A new nesting for two ‘odd bedfellow’ mammals

An ‘odd bedfellow’
is a taxon that doesn’t appear to fit where it currently nests. This has been called a “by default nesting” here because the more attractive true sisters are not present in the cladogram. That problem is called “taxon exclusion’ and often accompanies smaller more focused studies that do not glean their taxon lists from prior wide gamut, overarching studies, like this one. Some of those smaller studies include taxa that should not be included. Others exclude pertinent taxa.

For instance
pterosaurs don’t look like any archosaur. And fenestrasaurs (pterosaur sisters) are almost never included in pterosaur studies that usually favor archosaurs. Turtles don’t look like any archosauriform. Mesosaurs don’t look like any so-called anapsid(basal reptile). Caseasaurs don’t look like pelycosaurs. Vancleavea does not look like any archosaur. And the list goes on. These are traditional problems that continue to plague tetrapod paleontology.

This time in the large reptile tree
Vincelestes (Fig. 1) did not look enough like any basal placental and/or Carnivora. Ernanodon (Fig. 3) did not look enough like any pangolin or pangolin ancestor. The problem was, without more attractive taxa in the cladogram they nested by default in the large reptile tree where they did as ‘odd bedfellows.’ We’ve seen this before in a test when we deleted all current pterosaur and turtle sisters and they both nested with sauropterygians. The fact that there was massive convergence between certain marsupials and placentals just made the problem worse.

Figure 1. Vincelestes skull in two views. Now it nests with the creodont marsupial, Oxyaena (figure 2).

Figure 1. Vincelestes skull in two views. Now it nests with the creodont marsupial, Oxyaena (figure 2).

So
when creodonts were recently added to the large reptile tree, Hyaenodon and Oxyaena nested not with placentals, where traditional paleontologists thought should nest, but with marsupials, despite lacking four molars and epipubes. Their basal sister, Thylacinus, has very poorly developed epipubes and is losing molar 4. So that made the marsupial/placental convergence problem that much more difficult to solve.

Figure 2. Oxyaena, a credont/marsupial nests with Vincelestes.

Figure 2. Oxyaena, a credont/marsupial nests with Vincelestes. This marsupial lacks epipubes and has only three molars.

So now, with more taxa in the Marsupialia
Vincelestes now nests with the creodont/marsupial Oxyaena. Ernanodon now nests between the creodont/marsupial Hyaenodon and Oxyaena. Both look much more like their creodont sisters now. I think the ‘odd bedfellow’ problem has been solved.

Figure 3. Ernanodon used to nest poorly with pangolin ancestors, but not nests between the creodonts Hyaenodon (figure 4) and Oxyaena (figure 2).

Figure 3. Ernanodon used to nest poorly with pangolin ancestors, but not nests between the creodonts Hyaenodon (figure 4) and Oxyaena (figure 2).

The convergence problem in marsupials
has just gotten worse as four former placentals now nest with marsupials. We no longer can just count molars and look for epipubies. But we can still look for the septomaxilla, which were overlooked in these taxa. And the jugal extends back to the jaw joint, as in other marsupials (Oxyaena may be an exception, or the posterior jugal may be taphonomically missing).

Figure 1. Hyaenodon horrid us was the size of a large dog. This carnivorous marsupial was formerly considered a creodont.

Figure 4. Hyaenodon horrid us was the size of a large dog. This carnivorous marsupial was formerly considered a creodont. Now it nests with Ernanodon.

I have often encouraged detractors
to find poorly nested taxa. Well… here again, they had their chance and missed it. It’s good to clean up such problems. This is the process we call Science. And once again this demonstrates the importance of taxon inclusion in cladistic analysis. Every single taxon added to the large reptile tree (now at 798) reduces the chances that key sisters are missing when attempting to correctly nest new additions. It just keeps getting better and more complete. And problem solving, making little discoveries, like these, are wonderfully rewarding.

PS
I actually added a character to the LRT that scores the number of molars. We’ll look at that distribution in a future blogpost.

References
Bonaparte JF 1986. Sobre Mesungulatum houusayi y nuevos mamíferos Cretácicos de Patagonia, Argentina [On Mesungulatum houssayi and new Cretaceous mammals from Patagonia, Argentina]. Actas del IV Congreso Argentino de Paleontología y Biostratigrafía 2:48-61
Cope ED 1874. Report upon vertebrate fossils discovered in New Mexico, with descriptions of new species. Chief of Engineers Annual Report, Appendix, S. 589-606, U.S. Government Printing Office, Washington.
Ding SY 1979. A new edentate from the Paleocene of Guangdong. Vertebrata PalAsiatica 17:57–64. [Chinese 57–61; English 62–64].
Gaudrey A 1878. Les enchaînements du monde animal dans les temps géologiques mammifères tertiaires. F. Savy. (ed.) Paris 28pp. online here.
Horovitz I 2003. The type skeleton of Ernanodon antelios is not a single specimen. Journal of Vertebrate Paleontology. 23 (3): 706–8.
Kondrashov P and Agadjanian AK 2012. A nearly complete skeleton of Ernanodon (Mammalia, Palaeanodonta) from Mongolia: morphofunctional analysis. Journal of Vertebrate Paleontology. 32 (5): 983–1001. doi:10.1080/02724634.2012.694319.
Laizier L and de Parieu J 1838. Description et determination d’une machoire fossile appartenant a un mammifere jusqu’a pressent inconnu, Hyaenodon leptorhynchus. Comptes-rendus hebdomadaires des séances de l’Académie des Sciences, Paris 7:442.
Scott WM 1895. The osteology of Hyaenodon. Academy Natural Sciences Philadelphia Journal 9:499-536. online here.
Ting S, Wang B and Tong Y-S 2005. The type specimen of Ernanodon antelios. Journal of Vertebrate Paleontology 25(3):729-731.

wiki/Ernanodon
wiki/Vincelestes
wiki/Oxyaena
wiki/Hyaenodon

2 thoughts on “A new nesting for two ‘odd bedfellow’ mammals

  1. Congrats on adding a character. Seriously, that’s a step of progress. Did any non-synapsids count as having molars? Placodonts, Langobardisaurus, etc.? I’ve never considered that question before, but it seems worth asking since you can’t a priori restrict homology in a phylogenetic analysis.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s