Anebodon: another symmetrodont or kangaroo ancestor?

Updated Sept 23, 2016 with a new information on the septomaxilla in marsupials and a new basal mammal cladogram (Fig. 2).

Bi et al. 2016
reported on “Anebodon luoi (STM 38-4, Tianyu Museum of Nature, Shandong Province, China, Fig. 1) a new genus and species of zhangheotheriid symmetrodont mammal from the Lujiatun site of the Lower Cretaceous Yixian Formation, China. The fossil is represented by an associated partial skull and dentaries with a nearly complete dentition.”

Figure 1. Anebodon luoi subjected to DGS tracing and phylogenetic analysis nests with Macropus, the extant kangaroo, not with Zhangheotherium.

Figure 1. Anebodon luoi subjected to DGS tracing and phylogenetic analysis nests with Macropus, the extant kangaroo, not with Zhangheotherium. The kangaroo kink is just starting here with a concave/convex maxilla. The canines are present, but tiny. The anterior dentary teeth extend anteriorly, the first step toward the kangaroo’s ‘tusks’.

Bi et al. noted
Anebodon lacked the high molar count typical of derived symmetrodonts. Their diagnosis focused on dental differences compared to Maotherium and Kiyatherium.

By contrast
the large reptile tree (LRT) nested Early Cretaceous Anebodon with the extant kangaroo, Macropus (Fig. 1). A septomaxilla was identified in Anebodon and this cause me to reevaluate the septomaxilla situation. Apparently in continues in basal marsupials, but is lost several times in derived taxa. A septomaxilla has been figured for Vincelestes, but I cannot confirm a septomaxilla due to damage in the rostrum. Phylogenetic bracketing argues against it.

Revision to the earlier posted basal mammal cladogram. Here two wombats are added. Eomaia and Monodelphis switch places.

Figure 2. Revision to the earlier posted basal mammal cladogram. Here two wombats are added. Eomaia and Monodelphis switch places.

With this in mind, 
it’s worthwhile to look at intervening basal kangaroos (Fig. 3).

Figure 3. Anebodon, Nambaroo, Dendrolagus, Macropus, kangaroo ancestors. The purported 'wolf-like' fangs are not apparent here. Were they referring to the long dentary incisors? After phylogenetic analysis Nambaroo nested basal to Plesiadapis and rabbits.

Figure 3. Anebodon, Nambaroo, Dendrolagus, Macropus, kangaroo ancestors. The purported ‘wolf-like’ fangs are not apparent here. Were they referring to the long dentary incisors? After phylogenetic analysis Nambaroo nested basal to Plesiadapis and rabbits.

Nambaroo gillespieae (N. tarrinyeri Flannery & Rich, 1986; Kear et al. 2007; Late Oligocene, 25 mya) was reported to be the “granddaddy of kangaroos” and to have had fangs, ‘probably for display,’ and mostly ate soft food such as fruit and fungi (Kear 2007). It was found in Australia.

One wonders
if the fangs they refer to are indeed the lower incisors (Fig. 4), because I see no canine fangs here. And with that short rostrum, one wonders if a soft tissue tapir-like trunk extended its length.

One also wonders
whether Nambaroo is indeed a kangaroo at all given the short rostrum and comb-like surface of premolar 3. The jugal does not reach the jaw joint, as in other marsupials including kangaroos. Not sure why, but in their diagram of the skull and mandible with measurements Kear et al. reduced the scale of the mandible so the rostrum did not look so short relative to the dentary. Here (Fig. 4) that scaling problem is eliminated.

Figure 3. Nambaroo skull in several views. Colors added using DGS methods. Dentary scaled to skull.

Figure 4. Nambaroo skull in several views. Colors added using DGS methods. Dentary scaled to skull and the molars occlude. Using this data, Nambaroo nested with rabbits and Plesiadapis.

During the course of this writing
I added Nambaroo to the large reptile tree and it nested far from kangaroos, at the base of Plesiadapis + rabbits, close to the base of rodents + multituberculates. It’s not the first time paleontologists, including yours truly, have gotten things wrong, That’s why we need independent testing with larger taxon lists.

Figure 6. Nambaroo mandible. Note premolar 3 and its striking resemblance to the same tooth in multituberculates.

Figure 5. Nambaroo mandible. Note premolar 3 and its striking resemblance to the same tooth in multituberculates. Kangaroos don’t have such a tooth.

Figure 8. Nambaroo nests at the base of Plesiadapis + rabbits in the LRT.

Figure 8. Nambaroo nests at the base of Plesiadapis + rabbits in the LRT. See figure 2 for updated cladogram of basal mammals. 

Dendrolagus ursinus (Müller 1840, extant; 50 cm long + 60 cm tail) is the tree-kangaroo. More at home in trees than on the ground, Note the skull is very similar to that of the larger gray kangaroo and makes a good transitional taxon from Anebodon to Macropus.

Macropus giganteus (Shaw 1790, extant) is the eastern gray kangaroo. The forelimbs were elongated. The hind limbs even more so. The pedal digits were reduced, all but the second digit, which was robust. Kangaroos hop bipedally and rest tripodally with the tail. The canines were absent and a diastema separated the incisors from the molars, convergent with rodents and multituberculates. The lower incisors were elongated and procumbent.

The clade
Symmetrodonta is considered paraphyletic at Wikipedia. The clade is based on teeth characterized by the triangular aspect of the molars when viewed from above and the absence of a well-developed talonid. Perhaps such teeth were common to basal placentals.

When I first started
ReptileEvolution.com, about five years ago, I never thought I’d find so many new nestings for published taxa. It’s been an interesting run, but I really think I’m about out of taxa to examine (up to 780 at present for the LRT, plus 59 for therapsids and 229 for pterosaurs). Let me know of any enigmas that need testing.

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
Kear BP, Cooke BN, Archer M and Flannery TF 2007. Implications of a new species of the Oligo-Miocene kangaroo (Marsupialia: Macropodoidea) Nambaroo, from the Riversleigh World Heritage Area, Queensland, Australia, in Journal of Paleontology 81:1147-1167.
Shaw G 1790. Macropus giganteus. Nat. Miscell. plate 33 and text. kangaroo online pdf

wiki/Nambaroo
wiki/Tree-kangaroo
wiki/Kangaroo

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