Juramaia: Jurassic placental or monotreme?

Luo et al. 2011
brought us Juramaia sinensis, hailed as a Middle Late Jurassic (164mya) eutherian mammal. They said Juramaia “extends the first appearance of the eutherian–placental clade by about 35Myr from the previous record.” 

Figure 2. Juramaia (Late Jurassic, 160 mya) is more completely known and nests between monotremes and therians (marsupials + placentals).

Figure 1. Juramaia (Late Jurassic, 160 mya) is nests between monotremes and therians (marsupials + placentals) on the monotreme side in the LRT. (Beijing Museum of Natural History BMNH PM1143).

By contrast
the large reptile tree (LRT, 1265 taxa) nest Juramaia at the base of the Prototheria (egg-laying mammals). Nesting at the base also means it is a monotreme sister to all Therian mammals, including the equally long-snouted and long-legged Ukhaatherium (late survivor in the Late Cretaceous; Novacek et al. 1997) at the base of all therians. Ukhaatherium is traditionally considered a member of the Asioryctidae, close to placentals, but with epipubic bones.

The emergence of placentals in the LRT
is estimated to have occurred in the Early to Middle Jurassic, based on the Late Jurassic appearance of Shenshou, Maopatagium, and Vilevolodon, highly derived members of the Glires nesting with rodents.

The basalmost placental in the LRT,
the extant Caluromys, still has a pouch. But remember, it’s not one trait or two that tells you what a taxon is, but where the taxon nests in the cladogram. That’s why we have amphibian-like reptiles, mammal-like reptiles and the former ‘amphibian’ Diadectes nesting as a derived millerettid in the LRT. Remember, earlier we noted that the marsupial open pouch was enlarged and co-opted to form a nursery, then a parachute in members of the Volitantia, another basal placental clade. So we expect to see some sort of pouch in basalmost Eutherians, so it carries over into Volitantia.

Figure 2. Subset of the LRT focusing on basal Mammalia and the nesting of Juramaia with prototheres, not placentals.

Figure 2. Subset of the LRT focusing on basal Mammalia and the nesting of Juramaia with prototheres, not placentals. Compare to Luo et al. in figure 3.

Let’s take a closer look at Juramaia.
Luo et al. report, “This mammal has scansorial forelimb features, and provides the ancestral condition for dental and other anatomical features of eutherians.”

  1. Juramaia has 5 premolars and 3 molars, typical for Cretaceous eutherians. AND also typical of Jurassic prototherians. Evidently these were not considered, some of which postdate 2011.
  2. A long list of dental traits that the LRT does not consider. These must be basal to all mammals but lost in extant prototheres.
  3. Competing cladograms, including one with dozens of tooth-shape traits. The LRT has relatively few of those, so scores are not weighted toward dentition, which tends to converge in unrelated taxa.
  4. Juramaia is more closely related to extant placentals than all metatherians of the Cretaceous including Sinodelphys and Deltatheridium. In the LRT Sinodelphys also nests as a protothere. Deltatheridium nests as an unrelated derived marsupial carnivore. Juramaia nests closer to placentals in the LRT, too, but not closer than the small, tail-hanging didelphids, like Caluromys.

Competing cladograms (Fig. 3)

  1. Both the Luo et al. and LRT cladograms use or can use the cynodont, Thrinaxodon, and a number of other cynodonts as basal taxa.
  2. Luo et al. nest Haldanodon and Castorocauda as mammals within Megazostrodon. The LRT does not.
  3. Luo et al. include several mandible-only taxa. The LRT has very few.
  4. About half of the taxa appear to conform between the two cladograms (Fig. 3) demonstrating how powerful convergent dental traits can be. As some higher mammals lose their Y- and W-cusp-shapes, their cusps and/or teeth and end up with simple rows of cusps or single cusps.
  5. The LRT has the advantage of employing taxa from the last seven years unavailable to Luo et al. 2011. It is worthwhile to note the long list of pertinent taxa known from more or less complete skeletons in the LRT not employed by Luo et al.
  6. In the Luo et al. cladogram, metatherians and eutherians both arise from a last common ancestor, Aegialodon (Early Cretaceous, known only from teeth). In the LRT, eutherians arise from mink-sized arboreal metatherians with exposed nipples and open pouches.
  7. In the Luo et al. cladogram Vincelestes is an egg-laying mammal with maxillae that contact each other dorsally. In the LRT this fanged predator nests with other fanged predators with maxilla that contact each other dorsally, like Thylacosmilus.
Figure 2. On the left, the Luo et al. 2011 cladogram, colorized here for clarity. On the right clade colors are applied to Luo et al. taxa according to their nesting in the LRT. Not much correspondence, except on more completely known taxa. The LRT only tests more completely known taxa. 

Figure 3. On the left, the Luo et al. 2011 cladogram, colorized here for clarity. On the right clade colors are applied to Luo et al. taxa according to their nesting in the LRT. Not much correspondence, except on more completely known taxa. The LRT only tests more completely known taxa.

References
Luo Z-X, Yuan C-X, Men Q-J and JiQ 2011. A Jurassic eutherian mammal and divergence of marsupials and placentals. Nature 476: 442–445. doi:10.1038/nature10291.

Novacek MJ, Rogier GW, Wible JR, McKenna MC, Dashzev g D and Horovitz I 1997. Epipubic bones in eutherian mammals from the Late Cretaceous of Mongolia Nature 389: 483-486.

wiki/Juramaia

wiki/Ukhaatherium

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