Paragraph on the taxon, Shenshou, was added August 7, 2016. See below.
Only a few broken skull bones
and a less than complete dentary are known of Haramiyavia (Fig. 1, Jenkins et al. 1997, Luo et al 2015) a basal multituberculate mammal. The presence of these derived mammal teeth and bones in the Late Triassic (Norian/Rhaetian boundary, 207 mya) makes them important, because that’s early! That’s one reason why multis are considered pre-mammals by most paleontologists.
Note the small size
of the canines. And the large size of the procumbent premaxillary teeth. This is not a basal mammal, like Monodelphis is. Basal mammals don’t have derived features like this — unless they are extant monotremes, which have had a long time to evolve away from the plesiomorphic state. The grooves reported on the lower posterior inner dentary may be a reversal, or a trait that disappeared in all derived mammal taxa by convergence.
Luo et al. report:
“Reanalysis of the type specimen using high-resolution computed tomography (CT) has revealed new details, such as the presence of the dentary condyle of the mammalian jaw hinge and the postdentary trough for mandibular attachment of the middle ear—a transitional condition of the predecessors to crown Mammalia.” They found the trough, but not the bones that were supposed to fill it.
“On the mammal family tree Haramiyavia occupies a position crucial for dating the initial appearance of the major mammalian groups. Our reanalysis affirms that the earliest diversification of mammals occurred in the Jurassic.” This confirms what was recovered from the large reptile tree earlier. But wait. There’s more…
Now it appears
that the first radiation of basal mammals took place in the Late Triassic, and likely the earliest part of that period.
Remember, every fossil specimen recovered
probably (but not always) represents one of millions of specimens long after the initial separation and radiation in isolation at the clade genesis. Earliest representatives and phylogenetic analysis help us determine approximately when that origin was. In this case we can date it to tens of millions of years before the appearance of the derived mammal, Haramiyavis in the Late Triassic.
This should come as no surprise
The pre-mammal, Pachygenelus, is younger than the earlier mammals. This happens all the time. I would not be surprised to someday see the origin of mammals pushed back to the Middle Triassic.
Dinosaurs were diversifying then.
So why not mammals? Just because we haven’t found them yet, doesn’t mean they weren’t there. They were tiny! Just look at how few Jurassic and Triassic mammals we have found.
Perhaps a few mistakes were made
by the original and subsequent workers. What Luo et al. considered a premaxillary groove for the septomaxilla appears to be for the nasal. Thus the nasal can extend anteriorly further than the premaxilla, as in Rattus. The loss or fusion of the septomaxilla is a derived trait.
What Luo et al. considered the fourth premaxillary tooth appears to be the tiny canine. If so, then the premaxilla can be moved back to a point at which the maxillary canine is just behind the dentary canine, as in many other basal mammals. This reduces the preorbital length as compared to the postorbital length of the skull.
And the large reptile tree
still has had no major topological shifts with the addition of the last 70 or so mammals. If you don’t like it, you can still admire its architecture. It’s standing strong.
Added August 7, 2016
M. Mortimer suggested I take a look at the early mammal, Shenshou. Wikipedia reports: “The presence of a three-boned middle ear suggests these animals were mammals.”
“The remains of Shenshou were discovered along with fossils of two other extinct squirrel-like animals, Xianshou songae and Xianshou linglong, in strata dating back 160 million years. The animals were found to belong to a group referred to as haramiyids, now identifiable as early mammals. They lived alongside another group of small rodent-like animals, the multituberculates, which were already accepted as mammals. This discovery has pushed the date of the origin of mammals back to the Late Triassic, the period in which the haramiyid-multituberculate clade, Allotheria, originated, about 220 million to 200 million years ago. Until this was established, mammals were thought to have originated in the Middle Jurassic, which lasted from 174 million to 164 million years ago.”
Well, nice to get confirmation.
Jenkins FA, Jr, Gatesy SM, Shubin NH and Amaral WW 1997. Haramiyids and Triassic mammalian evolution. Nature 385(6618):715–718.
Luo Z-X, Gatesy SM, Jenkins FA, Jr, Amaralc WW and Shubin NH 2015. Mandibular and dental characteristics of Late Triassic mammaliaform Haramiyavia and their ramifications for basal mammal evolution. PNAS 112 (51) E7101–E7109.