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.

All research was conducted on living individuals of American alligator (Alligator mississippiensis) and was approved by the LSUHSC Institutional Animal Care and Use Committee (IACUC #6341). Data was collected with an ultrasound machine at  Louisiana State University Health Sciences Center (LSUHSC) in New Orleans and at the Rockefeller Wildlife Refuge in Grand Chenier, Louisiana. Two individuals were part of the collection of Dr. Raul Diaz (while he was faculty at Southeastern Louisiana University) and five individuals were collected by Dr. Ruth Elsey and staff (Rockefeller Wildlife Refuge). Individuals were relaxed and not sedated and videos were taken during both 1) relaxed breathing in normal atmospheric conditions and 2) breathing a CO_2, balance N₂ gas to induce deep breaths.

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