Basal placentals compared
When radiations occur
basal clade members tend to visually disperse on a cladogram grpahic as more and more taxa in each clade are added, just as branch tips on a real tree tend to disperse as they grow older and add more leaves.
On occasion it’s a good idea
to prune = delete the derived taxa in order to go back in time to when just the basal taxa existed. Graphically that brings basal branches back together facilitating comparisons (Fig 1). This helps one ‘eyeball’ trends that lump and split taxa.
Figure 1. Basal placental skeletons. Those in blue are proximal outgroups within the Marsupialia.
” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/06/leptictis-notharctus588-1.jpg?w=74″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/06/leptictis-notharctus588-1.jpg?w=253″ class=”size-full wp-image-86448″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/06/leptictis-notharctus588-1.jpg” alt=”Figure 1. Basal placental skeletons. Those in blue are proximal outgroups within the Marsupialia.” width=”584″ height=”2360″ />
Figure 1. Basal placental skeletons. Those in blue zone taxa are proximal outgroups within the Marsupialia. Arrows point in the direction of the clade Carnivora (above) and other basal placental clades (below).
Basal placentals likely lived and radiated
in the Middle Jurassic according to results recovered in the LRT, despite a paucity of fossils in the Mesozoic. None of these (Figs 1–4) are Mesozoic taxa, but we have Jurassic and Cretaceous fossils of their descendants.
But wait. this all gets much more interesting…
Figure 2. Basal placental skulls to scale.
” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/06/basal-placentalia-skulls588.jpg?w=252″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/06/basal-placentalia-skulls588.jpg?w=584″ class=”size-full wp-image-86449″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/06/basal-placentalia-skulls588.jpg” alt=”Figure 2. Basal placental skulls to scale. ” width=”584″ height=”695″ />
Figure 2. Basal placental skulls to scale. Placentals on the left tend to have a shorter snout and fewer teeth. Microcebus, the bat, is a miniaturized exception, retaining ‘cute’ juvenile traits into adulthood.
Along the way, I happened to see an overlooked resemblance
between the basal primate, Notharctus, and the basal condylarth Phenacodus previously linked to the oreodont, Merycoidodon (Figs 3, 4). Note the shared postorbital bar and every other skull element shared among these three previously unlinked taxa.
This is one hypothesis of what happened when dinosaurs disappeared and mammals could finally climb down from the trees.
Figure 3. Notharctus, Phenacodus and Merycoidodon skeletons compared.
” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/06/phenacodus.notharctus_skeletons588.jpg?w=203″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/06/phenacodus.notharctus_skeletons588.jpg?w=584″ class=”size-full wp-image-86456″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/06/phenacodus.notharctus_skeletons588.jpg” alt=”Figure 3. Notharctus, Phenacodus and Merycoidodon skeletons compared. ” width=”584″ height=”861″ />
Figure 3. Notharctus, Phenacodus and Merycoidodon skeletons compared.
Placentals always had two radiations.
One involved arboreal and burrowing nocturnal taxa starting at their origin in the Late Triassic. The other radiation, as everyone knows, is the one that involved the evolution of terrestrial taxa after the asteroid.
Figure 4. Notharctus, Phenacodus and Merycoidodon skulls compared.
” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/06/phenacodus_notharctus_skull588.jpg?w=129″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/06/phenacodus_notharctus_skull588.jpg?w=441″ class=”size-full wp-image-86458″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/06/phenacodus_notharctus_skull588.jpg” alt=”Figure 4. Notharctus, Phenacodus and Merycoidodon skulls compared. ” width=”584″ height=”1356″ />
Figure 4. Notharctus, Phenacodus and Merycoidodon skulls compared. These taxa have not been linked previously, despite their resemblance to one another.
So…
this is why mammal systematics and taxonomy have been difficult to understand. Today’s insight is a step in the right direction. The work is not done yet. Far from it.
At least you can see what issues have to be resolved.
References
Unfortunately, there has not been much attention paid to the transition between marsupials and placentals previously. Likewise the evolution of terrestrial herbivorous placentals with hooves has received scant attention in trait-based analyses.
Source: https://pterosaurheresies.wordpress.com/2024/06/06/basal-placentals-compared/
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