THE THERIAN CLADE

Monotremata 

1. Pascual, R., M. Archer, E.O. Jaureguizar, J.L. Prado, H. Godthelp, & S.J. Hand. 1992. First discovery of monotremes in South America. Nature 356:704-706. 
  Early Paleocene monotremes from Argentina are first record for Order outside of Australia, and provide evidence for a Gondwanan distribution of monotremes.
Marsupialia  2. Eaton, J.G. 1993. Marsupial dispersal. National Geographic Research & Exploration 9(4):436-443. 
  Succinct summary of geography of marsupial evolution and diversification.
Metathere-Euthere dichotomy 

[see papers in reader by Lillegraven and Hayssen et al.] 

3. Lillegraven, J.A., S.D. Thompson, B.K. McNab, and J.L. Patton. 1987. The origin of eutherian mammals. Biol. Jour. Linnean Soc. 32:281-336. 
  Summarizes biological (reproductive, energetic, behavioral, etc.) differences between metatheres and eutheres, and provides a 3-step hypothesis for derivation of eutherian condition from its common ancestor with metatheres.    4. Rougier, G. W., J. R. Wible, and M. J. Novacek. 1998. Implications of Deltatheridium specimens for early metatherian history. Nature 396: 459-463. 
  Asian Late Cretaceous group basal metatherians, but probably removed from the common ancestor of all living metatherians, and certainly not basal to crown-group Marsupialia. Does not provide support for mid-Cretaceous divergence dates for Marsupialia based on molecular data.   
Eutheres 

[in addition to paper by Novacek from your reader, the following are good statements of relationships among extant and extinct eutherian orders]: 

5. Benton, M.J. 1988. The relationships of the major groups of mammals: mew approaches. Trends in Ecology and Evolution 3:40-45. 

6. Szalay, F.S., M.J. Novacek, and M.C. McKenna (eds.). 1993. Mammal Phylogeny. Placentals. Springer-Verlag, New York. 
 

21 chapters detailing relationships among various groups of eutherian orders based on molecular and/or morphological data. Single most up-to-date reference available.    7. Simmons, N.B. 1993. The Importance of Methods: Archontan phylogeny and cladistic analysis of morphological data. In R.D.E. MacPhee (ed.) Primates and their relatives in phylogenetic perspective. Planum Press, New York. 
  Extremely thorough analysis of archontan (Primates, Scandentia, Dermoptera, and Chiroptera) relationships, with evaluation of hypothesis of chiropteran diphyly.    8. Meng, J., A.R. Wyss, M.R. Dawson, and R. Zhal. 1994. Primitive fossil rodent from Inner Mongolia and its implications for mammalian phylogeny. Nature 370:134-136. 
  Late Paleocene fossil confirms rodent monophyly (debated by interpretation of some molecular data) and the sister-group relationship of Rodentia and Lagomorpha (Simpson's Cohort Glires).
Time of diversification  9. Hedges, S.B., P. H. Parker, C. G. Sibley, and S. Kumar. 1996. Continental breakup and the ordinal diversification of birds and mammals. Nature 381: 226-229. 
  Estimates times of divergence among mammalian orders with a comprehensive set of genes that exhibit a constant rate of substitution, and argue that these molecular estimates place ordinal diversification between 120 and 100 million years ago, a time-period coincidental with continental breakup, and thus suggesting a causal linkage between that breakup and ordinal diversification.    10. Bromham, L., M. J. Phillips, and D. Penny. 1999. Growing up with dinosaurs: molecular dates and the mammalian radiation. Trends in Ecology and Evolution 14: 113-118. 
  Critiques the molecular data, and methods of analysis, that support the radiation of modern mammalian orders in the Cretaceous, long before the final extinction of the dinosaurs.
 
 
 
 
 
In the accompanying figures, we have represented hypothesized phylogenetic relationships among the orders of extant eutherian (or "placental") mammals, based on the classifications of Simpson (1945) and McKenna and Bell (1997).  We also provide a hypothesized phylogeny of living orders based on a consensus of molecular data published recently in the journal Systematic Biology (volume 48, number 1, March 1999). 
 
 
Simpson (1945)  -  remember that Simpson's classification is Linnean-based and reflects both ancestor-descent relationships and shifts in way of life (change of adaptive zone).  He also believed, in part based on the available fossil record at the time of his work, that modern eutherian mammals radiated explosively following the dinosaur extinction at the Cretaceous-Paleocene (K-T) boundary about 65 million years ago. 
 

 

 
 
 

 

 

McKenna and Bell (1997)  -  their classification is a node-based one, wherein the phylogeny can be read directly from the classification itself.  In theory, all nodes in the tree should be resolved into dichotomous branches but, in reality, some nodes remain polytomies because hierarchical relationships have not as yet become established.  Only Magnorder and Grandorder levels in their classification are given, and the ordinal names used are those that largely follow Simpson because of the familiarity of these names. 
 
 
 
 
 
The consensus molecular tree is "the best estimate of placental relationships based on our own extensive analyses of published and unpublished data" (Waddell et al., 1999, Syst. Biol. 48:1-5). There are three major branches in this molecular-based hypothesis, as follows: 
 

 

[1] The Atlantogenata (a newly named clade that couples the southern hemisphere South American Xenarthrans [sloths, anteaters, and armadillos] with the largely African golden moles [Chrysochloridae] and tenrecs [Tenricidae], elephant shrews [Macroscelidea], aardvarks [Tubulidentata], and Paenungulata [elephants (Proboscidea), dugongs and sea-cows (Sirenia), and hyraxes (Hyracoidea)]. This African clade is called the Afrotheria. The term Atlantogenata is based on the hypothesis that the opening of the Atlantic Ocean, and thus the breaking up of Gondwanaland, initially isolated one branch of this lineage in South America and another in Africa. The linkage of the African groups as a single phylogenetic clade is a unique perspective derived from molecular data; this hypothesis has not been proposed from morphological analyses. 
 

 

 

[2] An unnamed clade that consists of the Glires [Rodentia and Lagomorphs] and the typical archontan orders exclusive of the bats [tree shrews (Scandentia), and colugos (Dermoptera) plus primates (Primates)]. The archantan clade is termed Euarchonta to reflect the exclusion of the bats, and dermopterans and primates are collectively named the Primatomorpha to reflect their sister relationship. 
 

 

[3] The Laurasiatheria (so named because of the largely northern hemisphere origin and distribution) is a fully resolved clade with "true" lipotyphlans (Eulipotyphyla: shrews [Soricidae], moles [Talpidae], and hedgehogs [Erinaceidae]) on the first branch, followed by bats (Chiroptera), then the combination of cetaceans and artiodactyls (Cetartiodactyla) reflecting the molecular linkage of whales and hippos, then the odd-toed ungulates (Perissodactyla), and finally the pangolins (Pholidota) and carnivorans (Carnivora, including the Pinnipedia). 

 

 

Major deviations of this set of implied relationships from those most commonly hypothesized now include the following:  (1) cetaceans are the sister to a particular lineage [hippos] within artiodactyls, hence the use of the ordinal name "Cetartiodactyla"; (2) chiropterans are part of a larger group that includes the perissodactyls, carnivores, and cetartiodactyls, rather than part of the Archonta (with primates, dermopterans, and tree shrews); (3) and linkage of African taxa (the golden moles [Chrysochloridae] and tenrecs [Tenricidae] plus the elephant shrews [Macroscelidea] and aardvark [Tubulidentata]) along with elephants, sirenians, and hyraxes into a single lineage called the Afrotheria.