As in the Pterosauria, the Chelonia, the Serpentes and the Archosauria,
traditional paleontologists have searched in vain for the closest known relatives of multituberculate mammals.
According to Kielan-Jaworowsak and Hurum 2001
(citations removed here:), “Finding an outgroup for the Multituberculata, is the first difficulty which one encounters in an attempt at phylogenetic analysis of this order. The problem is that the origins of the Multituberculata are obscure and in recent phylogenetic analyses of early mammals, multituberculate relationships have been a subject of vigorous controversy.”
“Traditionally palaeontologists believed that multituberculates might have originated from cynodonts independently from all other mammals, or diverged from other mammals at a very early stage of mammalian evolution. The multituberculate structure was so radically distinctive throughout their history that it seems hardly possible that they are related to other mammals except by a common origin at, or even before, the class as such.
More recently the idea that multituberculates might be a sister taxon of all other mammals has been supported. On the other hand, studies of the last two decades on the skull structure of multituberculates and other early mammals demonstrated the homogeneity of the internal structure of the skull and vascular system of all mammals, including multituberculates. The same concerns the discovery of multituberculate ear ossicles, which display the same pattern as those of all other mammals. The notion that multituberculates might form a sister taxon of all other mammals is related to the idea that they are close relatives to the Haramiyidae, a family represented until recently only by isolated teeth, with numerous cusps arranged in longitudinal rows, known from the Late Triassic and Early Jurassic mostly in Europe.”
“Haramiyavia has been interpreted as having orthal jaw movement. On this basis Jenkins et al. excluded the Haramiyida from the Allotheria, which have propalinal (fore-and-aft) movement of the dentary and backward (palinal) power stroke. In turn Butler (2000) revised all known allotherians and argued that dental resemblance supports the hypothesis that the Multituberculata originated from the Haramiyida.”
“Finally, the most recent analyses of mammalian relationships, including analysis of the skeleton of a symmetrodont Zhangheotherium (Hu et al. 1997), and the skeleton of the eutriconodont Jeholodens (Ji et al. 1999), did not support multituberculate-therian sister-group relationship. In both of these papers the Multituberculata are placed between Monotremata (Ornithorhynchus) and Symmetrodonta (Zhangheotherium), being a sister taxon of all the Holotheria.”
“It should be pointed out that none of these analyses took into account the structure of the brain, which, as argued by Kielan-Jaworowska (1997), is one of several characters neglected in phylogenetic analyses of early mammals. The multituberculate brain, designated cryptomesencephalic (characterised by an expanded vermis, no cerebellar hemispheres, and lack of the dorsal midbrain exposure) is very different from that in Theria, which originally had eumesencephalic brains (characterised by a wide cerebellum with extensive cerebellar hemispheres and large dorsal midbrain exposure). The cryptomesencephalic brain characteristic of multituberculates otherwise occurs only in eutriconodonts.
The authors were not aware of most recent news on mammal brains.
Gilissen and Smith 2012 reported, “The terms cryptomesencephalic and eumesencephalic brains may have to be abandoned in light of a new interpretation of multituberculate and eutriconodontan endocasts. Our observations indicate that it is not the superior cistern but most probably the cerebellar vermis that makes a clearly visible impression on the posterior part of the endocranial casts of all extant and fossil mammals examined, including multituberculates and eutriconodonts.”
Kielan-Jaworowsak and Hurum 2001 continue:
“Another character neglected until recently in phylogenetic analyses of early mammals involves the foot structure. In the multituberculate foot the middle metatarsal is abducted from the longitudinal axis of the tuber calcanei, while the calcaneus contacts distally the 5fth metatarsal. This type of foot appeared at that time to be unique among mammals, but Ji et al. 1999 described a similar type of foot in the eutriconodont Jeholodens. It follows that there are two groups of characters related to brain and foot structure, which ally multituberculates with eutriconodonts.”
Re: the foot:
It looks like Jeholodens has a basal tarsus because distal tarsal 4 is not wide enough to double as a distal tarsal 5, as it does in the marsupial, Didelphis. A quick peek at Rattus, Vulpavus and Onychonycteris shows that these placental taxa likewise do not widen distal tarsal 4 to back up pedal digit 5. Comparable pedes among other tritylodontids are hard to find.
Dental traits can converge, just like any other traits.
(Remember ododontocetes have simple cones). Haramiyavia is tested along with 1360 other possible multituberculate sisters. In the large reptile tree Haramivavia nests basal to mammals apart from multituberculates. Multituberculates nest within Glires, closest to plesiadapids and rodents.
We looked at rodents
and multituberculates earlier here with lots of details (Fig. 3).
In private correspondence with an early mammal expert,
I received the following reply based on evidence that multituberculates were related to rodents, rather than Haramiyida: “Nice to hear from you. Honestly, this idea is so far out that I’m apoplectic. Yes, of course it’s worth reconsidering “conventional wisdom.” But the relationships of these groups within major clades are so well established that I see no point in considering the matter…beyond looking at relationships of plesiadapids within Eutheria. Anyway, good luck with your work!”
Sadly, this shows
some paleontologists are unwilling to test alternate hypotheses. Some would rather dismiss the obvious.
Kielan-Jaworowska Z and Hurum JH 2001. Phylogeny and Systematics of multituberculate mammals. Paleontology 44, 389–429.