It’s supposed to be the premiere scientific journal,
but Nature is failing more often lately. This time reporter John Pickrell (Sydney, Australia) is breaking with traditional hypotheses regarding mammal origins (Figs. 1–3) recovered decades before the advent of the large reptile tree (LRT, 1588 taxa). In figure one from the Nature article, the author flips the order of Liaconodon, a pre-mammal, with Morganucodon, a mammal. Pickerell does not understand the key trait: a dentary squamosal joint, marks the genesis of mammals. Liaoconodon does not have it, despite its late (more recent) appearance.
More to the point:
Liaoconodon and the two other taxa mentioned by Pickerell (see below) nest outside the last common ancestor of Mammalia.
Figure 1. Liaoconodon is a pre-mammal without a mammalian jaw joint (squamosal-dentary). Original art one frame one. Replacement art on frame two. Arrow points to jaw joint. For some reason teeth were omitted from the modern mammal jaw. Nature editor Hentry Gee and author John Pickrell should have noticed this.
Here’s what Nature.com reports and shows (Fig. 1)
“The latest finds are also offering clues to the evolution of key mammal features. For instance, the keen hearing of mammals is partly down to tiny bones in the middle ear — the malleus, incus and ectotympanic. But in the reptilian ancestors of mammals, these bones were part of the jaw, and were used for chewing instead of hearing. Mammal forerunners, such as shrew-like Morganucodon from 205 million years ago, sported a prototype of the mammal arrangement that allowed for both functions.”
Outside of this Nature.com article
Morganucodon has been recognized as a basal mammal since its discovery. Liaconodon (Fig. 1) was originally mistakenly considered a triconodont mammal. The LRT corrected that error here in 2016.
Figure 2. From Science News, Microdocodon ‘hyoids’ were misidentified fingers. See figure x.
Pickrell reported,
“In July, Luo published a paper revealing a 165-million-year-old vole-sized docodont — a close relative of true mammals — that had the hyoid bones of its throat preserved14. Microdocodon gracilis is the earliest animal known to have been able to suckle like a modern mammal.”
In counterpoint,
Microdocodon (Fig. 2) nests outside the last common ancestor of all mammals in the LRT (subset Fig. 5). So it is not a mammal and did not suckle like a metatherian or eutherian mammal. Microdocodon nests outside the Prototheria (= Monotremata, the egg-laying mammals), which lap milk, not suckle. Worse yet, we looked at Microdocodon earlier here. Those ‘hyoid bones’ were misidentified finger bones, otherwise not identified on the fossil.
Figure 3. Microdocodon throat region. Are those bones hyoids or fingers? If hyoids, then where are the fingers? Note the displaced radius (olive green) reaching toward the throat. Only impressions of once present fingers are present on the right limb.
Yet another misplaced pre-mammal,
Repenomamus (Fig. 4) went so far as to mimic the mammal jaw joint that rotated between the dentary and the squamosal. THAT DOESN”T MATTER! What matters is the last common ancestor of all living mammals, Megazostrodon, is a mammal. Repenomamus nests outside that clade, despite the fact that it had a convergent mammalian trait and survived into the Cretaceous. I know this can be confusing. The LRT resolves this issue.
Figure 4. Contra the caption, Repenomamus is not an early mammal. It is a pre-mammal.
Only a phylogenetic analysis
can determine what a taxon or clade is. Convergent traits do not determine this.
Figure 5. Subset of the LRT focusing on therapsids, like Repenomamus, leading to mammals.
If you know anyone over at Nature,
or independent reporter, John Pickrell, please pass this url over to them. They are reporting errors. This blogpost can help repair some of them.
References
Pickerell J 2019. How the earliest mammals thrived alongside dinosaurs. An explosion of fossil finds reveals that ancient mammals evolved a wide variety of adaptations allowing them to exploit the skies, rivers and underground lairs. Nature 574, 468-472. Nature.com News Feature Oct. 23, 2019. https://www.nature.com/articles/d41586-019-03170-7
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