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The phylogeny of mammals is dictated largely by their morphology, with molecular studies providing support (in most cases). The anatomy of placentals is therefore integral to elucidating phylogenetic relationships. When running a cladistic analysis anatomical and morphological features are coded as characters and each taxon given a number that corresponds to whether the character is present or absent. This method determines synapomorphies and symplesiomorphies in each resultant clade. The figure below shows the currently accepted phylogeny of the Placentalia (adapted from Benton, 2005) with the morphological synapomorphies that define each clade.
The following list of synapomorphies for each node of the cladogram are based on molecular and morphological studies from Novacek et al (1988).
A PLACENTALIA
Chorioallantoic placenta, prolonged gestation in uterus, median vagina, epipubic bones and pouch absent, shell membrane absent, narrow stylar shelves on upper molars, optic foramen widely spearated from sphenorbital fissure
B AFROTHERIA
No synapomorphies identified (except perhaps the trunk )
C No synapomorphies identified
D No synapomoprhies identified
E PAENUNGULATA
Amastoidy (mastoid process concealed by expansion and overlap of the squamosal), jugal extends posteriorly as a prominent ventral crest to anterolateral border of the glenoid fossa, carpals dorsoventrally compressed and serially arranged
F TETHYTHERIA
Bilophodont cheek teeth with tendancy to to form additional lobe on posterior part of cingulum, forward displacement of orbits, infraorbital canal very short, zygomatic process of squamosal robust and extends dorsally and laterally, premaxilla with strong posterior process extedning around reduced nasals and nearly contacting frontals
G No synapomorphies identified
H BOREOEUTHERIA
No synapomorphies identified
I LAURASIATHERIA
No synapomorphies identified
J No synapomorphies identified
K FERUNGULATA
No synapomorphies identified
L CETARTIODACTYLA
Trochlea (groove) on navicular bone in ankle, narrow calcaneum and elongate heel process
M No synapomorphies identified
N No synapomorphies identified
O EUARCHONTOGLIRES
P ARCHONTA
Sustentacular facet of astragalus in distinct medial contact with distal astragalar facets, pendulous penis suspended by reduced sheath between genital pouch and abdomen
Q No synapomorphies identified
R GLIRES
Posterior process of premaxilla long and contacts frontal, maxilla does not contact frontal, premaxilla and maxilla equally exposed in palate, glenoid fossa (jaw joint) set well dorsally of basicranium, upper and lower first incisor teeth absent, ever-growing incisors.
Author: Emma-Louise Nicholls
Last updated: 20th November 2005
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