A gravid fossil turtle from the Early Cretaceous reveals a different egg development strategy to that of extant marine turtles

Abstract

Extant sea turtles develop and lay pliable (flexible) eggs; however, it is unknown whether they inherited this reproductive strategy from their closer fossil relatives or if it represents an evolutionary novelty. Here, we describe the first undisputable gravid marine fossil turtle ever found, from the early Cretaceous of Colombia, belonging to Desmatochelys padillai Cadena & Parham, which constitutes a representative of the Protostegidae. Using thin sectioning of one of the eggs, as well as scanning electron microscopy coupled with elemental characterization, cathodoluminescence, and computer tomography, we established that Desmatochelys padillai produced rigid eggs similar to those associated with some extant and fossil freshwater and terrestrial turtles. At least 48 spherical eggs were preserved inside this gravid turtle. We suggest that the development of rigid eggs in the extinct marine turtle Desmatochelys padillai resulted as an adaptation for egg‐embryo requirements dictated by the physical attributes of the nesting site.

The diverse terminology of reptile eggshell microstructure and its effect on phylogenetic comparative analyses

“…This shell unit morphology, unique to turtle eggs, has allowed paleontologists to assign many fossil eggshells to Testudines (Hirsch, 1983; Lawver & Jackson, 2014; Mikhailov, 1997b; Moreno‐Azanza et al, 2021; Schleich & Kästle, 1988; but see Ke et al, 2021; Xu et al, 2022, for fossil turtle eggs with an unusual shell microstructure), with an earliest record in the Late Jurassic (Kohring, 1990) and empirical evidence of aragonitic shells in the Late Cretaceous (Choi, Kim, et al, 2022; Ferguson & Tapanila, 2022; Xu et al, 2022). The relative shape and size of turtle egg shell units compared with the thickness of the underlying membrana testacea varies significantly (Cadena et al, 2019; Lawver & Jackson, 2014), from thick and columnar in most terrestrial turtles to reduced and as thin as the membrane in some freshwater turtles (Hirsch, 1983; Packard et al, 1979; Packard & Packard, 1980, 1988; Schleich & Kästle, 1988).…”

A large and unusually thick-shelled turtle egg with embryonic remains from the Upper Cretaceous of China

“…Such identification is also possible when eggs are preserved inside a gravid adult, although few such fossil occurrences are known for turtles (i.e. Adocus, Desmatochelys and Basilemys) [12][13][14][15][16]. This scarcity of fossil eggs of turtles intimately associated with identifiable skeletal remains has limited our understanding of the evolution of their early development, reproduction and egg characteristics.…”

Exceptionally preserved ‘skin’ in an Early Cretaceous fish from Colombia

“…Despite the abundant recent discoveries of fossil vertebrates from the Cretaceous of Colombia ( Cadena, 2015 ; Cadena & Parham, 2015 ; Cadena et al, 2019 ; Carballido et al, 2015 ; Maxwell et al, 2019 ; Noé & Gómez-Pérez, 2020 ; Páramo-Fonseca et al, 2016 ; Vernygora et al, 2018 ), the exceptional preservation of soft tissue or their potential original components is still rarely reported for most of them, with the exception of the recently described gravid marine turtle from the Early Cretaceous of Villa de Leyva ( Cadena et al, 2019 ). Here we report a caudal fragment of an aspidorhynchid fossil fish recovered from the lower segment of the Paja Formation from Zapatoca, Santander, Colombia ( Figs.…”