PRELIMINARY ANALYSIS OF SACRAL RIB MORPHOLOGIES IN PHYTOSAURS

Phytosaurs are commonly seen as the Triassic equivalents of crocodylians: semi-aquatic ambush predators. However, crocodylians have not always occupied the same niche. Some extinct crocodylians walked with erect posture on land, as did many Triassic pseudosuchians. We sought to test the hypothesis that phytosaurs may also have varied in their niche by comparing the orientation of their sacral ribs with those of crocodylians and extinct archosauriforms. Vertically-oriented sacral ribs can support a ventrally-oriented acetabulum consistent with an erect posture, while laterally-oriented ribs support a laterally-facing acetabulum consistent with a sprawling posture.

We made four measurements on both sacral rib 1 (phytosaurs, n = 7; non-phytosaurs, n = 9) and sacral rib 2 (phytosaurs, n = 10; non-phytosaurs, n = 3): 1) the orientation of the rib relative to the sagittal plane, 2) the orientation of the rib relative to the articulation of the sacral rib with the vertebra, 3) the dip of the distal surface of the rib relative to the horizontal plane, and 4) the dip of the distal surface of the rib relative to the ventral margin of the rib. Quantitative measurements were made on photographs using ImageJ and principal component analysis (PCA) was performed using R version 4.3.2.

For both sacral ribs 1 and 2, phytosaurs clustered close together with non-phytosaur archosauriforms on the fringes of the cluster. The PCA plot for sacral rib 1 showed both Alligator and Batrachotomus as distant outliers, whereas the other non-phytosaurs were closer to the phytosaur taxa. Variation in Alligator is due to principal component 1 (PC1), which correlates mostly with measurements 3 and 4, while the variation in Batrachotomus is due to PC2, which correlates mostly with measurements 1 and 4. Although we had less non-phytosaur examples for sacral rib 2, a similar pattern can be observed where Alligator, as well as a crocodylid, are separated from the phytosaur cluster with variation due to both PC1 and PC2.

Our preliminary results suggest that the range of variation in sacral rib orientation and the dip of the distal surface of the ribs in phytosaurs is close to the variation between semi-aquatic and terrestrial archosauriforms. Future work evaluating a larger set of non-phytosaur specimens would aid in testing this hypothesis and determining if certain morphologies correspond to certain ecological niches.