American alligator ultrasound and microCT

Unlike the majority of sauropsids, which breathe primarily through costal
and abdominal muscle contractions, extant crocodilians have evolved the
hepatic piston pump, a unique ventilatory mechanism powered by the
diaphragmaticus muscle. This muscle attaches to the pelvis and the liver,
pulling the liver and viscera caudally during inspiration and lowering
pressure in the pleuroperitoneal cavity, helping to inflate the lungs. It
has been hypothesized that the hepatic piston pump is coupled to
modifications in the axial skeleton that result in a smooth interior
thoracic ceiling, facilitating craniocaudal translation of the viscera
during ventilation. We assess this using ultrasound video to visualize the
hepatic-piston apparatus in breathing sub-adult and adult individuals (n =
7) of the American alligator (Alligator mississippiensis). The magnitude
of displacement of the liver was calculated by measuring the distance
between the most cranial and most caudal positions of the liver during a
breath cycle. These data demonstrate that pleural tissues are freely
sliding cranially and caudally along the thoracic ceiling. Based on our
results, we suggest that features associated with the presence or absence
of a smooth thoracolumbar ceiling are viable osteological correlates for
reconstructing the evolution of the hepatic piston pump in extinct
crocodyliform archosaurs.