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Late Triassic mammaliaform Haramiyavia and basal mammal evolution (free pdf)

Ben Creisler

A new paper in open access:

Zhe-Xi Luo, Stephen M. Gatesy, Farish A. Jenkins Jr., William W.
Amaral, and Neil H. Shubin (2015)
Mandibular and dental characteristics of Late Triassic mammaliaform
Haramiyavia and their ramifications for basal mammal evolution.
Proceedings of the National Academy of Sciences (advance online publication)

Free pdf:


The origins and earliest evolution of mammals can be deciphered by
studying Late Triassic fossil relatives of modern mammals. The
computed tomography study of Haramiyavia from the Late Triassic has
revealed new information about the skull evolution and dental function
in the forerunners of mammals. Haramiyavia had a unique way of
chewing. Its teeth of multiple cusp-rows were adapted to omnivory or
herbivory and are distinctive from the teeth of other early mammal
relatives that are presumed to be insectivorous. 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.


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