Numbat genesis in the Early Jurassic

Coelocanth. Tuatara. Numbat.
Name three taxa that have not changed much in hundreds of millions of years.

Figure 1. Myrmecobius, the living numbat, has remained essentially unchanged for nearly 200 million years.

Figure 1. Myrmecobius, the living numbat, has remained essentially unchanged for nearly 200 million years based on the LRT. Note the loss of posterior molars and the simplification of the remaining anterior molars. Orange arrow point to palatal pits that receive the long lower canines.

Extant numbats
(genus: Myrmecobius, Fig. 1) nest in the large reptile tree (LRT, 1412 taxa; subset Fig. 2) basal to Early Cretaceous Anebodon, Middle Jurassic Docofossor and the extant marsupial mole (genus: Notoryctes). All arise from the extant Dasycercus (Fig. 3). So that provides an interesting cladogram with members that span from the Triassic to the present. That means some extant taxa had nearly identical ancestors that shared the planet with the first dinosaurs and pterosaurs.

Figure 2. Subset of the large reptile tree focusing on the basal phytometatheria, including extant numbats, basal to Middle Jurassic Docofossor.

Figure 2. Subset of the large reptile tree focusing on the basal phytometatheria, including extant numbats (Myrmecobius), basal to Middle Jurassic Docofossor.

Myrmecobius fasciatus (Waterhouse 1841) is the extant numbat. Here it nests between Dasycercus and Anebodon. Since an ancestral taxon, Docofossor, is known from the Middle Jurassic, a sister to Myrmecobius had its genesis in the Early Jurassic. The molars are narrow and simplified. This is a marsupial termite eater, convergent with placental termite- and ant-eaters. Over each orbit is the reappearance of an old bone, the postfrontal. The canine is smaller. The jugal is straighter.

Figure 5. Dasycercus, the extant mulgara, is the carnivorous phylogenetic ancestor to the clade that includes numbats, Docofossor and kin in the LRT.

Figure 3. Dasycercus, the extant mulgara, is the carnivorous phylogenetic ancestor to the clade that includes numbats, Docofossor and kin in the LRT.

What’s interesting are the molars in Myrmecobius.
Take a good look (Fig. 1). The molars are narrow and simplified because this taxon eats termites (or vice versa). A phylogenetic descendant, Docofossor (Fig. 5) was considered a docodont based on its simple tooth morphology. Another phylogenetic descendant, Anebodon, was considered a symmetrodont based on its tooth morphology.

The LRT results remind us
not to put so much emphasis on tooth morphology. The LRT makes mammal systematics so much simpler by nesting taxa according to all their tested traits, not just a few, rather plastic, dental traits.

Figure 4. Dasycercus in vivo. This is the extant mulgara, a carnivorous nocturnal basal marsupial.

Figure 4. Dasycercus in vivo. This is the extant mulgara, a carnivorous nocturnal basal phytomarsupial with origins in the Early Jurassic.

Dasycercus cristicauda (originally ‘Chaetocercus‘ Krefft 1867; Peters 1875; 22cm + 13 cm tail) is the extant mulgara, considered a dasyurid marsupial. Here carnivorous, nocturnal Dasycercus nests apart from Dasyurus between Anebodon and Myrmecobius at the base of the herbivorous clade of marsupials. The pouch is reduced to two lateral folds of skin.

Figure 1. Docofossor in situ with DGS tracings.

Figure 5. Docofossor in situ with DGS tracings. This Middle Jurassic taxon nests as a derived descendant of Dasycercus and Myrmecobius in the LRT.

Docofossor brachydactylus (Luo et al. 2015; Middle Jurassic, 160 mya; BMNH 131735; 9cm in precaudal length) was originally considered a member of the Docodontidae along with Docodon and Haldanodon outside of the Mammalia. Here it nests as a Jurassic sister to Anebodon and Notoryctes. Broad, short-fingered hands, larger than the feet, along with other traits mark Docofossor as a digging animal, similar to moles like Talpa and Chrysochloris.


References
Bi S-D, heng X-T, Meng J, Wang X-L, Robinson N and Davis B 2016. A new symmetrodont mammal (Trechnotheria: Zhangheotheriidae) from the Early Cretaceous of China and trechnotherian character evolution. Nature Scientific Reports 6:26668 DOI: 10.1038/srep26668
Gadow H 1892. On the systematic position of Notoryctes typhlops. Proc. Zool. Soc. London 1892, 361–370.
Luo Z-X, Meng QJ, Ji Q, Liu D, Zhang Y-G, Neande AI 2015.Evolutionary development in basal mammaliaforms as revealed by a docodontan. Science. 347 (6223): 760–764.
Peters WCH 1875. Sitzungsberichte der Gesellschaft Naturforschender Freunde zu Berlin 1875: 73.
Stirling EC 1888. Transactions of the Royal Society, South Australia 1888:21
Stirling EC 1891. Transactions of the Royal Society, South Australia 1891:154
Tate GHH 1951. The banded anteater, Myrmecobius Waterhouse (Marsupialia). American Museum Novitates 1521, 8 pp.
Waterhouse GR 1836. Myrmecobius fasciatus. Proc. Zool. Soc. London 4: 69–131.
Waterhouse GR 1841. Description of a new genus of mammiferous animals from Australia, belonging probably to the order Marsupialia. Trans. Zool. Soc., London2, aricle. 11, p 149.

wiki/Dasycercus
wiki/Myrmecobius

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