Yanoconodon alllini (Luo, Chen, Li and Chen 2007; Early Cretaceous, 122 mya; 13 cm in length; Figs. 1, 2) is known from a nearly complete and articulated crushed fossil. It is traditionally considered a eutriconodont, a clade that traditionally includes Spinoletes, Repenomamus, Gobiconodon, Liaoconodon, Jeholodens and Volaticotherium. Unfortunately that clade is paraphyletic in the large reptile tree (LRT). Here Yanoconodon was derived from a sister to Pachygenelus and nested between that clade and the Mammalia (Fig. 3).
Yanoconodon had a semi-sprawling posture
and a a long, robust torso with an unusually thick lumbar vertebrae provided with very short ribs. The limbs were short. The canines were quite narrow. The posterior jaw bones were still attached to the jaw. They had not yet become completely reduced to middle ear bones and completely separated from the jaw bones. So, by definition and cladogram (Fig. 3), Yanoconodon was not a true mammal. Wikipedia disagrees as that author reports, “Despite this feature Yanoconodon is a true mammal.”
See below for some thoughts on that.
from the Luo, Chen and Chen abstract
“Detachment of the three tiny middle ear bones from the reptilian mandible is an important innovation of modern mammals. Here we describe a Mesozoic eutriconodont nested within crown mammals (1) that clearly illustrates this transition: the middle ear bones are connected to the mandible via an ossified Meckel’s cartilage. The connected ear and jaw structure is similar to the embryonic pattern in modern monotremes (egg-laying mammals) and placental mammals, but is a paedomorphic feature retained in the adult, unlike in monotreme and placental adults. This suggests that reversal to (or retention of) this premammalian ancestral condition is correlated with different developmental timing (heterochrony) in eutriconodonts. (2) This new eutriconodont adds to the evidence of homoplasy of vertebral characters in the thoraco-lumbar transition and unfused lumbar ribs among early mammals. (3) This is similar to the effect of homeobox gene patterning of vertebrae in modern mammals, making it plausible to extrapolate the effects of Hox gene patterning to account for homoplastic evolution of vertebral characters in early mammals.” (4)
- The LRT nests Yanoconodon just outside the crown mammals. Not sure why the authors say this, given what they report about the posterior jaw bones as posterior jaw bones.
- Curious that the retention of “this pre-mammalian ancestral condition” does not indicate to the authors that Yanoconodon is indeed a pre-mammal.
- Yanoconodon does not nest as an early mammal in the LRT.
- …or…not, if Yanoconodon is indeed a non-mammalian trithelodont. Other non-mammalian cynodonts lived alongside Jurassic mammals. Only one purported eutriconodont listed above is a mammal, Volaticotherium. It nests as a basal placental. Triconodon is a mammal, too, a monotreme known from just a dentary and teeth.
Yanoconodon is a great transitional fossil.
You can call it a mammal, if you want to slightly stretch the current definition. You can call it a mammal if you want to describe it as the last common ancestor of all living mammals. But as soon as a better transitional candidate comes along, it will be demoted.
Luo Z, Chen P, Li G, and Chen M 2007. A new eutriconodont mammal and evolutionary development in early mammals. Nature 446:15. online Nature