Deltatheridium revisited

Deltatheridium
according to Wikipedia, is “an extinct species of metatherian. It lived in what is now Mongolia during the Upper Cretaceous, circa 80 million years ago. It was a basal metatherian, which places it near start of the lineage that led to the marsupials. It had a length of 15 cm (5.9 in). Its teeth indicate it was carnivorous.” 

The large reptile tree does not support that nesting, but nests Deltatheridium (Gregory and Simpson 1926, Rougier et al. 1998, Fig. 1) basal to Borhyaeana (Fig. 2), though much smaller. Ernanodon and Hyaenodon are outgroup taxa.

Figure 1. Deltatheridium skulls, PSS-MAE 132, 133 and AMNH 21706.

Figure 1. Deltatheridium skulls, PSS-MAE 132, 133 and AMNH 21706. The back of the skull in the 133 specimen is imagined twice. The elevated posterior rim matches sister taxa like Borhyaena. These skulls are almost twice life size based on a 72dpi monitor.

From the Rougier et al. 1998 abstract:
“We describe here two new specimens of the mammal Deltatheridium pretrituberculare from the Late Cretaceous period of Mongolia. These specimens provide information on tooth replacement in basal therian mammals and on lower jaw and basicranial morphology. Deltatheroidans, known previously from isolated teeth, partial rostra and jaws from the late Cretaceous of Asia and possibly North America have been identified variously as eutherians, as basal metatherians (the stem-based clade formed by marsupials and their extinct relatives) or as an outgroup to both eutherians and metatherians. Resolution of these conflicting hypotheses and 

Figure 2. Borhyaena compared to the smaller Deltatheridium.

Figure 2. Borhyaena compared to the smaller Deltatheridium.

understanding of the early evolution of the therian lineage have been hampered by a sparse fossil record for basal therians. The new evidence supports metatherian affinities for deltatheroidans and allows a comprehensive phylogenetic analysis of basal metatherians and marsupials. The presence of specialized marsupial patterns of tooth replacement and cranial vascularization in Deltatheridium and the basal phylogenetic position of this taxon indicate that these features are characteristic of Metatheria as a whole. Other morphological transformations recognized here secure the previously elusive diagnosis of Metatheria. The new specimens of Deltatheridium illustrate the effectiveness of fairly complete fossil specimens in determining the nature of early evolutionary events.”

Figure 1. The nesting of Deltatheridium according to Rougier et al. 1998. The LRT does not support this cladogram.

Figure 3. The nesting of Deltatheridium according to Rougier et al. 1998. The LRT does not support this cladogram. Pink highlights are taxa both cladograms share in common.

The large reptile tree 
subset (Fig. 3) includes a different set of taxa, none based on mandible only or tooth only taxa. Here Deltatheridium is a small ancestor to Didelphodon and Borhyaena and none of these taxa are basal metatherians, which more closely resemble Eomaia and Didelphis.

Figure 3. Subset of the LRT focusing on marsupials and Deltatheridium nesting with Borhyaena.

Figure 4. Subset of the LRT focusing on marsupials and Deltatheridium nesting with Borhyaena.

References
Rougier GW, Wible JR and Novacek JJ 1998. Implications of Deltatheridium specimens for early marsupial history. Nature 396, 459-463. doi:10.1038/24856
Gregory WK and Simpson GG 1926. Cretaceous mammal skulls from Mongolia. American Museum Novitates 225, 7-20.

wiki/Deltatheridium
wiki/Didelphodon
wiki/Borhyaena

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