From The Scientist :
In the grand scheme of geologic time, plus or minus a million years isn’t all that much. But new research suggests that the diversification of mammals occurred a full 30 million years later than previously thought, placing the rise the mammalian class in the Jurassic and not the Triassic period. The results were published today (November 16) in PNAS.
What the researchers discovered was an animal with clear postdentary trough, a feature present in primitive animals for hearing. Modern mammals, in contrast, have three bones in the middle ear, which support hearing and balance. The conclusion is that H. clemmenseni is a very primitive precursor to modern mammals.
Rich Cifelli, a zoology professor at the University of Oklahoma who was not involved with the study, said the new relative age of the mammalian class means that mammals likely evolved relatively quickly. “It makes a difference as to the rate of evolution. How long did it take for us to become the way we are?
It’s unusual to run a “much later than thought” story at a desk where “earlier than thought” is practically a macro.
“This fossil is a unique representative from an incredibly important era in the evolution of mammals; the ecosystem of the whole world changed as the Triassic transitioned into the Jurassic,” said study senior author Neil Shubin, PhD, Robert R. Bensley Distinguished Service Professor of Organismal Biology and Anatomy at the University of Chicago. “When you look at the entirety of the Haramiyavia jaw and its primitive features, it’s clear that this group sat at the very base of the mammalian family tree, much in the same way that Tiktaalik rosea sat at the base of the tetrapod tree.” More.
Here’s the abstract:
As one of the earliest-known mammaliaforms, Haramiyavia clemmenseni from the Rhaetic (Late Triassic) of East Greenland has held an important place in understanding the timing of the earliest radiation of the group. 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. Our tests of competing phylogenetic hypotheses with these new data show that Late Triassic haramiyids are a separate clade from multituberculate mammals and are excluded from the Mammalia. Consequently, hypotheses of a Late Triassic diversification of the Mammalia that depend on multituberculate affinities of haramiyidans are rejected. Scanning electron microscopy study of tooth-wear facets and kinematic functional simulation of occlusion with virtual 3D models from CT scans confirm that Haramiyavia had a major orthal occlusion with the tallest lingual cusp of the lower molars occluding into the lingual embrasure of the upper molars, followed by a short palinal movement along the cusp rows alternating between upper and lower molars. This movement differs from the minimal orthal but extensive palinal occlusal movement of multituberculate mammals, which previously were regarded as relatives of haramiyidans. The disparity of tooth morphology and the diversity of dental functions of haramiyids and their contemporary mammaliaforms suggest that dietary diversification is a major factor in the earliest mammaliaform evolution. (public access) – Z. Luo et al., “Mandibular and dental characteristics of Late Triassic mammaliaform Haramiyavia and their ramifications for basal mammal evolution,” PNAS, doi:10.1073/pnas.1519387112, 2015.
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