combined data from the skeleton, dentition, soft tissues, behavior, transposons, amino acid sequences, and genes clustered the extinct taxa †Hapalodectidae, †Mesonychidae, and †Eoconodon closest to living and extinct cetaceans, and in turn, this entire clade was deeply nested within a paraphyletic “Artiodactyla” (Figs 1B, 4, 5). Thus, our optimal cladograms showed a composite of relationships that were wholly consistent with many previous molecular trees for extant Cetartiodactyla (Fig. 7; Graur and Higgins, 1994; Gatesy et al., 1996, 1999a,b; Gatesy, 1998; Nikaido et al., 1999; Gatesy and Arctander, 2000; Madsen et al., 2001; Murphy et al., 2001a; Springer et al., 2003), but also included critical aspects of more traditional, paleontological interpretations of whale origins (VanValen, 1966; Prothero et al., 1988; Prothero, 1993; Thewissen, 1994; Geisler and Luo, 1998; Luo, 1998; O'Leary, 1998, 2001; Luo and Gingerich, 1999; O'Leary and Geisler, 1999).
Our results indicated, contraGingerich et al. (2001, p. 2241), that shared similarities in the dentitions of †archaeocete cetaceans and †mesonychids are homologous and were not independently derived. Furthermore, according to our total evidence MPTs, characters in the “artiodactylan” ankle that were assumed to be homoplasy-free (Luckett and Hong, 1998) have reversed or been lost in certain clades such as †Mesonychidae (Fig. 5). Schaeffer (1947, p. 22) called the artiodactylan astragalus “a basic ordinal character”; in the context of our results, some features of the astragalus remained ordinal characters, but supported a much more inclusive mammalian order, Cetartiodactyla.
In previous phylogenetic analyses of whale origins (e.g., Thewissen, 1994; O'Leary, 1998; O'Leary and Geisler, 1999; Geisler, 2001), dental characters provided critical support that grouped Cetacea with †Hapalodectes and †Mesonychidae (Figs 5 and 8), but the evidence for this relationship is not restricted to the dentition. For example, a detailed cladistic study of basicranial characters only (Luo and Gingerich, 1999) strongly grouped †Mesonychidae with Cetacea to the exclusion of “Artiodactyla” (bootstrap of 98%). Characters from all of these prior studies were incorporated into our matrix, and in combined analysis, both dental and cranial characters contributed support for a close relationship between Cetacea and †mesonychians (Figs 4 and 5). No “artiodactylan” that has been described to date possesses a dentition that is transitional to that seen in early whales, but this pronounced gap in morphology was bridged by †mesonychids and †Hapalodectes in our total evidence trees (Fig. 8 nodes C and D).
Our results might seem counter-intuitive, because several authors have interpreted the discovery of well-preserved hind limbs from early stem cetaceans as critical evidence that has closed the case on whale origins (e.g., Gingerich et al., 2001; Arnason et al., 2004). Others have suggested that the new cetacean ankle data tipped the balance of support toward exclusion of †Mesonychidae and †Hapalodectidae from Cetartiodactyla, but acknowledged that more thorough sampling of taxa and characters would be required to yield more robust conclusions (Thewissen et al., 2001; Geisler and Uhen, 2003, 2005; Boisserie et al., 2005a; Theodor and Foss, 2005).
One explanation for the difference in results between our study and many prior analyses is that the earlier studies did not maximize the full weight of character evidence from the literature. Of the studies listed above, Gingerich et al. (2001) did not execute a cladistic analysis to substantiate their phylogenetic conclusions. Thewissen et al. (2001), Geisler and Uhen (2003), Boisserie et al. (2005a), and Theodor and Foss (2005) excluded molecular evidence from their systematic studies, and Boisserie et al. (2005a) did not include †mesonychids or †hapalodectids, important taxa featured in nearly all previous discussions of whale origins (VanValen, 1966; Prothero et al., 1988; Prothero, 1993; Thewissen, 1994; Geisler and Luo, 1998; O'Leary, 1998, 2001; Luo and Gingerich, 1999; O'Leary and Geisler, 1999;Geisler, 2001; Gingerich et al., 2001; Thewissen et al., 2001; Geisler and Uhen, 2003, 2005; Theodor and Foss, 2005). The analysis of Geisler and Uhen (2005) merged a large array of molecular and phenotypic traits (38 018 characters (8229 informative) for 73 taxa). However, only a fraction of previously published morphological characters (208 informative) was incorporated into the analysis, and multiple “†anthracotheriids”, putative stem taxa to Hippopotamidae (Boisserie et al., 2005a,b), were not sampled. Here, we made a first attempt at combining the majority of logically independent phenotypic characters from the literature with the largest sample of molecular information ever compiled for Cetartiodactyla. By concatenating all of these data, we hoped to discern common support across character systems, and yield topologies that could be used to infer the polarities of key evolutionary events in this group (Figs 5 and 8).
Unlike our combined analysis of morphology and molecules, cladistic analysis of the osteological–dental partition alone did not produce a tree with Cetacea nested among extant “artiodactylans”, but instead favored a monophyletic clade of crown “artiodactylans” (Fig. 6; also see Thewissen et al., 2001; Theodor and Foss, 2005). Thus, despite a large increase in the number of phenotypic characters relative to previous studies and the discovery of well-preserved cetacean ankle bones (Gingerich et al., 2001; Thewissen et al., 2001), pronounced incongruence remained between characters that fossilize (Fig. 6) and those that do not (Fig. 7). The osteological–dental partition included more informative characters and a much lower percentage of missing data than any of the single gene partitions (Table 5). Only 23% of the characters in the molecular partition were parsimony informative across the entire taxonomic sample. Nonetheless, osteology and dentition accounted for just over 5% of informative characters in the combined matrix and did not overturn the signal in the molecular data for the majority of clades.
At the outset, we predicted that the topology so strongly supported by molecules (Fig. 7) might emerge from an expanded analysis of dental–osteological characters, but this was not the case. For such congruence to be realized in the future likely would require the discovery of early fossil taxa that differ quite drastically from currently known cetartiodactylan forms. In addition to a broader sample of Eocene and Paleocene ungulates, future phylogenetic tests of our overall tree will demand integration of data from Carnivora, †Creodonta, and related taxa. There are distinct similarities in the dentitions of †creodonts, carnivorans, early cetaceans, †mesonychids and †hapalodectids. Therefore, it will be critical to evaluate the impact of a more diverse sample of ancient taxa on the topologies supported by the present analysis.
The two papers are just part of a wide and continuing debate on the question of Cetacean origins. A sketch of that question is given
HERE.
Unsurprisingly, the ways in which a group of mammals (whose ancestors evolved on land) ended up spending their entire lives in and under the water are not easy to explain in simple terms, and people who have studied the evidence for many years still disagree with one another.
You can enjoy this situation in one of two ways:
1. Read their work (either directly or through the medium of good scientific journalism) to appreciate one aspect of how extremely versatile living forms can be, and of their apparently unlimited ability to adapt to and exploit new circumstances over the immense expanse of geological time. There may as yet be no agreement as to exactly how cetaceans evolved, but the question is a fascinating one.
2. Amuse yourself by caricaturing their work, and calling them names like 'evo-illusionists'. If this is what you enjoy doing, there is no need to feel guilty or embarrassed. Enjoy! Palaeontology will not be harmed or impeded in the least.
, so it re-evolved fins, shrank its hind legs down to small completely internal bones, disappeared its forelegs, and went back into the water with those pesky sharks, which must not have been so bad after all, to become the largest species ever known to man: a whale! It’s nasal openings migrated to the top of it’s head to become single or double blowholes.
