The Eutriconodonta is also paraphyletic in the LRT

Triconodon, Docodon, and Kuehneotherium are known form dentary bones with most of their teeth in place. Generally I avoid adding such partial specimens to the large reptile tree (LRT-updated at 848 taxa) because so few scores are generated for them with the current character list that they lead to loss of resolution at their nodes…

But curiosity won out
when wondering about members of the putative clade Eutriconodonta (Kermack et al. 1973), a clade that ostensibly replaces the paraphyletic Triconodonta. Some of these mandible-only taxa I added to the tree only to delete them later, just to see where they nested (often at the base of the Monotremata, as one would guess given their Mid-to Late Jurassic ages).

According to Wikipedia
“The Eutriconodonta  is a [presumeably monophyletic] order of mammals” broadly, though not exclusively characterized by molar teeth with three main cusps on a crown that were arranged in a row.  “Eutriconodonts retained classical mammalian synapomorphies like epipubic bonesvenomous spurs and sprawling limbs. Eutriconodonts had a modern ear anatomy, the main difference from therians being that the ear ossicles were still somewhat connected to the jaw via the Meckel’s cartilage.

“Phylogenetic studies conducted by Zheng et al. (2013), Zhou et al. (2013) and Yuan et al. (2013) recovered monophyletic Eutriconodonta containing triconodontids, gobiconodontids, Amphilestes, Jeholodens and Yanoconodon. The exact phylogenetic placement of eutriconodonts within Mammaliaformes is uncertain.”

“Traditionally seen as the classical Mesozoic small mammalian insectivores, discoveries over the years have ironically shown them to be among the best examples of the diversity of mammals in this time period, including a vast variety of bauplans, ecological niches and locomotion methods.”

Traditional Eutriconodont taxa (see Martin et al. 2015) presently included in the LRT nest in a variety of clades:

  1. Gobiconodon – pre-mammal tritylodontid
  2. Repenomamus – pre-mammal tritylodontid
  3. Spinolestespre-mammal tritylodontid
  4. Jeholodens – pre-mammal tritylodontid
  5. Volaticotherium basal placental 
  6. Triconodonbasal monotreme
  7. Trioracodon – basal monotreme
  8. Yanoconodonpre-mammal (Fig. 1).

We’ve seen this sort of splitting
of traditionally established clades based chiefly on tooth traits before with the Docodonta (Fig. 1). As in molecule trees, tooth trees are not replicated in the LRT, which recovers a distinctly new tree topology without the odd logic jumps that traditional clades, like Afrotheria, produce.

Figure1. Repeat of an early subset of the LRT, this time highlighting putative eutriconodonts and where they nest. No wonder they are described as a diverse clade!

Figure1. Repeat of an early subset of the LRT, this time highlighting putative eutriconodonts and where they nest. No wonder they are described as a diverse clade!

For the most part
eutriconodonts nest more or less together very close to the base of the Mammalia, whether in or out. Those slender posterior jaw bones are not typically preserved, but the long groove in which they are attached is typically preserved. Caution must be exercised, as fully mammalian taxa like Monodelphis, can also preserve a remnant of this groove despite the complete evolution of the post-dentary bones into tiny ear bones.

Figure 4. Mondelphis domestics with its posteromedial jaw groove highlighted in red. The ear bones are tiny and enclosed within the auditory bulla beneath the cranium.

Figure 2. Mondelphis domestics with its posteromedial jaw groove highlighted in red. The ear bones are tiny and enclosed within the auditory bulla beneath the cranium.

So.. about those venomous ankle spurs…
Ornithorhynchus, the platypus, has them and likely so do its sisters. The authors of the Volaticotherium paper make no mention of either venom nor spur and score it as a “?”. The Yanoconodon tarsi are a challenge to reconstruct based on their preservation. The authors and yours truly note no spur-like bones present.

A new evolution website has launched
Check out www.TimeTree.org for a tremendous amount of phylogenetic information. For instance, one can input two well-known taxa, like Gallus and Homo, and the tree will determine the estimated date of their last common ancestor, in this case Vaughnictis (which is a taxon not in their current database).

References
Editors: Carrano MT et al. 2006. Amniote Paleobiology: Perspectives on the Evolution of Mammals, Birds and Reptiles. University of Chicago Press.  online here.
Kermack KA, Mussett F, Rigney HW 1973. The lower jaw of Morganucodon. Zoological Journal of the Linnean Society.53 (2): 87–175.
Martin T et al. 2015. A Cretaceous eutriconodont and integument evolution of early mammals. Nature 526:380-384. online.

wiki/Eutriconodonta

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