Phylogeny of Placental Mammals: Consensus?

My last scholarly reference on vertebrate evolution, Michael Benson’s 1991 Vertebrate Paleontology, has this to say:

Essentially, the situation at that time was that individual orders were fairly clearly mapped out – the interrelationships of perissodactyls or of carnivores as amongst themselves were known – but that what orders were most closely associated with what other orders was still hotly debated. (For those confused, the placentals are the vast majority of living mammals, which bring forth their young in a relatively developed state after nurturing them with a fully developed placenta for much longer than do marsupials. Placentalia, AKA Eutheria, comprises a clade assigned Infraclass status in most traditionalist works, with a double dozen orders (Primates, Carnivora, Perissodactyla, Rodentia, Cetacea, etc.) assigned to it.

As of the time Benton wrote, there was a general agreement, supported by both skeletal/physiological and genetic/biochemical markers, that the Edentata/Xenarthra (sloths, anteaters, and armadillos) were an “outgroup” to the remainder. But which groups among the remainding 20-odd orders were most closely related to which other groups, forming intermediate-level clades of “Cohort” or “Superorder” status, was argued at great length by mammalogists, mammalian paleontologists, and other interested parties. Hyraxes, for example, were either most closely related to elephants and sirenians, or to perissodactyls. Whales and dolphins, either to the extinct mesonychids or to artiodactyls. Rabbits and pikas, either to rodents or to some of the primitive hoofed forms. For every given scientist with a clear theory, there were five equally skilled scientists prepared to refute his and advance their own in its place – and for the most part those five did not concur on a whole lot.

My question is if any consensus has been reached in the 15 years since Benson wrote, if so what it is, and if not, what progress has been made towards resolving some of the debated issues. Links and/or pointing me to intelligent discussion online would be appreciated; a post with a good less-than-totally-arcane summary of the situation today would be appreciated even more.

Have you read Dawkin’s book “The Ancestor’s Tale”? He’s not always clear as to whether he is speaking for himself or for “the concensus”, but he list Afrotheres as the most distant outgroup of all the placentals (splitting off about 105M years ago). These include the Hyraxes, Elaphants and Manatees as one subgroup and the Aardvarks, Tenrecs and golden moles and Elephant shrews as another subgroup.

At 95M years ago, he has the Xenarthans splitting off the remaining branch-- Sloths, Anteaters and Armadillos.

At 85M years ago, he has the Laurasiatheres splitting off the remaining branch. This is a large grouping which includes the Insectavores (shrews, moles etc), the Bats, the Cetartiodactyla (camels, pigs, deer, sheep, hiippos, whales, etc.), the Perissodactyla (horses, tapirs, rhinos), the Carnivores (bears, cats, dogs, seals, walruses, etc.) and the Pagolins.

At 75M years ago, he has the Rodents and Rabits splitting off (with the rabbits splitting off from the rodents about 70M years ago).

At 70M years ago, he has the Tree shrews and Colugos (Flying Lemurs) splitting off.

At 63M years ago he has the primates splitting off. I assume you aren’t much interested in the groupings after that, although he does put all the Gibbons and Siamangs together since the Crested Gibbons form an outgroup wrt to the Siamangs and the two other Gibbon genera.

I won’t have time to respond to this in detail since I am posting from a library, but the link below gives some recent molecular and morphological trees (2001) for eutherian mammals. As far as I know, these are close to the most current consensus.

http://www.sciencemag.org/cgi/content/full/291/5509/1786/DC1

As John says, the first group to split is Xenarthra.

The next is a group composed of elephants, hyraxes, sea cows (previously usually associated) plus the Aardvark and elephant shrews.

Next a group composed of most of the traditional “insectivore” groups (not including elephant shrews or tree shrews).

Next rabbits/pikas and rodents.

Flying lemurs, tree shrews, and primates together are a sister-group to the remaining orders.

From the remaining orders, the next group to branch are the bats.

Next, a group containing the pangolins and carnivores.

Finally a group containing the ungulates and whales, with perissodactyls branching first, then cetaceans, and finally artiodactlyls.

So is there any consensus on where uintatheres might have fallen? The last thing I read some years ago indicated there wasn’t general agreement. IANAP, but I’d guess perissodactyls.

Actually, I had those two reversed in my post, but keep in mid that I was just quoting verbatim from Dawkin’s book. I don’t have any independent sources to validate his claims.

John and Colibri, many thanks! This is going to get interesting! :slight_smile:

Earl, look for Dinocerata, the ordinal name for uintatheres and their relatives among paleontologists. The last I saw anything, they were considered an outgroup to the Tethytheres (elephants, sirenians, and perhaps hyraxes), though that’sa 1988 hypothesis. Before that, they were associated with the “condylarths” (Paleocene herbivores no longer considered a valid grouping) and with suggestions of affiliations to the extinct South American ungulates.

Here’s a very recent (2006) article that discusses some of the most recent thinking on the subject:

Pegasoferae, an unexpected mammalian clade revealed by tracking ancient retroposon insertions

Incidentally, the derivation of the clade name in the above article is interesting:

A relationship between bats and rhinos is perhaps even more unexpected than that now established between gazelles and whales.

This looks pretty much like what Dawkins proposes. No?

I am not familiar with Dawkins’ book, but the association of perissodactlyls most closely with bats and carnivores as opposed to artiodactyls within the Laurasiathere clade is what is most novel. I don’t see that particular relationship mentioned in your post.

OK. I was looking only at the higher level grouping in your post #7. Dawkins’ diagram implies a hierarchical grouping within the Laurasiatheres* as follows:

Pholidota + Carnivora (75M years)
Perissodactyla (77M years)
Cetartiodactila (78M years)
Chiroptera (80M years)
Insectavora (81M years)

However, the relationships, drawn as a “time since the lineages split” diagram is pretty darn flat-- spanning about 6M years, out of a total of about 80M years. Some of these could easily flip if the data were skewed just slightly differently. And he doesn’t show the time of split between the Cetacea and the Artiodactyla.

*from page 193 of “The Ancestor’s Tale”

When multiple clades have branched off within a short period of time (a few million years), it may be very difficult or impossible to resolve the exact branching pattern, and the answer you get may depend on the exact part of the genetic code sequenced. Although major branches among mammals have probably been identified, the final word is probably not in yet.

The problem is even worse among birds; many of the branching points between orders have not been resolved by genetic data. Some avian paleontologists refer to this as the “Wall of Death” since many orders seem to branch at essentially the same point (depicted as a long horizontal line in cladograms).

Yeah, I was just reading dates off a graph, and there were no error bars. I presume those would be at least on the order of a few percent, which is well within the range of the differences between the values themselves. 1 or 2M years isn’t much of a swing when you’re looking at 75M years total.

Funny, I was just thinking: Hey, someone should ask **Colibri **the same question about birds. :slight_smile: