
A study of fossil specimens reveals the eating and reproductive habits of sea turtles
23.05.2024
Published in PLOS ONE, the study entitled First evidence of marine turtle gastroliths in a fossil specimen: Paleobiological implication in comparison to modern analogues, was led by the University of Padua and the University of Modena and Reggio Emilia, in collaboration with the universities of Yale, Swansea and the Natural History Museum of Bamberg. The work offers surprising results that shed new light on the eating and reproductive habits of sea turtles, both at current and fossil level.
The international team, composed of Giovanni Serafini (UniMore), Caleb Gordon (Yale University), Luca Giusberti and Jacopo Amalfitano (UniPd), Oliver Wings (Bamberg SNM), Nicole Esteban and Holly Stokes (Swansea University) investigated the mysterious phenomenon of deliberately ingesting of sediment by animals known as geophagy. The behavior, commonly observed in all major vertebrate groups, had previously been poorly reported in sea turtles, and never reported in fossil specimens. The study combines paleontology, marine biology, and physiology, to highlight the phenomenon in this group of animals and tracing its evolution in the remote past.
Starting with an analysis of the fossil remains of a large sea turtle dating back to the Upper Cretaceous (about 90 million years ago) found near S. Anna d'Alfaedo (Verona), extracted from a quarry known as 'lastame' quarry in the early 2000s. The quarry is a stratigraphic unit famous in Veneto for its ornamental use and which yields marine fossils from the Upper Cretaceous, including gigantic sharks, mosasaurs (aquatic reptiles like monitor lizards and snakes), and turtles. During the description of the specimen preserved at the Municipal Theater of S. Anna d'Alfaedo, the team of paleontologists noticed the presence of ten pebbles grouped inside the turtle's skeleton.
The team hypothesized that the stones, mostly the same size, could be gastroliths ingested by the turtle and stored in its alimentary tract. The presence of gastrolith stones is common in many marine reptiles such as plesiosaurs but has never been observed in turtles, leading scientists to hypothesize that the group did not resort to this practice.
Confirmed by morphometric analyses, the nature of stones found inside the turtle offers dimensional values and rounded shapes perfectly consistent with other gastroliths described in the past. The fossil specimen of Sant'Anna d'Alfaedo is the first one in the world to be found with such gastroliths. Scanning Electron Microscopic (SEM) revealed that some of the ten stones found hold calcium carbonate, while others are made of silica.
Biologist and Paleontologist, Prof Giovanni Serafini, of the University of Modena and Reggio Emilia explains the findings, “The currently accepted theory is that the females ingest rocky substrates to supplement the calcium used in the formation of eggshells. We cannot exclude that the specimen from S. Anna d'Alfaedo ingested such stones accidentally or as an aid to digestion, however, the behavior of current sea turtles strongly suggests a link between geophagy and egg production.”
Such observations could imply that the fossil specimen was a female in the reproductive phase and that she had ingested the stones to replenish the calcium lost for egg production. It is no coincidence that the stones are of a continental origin, most likely from a beach or an estuary, visited by sea turtles during nesting.
The discovery has important implications for the evolution of this behavior with extinct protostegids and modern sea turtles of the Chelonioidea family, suggesting that geophagy linked to reproductive physiology is traceable to the Late Cretaceous period. The specificity of this behavior justifies the rarity of gastroliths in fossil sea turtles since ingested stones can only be found in sexually mature females in the breeding season and close to nesting.