Maelestes: a very basal tenrec/whale from the Late Cretaceous

Wible et al. (2007)
described Maelestes gobiensis (PSS-MAE 607), as a Late Cretaceous (75-71mya, Fig. 1) eutherian nesting outside of the Placentalia (Fig. 2). Wikipedia reports epipubes were present, but they are not listed by Wible et al. who list: “an incomplete skull, left mandible, atlas, axis, twelve thoracic vertebrae, eight partial ribs, incomplete scapula, clavicle, humerus, proximal radius and ulna, and incomplete astragalus.”

Figure 1. Maelestes skull with original and distorted lateral view to occlude uppers with lowers and match other skull landmarks.

Figure 1. Maelestes skull with original and distorted lateral view to occlude uppers with lowers and match other skull landmarks. The original dentary break at p0 is repaired here to create a convex ventral dentary. A descending squamosal is added to articulate with the dentary. The posterior mandible drawing doesn’t match the bone.

In contrast
the large reptile tree (LRT) nests Maelestes between Asioryctes and Onychodectes at the base of the tenrecs (Fig. 3). The Wible team tested many more mammals (Fig. 2), some far from complete. The Wible team nested Maelestes within a few nodes of Asioryctes + Ukhaatherium in a cladogram lacking many modern taxa. This nesting somewhat matches the LRT, which includes Leptictis + Ukhaatherium and Andrewsarchus at the base of the tenrecs. In the Wible et al. tree Leptictis nested several nodes away, but still not in the Placentalia. The Wible et al. tree (Fig. 2) also nested the wombat, Zalambdalestes deep with the Eutheria, but also not within the placentalia. So, as before, Wible et al. are finding clades between marsupials and placentals while the LRT (Fig. 3) does not.

Figure 2. In gray scale, the Wible et al. cladogram of mammal relations. Pink added for Maelestes and tenrecs. Gray added to Asioryctitheria and Leptictis, another tenrec, as they are placentals in the LRT. Blue added for marsupials incorrectly included here.

Figure 2. In gray scale, the Wible et al. cladogram of mammal relations. Pink added for Maelestes and tenrecs. Gray added to Asioryctitheria and Leptictis, another tenrec, as they are placentals in the LRT. Blue added for marsupials incorrectly included here. Several of these taxa are untested in the LRT and so are not colored.

According to Wible et al.
DNA studies postulate the origin of placentals between 129-78 mya. “This controversy has yet to be addressed by a comprehensive phylogenetic analysis that includes all well-known Cretaceous fossils and a wide sample of morphology among Tertiary and recent placentals. Here we report the discovery of a new well-preserved mammal from the Late Cretaceous of Mongolia and a broad-scale phylogenetic analysis. Our results exclude Cretaceous fossils from Placentalia, place the origin of Placentalia near the K/T boundary in Laurasia.”

Figure 3. Subset of the LRT showing Maelestes nesting at the base of the tenrecs / whales between Asioryctes and Onychodectes.

Figure 3. Subset of the LRT showing Maelestes nesting at the base of the tenrecs / whales between Asioryctes and Onychodectes.

By contrast,
the earliest placentals in the LRT include  Megaconus (170 mya) and the multituberculates. Based on comparisons to  Monodelphis and Ptilocercus the skull shape and size of Maelestes is more primitive than any of the above named Early Cretaceous through Jurassic placentals.

Molar count
Wible et al. count only three molars for the uppers and lower. I see four uppers and three lowers. Molar number is not a scored trait in the LRT. It just rides along with the other scored traits. Asioryctes has four upper and lower molars. Onychodectes also has four uppers. The lower molar data is not available at present.

To their credit
and distinct from several prior stuides, Wible et al. nest Carnivora as basal placentals and Xenarthra as derived and close to Paenunugulata (elephants and kin) matching the LRT. Unfortunately, they also nest whales with artiodactyls as sisters to Carnivora, which does not match the LRT.

Figure 4. Cimolestid jaws compared to Maelestes. Perhaps all of these taxa are basal tenrecs. They have not been tested yet.

Figure 4. Cimolestid jaws compared to Maelestes. Perhaps all of these taxa are basal tenrecs. They have not been tested yet.

References
Wible JR, Rougier GW, Novacek MJ and Asher RJ 2007. Cretaceous eutherians and Laurasian origin for placental mammals near the K/T boundary.” Nature, 447: 1003-1006.

wiki/Maelestes

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