Modified August 21, 2018 with the note that a procoracoid and coracoid were likely present in Jurassic rodents. These traits appear to be atavisms since taxa between prototheres and Jurassic rodents do not have these bones.
Another case of taxon exclusion…remedied.
Earlier we looked at the Jurassic mammal, Maiopatagium, a putative glider, surrounded by a deep halo of long, straight hair. Meng et al. 2017 nested Maiopatagium between Sinoconodon and Haldanodon, taxa more primitive than mammals.
Now with 24 more taxa,
and several new ones from the rodent clade, the LRT nests Maiopatagium with the only tested porcupine, the small arboreal Coendou.
With this nesting
that halo of long straight hair on Maiopatagium
(Fig. 4) takes on a new identity as a pelage of still soft pre-quills, similar to a closely related taxon, Chinchilla, nesting with a former enigma taxon, Neoreomys.
To no one’s surprise,
the guinea pig (genus: Cavia) nests with the pig-sized capybara (genus: Hydrochoerus). All but Maiopatagium are widely recognized members of the Hystricomorpha clade of rodents. The presence of Maiopatagium in this rodent clade supports the previously reported Jurassic radiation and dispersal of rodents (Fig. 1) currently represented by a few specimens not widely recognized as rodents, nor tested against rodents. Porcupines and chinchillas were not in the Meng et al. taxon list.
Shifting Maiopatagium in the LRT to Sinoconodon adds 49 steps. Shifting to the more primitive Haldanodon adds 58 steps.
Distinct from all extant and extinct rodents Maiopatagium was reported to have a small coracoid and pro-coracoid, traits that disappear in therian mammals. This could be an atavism (= reversal) or it could be a misinterpretation of a crushed process of the scapula that appears in other hystricomorphs (Fig. 2). Vilevolodon has a protothere-like pro-coracoid and coracoid and it is medial to the scapula, not lateral as shown below. Interesting that similar structures appeared medial and lateral to the shoulder joint by convergence.
When the skull of Maiopatagium
nests with rodents we should consider the possibility that it may have included a large braincase (Fig. 3) not figured or restored by Meng et al. 2017 (Fig. 4).
I was never able to see the gliding membrane
distinct from the halo of long hairs on Maiopatagium (Fig. 4) as described by Meng et al. 2017. No related taxa in the LRT are gliders.
The porcupine Coendou prehensilis
(Fig. 5) is the closest living relative to Maiopatagium in the LRT. Yes, the tooth shapes are distinctly different, but tooth shapes are highly variable and these taxa are separated by 160 million years. The limbs are longer in the Jurassic taxon and the hair has not yet turned into quills. The LRT does not test every trait. However, traits in the LRT nest Maiopatagium as a primitive porcupine and less likely to glide than originally figured.
The side-by-side alignment of the calcaneum and astragalus
figured by Meng et al. (Fig. 4) is yet another pre-therian trait (see Eomaia for the first shift to the therian state). Rodents don’t have this type of ankle (Fig. 5), so when you see it in the rodent clade we might count this as an atavism… possibly because Maiopatagium could have been hanging from branches or descending tree trunks head first and rotating the ankle, as squirrels do. The other possibility is a misinterpretation of the tarsals by Meng et al. An alternate reconstruction is shown here (Fig. 6).
In the porcupine pes,
please note the large flat bone arising from the medial tarsals (Fig. 5). The chinchilla does not have this disk, but Maiopatagium does (Fig. 6). It is an atavism arising from digit zero on the pes. Atavisms like this form the spur on the screamers.
Added almost a day later:
the pes of another specimen, BMNH1133 (from Meng et al. 2017, Fig. 7) compared to Rattus the rat. Pretty similar when reconstructed, aren’t they?
Kermack KA, Kermack DM, Lees PM and Mills JRE 1998. New multituberculate-like teeth from the Middle Jurassic of England. Acta Palaeontologica Polonica 43(4):581-606.
Meng Q-J, Grossnickle DM, Liu D, Zhang Y-G, Neander AI, Ji Q and Luo Z-X 2017.
New gliding mammaliaforms from the Jurassic. Nature (advance online publication)