A new eutriconodont mammal species, Yanoconodon, has been reported by a team of Chinese researchers.
The first point to make is that this is not an example of a transitional fossil. The animal has a number of specialised features and is described as "nested within crown mammals". The authors provide a cladogram of dental and skeletal characters of eutriconodonts, and the closest affinity is with Jeholodens. However, Yanoconodon has more thoracic ribs than Jeholodens, and Yanoconodon has lumbar ribs whereas Jeholodens does not. Whilst the authors discuss Hox gene mechanisms to alter the pattern of ribs, they do not attempt to locate Yanoconodon in any particular evolutionary lineage.
Rather, the new fossil has been hailed as illustrating an important evolutionary transition: detachment of the middle ear bones from the mandible. It is therefore better described as a fossil claimed to have a transitional structure associated with ear bones.
It is true that the authors favour the evolutionary transition scenario, but they are forced by the data to consider at least one alternative: that the definitive mammalian inner ear was present in "the common ancestor of monotremes, eutriconodonts and therians; but eutriconodonts re-evolved the middle ear attachment to mandible." (A polyphyletic approach introduces more options). The editors of Nature supplied a summary acknowledging that there is a legitimate debate about the significance of the find: "But the situation is not as clear-cut as it seems. The evolutionary relationships of the fossil suggest that either the 'modern' middle ear evolved twice, independently or that it evolved and was then lost in at least one ancient lineage." (In this comment, they are referring to monotremes and therians. The earbones of the duckbilled platypus emerge during development with a cartilaginous attachment to the jaw - much like Yanoconodon - but this attachment is reabsorbed as the animal grows thereby detaching the earbones).
It should be pointed out that this debate is not one where ID scholars might be expected to have a common view. Are evidences for design in the natural world affected by the earbones of Yanoconodon? The expectation would be that all the features of the animal would have functional significance, and that Darwinian mechanisms are not capable of achieving the specified complexity associated with the mammalian ear. Beyond that, let us explore the data without having to force everything into a Darwinian straitjacket! For those seeking a creationist comment, go here.
A new eutriconodont mammal and evolutionary development in early mammals
Zhe-Xi Luo, Peiji Chen, Gang Li, and Meng Chen
Nature 446, 288-293 (15 March 2007) | doi:10.1038/nature05627
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 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. This new eutriconodont adds to the evidence of homoplasy of vertebral characters in the thoraco-lumbar transition and unfused lumbar ribs among early mammals. 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.
Editor's summary, An early look at mammals, Nature 446, 15 March 2007, vii.
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