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Weird mammal cladograms, and dino implications
Despite the mammalian orientation of this post, I get to dinos at the end...
Just in to our library: the March 1999 (Vol. 48, Number 1) issue of
Systematic Biology. It features papers from the Symposium on the Origin of
Mammalian Orders, held in Hayama, Japan last July. Many interesting papers,
mostly molecular but some paleo/morphological. The summary paper is:
Waddell, P.J., Okada, N. & Hasegawa, M. 1999. Towards resolving the
interordinal relationships of placental mammals. Syst. Biol. 48:1-5.
The "best estimate" cladogram of this symposium is shown on the cover (this
is not a cladogram generated by a particular analysis, but their summary of
the papers presented. Not something I would like to see made the cover of a
journal devoted to explicit analyses of phylogenetic relationships, but I'm
not the editor...).
In any case, there are some weird things going on in placental
phylogenetics. Many molecular, and some morphological, studies are breaking
up the McKenna-type placental cladogram many of us are familiar with:
Xenarthra + (Insectivora + Glires (rodents & lagomorphs) + Ferae (Carnivora
+ Creodonta) + Archonta (primates, bats, tree shrews & dermopterans) +
Ungulata, with ungulates containing artiodactyls, whales, perissodactyls,
hyraxes and tethytheres (elephants, manatees, and extinct relatives).
The "new" view includes some big changes. The basal split is not between
xenarthrans and epitheres, but between (the ancestrally Gondwanan?) clade
Atlantogenata and a second assemblage containg Glires, Euarchonta (old
Archonta minus the bats), and the new Laurasiatheria. Atlantogenates are
the (ancestrally South American?) Xenarthra (sloths, anteaters, armadillos)
and the recently proposed Afrotheria, containing the golden mole-tenrec
clade Afrosoricida, elephant shrews, aardvarks, hyaxes, and tethytheres.
The laurasiatheres include many of the remaining traditional insectivores
(true moles, true shrews, soleodons, etc., but hedgehogs may lie outside all
other placentals!), bats, the cetartiodactyls (more below), and the new
Zooamata ("animal friends"): perissodactyls, carnivorans, and pangolins.
"Cetartiodacyla" is a name that wasn't needed: it is about as necessary as a
name Avireptilia. It might have been more useful to simply say that Cetacea
is a clade within Artiodactyla, which was one of the main points of
concensus from this meeting. The new, and weird, view from a lot of studies
seems to be camels + (pigs & peccaries + (ruminants + (hippos + whales))).
The hippo + whale clade is formally named Whippomorpha, in honor of John
Gatsey et al.'s whimsical term "whippo". Incidentally, Thewissen & Madar
have a paper in the issue describing the ankle of primitive cetaceans for
the first time, and they share more derived features with modern
artiodactyls than with the extinct mesonychians, long thought to be the
sister taxon/ancestral grade of whales.
Not addressed in these papers, and something I'd like to see new analyses of
in light of these topologies, are: the phylogenetic position of the South
American "ungulates": within Ungulata gone, where the heck do they go?
Sister group to Xenarthra would be too much to ask...; what about other
extinct "ungulates": phenacodonts, arctocyonids, etc.?
Okay, assuming the tree they give is a reasonable approximation of the One
Tree, what does it mean for dinosaur paleo?
Something that is annoying with this analysis: when Chiroptera was a firm
member of Archonta, we had the strongest case for arboreal origin of flight
in the history of life. Archontans are as a clade arboreal ancestrally
(indeed, the only really major clade of living placentals that can be
characterized as such), and there was even a nice functional series in the
form of tree shrews -> highly cursorial primates -> patagium-gliding
dermopterans -> bats. Now bats are not clearly nested among arboreal forms.
I would expect most people would still agree that bat flight probably began
from the trees down, but it was a lot nicer when they were grouped among all
the other real tree-climbing specialists.
Something that is appealing with this analysis: the evolution of placentals
as told by this tree more closely maps the paleogeographic history of
Pangaea than the traditional version. In its basic form, the break up of
Birth of North Atlantic, separating Laurasia (North America, Europe, Asia)
from Gondwana (southern continent). Divisions in Gondwana of South American
+ Africa from Indo-Madagascar from Antarctica-Australasia, then each of
those split (SAm & Africa in mid Cretaceous, India from Madagascar in Late
K, Antarctica from Australasia in late Eocene). Early Cenozoic sees faunal
links from Europe to North America from Greenland and Asia to North America
over the Bering land bridge. The former loses connection early on in the
Cenozoic, while the latter is still active during oceanic lowstands (i.e.,
during glacial advances).
This new tree shows a fundamental split between Gondwanan and Laurasian
clades, and within Gondwana between South American and African forms. Of
course, it isn't absolute: there are some important groups of
"laurasiatheres" in India (at least) during the Eocene (whales, for
instance!). Still, there is at initial glance a closer correspondance
between tree patterns and paleogeographic patterns in placental mammals (and
in recent studies of metatheres: marsupials and their extinct relatives)
than in dinosaurs. This probably has a lot to do with the fact that
placental and metathere diversification occurred much later than the
fundamental splits between the major groups of dinosaurs. Not to say that
dinosaur faunas don't reflect some aspects of paleogeography: they do, but
these seem to have as much to do with differential survival of once-global
populations or post-split origination of new groups as with more classical
vicariance biogeography (i.e., ancestral species becoming two separate
diverging lineages post-rifting).
Just some initial thoughts...
Thomas R. Holtz, Jr.
Vertebrate Paleontologist Webpage: http://www.geol.umd.edu
Dept. of Geology Email:firstname.lastname@example.org
University of Maryland Phone:301-405-4084
College Park, MD 20742 Fax: 301-314-9661