Musings on docodonts, monotremes and others
I always think about Mesozoic synapsids. If you don’t then fuck off to a Spinosaurus video or something. Anyways, three things in particular have been in my mind for a while:
- Apparently the traditional layout of mammalian evolution is out of whack. The highly specialised cranial anatomies of monotremes and allotheres apparently make their placement extremely hard, and as such relations among the various groups are rendered tenuous.
- Apparently “australosphenidans” have only three molars while the most basal monotreme, Teinolophos has five. Due to Dollo’s Law this makes it unlikely that monotremes are descendents from ‘australosphenidans’, though numbats may offer an example of more teeth being acquired being a possibility. Also, the authors of this paper ignored more recent papers that Kollikodon is actually a haramiyidan, so…
- Docodonts don’t show up beofre the Jurassic. While a ghost lineage is plausible, it is also likely that they are part of the Jurassic mammal radiation that included therians, monotremes, multituberculates, eutriconodonts among others. Do keep in mind that genetic studies overtly favor a split of crown Mammalian (monotremes and therians) in the Early Jurassic…
So A) the traditional mammalian cladogram is out of whack, B) monotremes may not be australosphenidans, C) docodonts could be crown-mammals.
You know what? I’m willing to wager that docodonts are stem-monotremes, particularly Shuotheriids (traditionally, but not always, considered related to monotremes) may actually be docodonts. Most have high molar numbers (up to six in some taxa) and they share losts of post-cranial anatomy and petrosal morphology.
With the possible exception of Gondtherium, all docodonts are northern hesmiphere animals. However, the fossil reccord of the southern continents during the Mesozoic remains patchy, seemingly including derived members of usually considered northern clades such as Corriebaatar, so southern docodonts may wait discovery. Likewise the “australosphenidans” have frequently been interpreted as stem-therians or even eutherians, and the Saint Bathans mammal seems to be closer to therians than to monotremes (albeit no recent studies have been done on it for a while), so this would be quite ironic.
Docodonts have what is called “pseudo-tribosphenic” teeth, similar enough to the tribosphenic teeth of crown mammals aside from a few cusp and talonid arrangements. Tribosphenic teeth could have evolved independently among mammals, but them being a synapomorphy of crown mammals would be pretty neat, especially given all the whacky alternatives like what allotheres came up with.
Part of me wants a monotreme + allothere clade, especially since both groups have deep history in the southern continents. But I think docodonts being stem platypi is the more realistic scenario, for now at least.
