New fossil sea turtle trackway morphotypes from the Pleistocene of South Africa highlight role of ichnology in turtle paleobiology
Martin G. Lockley, Hayley C. Cawthra, Jan C. De Vynck, Charles W. Helm, Richard T. McCrea, Ronel Nel
Summarized by Sophia Gutierrez, who is majoring in geology at the University of South Florida. She is currently a senior and plans to pursue some experience in the field before continuing to further her education with a graduate program majoring in sedimentary geology. When she’s not studying geology, she enjoys walking in nature and listening to music.
What data were used? Three coastal sites, showing never before seen fossilized sea turtle hatchling track marks were discovered on the south coast of South Africa originating from the late Pleistocene (~100,000 years ago). Two species of sea turtles were ultimately identified from their trace fossils at the sites as loggerhead (Caretta caretta) and leatherback (Dermochelyis coriacea) sea turtles. Some track marks showed multiple “footprints” coming out of a centralized area that could possibly represent the ancient nesting location.
Methods: This study used two dimensional (2D) and three dimensional (3D) photographed images of the trackways in track sites 1 and 2, not far from each other, showing the sea turtle hatchling trails that were preserved in the large slab of rock. Tracing diagrams were then drawn to illustrate detailed trackway patterns of the two hatchling species. Site 3 fossils were found on a portable boulder. The boulder was photographed and taken to be preserved and studied at a separate location.
Results: After the fossil traces had been properly examined, it was concluded that the sets of tracks found in site 1, on a large slab of rock that had fallen off the side of a cliff, were those from hatchling loggerhead sea turtles. Site 2 is positioned close to site 1 in a narrow cliff pass filled with fallen rock slabs and boulders. One of those slabs holds fossilized hatchling leatherback sea turtle tracks. The final track site was found about 100 km (~62 miles) away from the other two, on a portable boulder that showed newborn loggerhead tracks appearing suddenly from the ground, likely representing the moment the hatchlings traveled out of their nest in the sand. Site 3 is especially interesting because it shows two sets of track marks emerging from the sand but paddling in opposite directions, 180° from each other. Site 2 also shows indications of the hatchling emerging suddenly from the sand but in this case, it’s unclear to say if this suggests a nest or shows signs of inadequate trace fossil preservation. These fossilized loggerhead and leatherback hatchling sea turtle tracks have never been documented before and are fairly distinct from other marine and terrestrial turtle track sites recorded. Due to these reasons, the paleontologists who discovered these tracks may assign these ichnotaxa (a taxonomic group based on the trace fossils of an organism) new, original names: Australochelichnus agulhasii for the hatchling loggerhead tracks and Marinerichnus latus for the hatchling leatherback tracks. These ichnotaxa provide us with copious information about ancient sea turtle breeding ranges, given that both loggerheads and leatherbacks nest in very specific conditions. This gives an insight on how the climate conditions in the late Pleistocene may have been in this area where temperatures were calculated to be about 25° to 35° C with a water level up to 6 meters (20 ft) higher than they are today. The presence of the Marinerichnus latus tracks made by leatherbacks (site 2) so close to the Australochelichnus agulhasii tracks made by loggerheads (site 1) in the area they were found in suggests that the breeding areas for the leatherback were twice as extensive in the Pleistocene than they are today.
Why is this study important? This study allows scientists to combine what is known about modern sea turtle hatchings with what was discovered from fossil tracks from about 100,000 years or so ago. The location that the trace fossils are preserved in can reveal to scientists the environmental and climatic conditions of the late Pleistocene, which could broaden the understanding of naturally changing climates of the past and the rapidly increasing climate change in the present day. Along with that, due to the close range of the leatherback and loggerhead turtle fossil nest sites in sites 1 and 2, this study demonstrated that modern breeding ranges of both leatherback and loggerheads have been halved in the past 100,000 years.
The big picture: This paper studied the ichnological evidence (the trace fossils made by an organism at the time it is alive) made by two distinct species of hatchling sea turtles. Scientists related the breeding ranges of modern sea turtles to the ancient breeding ranges they observed at the fossil sites and suggested the climate of the late Pleistocene ranged from 25° to 35° C and had sea levels up to 6 meters (20 ft). As of 2021, loggerhead sea turtles are endangered and leatherback sea turtles are vulnerable species. This study could potentially help future populations of these species due to the new knowledge of ancient breeding ranges relative to the specific nesting conditions.
Citation: Lockley, M. G., Cawthra, H. C., De Vynck, J. C., Helm, C. W., McCrea, R. T., Nel, R. 2019. . Quaternary Research 1–15. https:// doi.org/10.1017/qua.2019.40