Current hypotheses of relationships among mammals
may cause one to develop a large cartoon question mark over one’s cranium. Traditional paleontologists still cannot recover the closest sisters to bats and they continue to suggest that hippos are the closest sisters to known pre-whales with legs.
very few fossil taxa are employed in these analyses (send them along if you know of any). DNA studies don’t completely match morphological studies and they ignore fossil taxa. Wikipedia sums up the current thinking dividing paleontologists from molecular biologists.
Molecular Biologists recover this tree (highly simplified):
- Atlantogenata =
Xenartha (sloth + anteater + armadillo)
Afrotheria (golden mole + elephant shrew + tenrec + aardvark + hyrax + elephant + sea cow) Afroinsectiphilia (golden mole + elephant shrew + tenrec),
- Boreoeutheria (Boreotheria) =
Euarchontoglires (rodent + rabbit + tree shrew + flying lemur + primate + Plesiadapis)
Laurasiatheria (hedge hog + mole + shrew + bat + pangolin + carnivores + odd-toed ungulates + even toed ungulates + whales)
Traditional paleontologists recover this tree (highly simplified):
- Xenarthra (sloth + anteater + armadillo)
- Afrotheria (hyrax + elephant + sea cow)
- Boreoeutheria (Boreotheria) =
Euarchontoglires (rodent + rabbit + tree shrew + elephant shrews + flying lemur + primate + Plesiadapis)
Laurasiatheria (hedge hog + mole + shrew + bat + pangolin + carnivores + odd-toed ungulates & aardvark + even toed ungulates & whales)
So there is broad agreement between the two camps.
The problem comes when one tries to replicate the experiment (Fig. 1). Adding mammal taxa to the large reptile tree delivers a different hypothesis of relationships. It preserves some traditional relationships and recovers some new ones. If you’re not sure why this keeps happening here, it happens on the professional level, too. Some prior workers employ suprageneric taxa. Others do not employ fossil taxa. I also wonder if there is substantial convergence in the dental traits of included taxa. Teeth vary greatly within the Mammalia.
The ReptileEvolution.com tree (built taxon by taxon)
presented here (Fig. 1) seems to make more sense (more sister taxa look similar to one another) with a basal split between large arboreal omnivores (that ultimately produced carnivores) and small arboreal omnivores (that ultimately produced large and small herbivores and some insectivores).
Similarities with traditional trees:
- Outgroups include cynodonts, monotremes and marsupials in that order.
- Eomaia is a basal placental (eutherian) despite retaining prepubic bones.
- Primates nest together and with a flying lemur
- Bats nest with carnivores
- Civets nest together
- Rodents nest with rabbits and one tree shrew
- Xenarthans nest together
- Ungulates nest together with Phenacodus (55 mya) at their base
- Elephant nests with hyrax
Differences with traditional trees:
- There are no geography-based divisions here, only morphology
- The basic division is between arboreal civet-like omnivores and arboreal rodent-like omnivores.
- Pangolin nests with primates and Plesiadapis does not
- Tree shrews are not a single clade. Tupaia nests with rodents, rabbits and Plesiadapis. Ptilocercus nests with flying lemur.
- Bats nest with a specific carnivore: Chriacus.
- Whales nest with tenrecs and pre-tenrecs, not ungulates
- Aardvark nests with armadillo derived from Pantolambda, which has a carnivore-like appearance).
- Ungulates nest with elephant + hyrax all derived from a sister to Phenacodus.
In the large reptile tree, all sister taxa share a long list of traits
and look similar to one another. You can’t say that about competing hypotheses which don’t always include fossil taxa. The mammals subset of the large reptile tree (725 taxa) has grown so far without tree topology changes. Taxa just drop in between existing nodes.
Very few tooth characters are used here.
And none distinguish one post-canine tooth from another other than [absent, blunt, sharp, multi cusp and multi cusp with constricted base] and another trait notes where the posterior maxillary tooth erupts relative to the orbit.
Wikipedia suggests that:
“Paleontologists naturally insist that fossil evidence must take priority over deductions from samples of the DNA of modern animals. More surprisingly, these new family trees have been criticised by other molecular phylogeneticists, sometimes quite harshly.”
We’ve come a long way…
Simpson 1945 was able to list several subclasses and orders, but was unable to show interrelationships. Novacek 1992 discussed several problems right before computer-assisted phylogenetic analysis came along.
Suggestions for pertinent mammal taxa that need to be added to the LRT?
Please, send them along.
Novacek MJ 1992. Mammalian phylogeny: shaking the tree. Nature Review Article. 356:121-125.
Simpson GG 1945. The principles of classification and a classification of mammals. Bulletin of the AMNH Bulletin of the American Museum of Natural History 85:1-350. online.
Songa S, Liub L, Edwards SW and Wub S 2012. Resolving conflict in eutherian mammal phylogeny using phylogenomics and the multi species coalescent model. PNAS 109 (37): 14942-14947.
Spaulding M, O’Leary MA and Gatesy, J 2009. Relationships of Cetacea (Artiodactyla) among mammals: Increased taxon sampling alters interpretations of key fossils and character evolution. PLoS ONE 4:e7062. doi:10.1371/journal.pone.0007062:1-14.