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Basilosaurus bite force + transition of Eocene whales from land to sea



Ben Creisler
bcreisler@gmail.com


New in PLoS ONE:

Eric Snively,  Julia M. Fahlke &  Robert C. Welsh (2015)
Bone-Breaking Bite Force of Basilosaurus isis (Mammalia, Cetacea) from
the Late Eocene of Egypt Estimated by Finite Element Analysis.
PLoS ONE 10(2): e0118380.
doi:10.1371/journal.pone.0118380
http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0118380


Bite marks suggest that the late Eocence archaeocete whale
Basilosaurus isis (Birket Qarun Formation, Egypt) fed upon juveniles
of the contemporary basilosaurid Dorudon atrox. Finite element
analysis (FEA) of a nearly complete adult cranium of B. isis enables
estimates of its bite force and tests the animal’s capabilities for
crushing bone. Two loadcases reflect different biting scenarios: 1) an
intitial closing phase, with all adductors active and a full condylar
reaction force; and 2) a shearing phase, with the posterior temporalis
active and minimized condylar force. The latter is considered probable
when the jaws were nearly closed because the preserved jaws do not
articulate as the molariform teeth come into occulusion. Reaction
forces with all muscles active indicate that B. isis maintained
relatively greater bite force anteriorly than seen in large
crocodilians, and exerted a maximum bite force of at least 16,400 N at
its upper P3. Under the shearing scenario with minimized condylar
forces, tooth reaction forces could exceed 20,000 N despite lower
magnitudes of muscle force. These bite forces at the teeth are
consistent with bone indentations on Dorudon crania,
reatract-and-shear hypotheses of Basilosaurus bite function, and
seizure of prey by anterior teeth as proposed for other archaeocetes.
The whale’s bite forces match those estimated for pliosaurus when
skull lengths are equalized, suggesting similar tradeoffs of bite
function and hydrodynamics. Reaction forces in B. isis were lower than
maxima estimated for large crocodylians and carnivorous dinosaurs.
However, comparison of force estimates from FEA and regression data
indicate that B. isis exerted the largest bite forces yet estimated
for any mammal, and greater force than expected from its skull width.
Cephalic feeding biomechanics of Basilosaurus isis are thus consistent
with habitual predation.

***
Alexandra Houssaye,  Paul Tafforeau,  Christian de Muizon &  Philip D.
Gingerich (2015)
Transition of Eocene Whales from Land to Sea: Evidence from Bone Microstructure.
PLoS ONE 10(2): e0118409.
doi:10.1371/journal.pone.0118409
http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0118409

Cetacea are secondarily aquatic amniotes that underwent their
land-to-sea transition during the Eocene. Primitive forms, called
archaeocetes, include five families with distinct degrees of
adaptation to an aquatic life, swimming mode and abilities that remain
difficult to estimate. The lifestyle of early cetaceans is
investigated by analysis of microanatomical features in postcranial
elements of archaeocetes. We document the internal structure of long
bones, ribs and vertebrae in fifteen specimens belonging to the three
more derived archaeocete families — Remingtonocetidae, Protocetidae,
and Basilosauridae — using microtomography and virtual
thin-sectioning. This enables us to discuss the osseous
specializations observed in these taxa and to comment on their
possible swimming behavior. All these taxa display bone mass increase
(BMI) in their ribs, which lack an open medullary cavity, and in their
femora, whereas their vertebrae are essentially spongious. Humeri and
femora show opposite trends in microanatomical specialization in the
progressive independence of cetaceans from a terrestrial environment.
Humeri change from very compact to spongious, which is in accordance
with the progressive loss of propulsive role for the forelimbs, which
were used instead for steering and stabilizing. Conversely, hind-limbs
in basilosaurids became strongly reduced with no involvement in
locomotion but display strong osteosclerosis in the femora. Our study
confirms that Remingtonocetidae and Protocetidae were almost
exclusively aquatic in locomotion for the taxa sampled, which probably
were shallow water suspended swimmers. Basilosaurids display osseous
specializations similar to those of modern cetaceans and are
considered more active open-sea swimmers. This study highlights the
strong need for homologous sections in comparative microanatomical
studies, and the importance of combining information from several
bones of the same taxon for improved functional interpretation.