We editors at Your Favourite Weekly Professional Science Magazine Beginning With N are masters of misrule. We revel in discord. We thrive on disagreement. Nothing pleases us more than witnessing the more cerebral branches of human misery. Nothing pushes our buttons quite as much as when studies on the same things blatantly disagree with each other.
‘Mwhahahaha’, we say, as we fluff out our velvet cape, pull out all the stops on the organ in our remote castle and give it some BWV 565.
Now, there are those whose agenda compels them to say that such disagreement is evidence of some deep malaise, as if it is science’s job to dish up uncontested facts for the delectation of the great unwashed. Au contraire, say I – disagreement is a sign of science in robust good health, evincing the kind of youthful, rosy-cheeked vigour in which new discoveries are being made, rather than the sere and pallid languour of age, in which the most one expect is the crossing of ‘t’s and the dotting of ‘i’s. Notwithstanding inasmuch as which that such disagreements also point up what is in my view the main aim of science – which is not about the exploration of knowledge, but the constant redefinition of ignorance, something I explore in my
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A case in point is the publication in this week’s issue of YFWPSMBWN of two papers, together, simultaneously, both at once, and at the same time, which disagree flatly with each another. The two concern discoveries about something hitherto sketchy and little known, and concerning which much remains to be discovered – so such disagreement is not only to be expected, but should be treasured.
The subject is the early evolution of mammals, the group of creatures which includes ourselves. This report gives an account of the story, and this one goes into a little more detail about the implications.
The proximate cause of the discord is this paper, from Zhou and colleagues, describing a beautiful skeleton of a fossil mammal, named Megaconus, which lived around 165 million years ago in what is now Inner Mongolia, China. Megaconus is extremely primitive, with a somewhat reptile-like jaw and middle ear and ankle, suggesting that it was rather far from the ancestry of modern mammals. Zhou and colleagues classify Megaconus as a member of a very poorly known group of extinct mammals called haramiyids.
Oh, yes, and then there’s this paper, from Zheng and colleagues, describing another beautiful skeleton of a fossil mammal, this time named Arboroharamiya, which is around 160 million years ago, and which is reconstructed as having scampered up trees in what is now Hebei Province, in China. Arboroharamiya was relatively advanced, with a lower jaw (and, presumably, a middle ear) rather similar to modern mammals. As the name suggests, Zheng and colleagues also refer their creature to the haramiyids.
This is a bit of a poser – teeth attributable to haramiyids date back to the Triassic, 50 million years older than any fossils attributable to mammals that might be related to any of the three extant mammal groups (placentals, marsupials and the egg-laying monotremes), and not long after the mammal lineage first appeared. The new fossils question what scientists mean by ‘haramiyid’ – and also imply rather different and mutually incompatible scenarios for mammalian evolution. Loving it so far? I am!
But first, some context.
Mammals really came into their own after the dinosaurs became extinct, 65 million years ago. But mammals have a very much longer history, back to times before the dinosaurs got going. For most of their long history, mammals have been rather small, living in the interstices of the dinosaurs’ world, their existence betrayed almost entirely by minute fossilized teeth. Various groups of mammals, all now extinct, have been proposed on the basis of these teeth, with very occasional skulls and skeletons. There are symmetrodonts and docodonts, tritylodonts and triconodonts, and who knows what else, each a tiny exclamation in a vastness of the unknown.
One of the least ill-known of these extinct mammals are the multituberculates, which evolved during the reign of the dinosaurs and, unusually, outlived them, becoming extinct in the Eocene epoch roughly 50 million years ago. Multituberculates are recognised by their distinctive teeth, which look a bit like the teeth of rodents. As anyone who’s been bitten by a guinea pig, or even a mouse, can attest, rodent teeth are among the greatest success stories of mammals. Most mammals living today are rodents. Having teeth that look a bit like rodent teeth would seem to be a good plan. Multituberculates, which invented rodent-like teeth long before rodents got in on the act, were conspicuously successful.
The relationships of multituberculates have been contentious, but most studies tend to show that they are more closely related to marsupials and placental mammals than are the monotremes (egg-laying mammals such as the platypus and echidna), which have their own, distinct history.
Enter haramiyids, a group of animal that evolved even earlier than multituberculates. Until recently, very little was known about them other than their teeth – which, quelle surprise, look somewhat rodent-like, prompting their alliance with multituberculates. This alliance poses several problems, not least the extremely and possibly unfeasibly ancient common ancestry of the three extant mammal groups. The discovery of jaws and skeletal elements of a haramiyid, Haramiyavia, suggested that haramiyids were a very ancient and distinct lineage of mammals whose relationship to more modern mammals, in particular multituberculates, was coincidental – a case of convergence.
So far, so good – except that up pops Arboroharamiya, which is ancient, has features suggesting a relationship with multituberculates – and therefore a common ancestry for modern mammal groups that stretches back to the beginning of the age of dinosaurs. Oh, and, lest we forget, Megaconus, which makes haramiyids primitive, with features only convergent on multituberculates, implying a much more recent common ancestry and divergence of modern mammals.
So, who’s right? My money is squarely on Neither Of The Above. The research makes it clear (at least to me) that the group known as the haramiyids is probably no more than an artificial assemblage of very ancient mammalian fossils forced together largely on the basis that their teeth all look rather similar. This similarity is very likely the result of the widespread adoption among small mammals, ancient and modern, of rodent-like teeth. In which case, Megaconus is a primitive mammal; Arboroharamiya a more advanced one; neither need be a close relative of the other; and the idea of a group called haramiyids disappears down the plughole of taxonomy.
Of course, both research groups have been working independently – the next step would be a comparison of both fossils in the same phylogenetic scheme to see where they pan out. Whatever the result, it is clear that the future is bright, happy, rumbustious and full of good, healthy argument.