SICB 2019: Crocodile Rocking (Different Snout Lengths) Is Something Shocking

Zachary Morris presents his research entitled “The role of craniofacial growth zones in shaping crocodylian snouts.”

Evolution of the crocodylian skull is driven by developmental changes. While embryos share many similarities, at some point within development they diverge into unique ecomorphs. Prior studies in American alligators (Alligator mississippiensis) showed that the snout is a source for early facial proliferation, wherein later stages lack growth plates that resemble post-hatching anole lizards. Snouts of crocodilians are described as moderate, blunt, and slender. Snout structure is related to dietary and ecological differences; for example, long, slender snouted crocodiles such as the Tomistoma (Tomistoma schlegelii) feed largely on fish. Heterochrony of slender-snouted crocodiles is responsible for continued elongation of the embryonic snout.

Zachary Morris of Harvard University following his great talk at SICB 2019.

To further understand forces behind these developmental changes, Zachary Morris, a current PhD student of Dr. Stephanie Pierce at Harvard University, began by asking the questions “When does this difference (in skulls) become apparent?” and “What are the cellular dynamics of snout elongation?” He presented his work on Saturday at the 2019 SICB conference and shared his experimental approach to answering such questions. To do this, he traveled to Imperial College London to work with Dr. Arkhat Abzhanov.

In answering his initial question, Morris incubated A. mississippiensis, dwarf crocodile (Osteolaemus tetraspis), and T. schlegelii embryos to developmental stages 14 and 17. These time-points were selected based on Ferguson staging. He could then examine snout/head length ratios to determine when skull differences, such as elongation, became apparent. At stage 14, no differences among blunt, moderate, or slender-snouted crocodilians were visible. However, at stage 17, he found that slender elongation began. To investigate his latter question, he followed the same procedure, but was able to calculate cell proliferation rate for tissue regions utilizing injection of EdU in ovo. He found that while elongation patterns such as facial shape were apparent during stage 17, early cell proliferation rate at the same stage was not apparent. His findings suggest that blunt species types (such as O. tetraspis) have decreased cell proliferation along the tip of the snout in comparison to slender species types (such as T. schlegelii).

In the future, Morris hopes to investigate whether cell proliferation at the tip of the snout is maintained in Tomistoma and if there is greater proliferation at lateral edges of facial structures in broader-snouted crocodilians. While these modern-day dinosaurs derived species-specific morphological differences from actual dinosaurs, the evolutionary processes by which these occurred remains the target of Zachary Morris’ interesting and exciting research.